CHAPTER 1_ PROCEDURES FOR REPORTING AERODROME PAVEMENT STRENGTH
1.1 Procedure for pavements
meant for heavy aircraft (ACN-PCN method)
1.1.1 Introduction
1.1.1.1
Ecar139, specifies that the bearing strength of a pavement intended for
aircraft of mans greater than
(a) the concept of the method; and
(b) how the ACNs of an
aircraft are determined.
1.1.2 Concept of the ACN-PCN method
1.1.2.1 Ecar139 defines ACN and PCN as
follows;
_ACN. A number expressing the relative effect
of an aircraft on a pavement for a specified standard subgrade strength.
_PCN. A number expressing the bearing
strength of a pavement for unrestricted operations.
At the outset, it needs to be noted that the
ACN-PCN method is meant only for publication of pavement strength data in the
Aeronautical Information Publications (AIPs). It is not intended for-design or
evaluation of pavements, nor does it contemplate the use of a specific method
by the airport authority either for the design or evaluation of pavements. In fact, the ACN-PCN method does
permit States to use any design/evaluation method of their choice. To this end, the method shifts the
emphasis from evaluation of pavements to evaluation of load rating of aircraft
(ACN) and includes a standard procedure for evaluation of the load rating of
aircraft. The strength of a pavement is reported under the method in terms of
the load rating of the aircraft which the pavement
can accept on an unrestricted basis. The
airport authority can use any method of his
choice to determine the load rating of his
pavement. If, in the absence of
technical evaluation, he chooses to go on the basis of the using aircraft
experience, then he would compute the ACN of the most critical aircraft using
one of the procedures described below, convert this figure into an equivalent
PCN and publish it in the AIP the load rating of his pavement. The PCN so
reported would indicate that an aircraft
with an ACN equal to or less than that figure
can operate on the pavement subject to any limitation on the tire pressure.
as
1.1.2.2 The
ACN-PCN method contemplates the reporting of pavement strengths on a continuous
scale. The lower end of the scale is zero and there is no upper end.
Additionally, the same scale is used to measure the load ratings of both
aircraft and pavements.
1.1.2.3 To
facilitate the use of the method, aircraft manufacturers will publish, in the
documents detailing the characteristics of their aircraft, ACNs computed at two
different masses; maximum apron mass, and a representative operating mass
empty, both on rigid
and flexible pavements and for the four
standard subgrade strength categories. Nevertheless, for the sake of
convenience Ecar139, Attachment B and Appendix 5 hereto include a table showing
the ACNs of a number of aircraft. It
is to be noted that the mass used in the
ACN calculation is a "static" mass
and that no allowance is made for an increase in loading through dynamic
effects.
1.1.2.4 The
ACN-PCN method also envisages the reporting of the following information in
respect of each pavement:
(a) pavement type;
(b) subgrade category;
(c) maximum tire pressure
allowable; and d) pavement evaluation method used.
The above data are primarily intended to
enable aircraft operators to determine the permissible aircraft types and
operating masses, and the aircraft manufacturers to ensure compatibility
between airport pavements and aircraft under development. There is, however, no
need to report the actual subgrade strength or the maximum tire pressure
allowable. Consequently, the subgrade strengths and tire pressures normally
encountered have been grouped into categories as indicated in 1.1.3.2 below. It
would be sufficient if the airport authority identifies the categories
appropriate to his pavement. (See also the
examples included under Ecar139, 2.5.6.)
1.1.3 How ACNs are determined
1.1.3.1 The
flow chart, below, briefly explains how the ACNs of aircraft are computed under
the ACN-PCN method.
1_1_3.2 standard
values used in the method and description of the various terms
(a) Sub grade category. In
the ACN-PCN method eight standard subgrade values i.e.,four rigid pavement k
values and four flexible pavement CBR values) are used, rather than a
continuous scale of subgrade strengths. The grouping of subgrades with a
standard value at the mid-range of each group is considered to be entirely adequate
for reporting. The subgrade strength categories are identified as high, medium,
low and ultra low and assigned the following numerical values;
Subgrade strength category
High strength; characterized
by k* -- 150 MN/m3 and representing all k values above 120 MN/m3 for rigid
pavements, and by CUR 15 and representing all GBR values above 13 for flexible
pavements.
Medium strength; characterized
by k = 80 MN/m3 and representing a range in k of 60 to 120 MN/m3 for rigid
pavements, and by GBR 10 and representing a range in CBR of 8 to 13 for
flexible pavements.
Low strength; characterized by
k = 40 MN/m3 and representing a range in k of 25 to 60 MN/m3 for rigid
pavements, and by CBR 6 and representing a range in CBR of 4 to B for flexible
pavements.
Ultra low strength;
characterized by k = 20 MN/m3 and representing all k values below 25 MN/m3 for
rigid pavements, and by CBR = 3 and representing all CBR values below 4 for
flexible pavements.
(b) Concrete working
stress for rigid pavements. For rigid pavements, a standard stress for
reporting purposes is stipulated (o = 2.75 MPa) only as a means of ensuring uniform reporting. The
working stress to be used for the design and/or evaluation of pavements has no
relationship to the standard stress for reporting.
(c) Tire pressure.
The results of pavement research and re-evaluation of old test results reaffirm
that except for unusual pavement Construction (i.e., flexible pavements with a
thin asphaltic concrete cover or weak upper layers), tire pressure effects are
secondary to load and wheel spacing, and may therefore be categorized in four
groups for reporting purposes as: high, medium, low and very low and assigned
the following numerical values:
High - No pressure limit
Medium - Pressure limited to
1.50 MPa Low - Pressure limited to 1.00 MPa Very low - Pressure limited to 0.50
Mpa Values determined using a
(d) Mathematically
derived single wheel load: The concept of a mathematically derived
single wheel load has been employed in the ACN-PCN method as a means to define
the landing gear/pavement interaction without specifying pavement thickness as
an ACN parameter. This is done by equating the thickness given by the
mathematical model for an aircraft landing gear to the thickness for a single
wheel at a standard tire pressure of 1.25 MPa. The single wheel load so
obtained is then used without further reference to thickness; this is so
because the essential significance is attached to the fact of having equal
thicknesses, implying "same applied stress to the pavement", rather
than the magnitude of the thickness. The foregoing is in accord with the
objective of the ACN-PCN method to evaluate the relative loading effect of an
aircraft on a pavement.
(e) Aircraft
classification number (ACN). The ACN of an aircraft is numerically
defined as two times the derived single wheel load, where the derived single
wheel load is expressed in thousands of kilograms.
As
noted previously, the single wheel tire pressure is standardized at 1.25 MPa.
Additionally, the derived single wheel load is a function of the subgrade
strength. The aircraft classification number (ACN) is defined only for the four
subgrade categories (i.e., high, medium, low, and ultra low strength). The
"two" (z) factor in the numerical definition of the ACN is used to
achieve a suitable ACN vs. gross mass scale so that whole number ACNs may be
used with reasonable accuracy.
(f) Because an aircraft
operates at various mass and centre of gravity conditions the folinwing
conventions have been used in ACN computations (see Figure 1-1).
(1) the maximum ACN of an aircraft is calculated at the mass and c.g
that produces the highest main gear loading on the pavement, usually the
maximum ramp mass and corresponding aft c.g. The aircraft tires are considered
as inflated to the manufacturers recommendation for the condition;
(2) relative aircraft ACN
charts and tables show the ACN as a function of aircraft gross mass with the
aircraft c.g. at a constant value corresponding to the maximum ACN value (i.e.,
usually, the aft c.g. for max ramp mass) and at the max ramp mass tire
pressure; and
(3) specific condition ACN
values are those ACN values that are adjusted for the effects of tire pressure
and/or c.g. location, at a specified gross mass for the aircraft.
1.1.3.3 Abbreviations
(a) Aircraft parameters
MRGM - Maximum ramp gross mass in kilograms
CBR - California Bearing Ratio in per cent
Tire Pressures
Ps - Tire pressure for derived single wheel
load - 1.25 MPa
Pq - Tire pressure for aircraft at maximum
ramp mass condition
1.1.3.4 Mathematical
models. Two mathematical models are used in the ACN-PCN method: the
Westergaard solution for a loaded elastic plate on a Winkler foundation
(interior load case) for rigid pavements, and the Boussinesq solution for
stresses and displacements in a homogeneous isotropic elastic half-space under
surface loading for flexible pavements. The use of these two, widely used,
models permits the maximum correlation to world-wide pavement design
methodologies, with a minimum need for pavement parameter values (i.e., only
approximate subgrade k, or CSR values are required).
1.1.3.5 Computer
programmes. The two computer programmes developed using, these
mathematical models are reproduced in Appendix 2. The programme for evaluating
aircraft on rigid pavements is based on the programme developed by Mr. R.G.
Packard * of Portland Cement Association, Illinois, USA and that for evaluating
aircraft OR flexible pavements is based on the US Army Engineer Waterways
Experiment Station Instruction Report S-77-1, entitled "Procedures for
Development of CBR Design Curves". It may, however, be noted that the
aircraft classification tables included in Ecar139, Attachment R and in
appendix 5 of this Manual completely eliminate the need to use these programmes
in respect of most or Clip aircraft currently in use.
1.1.3.6 Graphical
procedures. Aircraft for which pavement thickness requirement charts
have been published by the manufacturers can also be evaluated using the
graphical procedures described below_
1.1.3.7 Rigid
pavements. This procedure uses the conversion chart shown in Figure 1-4
and the pavement thickness requirement charts published) by the aircraft
manufacturers. The Portland Cement Association computer programme referred to
in 1.1.3.5 was used in developing Figure 1-4. This figure relates the derived
single wheel load at a constant tire pressure of 1.25 MPa to a reference
pavement thickness. It takes into account the four standard subgrade k values
detailed in 1.1.3.2 a) above, and a standard concrete stress of 2.75 MPa. The
figure also includes an ACN scale which permits the ACN to be raad directly.
The following steps are used to determine the ACN of an aircraft:
(a) using, the pavement
requirement chart published by the manufacturer obtain the reference thickness
for the given aircraft mass, k value of the subgrade, and the standard concrete
stress for reporting, i.e. 2.75 MPa;
(b) using the above reference
thickness and Figure 1-4, obtain a derived single wheel load for the selected
subgrade; and
Refer to document entitled "Design of
Concrete Airport Pavement" by R.G. Packard, Portland Cement Association,
(c) the aircraft classification number, at
the selected mass and subgrade k value, is two times the derived single wheel
load in
1.1.3.8 Flexible
pavements. This procedure uses the conversion chart shown in Figure 1-5
and the pavement thickness requirement charts published by the aircraft
manufacturers based on the United States Army Engineers CBR procedure. The former chart has been developed using the
following expression:
Where t = reference thickness in cm.
DSWL- a single wheel load with 1.25 MPa tire
pressure
Ps = I.25 Mra
GBR = standard subgrade (Note that the chart
uses four standard values 3, 6, 10 and 15)
C1 = 0.5695 C2
= 32.035
The reason for using the latter charts is to
obtain the equivalency between the "group of landing gear wheels
effect" to a derived single wheel load by means of Boussinesq Deflection
Factors. The following steps are used to
determine the AGN of an aircraft:
(a) Using the pavement
requirement chart published by the manufacturer determine the reference
thickness for the given aircraft mass, subgrade category, and 10 000 coverages;
(b) Enter Figure 1-5 with the
reference thickness determined in step a) and the CBR corresponding to the
subgrade category and read the derived single wheel load; and
(c) The ACN at the selected
mass and subgrade category is two times the derived single wheel load in
1.1.3.9
Tire pressure adjustment to ACN. Aircraft normally have their tires inflated
to the pressure corresponding to the maximum gross mass and maintain this
pressure regardless of the variations in take-off masses. There are times,
however, when operations at reduced masses and reduced tire pressures are
productive and reduced ACNs need to be calculated. To do this for rigid
pavements, a chart has been prepared by the use of the FCA computer programme
PDILB and is given in Figure 1-6. The example included in the chart itself
explains how the chart is used.
CHAPTER 2. - GUIDANCE ON OVERLOAD OPERATIONS
2.1 Criteria suggested in Ecar139,
Attachment B
2.1.1 Overloading
of pavements can result either from loads too large or from a Substantially
increased application rate or both. Loads larger than the defined (design or
evaluation) load shorten the design life whilst smaller loads extend it. With
the exception of massive overloading, pavements in their structural behaviour
are not subject to a particular limiting load above which they suddenly or
catastrophically fail. Rehaviour is
such that a pavement can sustain a definable load for an expected number of
repetitions during its design life. As a result, occasional minor overloading
is acceptable, when expedient, with only limited loss in pavement life
expectancy and
relatively small acceleration of pavement
deterioration. For those operations in
which magnitude of overload and/or the frequency of use do not justify a
detailed analysis the following criteria are suggested:
a) for flexible pavements occasional
movements by aircraft with ACN not exceeding 10 per cent above the reported PCN
should not adversely affect the pavement;
b) for rigid or composite pavements, in which
a rigid pavement layer provides a primary element of the structure, occasional
movements by aircraft with ACN not exceeding 5 per cent above the reported PCN
should not adversely affect the pavement;
c) if the pavement structure is unknown the 5
per cent limitation should apply; and
d) the annual number of overload movements
should not exceed approximately 5 per cent of the total annual aircraft
movements.
2.1.2 Such
overload movements should not normally be permitted on pavements exhibiting
signs of distress or failure. Furthermore, overloading should be avoided during
any periods of thaw following frost penetration or when the strength of the
pavement or its subgrade could be weakened by water. Where overload operations
are conducted, the appropriate authority should review the relevant pavement
condition regularly and should also review the criteria for overload operations
periodically since excessive repetition of overloads can cause severe
shortening of pavement life or require major rehabilitation of pavement.
2.2 State practices
2.2.1 Canadian practice
2.2.1.1 The
technical assessment of a proposed overload operation is based on the
"overload ratio" concept. The overload ratio is a measure of the load
imposed by the aircraft relative to the nominal design strength of the
pavement. For flexible pavements, the overload ratio imposed by an aircraft is
determined by calculating the subgrade bearing strength required for the
existing thickness of pavement, using the design equation given in 4.1.2.2.
This calculated subgrade bearing strength is then divided by the actual
subgrade bearing strength to form the overload ratio. For rigid pavements, the
overload ratio is defined as the flexural stress imposed in the slab by the
aircraft divided by the design flexural stress of 2.75 MPs.
2.2.1.2 On
the basis of these overload ratios, aircraft operations are classified as
follows=
Overload Ratio Operational Classification
less than 1.25 unrestricted
1.25 to 1.50
limited
1.50 to 2.00
marginal
greater than 2.00
emergency use only
2.2.1.3 The
approval of operations classified as limited or marginal involves the risk of
an accelerated rate of pavement deterioration and shortened service life. This
risk increases with increasing value of
overload ratio and frequency of operations. The decision to approve such
operations therefore depends on the willingness of the airport authority to
fund pavement rehabilitation measures earlier than may otherwise necessary.
Normal practice at airports operated by Transport
2.2.1.4 Similar
considerations apply to permitting operations by aircraft with tire pressures
higher than restrictions reported. Provided the overload ratio is less than
1.50, aircraft are normally permitted to operate with tire pressures one range
higher than the tire pressure range for which
the pavement was designed. These ranges are indicated in 4.1.2.6.
2.2.2 French practice
2.2.2.1 The
information published on the basis of one or the other method described in
4.2.8 does not permit a complete reflection of the operating condition of the
pavement. The following procedure should therefore be used to assess the
suitability of the pavement for the intended aircraft. Reference is made to the
flexible pavement or rigid pavement requirement graph for the subject aircraft
in Appendix 3. These graphs and the pavement data enable the exact authorized
load for the particular undercarriage to be determined. In the event that the
aircraft is not shown in Appendix 3, characteristics that are closest to the
subject aircraft will be selected. If the resulting load is higher than the
expected aircraft load, the allowable stresses of the pavement will not be
exceeded, and the aircraft can use the pavement without adverse effects. Should
the load established by means of the graph be less than the expected aircraft
load, there will be an effective overload and acceptance of the aircraft will
require special permission.
2.2.2.2 Concessions.
It should be noted that the risk to the aircraft itself when landing on a
runway without adequate bearing strength is minimal, unless the load it imposes
is considerably more than the bearing strength of the runway. Generally
speaking, the acceptance of an excessively heavy aircraft will undoubtedly
cause damage to the pavement, without detriment to the aircraft itself. The
user will in no case be held responsible for deteriorations of this type.
However, in no case should an aircraft load exceed by more than 50 per cent the
allowable load for the subject aircraft, in other words an actual overload
co-efficient P/Po higher than 1.5 for all pavements, except aprons for which
these values are limited to 20 per cent and 1.2,
respectively. (For runways, this rule does
not apply to emergency landings.) The decision to grant yr withhold the
concession to operate on weak runways can be arrived at as follows:
(a) the total equivalent
traffic supported by the pavement is calculated in accordance with the same principle
expressed in the optimized design method described in 4.2.6; however, this is
reduced to a daily traffic expressed in terms of movements per day; and
(b) if Lhe total equivalent
traffic exceeds ten movements daily, refusal of the concession would normally
be justified, unless more rapid wear of the runway is acceptable. Such a
decision might be acceptable on economic grounds with the intention of
increasing air traffic Without having to reinforce pavements, at least for the
time being
2.2.2.3 on
the other hand, it is recommended to limit the number of movements involving an
aircraft for which a concession has been granted and to undertake follow-up
action with regard to the pavement in accordance with the information provided
in the table below:
Remark: Instead of considering daily traffic, it would be more stringent to
consider cumulative traffic to take into account the actual magnitude of past
traffic. This could be done where it is justified for the sake of precision of
the study.
Example. A flexible runway has the following characteristics:
Total equivalent thickness e =
CBR of the subgrade CBR = 8
PCN 57/F/C/W/T
It receives four daily movements of Airbus
A-300 B2 with a load of 142 t (AGN = 55) and tour movements daily of B-727 with
a load of 78 t (ACN = 49). Under what conditions can it be used by the
B-747-200 with a load of 365 t?
Solution
Step 1.
Calculation of the ACN of the B-747-200
The ACN exceeds the published PCN: a
concession is therefore required for
the aircraft.
Step 2. Calculation of equivalent traffic:
For x = 1 mvt/d, the equivalent traffic is
9.3 mvt/d and less than 10 mvt/d: the
B-747-200 may require a concession.
For x greater than 1 mvt/d, the equivalent
traffic would exceed 10 mvt/d and the B-747-200 could not be accepted.
2.2.3
2.2.3.1 Individual
aerodrome authorities in the
(a) a 10 per cent difference
in ACN over PCN involves an increase in pavement working stresses which are
generally considered acceptable provided the following conditions are
satisfied:
(1) the pavement is more than
twelve months old;
(2) the pavement is not
already showing signs of loading distress; and
(3) overload operations do not
exceed 5 per cent of the annual departures and are spread throughout the year.
(b) overload operations
representing a difference in ACN over PCN of from 10 per cent to 25 per cent
justify regular inspections of the pavement by a competent person in addition
to satisfying the above criteria. There should be an immediate curtailment of
such overload operations as soon as distress becomes evident and the higher
loading should not be reimposed until appropriate pavement strengthening work
has been completed;
(c) overload operations
representing a difference in ACN over PCN of from 25 per cent to 50 per cent
may be undertaken under special circumstances. They call for scrutiny of
available pavement construction records as test data by a qualified pavement
engineer and a thorough inspection by a pavement engineer before and on
completion of the movement to assess any signs of pavement distress; and
(d) overload operations 1n
excess of an ACN over PCN of 50 per cent should only be undertaken in an emergency.
CHAPTER 3.- EVALUATION OF PAVEMENTS
3.1 General
3.1.1 The
purpose of this chapter is to present guidance on the evaluation of pavements
to those responsible for evaluating and reporting pavement bearing strength.
Recognizing that responsible individuals may range from experienced pavement
engineers to airfield managers not enjoying the direct staff support of
pavement behaviour experts, information will be included which attempts to
serve the various levels of need.
3.2 Elements of pavement evaluation
3.2.1 The
behaviour of any pavement depends upon the native materials of the site, which
after levelling and preparation is called the subgrade, its structure
including all layers up through the
surfacing, and the mass and frequency of using aircraft. Each of these three
elements must be considered when evaluating a pavement.
3.2.2 The
subgrade. The subgrade is the layer of material immediately below the
pavement structure which is prepared during
construction to support the loads transmitted by the pavement. It is prepared
by stripping vegetation, levelling or bringing to planned grade by cut and fill
operations, and compacting to the needed density. Strength of the subgrade is a
significant element and this must be characterized for evaluation or design of
a pavement facility or for each section of facility evaluated or designed
separately. Soil strength and therefore subgrade
strength is very dependent on soil moisture
and must be evaluated for the condition it is expected to attain in situ
beneath the pavement structure. Except in cases with high water tables, unusual
drainage, or extremely porous or cracked pavement conditions soil moisture will
tend to stabilize under wide pavements to something above 90 per cent of full
saturation. Seasonal variation (excepting frost penetration of susceptible
materials) is normally small to none and higher soil moisture conditions are
possible even in quite arid areas. Because materials can vary widely in type
the subgrade strength established for a particular pavement may fall anywhere
within the range indicated by the four subgrade strength categories used in the
ACN-PCN method. See Chapter 1 of this Manual and Ecar139, Chapter 2.
3.2.3 The
pavement structure. The terms "rigid" and "flexible" have
come into use for identification of the two principal types of pavements. The
terms attempt to characterize the response of each type to loading. The primary
element of a rigid pavement is a layer or slab of
to wide areas of the subgrade. The strength of the pavement depends on the
thickness and strength of the PCC and any underlying layers above the subgrade.
The pavement must be adequate to distribute surface loads so that the pressure
on the subgrade does not exceed its evaluated strength. A flexible pavement
consists of a series of layers increasing in strength from the subgrade to the
surface layer. A series such as select material, lower sub base, sub-base, base
and wearing course is commonly used. However,
the lower layers may not be present in a particular pavement. The pavements
meant for heavy aircraft usually have a bituminous bound wearing course. A
flexible pavement yields more under
surface loading merely accomplishing a widening of the loaded area and
consequent reduction of pressure layer by layer. At each level from the surface
to subgrade, the layers must have strength sufficient to tolerate the pressures
at their level. The pavement thus depends on its thickness over the subgrade
for reduction of the surface pressure to a value which the subgrade can accept.
A flexible pavement must also have thickness of structure above each layer to
reduce the pressure to a level acceptable by the layer. In addition, the wearing
course must be sufficient in strength to accept without distress tire pressures
of using aircraft.
3.2.4 Aircraft
loading. The aircraft mass is transmitted to the pavement through the
undercarriage of the aircraft. The number of wheels, their spacing, tire
pressure and size determine the distribution of aircraft load to the pavement.
In general, the pavement must be strong enough to support the loads applied by
the individual wheels, not only at the surface and the subgrade but also at
intermediate levels. For the closely spaced wheels of dual and dual-tandem legs
and even for adjacent legs of aircraft with complex undercarriages the effects
of distributed loads from adjacent wheels overlap at the subgrade (and
intermediate) level. In such
cases, the effective pressures are those combined from two or more wheels and
must be attenuated sufficiently by the pavement structure. Since the
distribution of load by a pavement structure is over a much narrower area on a
high strength subgrade than on a low strength subgrade, the combining effects
of adjacent wheels is much less for
pavements on high strength than on low
strength subgrades. This is the
reason why the relative effects of two aircraft types are not the same for
pavements of equivalent design strength, and this is the basis for reporting
pavement bearing strength by subgrade strength category. Within a subgrade
strength category the relative effects of two aircraft types on pavements can
be uniquely stated with good accuracy.
3.2.5 Load
repetitions and composition of traffic. It is not sufficient to consider the
magnitude of loading alone. There is a fatigue or repetitions of load factor
which should also be considered. Thus magnitude and repetitions must be treated
together, and a pavement which is designed to support one magnitude of load at
a defined number of repetitions can support a larger load at fewer repetitions
and a smaller load for a greater number of repetitions. It is thus possible to
establish the effect of one aircraft mass in terms of equivalent repetitions of
another aircraft mass (and type). Application of this concept permits the
determination of a single (selected) magnitude of load and repetitions level to
represent the effect of the mixture of aircraft using a pavement.
3.2.6 Pavement
condition survey. A particularly important adjunct to or part of evaluation is
a careful condition survey. The pavement should be closely examined for
evidences of deterioration, movement, or change of any kind. Any observable
pavement change provides information on effects of traffic or the environment
on the pavement. Observable effects of traffic along with an assessment of the
magnitude and composition of that traffic can provide an excellent basis for
defining the bearing capacity of a pavement.
3.3 Elements of the ACN-PCN method
3.3.1 Pavement
classification number. The pavement classification number (PCN) is an index
rating (1 /500th) of the mass which an evaluation shows can be borne by the
pavement when applied by a standard (1.25 MPa tire pressure) single-wheel. The
PCN rating established for a pavement indicates that the pavement is capable of
supporting aircraft having an ACN (aircraft classification number) of equal or
lower magnitude. The ACN for comparison to the PCN must be the aircraft ACN established
for the particular pavement type and subgrade category of the rated pavement as
well as for the particular aircraft mass and characteristics.
3.3.2 Pavement
t e. For purposes of reporting pavement strength, pavements must be classified
as either rigid or flexible. A rigid pavement is that employing a Portland
cement concrete (PCC) slab whether plain, reinforced, or prestressed and with
or without intermediate layers between the slab and subgrade. A flexible
pavement is that consisting of a series of layers increasing in strength from
the subgrade to the wearing surface. Composite pavements resulting from a PCC
overlay on a flexible pavement or an asphaltic concrete overlay on a rigid
pavement or those incorporating chemically (cement) stabilized layers of
particularly good integrity require care in classification. If the
"rigid" element remains the predominant structural element of the
pavement and is not severely distressed by closely spaced cracking the pavement
should be classified as rigid. Otherwise the flexible classification should
apply. Where classification remains doubtful, designation as flexible pavement
will generally be conservative. Unpaved surfaces (compacted earth, gravel,
laterite, coral, etc.) should be classified as flexible for reporting.
Similarly, pavements built with bricks, or blocks should be classified as
flexible. Large pre-cast slabs which require crane handling for placement can
be classified as rigid when used in pavements. Pavements covered with landing
mat and membrane surfaced pavements should be classified as flexible.
3.3.3 Subgrade
category. Since the effectiveness of aircraft undercarriages using
multiple-wheels is greater on pavements founded on strong subgrades compared to
those on weak subgrades, the problem of reporting bearing strength is
complicated. To simplify the reporting and permit the use of index values for
pavement and aircraft classification numbers (PCN and ACN) the ACN-PCN method
uses four subgrade strength categories. These are termed: high, medium, low and
ultra low with prescribed ranges for the categories. It follows that for a
reportd evaluation (PCN) to be useful the subgrade category to which the
subgrade of the reported pavement belongs must be established and reported.
Normally subgrade strength will have been evaluated in connexion with original
design of a pavement or later rehabilitation or strengthening. Where this
information is'not available the subgrade strength should be determined as part
of pavement evaluation. Subgrade strength evaluation should be based on testing
wherever possible. Where evaluation based on testing is not feasible a
representative subgrade strength category must be selected based on soil
characteristics, soil classification, local experience, or judgement. Commonly
one subgrade category may be appropriate for an aerodrome. However, where
pavement facilities are scattered over a large area and soil conditions differ
from location to location several categories may apply and should be assessed
and so reported. The subgrade strength evaluated must be that in situ beneath
the pavement. The subgrade beneath an aerodrome pavement will normally reach
and retain a fairly constant moisture and strength despite seasonal variations.
However, in the case of severely cracked surfacing, porous paving, high ground
water, or poor local drainage, the subgrade strength can reduce substantially
during wet periods. Gravel and compact soil surfaces will be especially subject
to moisture change. And in areas of seasonal frost, a lower reduced subgrade
strength can be expected during the thaw period where frost susceptible
materials are involved.
3.3.4 Tire
pressure category. Directly at the surface the tire contact pressure is the
most critical element of loading with little relation to other aspects of
pavement strength. This is the reason for reporting permissible tire pressure
in terms of tire pressure categories. Except for rare cases of spalling joints
and unusual surface deficiencies, rigid pavements do not require tire pressure
restrictions. However, pavements categorized as rigid which have overlays of
flexible or bituminous construction must be treated as flexible pavements for
reporting permissible tire pressure. Flexible pavements which are classified in
the highest tire pressure category must be of very good quality and integrity,
while those classified in the lowest category need only be capable of accepting
casual highway traffic. While tests of bituminous mixes and extracted cores for
quality of the bituminous surfacing will be most helpful in selecting the tire
pressure category, no specific relations have been developed between test
behaviour and acceptable tire pressure. It will usually be adequate, except
where limitations are obvious, to establish category limits only when
experience with high tire pressures indicates pavement distress.
3.3.5 Evaluation
method. Wherever possible reported pavement strength should be based on a
"technical evaluation". Commonly, evaluation is an inversion of a
design method. Design begins with the aircraft loading to be sustained and the
subgrade strength resulting from preparation of the local soil, then provides
the necessary thicknesses and quality of materials for the needed pavement
structure. Evaluation inverts this process. It begins with the existing
subgrade strength, finds thickness and quality of each component of the
pavement structure, and uses a design procedure pattern to determine the
aircraft loading which the pavement can support. Where available the design,
testing, and construction record data for the subgrade and components of the
pavement structure can often be used to make the evaluation. Or, test pits can
be opened to determine the thicknesses of layers, their strengths, and subgrade
strength for the purpose of evaluation. A technical evaluation also can be made
based on measurement of the response of pavement to load. Deflexion of a
pavement under static plate or tire load can be used to predict its behaviour.
Also there are various devices for applying dynamic loads to a pavement,
observing its response, and using this to predict its behaviour. When for
economic or other reasons a technical evaluation is not feasible, evaluation
can be based on experience with "using aircraft". A pavement
satisfactorily supporting aircraft using it can accept other aircraft if they
are no more demanding than the using aircraft. This can be the basis for an
evaluation.
3.3.6 Pavements
for light aircraft. Light aircraft are those having a mass of
3.4 Assessing the magnitude and composition
of traffic
3.4.1 General.
Pavement bearing strength evaluations should address not merely an allowable
load but a repetitions use level for that load. A pavement which can sustain
many repetitions of one load can sustain a larger load but for fewer
repetitions. Observable effects of traffic, even those involving careful
measurements in situ or on samples in controlled laboratory tests,
unfortunately do not (unless physical damage is apparent) permit a
determination of the portion of pavement's. In the case of evident physical
damage a pavement will already be in the last stages of its useful life.
3.5.3 Pavement
condition and behaviour. There must next be a careful examination of what
effect the traffic of using aircraft is having on the pavement. The condition
of the pavement in relation to any cracking, distortion or wear, and the
experience with needed maintenance are of first importance. Age must be
considered since overload effects on a new pavement may not yet be evident
while some accumulated indications of distress may normally be evident in a
very old pavement. In general, however, a pavement in good condition can be
considered to be satisfactorily carrying the using traffic, while indications
of advancing distress show the pavement is being overloaded. The condition
examination should take note of relative pavement behaviour in areas of intense
versus low usage such as in and out of wheel paths or most and least used
taxiways, zones subject to maximum braking, e.g., taxiway turn-off, etc. Note
should also be taken of behaviour of any known or observable weak or critical
areas such as low points of pavement grade, old stream crossings, pipe
crossings where initial compaction was poor, structurally weak sections, etc.
These will help to predict the rate of deterioration under extant traffic and
thereby indicate the degree of overloading or of underloading. The condition
examinations should also focus on any damage resulting from tire pressures of
using aircraft and the need for tire pressure limitations.
3.5.4 Reference
aircraft. Study of the types and masses of aircraft will indicate those which
must be of concern in establishing a reference aircraft and the condition
survey findings will indicate whether the load of the reference aircraft should
be less than that being applied or might be somewhat greater. Since load
distribution to the subgrade depends somewhat on pavement type and subgrade
strength, the particular reference aircraft and its mass cannot be selected
until those elements of the ACN-PCN method which are reported in addition to
the PCN have been established (see 3.3.2 and 3.3.3).
3.5.5 Determination
of the pavement type, subgrade strength and tire pressure categories. The
pavement type must be established as rigid or flexible. If the pavement
includes a Portland cement concrete slab as the primary structural element it
should be classified as rigid even though it may have a bituminous overlay
resurfacing (see 3.3.2). If the pavement includes no such load-distributing
slab it should be classified as flexible.
3.5.6 The
subgrade category must be determined as high, medium, low, or ultra low
strength. If CBR or plate bearing test data are available for the subgrade
these can be used directly to select the subgrade category. Such data, however,
must
represent in situ subgrade conditions. Similar data from any surrounding
structures on the same type of soil and in similar topography can also be used.
Soil strength data in almost any other form can be used to project an
equivalent CBR or modules of subgrade reaction k for use in selecting the
subgrade category. Information on subgrade soil strength may be obtainable from
local road or highways agencies or local agricultural agencies. A direct,
though somewhat crude or approximate, determination of subgrade strength can be
made from classification* of the subgrade material and reference to any of many
published correlations such as that shown in Figure 3-2. (Also see 3.3.3 and
3.2.2.)
* ASTPM D2487, D3282, and D2488.
3.5.7 The
tire pressure category must he determined as high, medium, low or very low.
Portland cement concrete surfacing and good to excellent quality bituminous
surfacing can sustain the tire pressures commonly encountered and should be
classified as high pressure category with no limit on pressure. Bituminous
surfacings of inferior quality and aggregate or earth surfacings will require
the limitation of lower categories (see 3.3.4). The applicable pressure
category should normally be selected based on experience with using aircraft. The
highest tire pressure being applied, other than rarely, by using aircraft,
without producing observable distress should be the basis for determining the
tire pressure category.
3.5.8 The
most significant element of the using aircraft evaluation is determination of
the critical aircraft and the equivalent pavement classification number (PCN)
for reporting purposes. Having determined the pavement type and the subgrade
category the next step would be the determination of the AGNs of aircraft using
the pavement. For this purpose, the aircraft classification table presented in
Appendix 5 or the relevant aircraft
characteristics document published by the manufacturer should be used.
Comparison of aircraft regularly using the pavements - at their operating
masses - with the above-mentioned table or the relevant aircraft
characteristics documents will permit determination of the most critical
aircraft using the pavement. If the using aircraft are satisfactorily being
sustained by the pavement and there are no known factors which indicate that
substantially heavier aircraft could be supported, the ACN of the most critical
aircraft should be reported as the PCN of the pavement. Thus any aircraft
having an ACN no higher than this PCN can use the pavement facility at a use
rate (as repetitions per month) no greater than that of presently supported
aircraft without shortening the use-life of the pavement.
3.5.9 In
arriving at the critical aircraft only aircraft using the pavement on a
continuing basis without unacceptable pavement distress should be considered.
The occasional use of the pavement by a more demanding aircraft is not
sufficient to ensure its continued support even if no pavement distress is
apparent.
3.5.10 As
Indicated, a PCN directly selected based on the evaluated critical aircraft
loading contemplates an aircraft use intensity in the future similar to that at
the time of evaluation. If a substantial increase in use (wheel load
repetitions) is expected, the PCN should be adjusted downward to accommodate
the increase. A basis for the adjustment, which relates load magnitude to load
repetitions, is presented in 3.4.
3.5.11 Pavements
for light aircraft. In evaluating pavements meant for light aircraft -
3.5.12 Because
the
3.6
Techniques and equipment for "technical" evaluation
3.6.1 Technical
evaluation is the process of defining or quantifying the bearing capacity of a
pavement through measurement and study of the characteristics of the pavement
and its behaviour under load. This can be done either by an inversion of the
design process, using design parameters and methods, but reversing the process
to determine allowable load from existing pavement characteristics, or by a
direct determination of response of the pavement to load by one of several
means.
3.6.2 Pavement
behaviour concepts for design and evaluation. Concepts of behaviour developed
into analytical means by which pavements can be designed to accommodate
specific site and aircraft traffic conditions are commonly referred to as
design methods. There are a variety of concepts and many specific design
methods. For example, several design and evaluation methods are explained in
some detail in Chapter 4 of this Manual.
3.6.2.1 The
early methods. The early methods for design and evaluation of flexible
pavements were experience based and theory extended. They made use of index type
tests to assess the strength of the subgrade and commonly to also assess the
strength or contributing strength of base and sub-base layers. These were tests
such as the CBR, plate bearing, and many others, especially in highway design.
These early methods, extensively developed, are still the methods in primary
use for aerodrome pavement design. The CBR method adopted for ACN
determinations as mentioned in Chapter 1 and Appendix 2 of this Manual is an
excellent example, and the French and Canadian methods described in Chapter 4
are further examples of CBR and plate loading methods, respectively.
3.6.2.2 Early
methods for design and evaluation of rigid pavements virtually all made use of
the Westergaard model (elastic plate on a Winkler foundation) but included
various extensions to treat fatigue, ratio of design stress to ultimate stress,
strengthening effects of subbase (or base) layers, etc. Westergaard developed
methods for two cases: loading at the centre of a pavement slab (width
unlimited) and loading at the edge of a slab (otherwise unlimited). While most
rigid pavement methods use the centre stab load condition, some use the edge
condition. These consider load transfer to the adjacent slab but means of
treating the transfer vary. Plate bearing tests are used to characterize
subgrade (or subgrade and sub-base) support which is an essential element of
these design methods. Here again the early methods, further developed, remain
the primary basis for aerodrome pavement design. The method adopted for ACN determination
(see Chapter 1 and Appendix 2) is an excellent example of these methods, and
several other examples are presented in Chapter 4.
3.6.2.3 The
newer - more fundamental - methods. Continuing efforts to base pavement design
on more fundamental principles has led to the development of methods using the
stress train response of materials and rational theoretical models. The
advances in computer technology have made these previously intractable methods
practical and led to computer-oriented developments not otherwise possible.
3.6.2.4 The
most popular theoretical model for the newer design methods is the elastic
layered system. Layers are of finite thickness and infinite extent laterally
except that the lowest layer (subgrade) is also of infinite extent downward.
Response of each layer is characterized by its modulus of elasticity and
Poisson's ratio. Values for these parameters are variously determined by
laboratory tests of several types, by field tests of several types with
correlations or calculated derivations, or merely by estimating values where
magnitudes are not critical. These methods permit the stresses, strains, and
deflexions from imposed loads to be computed. Multiple
loads can be treated by superimposition of single loads. Commonly, the magnitude
of strain at critical points (top of subgrade beneath load, bottom of surface
layer, etc) is correlated with intended pavement performance for use in design
or evaluation. While these methods have been applied mostly to flexible
pavements there have also been applications to design of rigid pavements.
3.6.2.5 While
the elastic layered models are currently popular it is recognized that the
stress-strain response of pavement materials is non-linear. The layering
permits variation o£ elastic modulus magnitude from layer tv layer, but not
laterally within each layer. There are developments which establish a stress
dependence of the modulus of elasticity and use this dependence in finite
element models of the pavement, through iterative computational means, to
establish the effective modulus - element by element in the grid - and thereby
produce a more satisfactory model. Here also strains
calculated for critical locations and
compared with Correlations to expected behaviour. Finite element models are
also being used to better model specific geometric aspects of rigid pavements
but these remain largely research applications.
3.6.2.6 Direct
load response methods. Theories applied earlier to pavement behaviour indicated
a proportionality between load and deflexion, thus implying that deflexion
should be an indicator of capacity of a pavement to support load. This also
implied that pavement deflexion determined for a particular applied load could
be adjusted proportionately to predict the deflexion which would result from
other loads. These were a basis for pavement evaluation. Field verification both from experience and research soon
showed strong trends relating pavement behaviour to load magnitude and
deflexion and led to the establishment of limiting deflexions for evaluation.
There have since been many controlled tests and much carefully analyzed field
experience which confirm a strong relation between pavement deflexion and the
expected load repetitions (to failure) life of the pavement subject to the load
which caused that deflexion. However, this relation, though strong, is not well
represented by a single line or curve. It is a somewhat broad band within which
many secondary factors appear to be impacting.
3.6.2.7 This
established strong relation has been and is being used as the basis for
pavement evaluation, but predominantly - until recently - applications have
been to flexible pavements. Methods based on plate tests have been most common
and the standard
examples (see Chapter 4). Deflexions under actual wheel loads (or
between the duals of two and four wheel gear) are the basis of some expedient
methods which closely parallel the plate methods. The Benkleman Beam methods, as well as other highway methods, are
applicable to evaluation of light aircraft pavements (see the Canadian practice
in Chapter 4).
3.6.2.8 There
are a number of reasons why dynamic pavement loading equipment
became popular. Static plate loads of wheel load magnitude are neither
transportable nor easily repositioned. Dynamic loading applies a pulse load
much more like the pulse induced by a passing wheel. Repeated dynamic loading better represents the repeated
loading of wheel traffic. But most important was the development of sensors
which could merely be positioned on the pavement or load plate and would
measure deflexion (vertical displacement). As a result, a variety of dynamic
load equipment has been developed. Initially there were devices for highway applications
and later heavier devices for aerodrome pavements. These range from light
devices inducing loads of less than
3.6.2.9 Essential
inputs to pavement design methods. The parameters which define behaviour of
elements (layers) of a particular pavement within the model upon which its
design is based vary from the CBR and other index type tests of the earlier
flexible pavement methods and plate load tests of Westergaard rigid pavement
and some flexible pavement methods to the stress-strain, modulus values
employed in the newer more fundamental methods.
3.6.2.10 CBR
tests for determining the strengths of subgrades and of other unbound pavement
layers Eor use in design or evaluation should be as described in the particular
method employed (French, United States/FAA, other), but generally will be as
covered in AST"1 D1883, "Bearing Ratio of Laboratory Compacted Soils
for Laboratory Test Determin-ations". Commonly, field in-place CBR tests
are preferable to laboratory tests whenever possible, and such tests should be
conducted in accordance with the following guidance (from United States
Military Standard 621A).
3.6.2.11 Field in-place CBR tests conditions:
(a)
These tests are used for design under any one of. the following
(1) when the in-place density
and water content are such that the degree of saturation (percentage of voids
filled with water) is 80 per cent or greater;
(2) when the material is
coarse-grained and cohesionless so that it is not affected by changes in water
content; or
(3) when construction was
completed several years before. In the last-named case, the water content does
not actually become constant but appears to fluctuate within rather narrow
ranges, and the field in-place test is considered a satisfactory indicator of
the load-carrying capacity. The time required for the water content to become
stabilized cannot be stated definitely, but the minimum time is approximately
three years.
(b) Penetration.
Level the surface to be tested, and remove all loose material. Then follow the
procedure described in ASTM D-1383.
(c) Number
of tests. Three in-place CBR tests should be performed at each elevation tested
in the base course and at the surface of the subgrade. However, if the results
of the three tests in any group do not show reasonable agreement, additional
tests should be made at the same location. A reasonable agreement between three
tests where the CBR is less than 10 permits a tolerance of 3; where the CBR is
from 10 to
(d) Moisture content and
density. After completion of the CBR test, a sample shall be obtained at the
point of penetration for moisturecontent determination, and 10 to
3.6.2.12 Plate
load tests for determination of the modulus of subgrade reaction (k) for Westergaard
analysis in evaluation or design should be made in accordance with procedures
of the method employed, or can be as presented in ASTM D1196,
"Non-Repetitive Static Plate Load Tests of Soils and Flexible Pavement
Components, for use in Evaluation and Design of Airport and Righway
Pavements" or in ASTM D1195, "Repetitive Static Plate Load Tests of
Soils and Flexible Pavement Components, for Use in Evaluation and Design of
Airport and Highway Pavements". The procedures also relate to flexible
pavement design, as indicated by ASTM standards' titles. The Canadian practice,
as described in Chapter 4, makes use of the ASTM method. The Canadian practice
also covers use of other plate sizes and the guidance for Canadian methods
described in Chapter 4 can be used for static or dynamic tests with
non-standard plate sizes for either determination of subgrade coefficient
values or for direct use in pavement evaluations.
3.6.2.13Conventional methods and values
pertaining to determination of modulus of elasticity, E, and Poisson's ratio,
p, are used in depicting structural behaviour of the concrete layer in
Westergaard analyses of rigid pavement. Commonly, U is taken to be 0.15. The
modulus, E, should be determined by test of the concrete and will normally be
in the range of 25 000 to 30 000 MPa.
3.6.2.14 Modulus
of elasticity and Poisson's ratio values are needed for each layer of an
elastic layered system, and these can be determined in a variety of ways.
Poisson's ratio is not a sensitive parameter and is commonly taken to be 0.3 to
0.33 for aggregate materials and 0.4 to 0.5 for fine grained or plastic
materials. Since means of determining modulus of elasticity vary and since the
stress-strain response of soil and aggregate materials is non-linear (not
proportional) the values found for a particular material, by the various means,
are not the same singular quantity which ideal theoretical considerations would
lead one to expect. Following are some of the ways in which modulus of
elasticity values can be determined for use in theoretical models (such as
elastic layered) of pavement behaviour.
(a) Hodulus of elasticity
values for subgrade materials particularly, but for other pavement layers as
well - excepting bituminous or cemented materials - can be determined from
correlations with index type strength tests. Most common has been correlation
with CBR where: E = 10 CBR MPa.
(b) Stress-strain response
(modulus) can be determined by direct test of prepared or field sampled
specimens, but these are nearly always unsatisfactory. Response is too greatly
affected by either preparation or sampling disturbance to he representative.
(c) It has been found that
prepared specimens, and in some cases specimens from field samples, can be
subjected to repeated loading to provide - after several to many load cycles -
a reasonably representative modulus or stress-strain response curve. Modulus of
elasticity so determined is referred to as resilient modulus and is currently
strongly favoured - in some form - for layered elastic analyses. Tests can be
conducted as triaxial tests, indirect tensile tests, even unconfined
compression tests, and there may be others. Loadings can be regular wave forms
(sinusoidal, etc) but are commonly of a selected load pulse shape with delays
between pulses to better represent passing wheels. Resilient modulus can be determined for bituminous
materials by some of these tests and other similar tests, but temperature is
most significant both for testing and application of the modulus for bituminous
layers. Moduli for the various pavement layers are taken from these type tests
and used directly in layered system analyses, but there are Frequently problems
or questions of validity.
(d) When dynamic plate load
testing is carried out on existing pavements it is possible to instrument to
measure the velocity of propagation of stress waves within the pavements. Means
have been developed for deducing the modulus of elasticity of each layer -
generally excepting the top layer or layers - of the pavement from these velocity
measurements. While moduli so determined are sometimes used directly in layered
analyses the determinations are for such small strains that values represent
tangent moduli for curved stress-strain relations while the moduli for higher
(working strain) stress levels should be lower. Determinations by this means
adjusted by judgement or some established analytical means are used.
(e) The subgrade modulus is
the most significant parameter aad some analyses use one of the above methods
to determine a modulus for the subgrade. and choose the moduli of other layers
either directly on a judgement basis or by some simple numerical process (such
as twice the underlying layer modulus or one-half the overlying layer modulus)
since precise values are not critical.
(f) By using selected or
simplistically derived moduli for all layers except the subgrade, it is
possible to compute a value for subgrade modulus using elastic layered analysis
and plate or wheel load deflexions. This is done for some analyses.
(g) There to; great interest
currently in using elastic layered theory and using field determined deflexions
from dynamic load pavement tests for off-set
positions means the moduli of the
computed. Such computed compute strains at performance. points beneath the centre
of load and at several from the load
centre. By iterative computer subgrade and several overlying layers can be
moduli are then used in the layered model to critical locations as predictors
of pavement
3.6.2.15Finite element methods permit
formulation of pavement models which not only can provide for layering but can
treat non-linear (curved) stress-strain response found for most pavement
materials. Here again there is a requirement for Poisson's ratios and moduli of
elasticity but these must now be determined for each pavement layer as a
function of the load or stress condition existing at any point in the model (on
any finite element). Moduli relations are established from laboratory tests and
most
rally, these are of the following form K1 Ok2
K3 o3k4
commonly by repeated triaxial load tests.but
there are variants. Generally, these are the following form but there are variants.
3.6.3 Evaluation
by inversion of design. To design a pavement one must select a design
method. Then determine the thicknesses and acceptable characteristics of
materials for each layer and the wearing surface taking into account the
subgrade upon which the pavement will rest and the magnitude and intensity of
traffic loading which must be supported. For evaluation, the process must be
inverted since the pavement is already in existence. Character of the subgrade
and thickness and character of each structural layer including the surfacing
must be established, from which the maximum allowable magnitude and frequency
of allowable aircraft loading can be determined by Part 3.- Pavements 3-37
using a chosen design method in reverse. It
is not necessary that the design method selected for evaluation be the method
by which the pavement was designed, but the esential parameters, which characterize
behaviour of the various materials (layers) must be those which the chosen
design method employed.
3.6.3.1 The
method and elements of design. The design method to be inverted for evaluation
must First be chosen. Next the elements of design inherent in the existing
pavement must be evaluated in accordance with the selected design method.
(a) Thickness of each layer
must be determined. This may be possible from construction records or may
require the drilling of core holes or opening of test pits to permit measuring
thickness.
(b) Subgrade strength and
character must be determined. Here also construction records may supply the
needed information either directly or by a translation of the information to
the form needed for the selected design method. Otherwise it will be necessary
to obtain the needed information from field studies. Reference to 3.6.2.9 to
3.6.2.14 will show the wide variety of ways in which subgrade behavior is
treated in the various design methods. Test pits may be necessary to permit
penetration or plate testing or sampling of subgrade material for laboratory
testing. Sampling or penetration testing in core holes may be possible. Dynamic
or static surface load-deflexion or wave propagation testing may be required.
Specific guidance must be gained from details of the design method chosen for
use in evaluation.
(c) The strength and character of layers
between the subgrade and surface must also be determined. Problems are much the
same as for the subgrade (see b) above) and guidance must come from the chosen
design method.
(d) Most procedures for the
design of rigid pavements require a modulus of elasticity and limiting flexural
stress for the concrete. It these are not available from construction records
they should be determined
by test on specimens extracted
from the pavement (see ASTM C 469 - modulus of elasticity and ASTM C683 -
flexural strength). For reinforced or prestressed concrete layers dependence
must be placed on details of the individual selected design method.
(e) Bituminous surfacing (or
overlay) layers must be characterized to suit the selected design method and to
permit determination of any tire pressure limitation which might apply.
Construction records may
provide the needed information
otherwise testing will be required. Pavement temperature data may be required
to help assess the stress-strain response or tire pressure effects on the
bituminous layer.
(f) Any special consideration
of frost effects by the selected design method or for the climate of the area
need to be treated and the impact upon the evaluation determined.
(g) The cumulative load
repetitions to which the pavement is subject is an imp
element of design and both past traffic
sustained and future traffic expected may be Factors in evaluation. See
From the chosen design method and established
quantities for the design elements, limiting load or mass can be established
for any aircraft expected to use the pavement.
3.6.4 Direct
or non-destructive evaluation. Direct evaluation involves loading a pavement,
measuring its response, usually in terms of deflexion under the load and
sometimes also at points offset from the load to show deflexion basin shape),
and inferring expected load support capacity from the measurements. Concepts
were discussed in 3.6.2.6, 3.6.2.7, and 3.6.2.8.
3.6.4.1 Static
methods. Static methods involve positioning plates or wheels, applying load,
and measuring deflexions_ Plate loads require a reaction against which to work
in applying load while wheels can be rolled into position and then away. The original LCN for flexible pavements,
developed by the
3.6.4.2 Expeditious
deflexion methods. Studies and observations by many researchers have shown a
strong general correlation between the deflexion of a pavement under a wheel
load and the number of traffic applications (repetitions) of that wheel load
which will result in severe deterioration (failure) of the pavement (see Figure
3-3). These provide the basis for a simple expeditious means of evaluating
pavement strength. References to several of these curves are listed below:
Transport and Road Research Laboratory Report
LR 375 (British);
Paper presented by Gschwendt and Poliacek at
the Third International Conference on Structural Design of Asphalt Pavements;
and
Paper presented by Joseph and Hall also at
the Third International Conference on Structural Design of Asphalt Pavements.
Figure 3-3
3.6.4.3 While
the pattern of these relations is quite strong, the scatter of specific points
is considerable. Thus either the conservatisms of a limiting curve or the low
confidence engendered by the broad scatter of points or some combination must
be accepted in using these relations for expeditious pavement evaluations. They
do provide a simple relatively inexpensive means of evaluation. The procedure
for such evaluation is as follows:
(a) measure deflexion under a
substantial wheel load in a selected critical pavement location. Single or
multiple wheel configurations can be used.
(1) position aircraft wheel in
critical area;
(2) mark points along pavement
for measurement as indicated in Figure 3-
(3) using "line of
sight" method, take ;;'A readings at each point;
(4) move aircraft away and
repeat rod readings
(5) plot difference in rod
readings as deflexions. See Figure 3-4 b); and
(6) connect points to gain an
estimate of maximum deflexion beneath tire.
(b) plot load versus maximum
deflexion as illustrated in Figure 3-4 c).
(c) combine the deflexion
versus failure repetitions curve with the above curve to provide an evaluation
of pavement bearing strength for the gear used to determine deflexion.
(1) determine the repetitions
of load (or equivalent repetitions as explained in 3.4) which it is intended
must use the pavement before failure;
(2) from a correlation of the type shown in Figure 3-3 determine the
deflexion for the repetitions to failure; and
(3) from the established relation
of load to deflexion of the type shown in Figure 3-4 determine the pavement
bearing strength in terms of the magnitude of load allowable on the wheel used
for the deflexion measurements.
(d) use the procedure
described in Chapter 1 to find how the evaluated pavement bearing strength
relates to the FCN. Aircraft with ACN no greater than this PCN can use the
pavement without overloading .it. See Appendix 5 for ACN versus mass
information.
Figure 3-4
3.6.4.4 beneath a
complete pattern of load versus deflexion
long reference beam can be used instead of
optical survey methods.
suitable access aperture the deflexion
directly beneath the centre
measured. Results can be treated on the same
lines as those for a single wheel load.
A similar procedure can be followed using a
jack and loading plate working jacking point of an aircraft wing or some
equally suitable reaction load. The can
be determined and dial gauges mounted on a With provision of a of the load can be
3.6.4.5 Methods
used for highway load deflexion measurements, such as the Benkleman Beam
methods, can be used to develop deflexion versus load patterns. Results are
treated as indicated in Figure 3-4 to extrapolate loads to those of aircraft
singlewheel loads, which with a relation as in Figure 3-3, permits evaluation
of pavement bearing strength for single-wheel loads. From this the limiting
aircraft mass on pavements for light aircraft can be determined directly and
reported in accordance with Chapter 1, 1.2. If unusually large loading plate or
tire pressures are involved it may be necessary to adjust between the single
load characteristics used in the determination of the type indicated in Figure
3-4 (3.6.4.3a)) and the reported limiting aircraft mass allowable or critical
vehicle loads being compared to the limiting mass. Such adjustments can follow
the procedures in Appendix 2 or a selected pavement design method. Limits on
pavements for heavier aircraft can be determined as indicated in 3.6.4.3d), It
should be noted that recent findings indicate extrapolation of load deflexion
relations (as in Figure 3-4 c)) from small load data taken on high strength
pavements do not give good results. Unfortunately, the limits of extrapolation
for good results are not established.
3.6.4.6 Dinamic
methods. These methods involve a dynamic loading device which is mounted for
travel on a vehicle or trailer and which is lowered, in position, onto the
pavement. Devices make use of counter rotating masses, hydraulically actuated
reciprocating masses, or falling weights (masses) to apply a series of pulses
either in steady state by the reciprocating or rotating masses or attenuating
by the falling mass. Most apply the load through a loading plate but some
smaller devices use rigid wheels or pads. All methods make use of inertial
instruments (sensors) which when placed on the pavement surface or on the
loading plate can measure vertical displacement (deflexion), The dynamic
loading is determined, usually by a load cell through which the load is passed
on to the load plate. Comparison of the load applied and displacements measured
provides load-deflexion relations for the pavement tested. Displacements are
always measured directly under the load but are also measured at several
additional points at specific distances from the centre of the load. Thus load deflexion relations are
determined not only for the load axis (point of maximum deflexion) but also at
offset points which indicate the curvature or shape (slope) of the deflexion
basin. The devices vary in size from some highly developed, highway oriented,
units which apply loadings of less than
3.6.4.7 It
is possible to measure the time for stress waves induced by the dynamic loading
to travel from one sensor to the next, and to Compute the velocity from this
time and distance between sensors. Some dynamic methods make use of these
velocity measurements to evaluate the strength or stress-strain response of the
subgrade and overlying pavement layers for use in various design methods. Shear wave velocity, v, is related
to Modulus of Elasticity, E, by the relation:
Where Poisson’s Ratio, u, can satisfactorily
be estimated (see 3.6.2.13 and 3.6.2.14),
and density, p, of the subgrade or pavement
layer (sub-base-base) can be determined by measurement or satisfactorily
estimated. Modulus values thus determined are used,
either directly or with modification, in
theoretical design models, or they are used with correlations to project
subgrade and other layer strengths in terms of CBR, subgrade coefficient k, and
similar strength index quantities. Sensors used in the velocity measurements
may need to be located at greater distances from the load than when used to
determine deflection basin shape. Also, the dynamic device must be capable of
frequency variation since the various pavement layers respond at preferred
frequen- and these must be found and dynamic load energy induced at the
preferred frequency for determination of each layer's velocity of wave energy
propagation.
'Where Poisson's
3.6.4.8 Application
of dynamic methods measurements. The central and offset positions deflexions
and stress-wave velocities variously determined by the variety of dynamic
equipment and methods in use are being applied for pavement evaluation in a
number of ways.
a) Direct correlations are made between the
load-deflexion in response of pavement to dynamic loading and pavement
behaviour. The correlations are developed from dynamic load testing of
pavements for which
behaviour can be established. The United
States FAA non-destructive evaluation methodology presented in 3.6.5 is an
excellent example.
b) Measurements from dynamic methods, either
directly or with extrapolation, can provide plate load information. This can
serve as input - with suitable plate size or other conversions - to methods
such as the LCN or Canadian procedures. Used directly on subgrades or on other
layers with established correlations subgrade coefficients can be determined
for Westergaard analyses.
c) Shape of the deflection basin established
from sensors placed at offsets from the load axis are used in some methods -
especially for highways - to reflect over-all stiffness, and thereby load
distributing character, of the pavement
structure. But direct use in establishing evaluation of load capacity has not found
success.
d) Measured deflection under dynamic load is
used to establish the effective modulus of elasticity of the subgrade in
theoretical paveucnt models. The elastic constants (modulus and Poisson's
ratio) for other layers are established by assumption or test and the subgrade
modulus calculated using the load, the deflection measured, and the pavement
model, commonly the elastic layered theory,
e) More recent developments involve the use
of the elastic layered computer programmes. With an appropriate load applied,
deflections are measured in the centre and at several offset locations. Then
iterative computation means are used to
establish elastic moduli for all layers of the pavement modelled.
f) Theoretical models with elastic constants
as in d) and e) above are used to calculate strain in flexure of the top layer
beneath the load or vertical strain at the top of subgrade beneath the load;
which locations are considered critical for flexible pavements. Stress or
strain in flexure of a rigid pavement slab can be similarly calculated. These
are compared to values of strain (or stress) from established correlations with
pavement performance. The literature provides many examples of these
correlations.
1) 1977 International Air Transportation
Conference, ASCE Proceedings - paper by Monismith.
2) The Design and Performance of Road
Pavements by D. Croney - Tranoport and Road Research
3) Fatigue of Compacted Bituminous Aggregate
Mixtures, ASTM - STP508.
4) Symposium on Nondestructive Test and
Evaluation of Airport Pavement - Nov 1975,
Army Engineer - WES paper by Nielsen and
Baird.
5) Other examples should be easily found in
the pavement literature
from the last 10 years.
Stress-wave velocity measurements are used to
establish pavement layer characteristics without sampling. Moduli of elasticity
of pavement layers are derived from these measurements and used directly in
theoretical models or adjusted to better represent moduli at larger strains and
used in the models. CBR values are derived from correlations between CBR and
derived elastic moduli, commonly from E=10 CRR in MPa. Modulus of subgrade
reaction, k, and other such strength values could be similarly derived.
3.6.4.9 Pavement
strength reporting. For reporting information on pavement bearing strength the
four elements specified in Ecar139 and the PCN mus[ be established.
a) Pavement type. The pavement will be
considered rigid (code-R) if its primary load distribution capability is
provided by a plain, reinforced, or pre-Stressed Portland cement concrete (PCC)
layer, and this layer is not so shattered that it can no longer perform as a
load distributing slab. A pavement which makes primary use of a thick and strongly
stabilized layer and which, as a result, is substantially thinner than an
equivalent flexible pavement using no stabilized layer (such as the LCF
structures at Newark) night also be considered rigid. All other pavements
should be reported as flexible (code-F). This includes aggregate or
earth-surfaced strips and expedient surfacings of military landing mat.
(b) gubgrade strength. The subgrade strength
category must be evaluated as high strength (A), medium strength (B), low
strength (C), or ultra low strength (o). Lf CBR or coefficient of subgrade
reaction are directly involved, selection of category can be made directly from
the prescribed Limits in Ecar139. Otherwise the category must be determined
from a correlation between the subgrade strength parameter used for evaluation
and CBR or subgrade coefficient, or it must be determined directly by
judgement. For subgrade strengths on the borderline between categories,
selection of the lower (weaker) strength category will generally be more
conservative in relation to protection of the pavement from overload.
(c) Tire pressure. The tire pressure category
must be evaluated as high W). medium (X). low (Y) or very low (Z), Where a
surfacing is PCC the high category is virtually always pertinent. High quality
bituminous surfacings or overlays should readily accept high category tire
pressures while the very low category need only be able to sustain normal truck
tire pressures. The medium and low categories fall below and above these two
limits respectively. Some design methods set minimum bituminous layer
thicknesses in relation to tire pressures (see the Canadian method in Chapter
4) and these may help in selecting the tire pressure category. some methods
prescribe tire pressure directly in relation to surfacing characteristics and
these can be directly applied for category selection. Otherwise selection must
depend on experience and judgement In relation to surfacing characteristics,
tire pressures of using aircraft. and condition surveys of pavements.
(d) Evaluation method. This will be a
technical evaluation reported ascode T.
(e) Reported PCN. The PCN to be reported can
be determined from the aircraft loads (masses) which the evaluation has
established as maximum allowable for the pavement. By using the evaluation load
for one of the heaviest type aircraft using the pavement and information shown
in Appendix 5, and interpolating as necessary, the
PCN can be found. This can be done for a
selected representative aircraft or for several aircraft for which evaluation
of allowable load has been made. All such determinations should yield the same
PCN value, or very nearly so. If there are large differences it would be well
to recheck both the translation from the evaluation load and the evaluation. If
differences are small an average or lower range value should be selected for
reporting. If needed information is not provided in Appendix 5 they can be
obtained from the aircraft manufacturer, ICAO, or by analysis using the
prescribed ACN-PCN methods (see Appendix 2).
3.6.4.10 Reporting strength of pavements
type, subgrade strength category, and type of aircraft pavements, so only the limiting
aircraft mass reported. The foregoing
methods for load and tire pressure limitation determinations apply to pavements
meant for light aircraft as well. Highway evaluation or design methods might
also be used. All the precautionary measures discussed in 3.5.7 are equally
applicable here. meant for light aircraft. The pavement evaluation are not
required for light and tire pressure need
be
1.6.5 United States Fudural Aviation
Administration non-destructive evaluation method
3.6.5.1 Introduction.
This report describes a procedure for the determination of the luad-carrying
capacity of airport pavement systems using non-destructive testing (NDT)
techniques. The equipment and procedures have been developed by the United
States Corps of Engineers in response to a need of the Federal Aviation
Administration (FAA) and United States Army for making rapid evaluations of
pavement systems with a minimum of interference to normal airport operations.
3.6.5.2 Little
research was conducted in the field of NDT until about the mid-1950s when Royal
Dutch Shell Laboratory researchers began a study of vibratory loading devices
to evaluate flexible pavements. Many other agencies have since investigated the
use of NDT techniques to evaluate pavements. The United States Army Engineer
Waterways Experiment Station (WES) conducted minimal research using various
types of vibratory equipment during the 1950s and 1960s. Much of the early WES
work emphasized attempts to measure the elastic properties of the various
layers of pavement materials using wave propagation measurements. The basic
approach involved use of these elastic constants along with multilayered theory
for computation of allowable aircraft loadings. In 1970, an improved vibratory
loading device was developed by the Army, and, in 1972, WES began a study for
the FAA to develop an NDT evaluation procedure. To meet the FAA time frame, the
primary effort has been directed toward developing a procedure based upon
measuring the dynamic stiffness modulus (DSM) of the pavement system and
relating this value to pavement performance data. Work is continuing on the
development of a methodology for measuring the elastic constants of the various
layers using NDT techniques; however, this method
has not yet been developed to an acceptable
level of confidence.
3.6.5.3 Applications.
The NDT evaluation procedure reported herein is applicable only to conventional
rigid and flexible pavement systems. A conventional rigid pavement consists of
a non-reinforced concrete surfacing layer on non-stabilized base and/or
subgrade materials. A conventional flexible pavement consists of a thin (
or lpgq) bituminous surfacing layer on
non-stabilized layers of base, sub-base. and subgrade materials. Work is
currently under way to extend the NDT procedure to other types of pavement
systems which incorporate such other variables as thick bituminous surfacings
and stabilized layers.
3.6.5.4 Equipment.
The evaluation procedure contained herein requires the determination of the
response of the pavement system to a specific steady state vibratory loading.
Inasmuch as the response of materials making up the pavement system to loading
is generally non-linear, the determination of
the pavement response of use in the evaluation procedure contained herein
requires a specific loading system. The loading device must exert a static load
of 16 kips**on the pavement and be capable of producing 0 to 15-kip peak
vibratory loads at a frequency of 15 Hz. The load is applied to the pavement
surface through a
in a tractor-trailer unit as shown in Figure
3-5.
* The material included in this section was
taken from the Federal Aviation Administration United States, Airport Pavement
Bulletin No. FAA-74-1 of September 1974.
** 1 kip =
Figure
3-5. Waterways Experiment Station non-destructive testing equipment
3.6.5.5 Data
collection. In the evaluation procedure, the response of the pavement system to
vibratory loading is expressed in terms of the DSM. Since the time required to
measure a DSM at each testing point is short (2 to 4 min), a large number of
DSM measurements can be made during the normal evaluation period. On runways
and primary and high-speed taxiways, DSM tests should be made at least every
be selected (as described below) for
computation of the allowable loading.
3.6.5.1 Ar
each test sire, the loading equipment ic positioned, and the dynamic force is
varied from 0 to 15 kips at 2-kip intervals at a constant frequency of 15 Hz,
The deflexion of the pavement surface, measured by the velocity Lrdnbducerb, i5
plotted versus the applied load as shown in Figure 3-6. The DSM (corrected as
described below) is the inverse of the slope of the deflexion versus load plot
(see Figure 3-6).
3.6.5.7 In
addition to the DSM measurement, it is necessary to know the pavement type
(rigid or flexible) and the thicknesses and material classifications of each
layer malting up the pavement section. These parameters can be determined from
the construction (as-built) drawings or by drilling small-diameter holes
through the pavement.
3.6.5.8 when
the evaluation is for flexible pavement, the temperature of the bituminous
material must be determined at the time of test. This can be determined by
directly measuring the temperatures with thermometers installed
3.6.5.9 Data
correction. The load-deflexion response of many pavements, particularly
flexible pavements, is non-linear at the lower force levels but becomes more
linear at the higher force levels (12 to 15 kips). In such cases, a correction
is applied to the load-deflexion curve so that the DSM is obtained from the
linear portion of the curve
(see Figure 3-6).
3.6.5.10 The
modulus of bituminous materials is highly dependent upon temperature, so an
adjustment in the measured DSM must be made if the temperature of the
bituminous material at the time of test is other than
3.6.5.11 The
DAM and load-carrying capacity of a pavement system can be significantly
changed by the freezing and thawing of the materials, especially when frost
penetrates a frost-susceptible layer of material. Correction factors to account
for these conditions have not been developed. Therefore, the evaluation should
be based on the normal temperature range, and, if a frost evaluation is
desired, the DSM should be determined during the frost melting period.
3.6.5.12 A
representative DSM value must be selected for each pavement group to be
evaluated. Although a section of pavement may supposedly be of the same type
and construction, it should be treated as more than one pavement group when the
DSM values measured in one section of the pavement are greatly different from
those in another section. The DSM value to be assigned to a pavement group for
evaluation purposes will be determined by subtracting one standard deviation
from the statistical mean.
3.6.5.13 Determination
of allowable aircraft load. After determination and correction of the measurement
of the DSM, the evaluation procedure depends upon the type of pavement, rigid
or flexible.
3.6.5.14 Rigid
pavement evaluation.
Step 1
The corrected DSM is used to enter Figure 3-9
and determine the allowable single-wheel load.
step 2
The radius of relative stiffness Q is
computed as
k = 24.2
Where
h = thickness of the concrete slab, in.
FF = foundation strength factor determined
from Figure 3-10 using the FAA subgrade soil group classification.
Step 3
Using Q, determine the load factor FL from
Figure 3-11, 3-12, 3-13 or 3-14, depending upon the gear configuration of the
aircraft for which the evaluation is being made.
Step 4
Multiply the allowable single-wheel load from
Step 1 by the FL value determined from Step 3 to obtain the gross aircraft
loading.
Step 5
Multiply the gross aircraft loading from Step
4 by the appropriate traffic factor from Table 3-1 to obtain the allowable
aircraft gross loading for critical areas for the pavement being evaluated. For
the Case of high-speed exit taxiways, the computed allowable gross load should
be increased by multiplying by a factor of 1.18.
Step 6
The allowable loading obtained from Step 5
assumes that the rigid pavement being evaluated is structurally sound and
functionally safe. The computed allowable loading should be reduced if one or
more of the following conditions exist at the time of the evaluation:
1) the allowable load should be reduced by 10
per cent if 25 per cent or more of the slabs show evidence of pumping;
2) the allowable load should be reduced by 25
per cent if 30 to 50 per cent of the slabs have structural cracking associated
with load (as opposed to shrinkage cracking, uncontrolled contraction cracking,
frost heave, swelling soil. etc.). If more than 5n per cent of the slabs show
load-induced cracking, the pavement should be considered failed;
3) the allowable loading should be reduced by
25 per cent if there is evidence of excessive joint distress such as continuous
spalling along longitudinal joints, which would denote loss of the
load-transfer mechanism.
3.6.5.15
Flexible pavement evaluation
Step 5
Evaluate the pavement for any aircraft
desired as follows:
1) select the aircraft or aircraft main gear
configuration for which the evaluation is being made and determine the tire
contact area A of one wheel of the main landing gear (see Table 3-2);
2) select the annual departure level for each
aircraft for which the evaluation is being made and determine the traffic
factor a for each aircraft from Table 3-1;
3) compute the factor Ft for each aircraft
for which the evaluation is being made for critical pavements as
6) multiply sp by the tire contact area A
from Table 3-2 to
equivalent single-wheel load (ESWL) of each
aircraft for which the evaluation is being made;
7) enter Figure 3-17, 3-18, or 3-19 with the
total pavement thickness t and determine the percentage of ESWL for the
controlling number of wheels of the aircraft for which the evaluation is being
made, i.e., if the aircraft has a dual-wheel assembly with a dual spacing of
3-17 or, if the evaluation is for the the
Boeing 747 STR curve in Figure 3-19;
8) the allowable gross aircraft load for the
pavement being evaluated
and for the traffic volume selected is then
obtained from
where
ESWL = determined by Substep 6)
Per cent ESWL = determined by Substep 7)
CHAPTER
4.- STATE PRACTICES FOR DESIGN AND EVALUATION OF PAVEMENTS
4.1 Canadian practice
4.1.1 Scope
4.1.1.1 This
section briefly outlines Transport
and evaluation of airport pavements. Further
details are available in Transport
in
4.1.2 Pavement design practices
Partial frost protection
4.1.2.1 Unless
otherwise justified by a life cycle cost analysis, the thickness of pavements
constructed on frost susceptible subgrades must not be less than the partial
frost protection requirement given in Figure 4-l. The frost susceptibility of
subgrades is assessed on the basis of subgrade soil gradation as shown in
Figure 4-2. The partial frost protection requirement given in Figure 4-1 is a
function of site freezing index. For a given winter period, this index in
oC-days is calculated as the sum of average daily temperatures in OC, for each
day over the freezing season, with below 0OC temperatures taken as positive and
above
Flexible pavement design curves
4.1.2.2 A
flexible pavement design curve for a given aircraft is a plot of pavement
thickness required to support the aircraft loading as a function of subgrade
bearing strength. The equation utilized to generate this design curve is:
Construction materials and specifications
4.1.2.9 The
pavement design practices outlined above, and the evaluation practices outlined
below, assume that the pavement is constructed to standard specifications
governing the quality of pavement construction materials and workmanship. Tf
standard specification requirements are not met, some adjustments based on
engineering judgement may be required to the design and evaluation practices
outlined. Tables 4-2, 4-3 and 4-4 provide some construction requirements
considered essential to normal design and evaluation practices.
4.1.3 Pavement
evaluation practices
Pavement thickness and equivalent granular
thickness
4.1.3.1 The
evaluation of pavement structures for aircraft loadings requires accurate
information on the thickness of layers within the structure, and the physical
properties of the materials in these layers. A bore hole survey is conducted to
determine this information when it is not available from existing construction
records. Equivalent granular thickness is a term applied to flexible pavement
structures, and is the basis for comparing pavements constructed with different
thicknesses of materials having different load distribution characteristics.
The equivalent granular thickness is computed through the use of the granular
equivalency factors for pavement construction materials listed in Table 4-5. The granular equivalency factor of a material
is the
granular base in centimetres considered
equivalent to one centimetre of the
on the basis of load distribution
characteristics. The values given in Table 4-5
conservative and actual granular equivalency
factors are normally higher than the
values listed. To determine the. equivalent
granular thickness of flexible pavement structure, the depth of each layer in
the structure is multiplied by the granular equivalency factor for the material
in the layer. The pavement equivalent granular
thickness is the
depth of material
are
sum of these converted layer thicknesses.
Table 4-2. Compaction requirements
Pavement bearing strength measurements
4.1.3.2 Transport
of bearing strength defined above.
Subgrade bearing strength
4.1.3.3 When
a bearing strength measurement has been made on the surface of flexible
pavement, and the equivalent granular thickness of the pavement structure is
known, the subgrade bearing strength at that location may be estimated from
Figure 4-4. Subgrade bearing strength varies from location to location throughout
a pavement area. In pavements subject to seasonal frost penetration, variation
also occurs with time of year, with the lowest values reached during the spring
thaw period. The subgrade bearing strength used to characterize a pavement area
is the lower quartile, spring reduced value. The lower quartile value of
several bearing strength measurements made throughout a pavement area is that
value for which 75 per cent of the measurements are greater in magnitude. It is
calculated as x - 0.675s, where x is the average of measurements made and s is
their standard deviation. For pavements subject to seasonal frost penetration,
spring thaw conditions are estimated by applying a reduction factor to lower
quartile subgrade bearing strengths derived from summer and fall measurements.
The reduction factor applied depends on gradation of the subgrade soil as shown
in Figure 4-2, and typical spring reduction factors based on soil
classification are listed in Table 4-6.
When the ground water table is within
when designing new pavement facilities at the
airport provided subgrade soil conditions are similar throughout the site. When
designing or evaluating pavements at an airport where strength measurements
have not been made, a value of subgrade bearing strength is selected from Table
4-6 on the basis of subgrade soil classification.
Rigid pavement bearing modulus
4.1.3.4 Bearing
modulus is based on the load in meganewtons which will produce a deflection of
or evaluation purposes. instead, bearing
modulus at the top of the base course is estimated from Figure 4-4 on the basis
of a uubgrade bearing strength determined as discussed in 4.1.3.3, and the
equivalent granular thickness of sub-base and base course provided between
subgrade and concrete slab.
Pavement strength reporting
4.1.3.5 The
two parameters governing strength of a flexible pavement are pavement
equivalent granular thickness (t) as discussed in 4.1.3.1 and subgrade bearing
strength (S) as discussed in 4.1.3.3. Pavement strength is reported in terms of
the Pavement Toad Rating (PLR) which is determined by plotting the point on
Figure 4-5 using the pavement
t and S values as co-ordinates. The load
rating reported for the pavement is the numerical value of the standard gear
loading whose design curve falls immediately above this point. The two parameters
governing the strength of a rigid pavement are bearing modulus (k) as discussed
in 4.1.3.4, and concrete slab thickness (h). These values are plotted on Figure
4-6 to determine the load rating of rigid pavements in a manner similar to that
for flexible pavements. A tire pressure restriction may be applied to flexible
pavements. The restriction applied is the tire pressure for which the pavement
asphalt and base course thicknesses satisfy design requirements, as given in
4.1.2.6. No tire pressure restrictions are applied for concrete pavements.
Aircraft having a load
rating (ALK) and tire pressure equal to or
less than the values reported for a pavement structure are authorized to
operate on the pavement without restriction. Proposed operations by an aircraft
with a load rating or tire pressure exceeding reported values must be referred
to the airport operating authority for an engineering and management
assessment.
Composite pavement structures
4.1.3.6 A
composite pavement structure is created when an existing pavement structure is
overlaid for strengthening or resurfacing purposes. Composite pavement
structures are evaluated as flexible or rigid pavements in accordance with the
procedures below.
a) Asphalt overlay on flexible pavement
A flexible pavement overlaid with additional
asphalt pavement lavers is evaluated as a flexible pavement having an
equivalent granular thickness determined as outlined in 4.1.3.1.
b) Asphalt overlay on rigid pavement
A rigid pavement receiving an asphalt overlay
less than
c) Concrete overlay on flexible pavement
A flexible pavement overlaid with a concrete
slab is evaluated as a rigid pavement with the flexible pavement structure
forming the base for the concrete slab.
d) Concrete overlay on rigid pavement
A rigid pavement overlaid by a concrete slab
is evaluated as a rigid pavement with the two slabs converted to an equivalent
slab thickness as given in Figure 4-7, except when a separation course greater
than
Surface condition evaluation
4.1.3.7 In
addition to pavement bearing strength evaluation and reporting, airport
pavements are subject to an evaluation of surface conditions yearly at
international airports and biennially at other airports. The surface condition
evaluation programme consists of a visually based structural conditions survey,
and quantitative measurements of roughness and friction levels on runway
surfaces.
4.1.3.8 Structural
condition surveys are conducted by an experienced pavements engineer or
technician who visually inspects the pavements and reports on the extent and
severity of observed pavement defects and distress features. On the basis of
traffic levels and observed defects and distress features, an estimate is also
provided for the year in which pavement rehabilitation should be programmed. A
typical structural condition survey report is shown in Figure 4-8.
4.1.3.9 Runway
roughness measurements are conducted with a Roadmeter, a device which records
vertical movements in an automobile as the vehicle is driven along the runway
at
4.1.3.10 Runway
surface friction measurements (normal wet state) are currently conducted with a
SAAB Surface Friction Tester. Measurements are conducted at a vehicle speed of
4.2 French practice
4.2.1 General
4.2.1.1 Definitions
a) Structure of pavement. A pavement normally
comprises the following from top to bottom:
- a "surface layer" consisting of a
"wearing course"' and possibly a "binder course";
- a "base";
- a "sub-base"; and
- possibly a lower sub-base or an improved
subgrade.
b) Types of structures.
- A "flexible structure" consists
only of courses of materials that have not been bound or treated with
hydrocarbon binders.
a "rigid structure" offers a
wearing course made up of a portland cement slab;
a "semi-rigid structure" comprises
a base treated with hydrocarbon binders; and
a "composite (or mixed) structure"
results from reinforcing a rigid structure with a flexible or semi-rigid
structure.
c) Pavement types. For the sake of
simplification a distinction is made hereinafter only between the two major
pavement types, referred to in general terms as follows:
"flexible pavements" include
flexible and semi-rigid structures, as well as certain types of composite
structures (e.g.,a formerly rigid, badly cracked pavement reinforced with
material treated with hydrocarbon binders); and
"rigid pavements" include rigid
structures and certain types of comp osite structures (e.g.,a rigid pavement
renewed by applying a wearing course treated with hydrocarbon binders).
d) Bearing strength. The "bearing
strength" or "bearing capacity" is the ability of a pavement to
accept the loads imposed by aircraft while maintaining its structural
integrity.
e) Pavement life. This is the period at the
end of which the bearing strength of the pavement becomes inadequate to bear,
without risk, the same traffic in the course of the following year, thus necessitating
a general reinforcement or a reduction in traffic. The "normal life"
of
a pavement is ten years and pavements are
generally designed for that period. However. in the circumstances described
later on in these guidelines, another value may be established for the life of
a pavement.
f ) Traffic
One "movement (actual)" is the
application to the pavement of a load by an actual undercarriage leg during one
manoeuvre (take-off,
landing. taxiing). The number o£ actual
movements is generally higher than the number of movements accounted for by the
operator (takeoffs and landings).
An "actual load p" is the load
actually applied by an aircraft undercarriage leg.
"Actual traffic" consists of
different movements of varying actual loads applied by actual undercarriage
legs of different categories.
The "normal design load p" is the
load taken into account in formulas or graphs for the purpose of designing the
pavement. It may be "weighted" or not, depending on the function of
the pavement involved.
- "
The "allowable load Po" of a
pavement is the load on an undercarriage leg (actual or fictitious) calculated
according to the design concept as being allowable at the rate of ten movements
per day over ten years.
An "equivalent movement" is the
application of a reference load by an undercarriage leg (actual or fictitious).
"Equivalent traffic" corresponds to
actual traffic reduced to a number of equivalent movements.
- The "potential" of a pavement on
a given date is represented by the number of equivalent movements which it can
accept during its residual life.
g) Types of design
"optimized design" (or optimized
design method): design which takes into account all aircraft types having a
significant effect on the pavement. This method is preferable if sufficiently
reliable and accurate traffic forecasts are available throughout the expected
life of the pavement.
"general design" (or general design
method): design in terms of a reference load which the pavement must support.
In practice, this method is mainly used at the level of preliminary studies or
in the absence of accurate data. The reference load is evaluated in terms of
the anticipated utilization of the aerodrome, the characteristics of aircraft
in service or at the planning stage, and the specific role of the pavement in
question.
4.2.2 Choice of the design load
4.2.2.1 Aircraft
characteristics affecting the design
a) Aircraft mass. There is a need to list for
each aircraft: - in the case of the general design method: take-off mass
- in the case of the optimized design method:
take-off mass, landing
Lad 55
Collection of data on the mass of the various
aircraft to be considered in a design is a difficult task bearing in mind:
- the variations in payload
- the uncertainty of forecasting traffic
composition (aircraft, stages) and developments in regard to aircraft fleets.
For the purpose of studying an optimized
design, one useful method consists of establishing mass histograms in respect
of each aircraft. Selecting a category width of 1/20th of the maximum mass
provides sufficient accuracy.
b) Undercarriage leg. Wheel assembly mounted
on one leg. The complete set of undercarriage legs constitutes the
undercarriage. A "typical undercarriage leg" which is representative
of each of the three most widely used categories of undercarriages (single
wheel, dual wheels, dual tandem wheels) is introduced. The characteristics of
the typical undercarriage legs are as follows:
undercarriage leg Track(cm) Base(cm) Tirepressure
Single wheel -- -- 0.6
MPa
Dual wheels 70
-- 0.9 MPa
Dual tandem wheels 75 1 40 1.2 MPa
c) Distribution of the mass over the
undercarriage legs
1) Static distribution. The over-all
distribution of the aircraft mass between the nose leg and the main
undercarriage legs is dependent upon the load distribution of the aircraft
(i.e.,the
position of the centre of gravity) and varies little. In
the
absence of data, one would assume that the distribution is
10 per
cent on the nose leg (maximum forward load distribution) 95
per
cent on the main undercarriage legs (maximum rearward load
distribution) for conventional
undercarriages.
2) Braking action. The effect of braking
action is not taken into account to designing pavements. It plays a role only
in specific studies (example: structures underneath the runway).
d) Loads used in the calculations. In the
case of the undercarriages of current aircraft, the distance between the legs
is such as to justify a separate study of the action of each undercarriage leg.
The main undercarriage leg generally causes the greatest stress. In some cases.
the secondary undercarriage leg may well be the most critical for the pavement
(examples: nose leg of 8-747, centre leg of DC-1030). The load is taken into
account in the calculations in the form of a load per undercarriage leg. The graphs
in respect of the main aircraft examined (Appendix 3) are produced in
accordance with this concept. Those cases where the secondary undercarriage leg
is likely to be more critical than the main undercarriagp leg are identified
and additional graphs provided.
4.2.2.2 Weighting
of load according to the function of the pavement. Each type of facility
(runways. taxiway, aprons, maintenance areas, etc.) must be designed separately
to take into account differing stress conditions. Although subjected to the same
loads, some pavements may experience different fatigue conditions. For example:
a) traffic is slow and concentrated on aprons
and, conversely, rare and dispersed on shoulders and stopways; and
b) consequences of dynamic effect. When an
aircraft rolls at high speed (such as the middle part of the runway at take-off
and the first
4.2.2.3 Loads
other than those produced by aircraft. Some areas (such as those in front of
airport buildings) are not accessible to the undercarriage legs. On the other
hand. aerodrome pavements do not only support aircraft. but also other vehicles
and machinery (e.g.,ground transportation vehicles - buses, trucks, baggage
towtrolleys, container carriers, fire fighting vehicles, aerobridges, etc.)
which sometimes
4.2.3 Designing flexible pavements
4.2.3.1 The
design of a flexible pavement involves two stages: a) Collection of data: -
traffic (loads, movements)
- characteristics of the natural soil.
b) Calculation of the thickness, which also
comprises two stages:
- the determination of an "equivalent
pavement thickness" e using either the general design or optimized design
methods.
- the selection of a pavement structure which
provides an equivalent thickness corresponding to or greater than the thickness
determined above.
4.2.3.2 Bearing
strength of the subgrade
a) General case! The bearing strength of the
subgrade is denoted by its California Bearing Ratio (CBR). The CBR value
adopted is the lowest one obtained during the test series in which the total
number of
samples is compacted to 95 per cent of
Modified Proctor Optimum Density after having been immersed in water for four
days.
b) Gravelly soils and pure sand: In the case
of gravelly soils and pure sand, the CBR measurement is meaningless and general
values will be adopted as shown in the following table:
c) Improved subgrade. Where the pavement
comprises an improved subgrade (considerable thickness of added materials of
average or non-homogeneous quality), this will be taken into account in the
calculations in
the following manner. Let it be assumed that
the bearing strengths of the untreated and improved subgrades are,
respectively, CBR1 and CBR2 and that h1 and h2, which will be calculated
according to the design method selected (general or optimized) correspond to
one of these CBRs. If h is the thickness of the improved subgrade, the required
thickness of the pavement above this subgrade, i.e.1"e can be calculated
by applying the formula:
providing e exceeds or is at least equal t0
h2. Should e be less than h2 then the thickness of the pavement is fixed at h2.
This also
applies to cases where the natural soil
comprises a substratum that covered by a relatively thin soil layer of better
bearing strength. This top layer may then be regarded as an improved subgrade
so that the above method can still be used.
4.2,3,3 Calculating the equivalent pavement
thickness
- Ceneral design - see 4.2.S
- Optimized design - see 4.2.6
4.2.3.4 Structure
of the pavement. A flexible pavement is normally made up of three different
courses of increasing quality from bottom to top: the sub-base, the base and the surface course. The concept of
equivalent thickness is introduced to take into account the different
mechanical qualities of each course. The equivalent thickness e of a course is
equal to its actual thickness er multiplied by a numerical coefficient c or
equivalence coefficient. The equivalent thickness of the pavement is equal to
the sum o£ the equivalent thicknesses of. its courses. The values shown in the
table below may be used as a reference in the case of new materials:
In a properly constituted pavement, the
equivalence coefficients of necessity increase From bottom to Lop.
4.Z.3.) Choice
of a structure. The choice of a structure must take into account
two general concepts:
a) Construction concepts which relate to the
nature of the materials to be used, the quality and formulation of components,
the minimum and maximum thicknesses involved, sound bonding of courses, etc.;
and
b) Mechanical concepts which define the
values of equivalence coefficients, proscribe or advise against the use of
certain materials in the different courses, indicate the thicknesses of the
treated materials needed for the normal mechanical behaviour of the pavement,
etc. These directives have the following effect on the different courses:
Surface course (wearing course and possibly
binder course). The surface course must consist of bituminous concrete. (some
directives. especially as regards formulation and compactness to be achieved at
the work site, differ considerably from those applicable to road pavements.)
Base and sub-base. The choice of materials
for the base and subbase is subject to the applications specified in the
following table:
Frequently. economic considerations make it
necessary to envisage the use of materials that have been treated with
hydraulic binders (coarse-aggregate concrete. slag based on sand-gravel mix,
sand-gravel fly-ash mix, etc,) in the base or sub-base. However, the magnitude
of the loads applied to aerodrome pavements creates much greater stresses than
those produced on road pavements. The risks and consequences, among others,
are:
- for the pavements: rapid signs of
deterioration (cracks in wearing course, crumbling, tearing, pumping up of
particles or reappearance of fines of laitance);
- for aircraft: ingestion by jet engines of
aggregate particles,
evenness; and
- for management: higher maintenance costs
(filling cracks).
Consequently, the use of materials treated
with hydraulic binders is proscribed for the base and not advised for the
sub-base. In the case of the latter, an actual thickness measuring at least
4.2.3.6 Thickness
of treated materials. An adequate thickness of treated materials is necessary
to ensure an acceptable behaviour of the upper pavement layers. Figure 4-12
shows, for guidance, the optimum equivalent thickness of treated materials with
respect to the total equivalent thickness of the pavement and the CBR of the
natural soil.
4.2.3.7 Influence
of climatic factors. In regions that are subject to significant seasonal
climatic variations, possible changes in the bearing strength of the soil shall
be taken into account. Despite the considerable influence which temperature has
on bituminous mix pavements, no correction for thickness will be made to account
for this parameter: the values indicated for the equivalence coefficients for
the coating mixes suggested previously represent a weighted average. It is
recommended that testing for frost-thaw be performed in accordance with the
information contained in 4.2.7.
4.2.4 Designing
rigid pavements
4.2.4.1 The
design of rigid pavements involves the following two stages:
a) Collection of data:
- traffic (loads, movements)
- characteristics of the subgrade and of the
hydraulic cement concrete; and
b) Calculation of the thickness of the
concrete slab (only the most general case of non-reinforced and non-prestressed
pavements is examined).
4.2.4.2 Evaluation
of the sub-base. A rigid pavement normally consists of two courses on top of
the natural soil, i.e.,a sub-base and hydraulic cement concrete slab. The
bearing strength of the natural soil is expressed in the form of its
"modulus of reaction" ko. This is corrected in accordance with the
equivalent thickness of the subbase. The modulus thus corrected (i.e.,modulus
of sub-base reaction) makes it possible to account for the soil and sub-base as
one single parameter in the calculations.
4.2.4.3 Bearing
strength of natural soil (subgrade). The modulus of subgrade
reaction ko of the soil is evaluated by means
of a plate bearing test carried out in situ
on soil compacted to 95 per cent of the
Modified Proctor Optimum density. It is
desirable for a certain time to elapse
between compacting and testing to allow the soil to regain its free moisture
content. The number and distribution of test points must be such as to make the
results meaningful.
4.2.4.4 Bearing
strength of the sub-base. The modulus of subgrade reaction of natural soil is
subsequently corrected in regard to the equivalent thickness of the subbase.
Figure 4-13 is used for this purpose. The definition of equivalent thickness is
given in 4.2.3.4.
Important Note: The corrected k should be
used in these calculations. Using the k measured at the top of the sub-base
course would result in optimistic figures.
Although the sub-base affects the calculation
only slightly (as a corrective term of
modulus k which itself has only a minor
impact), it has an important multiple role:
it ensures a continuous support for the slab,
particularly at its joints and participates in the transfer of loads;
because of its weight it opposes a possible
swelling of the sub-grade soil and protects it against frost;
it offers a stable surface for subsequent
concreting operations; and
it prevents pumped up particles from rising
at the joints.
4.2.4.5 Structure
of the sub-base. It is important to have a high quality subbase. The following
rules must be applied:
the sub-base course must be treated;
the use of coarse aggregate concrete is
advisable;
lean cement concrete is not really
recommended (higher risk of cracking);
the actual thickness of the sub-base must be
at least
the specifications for materials that may be
used in a sub-base are similar to those for road pavements.
The sub-base can rest on an improved subgrade
which may or may not consist of stabilized materials. The total equivalent
thickness of the two courses is subsequently taken into account to correct the
modulus of subgrade reaction K. It is feasible to place a layer of porous
concrete between the concrete slab and the treated sub-base in order to improve
the drainage and to reduce the pumping effect.
4.2.4.6 Designing
the thickness of the concrete slab. Due to the rigidity of the concrete, the
vertical stresses applied to the sub-base by a loaded concrete slab are always
very low; the slab ensures the distribution of stresses due to loading by
mobili
zing its flexural strength. Consequently,
contrary to what happens in the case of a flexible pavement, the design
criterion for a rigid pavement is not maximum pressure at subgrade level, but
permissible flexural moment of the slab. In the design, constant values are
adopted to describe the concrete as follows:
modulus of elasticity: E = 30 000 MPa
Poisson's ratio = 0.15
4.2.4.7 Stresses
of concrete. Account is taken in the calculations of the permissible flexural
stress on the concrete which equals the flexural breaking strength divided by a
safety factor. The flexural breaking strength is measured on prismatic
specimens after 90 days. The final value to be retained is the mean of the
measured values reduced by a standard deviation which corresponds to the
foreseeable scatter over the site (varying between a minimum of 10 per cent for
a closely supervised construction site and 20 per cent). If. the results of
tests performed after 28 days' curing only are available, it may be assumed
that the flexural strength of the concrete increases by 10 per cent between 28
and 90 days.
4.2.4.8 Safety
factors. The safety factor depends on the type of joints used between the slabs
of the pavement. It is established at 1.8 where joints are equipped with
devices for the efficient transfer of loads and at
unfavourable conditions
- poor subgrade (k ZO MN/m3) or
non-homogeneous or frost susceptible
- thin sub-base (e <
heavy traffic consisting of wide-bodied
aircraft (D-747, DC-10, etc. )
significant daily temperature gradient
absence of tie bars across joints
4.2.4.9 Construction
rules - see 4.2.4.11
4.2.4.10
Thickness of concrete slab
general design (see 4.2.5)
optimized design (see 4.2.6)
Comment. The general design method is
generally adequate for studying rigid pavements.
4.2.4.11 Construction
rules
a) Joints. A correctly designed rigid
pavement must respect the main construction rules laid down in Figure 4-14.
b) Efficient transfer of loads. None of the
devices described provides complete efficiency. The tongue and groove systems
and the coatraction-expansion joints are efficient only where the joints are
not too open under the combined effect of hydraulic contraction (definitive)
and thermic contraction (periodic); also, with time they lose some of this
efficiency due to the fact that the two surfaces in contact show wear from the
effects of traffic and the thermic cycles. The efficiency of dowelled joints is
not closely linked to their openings. However, the transfer of loads is also
likely to diminish with time, mainly due to the fact that the cyclindrical
cavity in which the dowel moves in a longitudinal direction becomes enlarged
and more oval in shape. As pointed out, the sub-base may improve the transfer
of loads, provided it is sufficiently rigid. However, its beneficial action
also decreases with time, particularly because of surface erosion.
4.2.4.12 Influence
of climatic factors
a) Factors of thermic or hygrometric origin.
As a general rule it is accepted that, provided appropriate methods are used
for the joints, stresses which have a thermic or hygrothermic origin need not
be taken into account in the design. Flexural stresses produced by loads during
use of the pavement are not the only tensile stresses to which the concrete may
be subjected. Stresses may, first of all, result [cum differential expansions
between the top and bottom surfaces of the concrete because of differences
between these two faces:
- in the temperature (temperature gradient)
- water content
Other stresses may also be caused by friction
on the sub-base which resists a variation in length of the slab as a whole when
a change in the temperature or in the water content occurs. These changes are
assumed to be of a sufficient duration to enable the slab to achieve a state of
hygrothermic equilibrium. Consequently, they are changes that may be described
as seasonal as opposed to those (daily) changes that are produced by
hygrothermic gradients in the slab. In all cases, the existence of joints which
limit the lengths of the basic slabs, has the effect of reducing the magnitude
of the different types of stresses. Moreover, the stresses of the first
category largely tend to compensate each other due to the fact that temperature
gradients and water content are normally opposite characteristics. Finally,
these different stresses do not appreciably increase the stresses imposed by
loads.
b) Frost. An inspection for frost-thaw in
accordance with the explantions contained in 4.2.7 is recommended.
4.2..5 General design
4.2.5.1 Principle.
The general design method enables a pavement to be designed according to a
reference load. For example:
the maximum load of the aircraft considered
to produce the greatest stress; and
the desired load for a typical category of
undercarriage.
The design is based on normal traffic
conditions, i.e.,ten movements per day over ten years at the design load.
However, where the actual traffic clearly differs from this
basic assumption, it is possible to apply a
correction factor to take account of the actual traffic intensity. Examples of
using the general design method are!
study of an aerodrome used for operations
with an aircraft type that clearly produces greater stress than others;
rigid pavements (the accuracy of the method
is generally sufficient); and
- preliminary
studies in the absence of reliable traffic forecasts.
4.2.5.2 Determination
of pavement thickness
Data required
Normal design load P'
CBR of the natural soil (flexible pavements)
Modulus of subgrade reaction k and the
permissible flexural stress of the concrete (rigid pavements)
Graphical method
Depending on the case under study, one uses
either the graph for typical undercarriage (Figures 4-15 to 4-27) or the
specific graph for the aircraft
(Appendix 3).
Note.- If one intends to determine pavement
thickness for an aircraft or, more generally, an undercarriage Zeg not included
in the graphs in Appendix 3, it is possible to use the graphs for an aircraft
whose main undercarriage Zeg (track, base)
has characteristics that most closely
resemble those of the aircraft under study.
4.2.5.3 Traffic
intensity. Ten movements per day over 10 years represents an entirely
reasonable and conservative assumption for the purpose of designing a new
pavement. Nevertheless, it is conceivable that this figure is either clearly
below the foreseeable traffic volume for the aerodrome (e.g.,a major aerodrome)
or considerably higher (e.g.,an alternate aerodrome). It is necessary in those
cases to take account of the actual traffic intensity appropriately adjusted.
The correction is based on a relationship between the pairs (P, n), where P is
the load and n the number of applications in movements/day and the pair (P',
10) where P' is the normal design load (by definition applied 10 times per day
for):
with
P
P' = - The
graph in Figure 4-28
C translates
relationship 1
C = 1.2 - 0.2 log n
Important remark: Relationship [1] is only
valid for a pavement life of ten years. For any other period, it would be
appropriate to relate the figure to ten years (example: 4 movements/day over 20 years are equivalent to 8 movements/day
over ten
years). The value of factor C is limited to
1.2 at the top end of the scale (minimum assumption of 1 movement/day) and to
0.8 at the bottom end of the scale (maximum assumption of 100 movements/day).
4.2.6 Optimized design
4.2.6.1 Principle.
The optimized design method enables a pavement to be designed by taking into
account several aircraft types at different frequencies. This method has the
advantage that the actual movements of each actual load considered can be
converted into equivalent movements of the same reference load. It is thus
possible to compare the relative effect of different aircraft. In practice,
therefore, the optimized design method is used when several types of aircraft
producing approximately the same stresses must be considered (e.g. at major
aerodromes), as well as for the purpose of granting concessions (see 2.2.2.2
and 4.2.8). Detailed traffic forecasts according to aircraft type serve as the
basis for the design. Bearing in mind that it is sometimes difficult to
establish accurate data (particularly for the actual loads), it is recommended
that two calculations be made, i.e. one assuming a low traffic volume and the
other a high one, with a view to assessing the sensitivity of the different
parameters and the error margin for the calculation. Any pavement life may be
selected (see 4.2.6.2). The optimized design takes into account the precise
number of actual movements of each aircraft for the expected pavement life.
Contrary to the general design method there is no minimum assumption (1
movement/day or 3 650 movements over ten years): the calculated pavement is more sensitive to traffic
variations.
4.2.6.2 Pavement
life. The life of a pavement (see definition in 4.2.1.1) is normally selected
on the basis of the table below:
A period of ten years is normally adopted
which corresponds to the practice most widely used. The optimized design method
takes into account a number of actual movements over a fixed pavement life. Any
value may thus be chosen for the latter.
4.2.6.3 Determination
of pavement thickness
a) Data required
- Traffic forecasts (for method used to
establish these, see 4.2.1.1)
- CBR of natural soil (flexible pavements)
- Modulus of subgrade reaction k and the
permissible flexural stress of the concrete (rigid pavements)
b) Calculation method. The calculation
consists of applying an "iterative method" which permits the
structural integrity under
expected traffic to be checked in respect of
successive thickness values:
Step 1 - An initial thickness is established.
Step 2 - The equivalent traffic of the
expected actual traffic, equalling a number of equivalent movements of the
allowable load
Step 3 - Depending on whether the result is
less than or more than
36 500 equivalent movements, steps 1 and 2
are repeated with a smaller or greater thickness respectively, until a
thickness is found where the equivalent traffic is equal or as close as possible
to the 36 500 equivalent movements.
c) Practical calculation. In this way one can
calculate for each aircraft considered as the most critical, the thickness
required by its maximum expected mass, taking into account the number of actual
movements anticipated at this mass and assuming that it would be the only
aircraft using the pavement under study. The maximum thickness thus obtained,
plus a few centimetres, usually produces an initial thickness that is fairly
close to the final value. The effects of some aircraft quickly become
negligible as the thickness is increased in the iterations (as soon as P/Po is
less than 0.8). They can be deleted from the tables to simplify the
calculations. The minimum increments in the iterations are generally
Remark
The optimized design method can be used for
purposes other than calculating thicknesses, e.g.,
1 - Granting of concessions (see 2.2.2.2 and
4.2.8); and
2 - Potential of remaining pavement life (by
comparing total and past traffic equivalents for an existing pavement).
4.2.7 Frost
4.2.7.1 It
is recommended that structures be tested for the effects of frost-thaw as
follows:
a) Classification of soil according to frost
susceptibility. The classification of the Laboratoire Central des Ponts et
Chaussees* (Ministere des Transports,
* Abbreviated as LCPC.
b) Determination of frost penetration. Frost
penetration is determined using the modified Berggren method adapted to the
multi-layer case. The frost indices and thermic parameters are defined in the
same manner as the LCPC.
c) Protecting pavement from frost. There are
three feasible protection levels, as follows:
1) Total protection. Protection is calculated
so as to ensure that the frost penetration determined for the exceptionally
severe
winter cannot reach soil layers that may be
susceptible to frost.
2) High protection. Same principle as total
protection; however, the frost penetration is calculated for a not
exceptionally severe winter.
3) Low protection. It is recognized that
frost under severe winter conditions may penetrate a few centimetres into the
courses or Into frost-susceptible soil. The acceptable depth of penetration
largely depends on the individual case and will be determined in consultation
with the Administration. The table hereunder shows the recommended protection
levels for information:
4.2.8 Allowable loads
4.2.8.1 Determining
the allowable loads for existing pavements is a reciprocal problem of the
design process. Actually, three types of questions are covered by this heading,
namely:
a) as regards a specific pavement, how to
publish information on its bearing strength in terms of its characteristics;
b) conversely, how can the allowable load for
every aircraft be determined from this information (which has been established
in a synthetic manner); and
c) under what conditions should concessions
be granted if the actual loads exceed the allowable loads.
Moreover, in
strength exist side by side, i.e.,
the method based on a typical undercarriage
leg applied in
the ACN-PCN method.
4.2.8.2 It
is intended in this section to:
a)
describe each of the two methods and the conditions in which they are used;
b)
specify interim measures required as a result of using the two methods side
by
side; and
c)
indicate the calculation process used in deciding when concessions should
be
granted.
4.2.8.3 Publishing
information on runway bearing strength
a) Method based on typical undercarriage leg.
Since practically all modern aircraft are equipped with undercarriages with
single, dual or dual tandem wheel arrangements, the maximum load allowable on
each pavement will have to be fixed for each of the three typical
undercarriages on the basis of ten movements per day over ten years.
Example: 20 t in respect of the single wheel, 35 t in
respect of the dual wheel and 50 t in respect of the dual tandem wheel
arrangements are expressed symbolically as follows:
20 T/SWL - 35 T/DW - 50 T/DTW
The characteristics of the typical
undercarriage legs are selected from the most critical landing gear
characteristics of current aircraft (see 4.2.2). This method of fixing the
allowable loads has the disadvantage of ignoring the variations which in fact
exist within the same category of undercarriage. For example, if the track of
the dual wheels or the tire pressure is different from that of the typical
undercarriage, the effect on the pavement will differ considerably for the same
mass of aircraft. Strictly speaking, therefore, an allowable load according to
aircraft type should be established for a given pavement. Obviously, this
method cannot be applied in practice. However, whenever such a precise
calculation is justified (e.g., for the purpose of concessions), the exact
landing gear characteristics are taken into account, so that this does not
deprive certain aircraft of the advantages they derive from the design of their
undercarriage.
b) ACN-PCN method
Note.- This method is described in Ecar139
and in Chapter 1 of this manual.
4.2.8.4 Choice
of a method. The ACN-PCN method came into force for AIPs on 26 November 1981
and is gradually replacing the method based on a typical undercarriage leg.
a) Existing pavements
A final PCN will be published following the
complete evaluation of pavements under the conditions described in Section
4.2.9, and this will replace publications based on a typical undercarriage leg.
An interim PCN will be published pending an
evaluation, together with the existing method of reporting data based on a
typical undercarriage leg.
b) Reinforced pavements
- A final PCN will be published following the
complete reinforcement of a pavement; this will replace publications based on a
typical undercarriage leg for the old pavement.
c) New pavements
- A final PCN will be published for new
pavements.
Remark: In areas subject to pronounced seasonal climatic changes, the bearing
strength of the subgrade can vary considerably in the course of the year. This
may necessitate reporting two sets of PCN values, one for the dry and one for
the wet season.
4.2.8.5 Calculating
the values to be published
a) Required data. The data required for
publishing information on pavement strength consist of:
- total equivalent thickness and the CBR of
the subgrade for flexible pavements.
- thickness of the slab, permissible flexural
stress, modulus of subgrade reaction k for rigid pavements.
Such data are obtained in the case of:
-
old pavements:
from an evaluation of bearing strength under conditions described in 4.2.9
-
reinforced pavements: from the evaluation of the bearing strength prior to reinforcement and
from the characteristics adopted in designing the reinforcement.
-
new pavements: from the characteristics adopted for the design with
possible corrections to take account of the
actual construction.
b) Calculation
Method based on a typical undercarriage leg.
The permissible load
- ACN-PCN Method. Determining the PCN is a long and complex
operation. The calculations involve the following successive steps:
Step 1 - Establishing a list of aircraft
using or Likely to use the pavement under study.
Step 2 - Calculating, with the aid of the
reverse design method,
the permissible Poi of the various aircraft
in terms of the characteristics of the subgrade and the pavement.
Step 3 - Calculating for each typical soil
category the ACN which corresponds to the permissible load Poi. Subsequently,
in each category one considers the PCN included between the maximum and minimum
ACN values obtained. The PCN is expressed by two significant figures.
Step 4 - Searching, among the couples (soil
category, PCN) for the value that will produce permissible loads P'oi that are
closest to Poi.
Usually the calculation results in a subgrade
category that contains the CBR or modulus k value of the pavement under study.
However, it is not unusual to obtain an adjacent subgrade category and the
classification thus determined must be interpreted "within the meaning of
the ACN-PCN method".
c) The four code letters which follow the PCN
are selected in the following manner:
- type of pavement: the classification is established according
to the criteria in 4.2.1.1.
- category of subgrade strength: this is provided at the same time
as the PCN by the calculation described
above.
- maximum allowable tire pressure: Code W (no pressure limitation) will
generally be adopted. Code X (pressure limited to 1.5 HPa) is adopted where
there is a proven risk of surface damage.
- evaluation method: the PCN is calculated
following a complete evaluation: Code T will normally be adopted. Code U can only be applied for an
interim publication of the PCN of a pavement for which there are no reliable
results obtained by detailed evaluation and whose behaviour has been judged on
the basis of its ability to accept existing traffic.
Remarks: 1) For a runway for which several
homogeneous areas can be distinguished in regard to bearing strength, the
values to be published are the lowest obtained over the entire pavement area.
2) If an area is amenable to a
reduction in the normal design load (see 4.2.2.2), weighting is also used in
calculating the allowable loads.
4.2.8.6 Using
the published values
a) Determination of allowable loads:
1) ACN-PCN method. The allowable load
max ACN: ACN value corresponding to the
maximum mass*
min ACN: ACN value corresponding to the
minimum mass (operating mass empty)
* See Appendix 5, Table 5-1.
2) Typical undercarriage leg method.
The allowable load
Remark: In the case of the
pavement for which both the load per
typical undercarriage leg and a PCN are
published, one adopts the highest value obtained by using one or the other
method.
b) Use of allowable loads:
- if the actual load P is less than the
allowable load Po there is no restriction (load, number of movements) for the
aircraft under study within the over-all fatigue limit of the pavement.
- if the actual load P exceeds load
- no restriction
- limited operation* (as regards mass or
number of movements under a concession)
- refusal of access
Example
Determination of PCN of a flexible runway
with the following characteristics:
total equivalent thickness e =
CBR of subgrade CBR = 8
The pavement receives traffic consisting
almost exclusively of B-727-200, Standard, Airbus A-300 B2, B-747-100.
Solution
Step 1. The subgrade may be classified in Category B (medium strength) as well
as in Category C (low strength). These two categories will then be tested in a
subsequent calculation.
* See 2.2.2.2 for guidance on this issue.
Step 2. Calculation of allowable loads based
on French practice (use of graphs in Appendix 3):
Step 3. Calculation of the ACN corresponding
to the allowable load determined for each aircraft.
CATEGORY B
Step 4. The PCN value to be determined ranges from 45 to 49 if one adopts
Category B and between 53 and 59 for Category C. It is noted, however, that the
B-727 is acceptable in both cases at a load exceeding the maximum all-up mass.
When considering the
A-300B2 and the B-747-100 only, the choice is
limited within the range 55 to 59 for Category C.
* See Appendix 5, Table 5-l.
Step 5. The final choice is made between the mean values PCN = 47 and PCN = 57
obtained for Categories B and C respectively.
The difference between the allowable loads
calculated by means of the two methods is less in the second case.
Step 6. Publication PCN 57/F/C/W/T
4.2.9
Evaluation of pavements
4.2.9.1 General.
Evaluation of existing pavements is an indispensable tool in ensuring efficient
utilization of their potential. It
fulfils three main objectives, as follows:
a) to determine when maintenance operations
or more extensive work must
be undertaken;
b) at the time such work has to be
undertaken, to assess the residual qualities of the pavement with a view to
enabling a technical and economic solution to be found and the design for a
possible reinforcement to be determined; and
c) to determine, at any time, which aircraft
types can use a particular pavement, and their mass and maximum movement
frequency (allowable loads described in 4.2.8).
4.2.9.2 Pavement
evaluation must take into account both the structural and functional
characteristics of the pavements. The structural characteristics of the
pavement /subgrade complex govern its bearing strength, i.e.,its ability to
bear loads imposed by aircraft while retaining its structural integrity during
a certain life. The functional characteristics affect the state of the pavement
surface and to what extent the pavement can be safely used by aircraft. They
are:
a) the quality of the longitudinal profile
and, in particular, the evenness which determine the degree of vibrations
produced in aircraft during roll out;
b) slipperiness, which determines the degree
of directional control and braking of the aircraft; and
c) quality of the surface (crumbling,
breaking up of the asphalt, etc.), since detects can damage aircraft (ingestion
of small stones by jet engines, tire bursts).
Moreover, the structural and functional
characteristics are not independent: thus, the state of the surface can reveal
possible structural defects and, conversely, a structure unsuited to the
traffic causes deterioration of the surface.
4.2.9.3 Evaluation
of pavements is a very complex procedure which calls for a synthesis by a
specialist team of the following elements;
a) data on the design of the pavement and of
the subsoil, as well as on possible subsequent work (maintenance,
reinforcement, etc.);
b) study of the aerodrome site;
c) climatological data (hydrology, ground
water, frost, etc.);
d) visual inspections of the state of the
pavement, surveying the deterioration and examining the drainage;
e) various measurements which enable certain
parameters associated with the pavement characteristics (evenness,
slipperiness, bearing strength) to be determined: and
f) measurement of the thickness and
qualitative assessment of the pavement courses and the characteristics of the
subgrade.
4.2.9.4 The
following paragraphs deal only with the evaluation of the pavement bearing
strength. The purpose of this evaluation is to assign the following
representative structural parameters to an existing pavement to represent its
current bearing strength which can be directly applied to determine the
allowable load and any reinforcement required;
a) the CBR of the subgrade and the total
equivalent thickness for a
flexible pavement; and
b) the modulus of reaction k of the subgrade,
thickness of the concrete slab and the permissible flexural stress of the
concrete in the case of a rigid pavement.
4.2.9.5 Two
approaches may be used to determine these parameters, as follows:
a) by a procedure which is the exact reverse
of the design process, the so-called "reverse design method";
and
b) by means of non-destructure plate
loading tests on the surface of the pavement which indicate the actual
allowable load in the case of a single wheel leg.
In practice, the evaluation of a pavement
bearing strength must be made by synthesizing the results of these two
complementary approaches.
4.2.9.6
Reverse design method. The purpose of the design method described
previously which uses the subgrade data, is to determine a pavement structure
that can bear a given traffic over a certain life, provided "normal"
maintenance is performed. Conversely, once the characteristics of the subgrade
and of the pavement structure are known, this method enables the traffic which
can be accepted during a given time to be determined. The foregoing is the
basis for evaluation bearing strength by means of the reverse design method.
When this method is used by itself, however, considerable difficulties are
encountered in determining the structural parameters that must be taken into
account in evaluating an existing pavement and its subgrade. Even if records
are available of the construction of the pavement, of any maintenance and
reinforcement work performed in the past, and of the traffic accepted, this
method requires many trial borings and testings of the pavement. Moreover,
there will usually be some uncertainty concerning the results because of the
difficulty of evaluating certain parameters (equivalence coefficients of the
courses of a flexible pavement, load transfers between concrete slabs, etc.).
Remark: The reverse design method can only be used for a pavement that is
correctly constituted (for flexible pavements, the courses must be of
increasing quality from bottom to top and adhere closely).
4.2.9.7
Non-destructive plate tests. When interpreted by qualified personnel,
non-destructive plate tests can directly provide the allowable load for a
single wheel at a large number of points on a flexible pavement and the
allowable load at the corners of slabs in the case of a rigid pavement. These
tests are insufficient to determine the allowable load for aircraft with
multiple wheel undercarriages or to serve as the basis for designing a
reinforcement, in which case the reverse design method must be adopted.
Nevertheless, the plate tests considerably reduce the number of destructive
tests required in order to apply a reliable cross-check in the case of flexible
pavements and enable the quality of the load transfer to be evaluated in the
case of rigid pavements, as explained in the following paragraph.
4.2.9.8 Test
programme to evaluate bearing strength. The amount of equipment required
depends on the particular objective and how mach is already known about the
pavement:
a) If the pavement is old and little is known
of its characteristics, all the equipment described below must be used.
b) If the pavement is of recent construction
and adequate records are available or the pavement has already been the subject
of a comprehensive evaluation of the type described above and changes in
bearing strength only are to be determined, non-destructive plate tests are
usually adequate. This also applies to a pavement which has undergone a complete
evaluation followed by reinforcement work, where the results of such work are
to be checked.
The following paragraphs deal with the first
case, i.e.,a complete study.
4.2.9.9 Delineation
of homogeneous zones
a) The first phase of the study is intended
to delineate the zones whose structure and state are identical and to assess
their homogeneity in order to reduce the number of other tests needed to
determine the pavement structure. To complete the information available from
the records, a detailed visual inspection of the pavement must first be
performed, including a survey and classification of its deterioration, as well
as an inspection of the drainage system.
b) During a second stage, the following may
be used:
For flexible pavements: either the Lacroix deflectograph of the
LGPc, or the influograph of the STBA*.
For rigid pavements: the equipment for measuring vibration of
slabs
DMBD) of the LCPC.
c) Finally, a relatively large number of
non-destructive plate tests (from 80 to 100 on a medium-size aerodrome) are
performed which not only enable the homogeneity of pavement behaviour to be
assessed, as in the case of the above-mentioned equipment, but which also give
the value of the allowable load for a single wheel at each of these points.
4.2.9.10 Description
of the homogeneous zones. All the above-mentioned equipment is used to
define the homogeneous zones on the basis of their structure and behaviour.
Having determined the allowable load
must cover a surface area of approximately
a) to determine the structure of the
pavement, particularly the thickness of the courses and to check the quality of
the materials encountered, if necessary in the laboratory;
b) to undertake CBR tests in situ or
tests of the modulus of subgrade reaction k whenever possible; and
c) to
measure the moisture content and dry density of the subgrade and to take intact
or treated samples for laboratory analysis and tests.
4.2.9.11 Interpretation
and synthesis of the results. The results for each homogeneous zone are
interpreted in the light of the data in respect of the pavement and traffic it
has accepted, the surveys of its deterioration, the results of the inspection
of the drainage system and all the measurements performed. This synthesis must
be carried out by a specialist team, in practice the STBA. Cross-checking of
the different measurement values permits making a final choice of the different
characteristics required to calculate the allowable loads (see 4.2.8).
STBA: Service Technique des Bases Aeriennes,
Ministere des Transports,
4.2.10
Reinforcement of pavements
4.2.10.1 General.
The problem of reinforcement of aerodrome pavements can arise when manoeuvring
areas must be adapted to meet the future requirements of heavier aircraft or
when pavements require strengthening to meet immediate needs of current
traffic. In practice, these two concerns are frequently confused. Reinforcement
is not the only solution, however, if a particular pavement is not suited to
the present or future traffic:
-It may at times be preferable to build a new
pavement somewhere else. This solution obviates the difficulty of maintaining
the flow of traffic during the reinforcing work; it also allows for the introduction
of an improved layout more adapted to new operating conditions.
-The "substitution" method could
also be adopted. This consists of removing the existing pavement and rebuilding
a new one at the same level. This solution, which in the case of a runway can
be limited to
The text below deals with the actual
reinforcement of pavements; it describes a method for determining the thickness
of the reinforcement and deals with certain relevant problems encountered
during construction.
4.2.10.2 Choice
of solution. The reinforcement for a particular pavement (flexible or rigid)
can be of the same type or different. The choice is governed by technical and
the restrictions imposed by the solution on
the use of the being carried out and by the bond between the reinforcement
economic considerations, by aerodrome while
the work is and the existing pavement.
4.2.10.3 Choice
of the cross-sectional profile. Appreciable savings can be made in the cost of
reinforcing a runway by reducing the thickness of the pavement outside a
4.2.10.4 The
thickness of the flexible reinforcement may be obtained using the following
relationship:
e = 3.75 (Fht - h)
in this relationship, e is the equivalent
thickness in accordance with the definition given in 4.2.3.4. It should be
noted that the materials used for a reinforcement must be at least equal in
quality to those used for the sub-base course, i.e.,the coefficient of
equivalence must be at least 1;
- h is the thickness of the existing concrete
slab;
- ht is the theoretical thickness of the new
slab less the existing slab. This thickness is calculated taking into account
the allowable stress and the corrected k applicable to the existing slab;
- F is a coefficient of reduction of the
thickness ht> the value of which is given in Figure 4-31 as a function of
the Modulus k already mentioned (the theoretical thickness of the concrete slab
is reduced because it is assumed that the slab will crack to a certain extent
in service, in contract with the assumption made in connexion with the
calculation for slabs used in the wearing course);
The equivalent thickness of the reinforcement
must not be less than
- Moreover, the relationship at [5] is
applicable only to values resulting in an equivalent thickness e exceeding
Figure 4-31. Flexible reinforcement on rigid
pavement - Factor F
4.2.10.5 Construction
rules. The most pressing problem - and one which has not yet been
satisfactorily resolved - associated with the direct reinforcement of concrete
with a bituminous mix is that of the reappearance of the joint in the rigid
pavement at the surface of the reinforcement. Attempts are made to prevent this
damage by reinforcing the pavement at these joints by means of metal lattices,
plates, fabrics, etc., or at least by separating the course of bituminous mix
from the slab over a certain distance on either side of the joint (e.g.,by
interposing a layer of sand). It is also possible to provide saw cut joints on
the surface of the reinforcement to avoid irregular cracking. This solution
facilitates maintenance, but reduces the bearing strength of the pavement.
4.2.10.6 Although
seldom encountered, another possible difficulty is caused by the affinity of
certain jointing compounds for the bitumen, which can result in swelling of
the pavement at the joint of the reinforced
slab. If in doubt, it will then be advisable to remove the jointing compound
before the reinforcement is applied and to refill
the joints with a mixture of sand and binder
compatible with the one used in the
reinforcing course. These rules cannot be
applied in the case of reinforcement with concrete, unless the concrete is
limited to the central portion of the runway and a "flexible"
solution is adopted in the case of the lateral parts.
4.2.10.7 Preliminary
studies. An evaluation of the existing pavement is required (see 4.2.9). Of
prime importance is a systematic boring of the pavement in view of the frequent
discrepancies in thickness, constitution, etc. of the old pavements.
4.2.10.8 Reinforcement
of flexible pavements
a) Flexible reinforcement. The
thickness of the reinforcement is determined by the difference between the
equivalent thickness required for a new pavement and that of the existing
pavement. When determining the latter, the following should be taken into
account:
1) the equivalence coefficients have to be
corrected according to the
actual condition of the pavement courses; and
2) the equivalence coefficient of a pavement
course at a given level cannot be greater than that of the course above it. For
instance, if a bituminous mix in good condition (coefficient 2) is covered
by a coarse-aggregate cement (coefficient
1.5), the coefficient of the former also becomes 1.5
b) Rigid reinforcement. When a
flexible pavement is reinforced with a concrete slab, the former is only
considered as a sub-base course in the calculations. The k value which is
attributed to this course is determined by reference to Figure 4-13. The
thickness of the slab is then established in accordance with 4.2.4, 4.2.5, and
4.2.6.
4.2.10.9 Reinforcement
of rigid pavements
a) Flexible reinforcement. If the
existing pavement is appreciably fragmented, it is advisable to consider it as
a flexible pavement of the same thickness when computing the thickness of the
reinforcement. It thus amounts to the same case as described above. The
description below presupposes that the existing rigid pavement is still sound
(in that case it is still possible to consider the existing rigid pavement as a
flexible pavement of the same thickness if this is favourable to the
calculations).
b) Rigid reinforcement. The thickness of the
reinforcing slab is obtained by applying the formula:
- ht is the theoretical thickness of a new
slab determined using the permissible stress in the new concrete and the
corrected modulus of reaction for the existing subgrade.
- h is the thickness of the existing concrete
slab.
- C is a coefficient introduced in order to
take account of the quality of the existing pavement:
C = 1 for a pavement in good condition,
C = 0.75 for a pavement exhibiting some
cracking at the corners, but not appreciably deteriorated,
C = 0.35 for a badly fragmented pavement.
In practice one of the two latter values are
generally applied.
The above relationship only applies if the
reinforcing slab is laid directly on top of the existing pavement. If a layer
of material (usually bituminous mix) is interposed between the two slabs, e.g.
in order to alter the profile of the existing pavement, the formula for
calculating the thickness of the reinforcement becomes:
In this expression, the significance of the
parameters and the values for coefficient C are the same as detailed
previously. This formula results in slightly increased thick-nesses of the
reinforcement.
4.2.10.10 Construction
rules. To avoid the reappearance of the joints in the existing pavement in
the form of cracks in the reinforcing slab, it is essential that the joints be
superimposed as accurately as possible. Moreover, all the joints in the
existing pavement must have new joints (of
any type) above them. In
particular, since the old slabs are generally smaller in width than those
currently adopted, additional longitudinal contraction-expansion joints may be
necessary in the reinforcing slab. The placement of the different reinforcing
joints thus calls for a preliminary in-depth study if one wishes to avoid
miscalculations.
4.2.11 Light
pavements
4.2.11.1 Light
pavements are intended exclusively for aircraft whose total mass does not
exceed 5.7 tonnes. Figure 4-32 may be used to calculate the pavement thickness
in relation to the CBR of the natural soil.
4.2.11.2 Allowable
loads. The allowable load on a light pavement is
4.3
4.3.1 Design and evaluation of pavements
4.3.1.1 It
is the
4.3.1.2 While
there are now available a number of computer programmes based on plate theory,
multilayer elastic theory and finite element analysis, for those wishing to
have readily available tabulated data for pavement design and evaluation, the
Reference Construction Classification (RCC) system has been developed from the
Rritish Load Classification Number (LGN) and Load Classification Group (LGG)
systems. Pavements are identified as dividing broadly into rigid or flexible
construction and analysed accordingly.
4.3.1.3 For
the reaction of aircraft on rigid pavements, a simple two layer model is
adopted. To establish an aircraft's theoretical depth of reference construction
on a range of subgrade support values equating to the ICAO ACN/PCN reporting
method, the model is analysed by Westergaard centre case theory. Account is
taken of the effect of adjacent landing gear wheel assemblies up to a distance
equal to three times the radius of relative stiffness. This is considered essential in any new system in view of
the increasing mass of aircraft, complexity of landing gear layouts and the
possible interaction of adjacent wheel assemblies on poor subgrades especially.
4.3.1.4 To
resolve practical design and evaluation problems, a range of equivalency
factors appropriate to the relative strengths of indigenous construction
materials is adopted to convert between theoretical model reference
construction depths and actual pavement thickness.
4.3.1.5 Aircraft
reaction on flexible pavements follows the same basic pattern adopted for rigid
pavement design and evaluation. In this case a four pavement model is analysed
using the United States Corps of Engineers' development of the California
Bearing Ratio (CBR) method. This includes Boussinesq deflection factors and
takes into account interaction between adjacent landing gear wheel assemblies
up to 20 radii distance. Practical design and evaluation problems are resolved
using equivalency factors to relate materials and layer thicknesses to the
theoretical model on which the reference construction depths for aircraft are
assessed.
4.3.2 Reporting pavement strength
4.3.2.1 It
is the
4.3.2.2 Though
not revealed by the ICAO ACN/PCN reporting method, when interaction between
adjacent landing gear wheel assemblies affects the level of leading imposed by
an aircraft,
4.4
Note.- The specifications in this Section,
and the calculations upon which they are based, were quoted to ICAO in inches
and fractions thereof. Although metric equivalents are provided, in accordance
with standard ICAO practice, they cannot be
taken as being as precise as the figures
quoted in inches.
4.4.1 Introduction
4.4.1.1 The
United States Federal Aviation Administration
adopted the California Bearing Ratio (CBR) method of flexible pavement design,
edge loading assumption for the design of rigid pavements and the Unified Soil
Classification System. This section presents a detailed outline of current
procedures and criteria which the United States Federal Aviation Administration
has found necessary to follow in pavement design and in conducting a pavement
strength evaluation.
4.4.2 Basic investigations and
considerations
4.4.2.1 The
are all essential elements of any evaluation
technique. The following basic investigations should be included in any
meaningful evaluation:
a) pavement condition surveys showing how the
existing pavements are holding up under traffic must be conducted in detail.
All areas of failure must be accurately mapped and causes of such failures
ascertained. It is extremely important that failures due to traffic and load be
differentiated from failures due to climate, drainage, and/or poor material, and
workmanship;
b) a soil survey must be completed to
disclose important variations in soil structure, changes in moisture content,
water-bearing layers, water table, and similar determinations;
c) adequate tests, both field and laboratory,
should be employed in evaluating the pavement foundation and the pavement's
component parts;
d) drainage conditions at the site shall be
analysed to ascertain the need for corrective measures prior to any
rehabilitation work;
e) an analysis of the traffic history of the
airport with regard to both weight of aircraft and number of operations
associated with traffic density for the particular area under study must be
undertaken and appropriately correlated with pavement performance; and
f) the quality of pavement materials and
adequacy of construction methods and practices must be evaluated to determine
the degree of conformance with required standards and specifications.
4.4.2.2 The
soil survey is not confined to soils encountered in grading or necessarily to
the area within the boundaries of the airport site. Possible sources of locally
available material that may be used as borrow areas or aggregate sources should
be investigated.
4.4.2.3 Samples
representative of the different layers of the various soils encountered and
various construction material discovered should be obtained and tested in the
laboratory to determine their physical and engineering properties. Because the
results of a test can only be as good as the sampling, it is of utmost
importance that each sample be representative of a particular type of soil
material and not be a careless and indiscriminate mixture of several materials.
4.4.2.4 Pits,
open cuts, or both may be required for making inplace bearing tests, for the
taking of undisturbed samples, for charting variable soil strata, etc. This
type of supplemental soil investigation is recommended for situations which
warrant a high degree of accuracy or when in situ conditions are complex and
require extensive investigation.
4.4.3 Soil tests
4.4.3.1 _Physical
soil properties. To determine the physical properties of a soil and to
provide an estimate of its behaviour under various conditions, it is necessary
to conduct certain soil tests. A number of field and laboratory tests have been
developed and standardized. Detailed methods of performing soil tests are
completely covered in publications of the American Society for Testing and
Materials.
4.4.3.2 Testing
requirements. Soil tests are usually identified by terms indicating the
soil characteristics which the tests will reveal. Terms which identify the
tests considered to be the minimum or basic requirement for airport pavement,
with their ASTM designations and brief explanations, follow:
a) Dry preparation of soil samples for
particle-size analysis and determination of soil constants (ASTM D-421) or wet
preparation of soil samples for grain-size analysis and determination of soil
constants (ASTM D-2217). The dry method (D-421) should be used only for
clean, cohesionless granular materials. The wet method (D-2217) should be used
for all cohesive or borderline materials. In case of doubt, the wet method
should be used.
b) Particle-size analysis of soils (ASTM
C-422). This analysis provides
a quantitative determination of the
distribution of particle sizes in soils.
c) Plastic limit of soils (ASTM D-424). The
plastic limit of a soil is defined as the lowest moisture content at which a
soil will change from a semi-solid to a plastic state. At moisture contents
above the plastic limit, there is a sharp drop in the stability of soils.
d) Liquid limit of soils (ASTM D-423). The
liquid limit of a soil is defined as the lowest moisture content at which a
soil passes from a plastic to a liquid state. The liquid state is defined as
the condition in which the shear resistance of the soil is so slight that a
small force will cause it to flow.
e) Plasticity index of soils (ASTM D-424).
The plasticity index is the numerical difference between the plastic limit and
the liquid limit. Tt indicates the range in moisture content over which a soil
remains in a plastic state prior to changing into a liquid.
f) Moisture density relations of soils (ASTM
D-698, D-1557). For purposes of compaction control during construction, tests
to determine the moisture-density relations of the different types of soils
should be
performed.
1) For pavements designed to serve aircraft
weighing
2) For pavements designed to serve aircraft
weighing less than
4.4.3.3 Supplemental
tests. In many cases additional soil tests will be required over those listed
in 4.4.3.2 above. It is not possible to cover all the additional tests which
may be required; however, a few examples are presented below. This list is not
to be considered a complete list by any means.
a) Shrinkage factors of soils (ASTM
D-427). This text may be
in areas where swelling soils might be
encountered.
required
test may be
b) Permeability of granular soils (ASTM
D-2434). This
needed to assist in the design of subsurface
drainage.
c) Determination of organic material in
soils by wet combustion (AASHTO T-194). This test may be needed in areas
where deep pockets of organic material are encountered or suspected.
d) Bearing ratio of laboratory-compacted
soils (ASTM D-1883). This test is used to assign a California Bearing Ratio
(CKK) value to subgrade soils for use in the design of flexible pavements.
e) Modulus of soil reaction (AASHTO T
222). This test is used to determine the modulus of soil reaction, K, for
use in the design of rigid pavements.
f)
4.4.4 Unified soil classification system
4.4.4.1 The
standard method of classifying soils for engineering purposes is ASTM D-2487,
commonly called the Unified system. The change from the FAA system to the
Unified system is based on the results of a research study which compared three
different methods of soil classification. The research study concluded the
Unified system is superior in detecting properties of soils which affect
airport pavement performance. The primary purpose in determining the soil
classification is to enable the engineer to predict probable field behaviour of
soils. The soil constants in themselves also provide some guidance on which to
base performance predictions. The Unified system classifies soils first on the
basis of grain size, then further subgroups soils on the plasticity constants.
Table 4-7 presents the classification of soils by the Unified system.
4.4.4.2 As
indicated in Table 4-7, the initial division of soils is based on the
separation of course and fine-grained soils and highly organic soils. The
distinction between coarse and fine grained is determined by the amount of
material retained on the No. 200 sieve. Coarse-grained soils are further
subdivided into gravels and sands on the basis of the amount of material
retained on the No. 4 sieve. Gravels and sands are then classed according to
whether or not fine material is present. Fine-grained soils are subdivided into
two groups on the basis of liquid limit. A separate division of highly organic
soils is established for materials which are not generally suitable for
construction purposes. The final
classification of soils subdivides materials into 15 different groupings. The
group symbols and a brief description of each is given below:
4.4.4.3 Determination
of the final classification group requires other criteria in addition to those
give in Table 4-7. These additional criteria are presented in Figure 4-33 and
have application to both coarse and fine-grained soils.
4.4.4.4 A
flow chart which outlines the soil classification process has been developed
and is included as Figure 4-34. This flow chart indicates the steps necessary
to classify soils in accordance with ASTM D-2487.
4.4.4.5 A
major advantage of the ASTM D-2487 Unified system of classifying soils is that
a simple, rapid method of field classification has also been developed; see
ASTM D-2488, Descpiption, of soils
(Visual-manual procedure). This
procedure enables field personnel to classify soils rather accurately with a
minimum of time and equipment.
4.4.4.6 A
table of pertinent characteristics of soils used for pavement foundations is
presented in Table 4-8. These characteristics are to be considered as
approximate, and the values listed are generalizations which should not be used
in lieu of testing.
4.4.5 Soil
classification examples
4.4.5.1 The
following examples illustrate the classification of soils by the Unified
system. The classification process progresses through the flow chart shown in
Figure 4-34.
Example 1
Assume a soil sample has the following
properties and is to be classified in accordance with the Unified system.
Percentage passing No. 200 sieve - 98 per
cent.
Liquid limit on minus 40 material - 30 per
cent.
Plastic limit on minus 40 material - 10 per
cent.
Solution
See above "A" line, Figure 4-33.
The soil would be classified as CL, lean clay of low to medium plasticity.
Table 4-8 indicates the material would be of fair to poor value as a foundation
when not subject to frost action. The potential for frost action is medium to
high.
Example 2
Assume a soil sample with the following
properties is to be classified by the Unified system.
Percentage passing No. 200 sieve - 48 per
cent.
Percentage of coarse fraction retained on No.
4 sieve - 70 per cent. Liquid limit on minus 40 fraction - 60 per cent.
Plastic limit on minus 40 fraction - 20 per
cent.
Solution
Compute plasticity index LL-PL - 40 per cent.
See above "A" line, Figure 4-33.
This sample is classified as GC, clayey
gravel. Table 4-8 indicates the material is good for use as a pavement
foundation when not subject to frost action. The potential for frost action is
slight to medium.
4.4.6 Frost and permafrost
4.4.6.1 The
design of pavements in areas subject to frost action or in areas of permafrost
is a complex problem requiring detailed study. The detrimental effects of frost
action may be manifested in frost heave or in loss of foundation support
through frost melting.
4.4.6.2 The
design of pavements for seasonal frost conditions can be accomplished in four
different ways.
a) Complete protection method involves the
removal of frost susceptible material to the depth of frost penetration and replacing
the material with nonfrost susceptible material.
b) Limited subgrade frost penetration method
allows the frost to penetrate a limited depth into the frost susceptible
subgrade. This method holds deformations to small acceptable values.
c) Reduced subgrade strength method usually
permits less pavement thickness than the two methods discussed above and should
be applied to pavements where aircraft speeds are low and the effects of frost
heave are less objectionable. The primary aim of this method is to provide
adequate structural capacity for the pavement during the frost melt period.
Frost heave is not the primary consideration in this method.
d) Reduced subgrade frost protection method
provides the designer a method of statistically handling frost design. This
method should only be used where aircraft speeds are low and some
frost heave can be tolerated. The statistical
approach allows the designer more latitude than the other three methods
discussed above.
4.4.6.3 The
design of pavements in permafrost areas requires efforts to restrict the depth
of thaw. Thawing of the permafrost can result in loss of bearing strength. If
thawed permafrost is refrozen, heaving can result and cause pavement roughness
and cracking. Two methods of design are available for construction in
permafrost areas, complete protection method and the reduced subgrade strength
method. These methods are somewhat similar to the methods discussed under
4.4.6.2 for seasonal frost design.
4.4.6.4 The
depth of frost penetration can be computed using the modified Berggren
equation. The Berggren equation requires several inputs concerning local soil
conditions and local temperature data. Utility companies near the site can also
provide valuable data concerning frost depth. The designer should be cautioned
that the depths of cover required to protect utility lines are conservative and
generally exceed the depths of frost penetration.
4.4.6.5 The
frost design procedures discussed herein can be found in FAA Research Report
FAA-RD-74-30, Design of civil airfield pavement for seasonal frost and
permafrost conditions. Another valuable reference for frost and permafrost
design is United States Army Corps of Engineers Technical Manual TM 5-818-2,
Pavement design for frost conditions.
4.4.7 Soil strength tests
4.4.7.1 Soil
classification for engineering purposes provides an indication of the probable
behaviour of the soil as a pavement subgrade. This indication of behaviour is,
however, approximate. Performance different from that expected can occur due to
a variety of reasons such as degree of compaction, degree of saturation, height
of overburden, etc. The possibility of incorrectly predicting subgrade
behaviour can be largely eliminated
by measuring soil strength. The strength of
materials intended for use in flexible pavement structures is measured by the
California Bearing Ratio (CBR) tests. Materials intended for use in rigid
pavement structures are tested by the plate-bearing method
of test. Each of these tests is discussed in
greater detail in the subsequent paragraphs.
4.4.7.2
the two forces. Thus a material with a CBR
value of 15 means the material in question offers 15 per cent of the resistance
to penetration that the standard crushed stone offers. Laboratory. CBR tests should be performed in accordance with
ASTM D-1883, Bearing ratio of laboratory-compacted soils. Field CBR tests should be conducted in
accordance with the procedures given in Manual Series No. 10 (MS-10) by The
Asphalt Institute.
a) Laboratory CBR tests are conducted on
materials which have been obtained from the site and remoulded to the density
which will be obtained during construction. Specimens are soaked for
four days to allow the material to reach
saturation. A saturated CBR test is used to simulate the conditions likely to
occur in a pavement which has been in service for some time. Pavement
foundations tend to reach nearly complete saturation after
about three years. Seasonal moisture changes
also dictate the use of a saturated CBR design value since traffic must be
supported during periods of high moisture such as spring seasons.
b) Field CBR tests can provide valuable
information on foundations which have been in place for several years. The materials
should have been in place for a sufficient time to allow for the moisture to
reach an equilibrium condition. An example of this condition is a fill which
has been constructed and surcharged for a long period of time prior to pavement
construction.
c) CBR tests on gravelly materials are
difficult to interpret. Laboratory CBR tests on gravel often yield CBR results
which are too high owing to the confining effects of the mould. The assignment
of CBR values to gravelly subgrade materials may be based on judgement and
experience. The information given in Table 4-8 may provide helpful guidance in
selecting a design CBR value for a gravelly soil. Table 4-8 should not,
however, be used indiscriminately as a sole source of data. It is recommended
that the maximum CBR for unstabilized gravel subgrade be 50.
d) The number of CBR tests needed to properly
establish a design value cannot be simply stated. Variability of the soil
conditions encountered at the site will have the greatest influence on the
number of tests needed. As an approximate "rule of thumb" three CBR
tests on each different major soil type should be considered. The preliminary
soil survey will reveal how many different soil types will be encountered. The design CBR value should be conservatively
selected. Common paving engineering practice is to select a value which is one
standard deviation below the mean.
4.4.7.3 Plate
bearing test. As the name indicates, the plate bearing test measures the
bearing capacity of the pavement foundation. The plate bearing test result is
expressed as a k value which has the units of pressure over length. The k value
can be envisioned as the pressure required to produce a unit deformation of a
bearing plate into the pavement foundation. Plate bearing tests should be
performed in accordance with the procedures established in AASHTO T 222.
a) Rigid pavement design is not too sensitive
to the k value. An error in
establishing a k value will not have a drastic impact on the design thickness
of the rigid pavement. Plate bearing tests must be conducted in the field and
are best performed on test sections which are constructed to the design
compaction and moisture conditions. A correction to the k value for saturation
is required to simulate the moisture conditions likely to be encountered by the
in-service pavement.
b) Plate bearing tests are relatively
expensive to perform and thus the number of tests which can be conducted to
establish a design value is limited. Generally, only two or three tests can be
performed for each pavement feature. The design k value should be
conservatively selected.
c) The rigid pavement design and evaluation
curves presented in this material are based on a k value determined by a static
plate load test using a
d) It is recommended that plate bearing tests
be conducted on the subgrade and the results adjusted to account for the effect
of sub-base. Figure 4-35 shows the increase in k value for various thicknesses
of sub-base over a given subgrade k. Plate bearing
tests conducted on top of sub-base courses
can sometimes yield erroneous results since the depth of influence beneath a
e) The determination of k value for
stabilized layers is a difficult problem. The k value normally has to be
estimated. It is recommended that the k value be estimated as follows. The
thickness of the stabilized layer should be
multiplied by a factor ranging from 1.2 to 1.6 to determine the equivalent
thickness of well-graded crushed aggregate. The actual value in the 1.2 to 1.6
range should be based on the quality of the stabilized layer and the thickness
of the slab relative to the thickness of the stabilized layer. High-quality
materials
which are stabilized with high percentages of
stabilizers should be assigned an equivalency factor which is higher than a
lowerquality stabilized material. For a given rigid pavement thickness, a
thicker stabilized layer will influence pavement performance more than a thin
stabilized layer and should thus be assigned a higher equivalency factor.
f) It is recommended that a design k value of
500 1b/in3 (136 MN/m3) not be exceeded for any foundation. The information
presented in Table 4-8 gives general guidance as to probable k values for
various soil types.
4.4.8 Pavement design philosophy
4.4.8.1 The
FAA policy of treating the design of aircraft landing gear and the design and
evaluation of airport pavements as three separate entities is described in
4.4.1 of this Manual. The design of airport pavements is a complex engineering
problem which involves a large number of interacting variables. The design
curves presented in this Section are based on the CBR method of design for flexible
pavements and a jointed edge stress analysis for rigid pavements. These procedures represent a change from
prior FAA design methods and will result in slightly different pavement
thicknesses. Because of thickness variations, the evaluation of existing
pavements should be performed using the same method as was employed in the
design. Details on how the new FAA methods of design were developed are as
follows:
4.4.8.2 Flexible
pavements. The flexible pavement design curves presented in this
Section are based on the California Bearing
Ratio (CBR) method of design. The
method is basically empirical; however, a
great deal of research has been done method and reliable correlations have been
developed. Gear configurations are
using theoretical concepts as well as
empirically developed data. The design curves provide the required total
thickness of flexible pavement (surface, base, and sub-base) needed to support
a given weight of aircraft over a particular subgrade. The curves also show the
required surface thickness. Minimum base course thicknesses are shown on a
separate curve. A more detailed discussion of CBR design is presented in
Appendix 4.
CBR design with the related
4.4.8.3 Rigid
pavements.
The rigid pavement design curves in this
Section are based on the Westergaard analysis of edge loading. The edge loading
analysis has been modified to simulate a jointed edge condition. Design curves
are furnished for areas where traffic will predominantly follow parallel to the
joints and for areas where FAA rigid pavement stresses are higher and field
performance show practically all load induced cracks develop at the jointed
edge and migrate towards the slab interior. For these reasons the basis of
design was changed from interior to jointed edge. The design curves contain
lines for five
The thickness of pavement determined from the
curves is thicknesses are determined separately. A more detailed pavement
design is presented in Appendix 4.
traffic is likely to cross joints at some
acute angle. Previous criteria were based on an interior loading assumption.
Pavement at the jointed edge than at the slab interior. Test validations
different annual traffic volumes. for slab thickness only. Sub-base discussion
of the basis for rigid
4.4.9 Background
4.4.9.1 An
airfield pavement and the operating aircraft represent an interactive
system which must be recognized in the
pavement design process. Design
considerations associated with both the aircraft and the pavement must be
satisfied in order to produce a satisfactory design. Careful construction
control and some degree of maintenance will be required to produce a pavement
which will achieve the intended design life. Pavements are designed to provide
a finite life and fatigue failures are anticipated. Poor construction and lack
of preventative maintenance will usually result in disappointing performance of
even the best designed pavement.
4.4.9.2 The determination of pavement
thickness requirements is a complex engineering problem. Pavements are subject
to a wide variety of loadings and climatic effects. The design process involves
a large number of interacting variables which are often difficult to quantify.
Although a great deal of research work has been completed and more is underway,
it has been impossible to arrive at a direct mathematical solution of thickness
requirements. For this reason the determination of pavement thickness must be
based on the theoretical analysis of load distribution through pavements and
soils, the analysis of experimental pavement data, and a study of the
performance of pavements under actual service conditions. Pavement thickness
curves presented in this Section have been developed through correlation of the
data obtained from these sources. Pavements designed in accordance with these
standards are intended to provide a structural life of 20 years that is free of
major maintenance if no major changes in forecast traffic are encountered. It
is likely that rehabilitation of surface grades and renewal of skid resistant
properties will be needed before 20 years owing to destructive climatic effects
and deteriorating effects of normal usage.
4.4.9.3 The
structural design of airport pavements consists of determining both the
over-all pavement thickness and the thickness of the component parts of the
pavement. There are a number of factors which influence the thickness of
pavement required to provide satisfactory service. These include the magnitude
and character of the aircraft loads to be supported, the volume of traffic, the
concentration of traffic in certain areas, and the quality of the subgrade soil
and materials comprising the pavement structure.
4.4.10 Aircraft considerations
4.4.10.1 Load.
The pavement design method is based on the gross weight of the aircraft. For
design purposes the pavement should be designed for the maximum take-off weight
of the aircraft. The design procedure assumes 95 per cent of the gross weight
is carried by the main landing gears and 5 per cent is carried by the nose
gear. The maximum take-off weight should be used in calculating the pavement
thickness required. Use of the maximum take-off weight is recommended to
provide some degree of conservatism in the design and is justified by the fact
that changes in operational use can often occur and recognition of the fact
that forecast traffic is approximate at best. By ignoring arriving traffic some
of the conservatism is offset.
4.4.10.2 Landing
gear type and geometry
a) The gear type and configuration dictate
how the aircraft weight is distributed to the pavement and determine pavement
response to aircraft loadings. It would have been impractical to develop design
curves for each type of aircraft. However, since the thickness of both rigid
and flexible pavements is dependent upon the gear dimensions and the type of
gear, separate design curves would be necessary unless some valid assumptions
could be made to reduce the number of variables. Examination of gear
configuration, tire contact areas, and tire pressure in common use indicated
that these follow a definite trend related to aircraft gross weight. Reasonable
assumptions could therefore be made and design curves constructed from the
assumed data. These assumed data are as follows:
1) Single
gear aircraft. No special
assumptions needed.
2) Dual
gear aircraft. A study of the spacing between dual wheels for these aircraft
indicated that a dimension of
3) Dual tandem gear aircraft. The
study indicated a dual wheel spacing of
4) Wide body aircraft. Wide body aircraft,
i.e.,B-747, DC-10, and L-1011 represent a radical departure from the geometry
assumed for dual tandem aircraft described in 3 above. Owing to the large
differences in gross weights and gear geometries, separate design curves have
been prepared for the wide body aircraft.
b) Tire pressure varies between 75 and 200
psi (0.52 to 1.38 MPa) depending on gear configuration and gross weight. It
should be noted that tire pressure asserts less influence on pavement stresses
as gross weight increases, and the assumed maximum of 200 psi (1.38 MPa) may be
safely exceeded if other parameters are not exceeded.
4.4.10.3 Traffic
volume. Forecasts of annual departures by aircraft type are needed for pavement
design. Information on aircraft operations is available from Airport
Master Plans, Terminal Area Forecasts, the
4.4.11 Determination of design aircraft.
4.4.11.1 The
forecast of annual departures by aircraft type will result in a list of a
number of different aircraft. The design aircraft should be selected on the
basis of the one requiring the greatest pavement thickness. Each aircraft type
in the forecast should be checked to determine the pavement thickness required
by using the appropriate design curve with the forecast number of annual
departures for that aircraft. The aircraft type which produces the greatest
pavement thickness is the design aircraft. The design aircraft is not
necessarily the heaviest aircraft in the forecast.
4.4.12 Determination of equivalent annual
departures by the design aircraft
4.4.12.1 Since
the traffic forecast is a mixture of a variety of aircraft having different
landing gear types and different weights, the effects of all traffic must be
accounted for in terms of the design aircraft. First, all aircraft must be
converted to the same landing gear type as the design aircraft. The following
conversion factors should be used to convert from one landing gear type to
another:
To convert from To Multiply
departures by
single wheel dual wheel 0.8
single wheel dual tandem 0.5
dual wheel dual tandem 0.6
double dual tandem dual tandem 1.0
dual tandem single wheel 2.0
dual tandem dual wheel 1.7
dual wheel single wheel 1.3
double dual tandem dual wheel 1.7
Secondly, after the aircraft have been
grouped into the same landing gear configuration, the conversion to equivalent
annual departures of the design aircraft should be determined by the following
formula:
where R1 = equivalent
annual departures by the design aircraft
R2 = annual
departures expressed in design aircraft landing gear
Wl = wheel
load of the design aircraft
W2 = wheel
load of the aircraft in question
For this computation 95
per cent of the gross weight of the aircraft is assumed to be carried by the
main landing gears. Wide body aircraft require special attention in this
calculation. The procedure discussed above is a relative rating which compares
different aircraft to a common design aircraft. Since wide body aircraft have
radically different landing gear assemblies than other aircraft, special
considerations are needed to maintain the relative effects. This is done by
treating each wide body as
a
After the equivalent annual departures are
determined, the design should proceed using the appropriate design curve for
the design aircraft. For example, if a wide body is the design aircraft, all
equivalent departures should be calculated as described above, then the design
curve for the wide body should be used with the calculated equivalent annual
departures.
4.4.12.2 Example: Assume an airport pavement
is to be designed for the following forecast traffic:
Solution
a) Determine design aircraft. A
pavement thickness is determined for each aircraft in the forecast using the
appropriate design curves. The pavement input data, CBR, k value, flexural
strength, etc., should be the same for all aircraft. Aircraft weights and
departure levels must correspond to the particular aircraft in the forecast. In
this example the 727-200 requires the greatest pavement thickness and is thus
the design aircraft.
b) Group forecast traffic into landing
gear of design aircraft. In this example the design aircraft is equipped
with a dual wheel landing gear so all traffic must be grouped into the dual
wheel configuration.
c) Convert aircraft to equivalent annual
departures of the design aircraft. After the aircraft mixture has been
grouped into a common landing gear configuration, the equivalent annual
departures of the design aircraft can be calculated.
Wheel loads for wide body aircraft will be
taken as the wheel load for a
(
d) For this example the pavement would be
designed for 16 000 annual departures of a dual wheel aircraft weighing
4.4.13 Designing the flexible pavement
4.4.13.1 Flexible
pavements consist of a bituminous wearing surface placed on a base course and,
when required by subgrade conditions, a sub-base. The entire flexible pavement
structure is ultimately supported by the subgrade. Definitions of the function
of the various components are given in the following paragraphs. For some
aircraft
the base and sub-base have to be constructed
of stabilized materials. The requirements for stabilized base and sub-base are
also discussed in 4.4.15.
4.4.13.2 Use
of the design curves for flexible pavements requires a CBR value for the
subgrade material, a CBR value for the sub-base material, the gross weight of
the design aircraft, and the number of annual departures of the design
aircraft. The design curves presented in Figures 4-36 to 4-44 indicate the
total pavement thickness required and the thickness of bituminous surfacing.
Figure 4-45 indicates the minimum thickness
of base course for given total pavement
thicknesses and CBR values. For annual departures in excess of 25 000 the total
pavement thickness should be increased in accordance with 4.4.24 and the
bituminous surfacing increased by
4.4.14 Critical and non-critical areas
4.4.14.1 The
design curves, Figures 4-36 to 4-44, are used to determine the total critical pavement
thickness, T and the surface course thickness requirements. The 0.9T factor for
the non-critical pavement applies to the base and sub-base courses; the surface
course thickness is as noted on the design curves. For the variable section of
the transition section and thinned edge, the reduction applies only to the base
The 0.7T thickness for base shall be minimum
permitted, and the sub-base
drainage from the use the next higher
course.
thickness shall be increased or varied to
provide positive surface entire subgradc surface. For fractions of an inch of
0.5 or more, whole number; for less than 0.5, use the next lower number.
4.4.15 Stabilized base and sub-base
4.4.15.1 Stabilized
base and sub-base courses are necessary for new pavements
designed to accomodate jet aircraft weighing
factor is sensitive to a number of variables
such as layer thickness, stabilizing agent type and quantity, location of
stabilized layer in the pavement structure, etc.
4.4.15.2 Exceptions
to the policy requiring stabilized base and sub-base should be based on proven performance
of a granular material. Proven performance in this instance means a history of
satisfactory airport pavements using the materials. This history of
satisfactory performance should be under aircraft loadings and climatic
conditions comparable to those anticipated.
4.4.15.3 Other
exceptions may be made on the basis of superior materials being available, such
as 100 per cent crushed, hard, closely graded stone. These materials should
exhibit a remoulded soaked CBR minimum of 100 for base and 35 for sub-base. In
areas subject to frost penetration the materials should meet permeability and
non-frost susceptibility tests in addition to the CBR requirements.
4.4.15.4 The
minimum total pavement thickness should not be less than the total pavement
thickness required by a 20 CBR subgrade on the appropriate design curve.
Reflection cracking is sometimes encountered
when cement treated base is used.
The thickness of the bituminous surfacing
course should be at least
4.4.16 Stabilized sub-base and base
equivalency factors
4.4.16.1 Stabilized
sub-base courses offer some structural benefits to a flexible pavement. The
benefits can be expressed in the form of equivalency factors which indicate the
substitution thickness ratios applicable to various stabilized layers. The
thickness of stabilized material can be computed by dividing the granular
sub-base thickness requirement by the equivalency factor. The equivalency factor
ranges are presented in Table 4-9 below.
In establishing the equivalency factors shown
above, the CBR of the gravel sub-base course was assumed to be 20.
4.4.16.2 Stabilized
base courses offer structural benefits to a flexible pavement in much the same
manner as stabilized sub-base. The benefits are expressed as equivalency
factors similar to those shown for stabilized sub-base. These ratios are used
to compute the thickness of stabilized base by dividing the granular base requirement
by the equivalency factor. The equivalency factor ranges are presented in Table
4-10 below.
Table 4-10. Recommended equivalency factor
range stabilized base
The equivalency factors shown above assume a
aggregate base course.
CBR value of 80 for crushed
4.4.17 Design example
4.4.17.1 As
an example of the use of the design curves, assume a flexible pavement is to be
designed for a dual gear aircraft having a gross mass of
the sub-base and subgrade are 20 and 6,
respectively.
4.4.17.2 Total
pavement thickness. The total pavement thickness required is
determined from Figure 4-37. Enter the upper abscissa with the subgrade
CBR value, 6. Project vertically downward to the gross mass of the design
aircraft,
(
4.4.17.3 Thickness
of sub-base course. The thickness of the sub-base course is determined in a
manner similar to the total pavement thickness. Using Figure 4-37 enter the
upper abscissa with the design CBR value for the sub-base, 20. The chart is
used in the same manner as described in 4.4.17.2 above, i.e.,vertical
projection to aircraft gross weight, horizontal projection to annual
departures, and vertical projection to lower abscissa. In this example the
thickness obtained is
20 CBR sub-base is
4.4.17.4 Thickness
of bituminous surface. As indicated by the Note in Figure 4-37,
the thickness of bituminous surface for
critical areas is
4.4.17.5 Thickness
of base course. The thickness of base course can be computed by subtracting
the thickness of bituminous surface from the combined thickness of surface and
base determined in 4.4.17.3 above; in this example 8.6 - 4.0 =
in this example, 6. From the intersection of
the horizontal projection and the subgrade CH line, make a vertical projection
down to the lower abscissa and read the minimum base course thickness, in this
example the minimum thickness of
4.4.17.6 Thickness
of non-critical areas. The total pavement thickness for non-critical areas
is obtained by taking 0.9 of the critical pavement base and sub-base thickness
plus the required bituminous surface thickness given on the design charts. For
the thinned edge portion of the critical and non-critical pavements, the 0.7T
factor applies only to the base course because the sub-base should allow for
transverse drainage.
4.4.17.7 Summary.
The thickness calculated in the above paragraphs should be rounded off to even
increments. If conditions for detrimental frost action exist, another analysis
is required. The final design thicknesses for this example would be as follows:
Thickness Requirements
Critical Non-critical
in (cm) in (cm)
Bituminous surface 4 (10) 3 (8)
Base course
6 (15)
5 (13)
Sub-base course 11 (28) 10 (25)
Transverse drainage 0 (0) 3 (8)
Since the design aircraft in this example
weighs less than 100 000 1b
(
used if desired.
4.4.18 Designing the rigid pavement
4.4.18.1 Design
curves have been prepared for rigid pavements similar to those for flexible
pavements; i.e., separate curves for single, dual, and dual tandem landing gear
assemblies and separate design curves for
wide-body jet aircraft. See Figures 4-46 to
4-54. These curves are based on a jointed
edge loading assumption where the load is tangent to the joint.
Use of the design curves requires four design
input parameters: concrete flexural strength, subgrade modulus, gross weight of
the design aircraft, and annual departure of the design aircraft. The rigid
pavement design curves indicate the thickness of concrete only. Thicknesses of
other components of the rigid pavement structure must be determined separately.
4.4.18.2 Concrete
flexural strength. The required thickness of concrete pavement is related
to the strength of the concrete used in the pavement. Concrete strength is
assessed by the flexural strength method as the primary action of a concrete
pavement slab is flexure. Concrete flexural strength should be determined by ASTM C-78 test
method. Normally a 90-day flexural strength
is used for design. The designer can
safely assume the 90-day flexural strength of
concrete will be 10 per cent
higher than
the 28-day strength.
4.4.18.3 k
value. The k value is, in effect, a spring constant for the material
supporting the rigid pavement and is indicative of the bearing value of the
supporting material.
4.4.18.4 Gross
weight of aircraft. The gross weight of the design aircraft is shown on each
design curve. The design curves are grouped in accordance with main landing
gear assembly type except for wide body aircraft which are shown on separate
curves. A wide range of gross weights is shown on all curves to assist in any
interpolations which may be required. In all instances, the range of gross
weights shown is adequate to cover weights of existing aircraft.
4.4.18.5 Annual
departure of design aircraft. The fourth input annual departures of the design
aircraft. The departures should be the procedure explained in 4.4.12.
parameter is computed using
4.4.18.6 Use
of design curves. The rigid pavement design curves are constructed such that
the design inputs are entered in the same order as they are discussed above.
Concrete flexural strength is the first input. The left ordinate of the design
curve is entered with concrete flexural strength. A horizontal projection is
made until it intersects with the appropriate foundation modulus line. A
vertical projection is
made from the intersection point to the
appropriate gross weight of the design aircraft. A horizontal projection is
made to the right ordinate showing annual departures. The pavement thickness is
read from the appropriate annual departure line. The pavement thickness shown
refers to the thickness of the concrete pavement only, exclusive of
the sub-base.
4.4.19 Sub-base requirements
4.4.19.1 The purpose of a sub-base under a
rigid pavement is to provide uniform
stable support for the pavement slabs. A
minimum thickness of 4 in (
is required under all rigid pavements, except
as shown in Table 4-11 below:
4.4.19.2 Sub-base thickness in excess of
savings in concrete thickness. The materials
suitable for sub-base courses under rigid
pavements are listed below:
Gravel sub-base course
Bituminous base course
Aggregate base course
Crushed aggregate base course
Soil cement base course
Cement treated base course
4.4.19.3 Determination
of k value for granular sub-base. The probable increase in k value
associated with various thicknesses of different sub-base materials is shown in
Figure 4-35. Figure 4-35 is intended for use when the sub-base is composed of
unstabilized granular materials. Values shown in Figure 4-35 are to be
considered guides and can be tempered by local experience.
4.4.21.2 Determination
of k value for stabilized sub-base. The effect of stabilized sub-base is
reflected in the foundation modulus. The difficulty in assigning a foundation
modulus is that test data will not be available during the design phase. Figure
4-55 shows the probable increase in k value with various thicknesses of
stabilized sub-base located on subgrades of varying moduli. Figure 4-55 is
applicable to cement stabilized and bituminous stabilized layers. Figure 4-55
was developed by assuming a stabilized layer is twice as effective as a
well-graded crushed aggregate in increasing the subgrade modulus. Stabilized
layers of lesser quality should be assigned somewhat lower k values. After k
value is assigned to the stabilized sub-base, the design procedure is the same
as described in 4.4.18.
4.4.22 Design
example
4.4.22.1 As
an example of the use of the design curves, assume that a rigid pavement is to
be designed for dual tandem aircraft having a gross weight of
(
equivalent annual departures of 6 000
includes 1 200 annual departures of B-747 aircraft
weighing
(25 MN/m3) with poor drainage and frost
penetration is
4.4.22.2 The
gross weight of the design aircraft dictates the use of a stabilized sub-base.
Several thicknesses of stabilized sub-bases should be tried to determine the
most economical section. Assume a cement stabilized sub-base will be used. Try
a sub-base thickness of
4.4.23 Optional rigid pavement design
curves
4.4.23.1 When
aircraft loadings are applied to a jointed edge, the angle of the landing gear
relative to the jointed edge influences the magnitude of the stress in the slab.
Figures 4-46 and 4-47, single wheel and dual wheel landing gear assemblies, are
at the maximum stress when the gear is located parallel to the joint. Dual
tandem assemblies do not produce the maximum stress when located parallel to
the joint. Locating the dual tandem at an acute angle to the jointed edge will
produce the maximum stress. Design curves, Figures 4-56 through 4-62, have been
prepared for dual tandem gears located tangent to the jointed edge but rotated
to the angle causing the maximum stress. These design curves can be used to
design pavements in areas where aircraft are likely to cross the pavement
joints at angles at low speeds such as runway holding aprons, runway ends,
runway-taxiway intersections, aprons, etc. Use of Figures 4-56 to 4-62 is
optional and should only be applied in areas where aircraft are likely to cross
pavement joints at an angle and at low speeds.
4.4.24 High traffic volumes
4.4.24.1 There are a number of airports which
experience traffic intensities far in excess of those indicated on the design
curves. Tn these situations, maintenance is nearly impossible due to traffic
intensity and makes initial construction even more important. Unfortunately,
little information exists on the performance of airport pavements under high
traffic intensities except for the experience gained through observation of
in-service pavements. Rigid pavements designed to serve in situations where
traffic intensity is high should reflect the following considerations.
4.4.24.2 Foundation.
The foundation for the pavement provides the ultimate support to the structure.
Every effort should be made to provide a stable foundation as problems arising
later from an inadequate foundation cannot be practicably corrected after the
pavement is constructed. The use of stabilized sub-base will aid greatly in
providing a uniform, stable foundation. Generally speaking, the most efficient
combination of rigid pavement thickness and stabilized sub-base thickness for
structural capacity is a 1:1 ratio.
4.4.24.3 Thickness.
Pavements subjected to traffic intensities greater than the 25 000 annual
departure level shown on the design curves will require more thickness to
accommodate the traffic volume. Additional thickness can be provided by
increasing the pavement thickness in accordance with Table 4-12 shown below:
Table 4-12. Pavement thickness for high
departure level expressed as a percentage of the 25 000 departure thickness
Annual departure level
Percentage of25 000 departure thickness
50 000
104
100 000 108
150 000 110
200 000 112
The values given in Table 4-12 are based on
extrapolations of research data and observations of in-service pavements. Table
4-12 was developed assuming a logarithmic relationship between percentage of
thickness and departures.
4.4.24.4 Panel
size. Slab panels should be constructed to minimize joint movement. Small
joint movement tends to provide for better load transfer across joints and
reduces the elongation the joint sealant materials must accommodate when the
slabs expand and contract. High-quality joint sealants should be specified to
provide the best possible performance.
4.4.25 Reinforced concrete pavement
4.4.25.1 The
main benefit of steel reinforcing is that, although it does not prevent
cracking, it keeps the cracks that form tightly closed so that the interlock of
the irregular faces provides structural integrity and usually improves pavement
performance. By holding the cracks tightly closed, the steel minimizes the
infiltration of debris into the cracks. The thickness requirements for
reinforced concrete pavements are the same as plain concrete and are determined
from the appropriate design curves. Steel reinforcement allows longer joint
spacings, thus the cost benefits associated with fewer joints must be
determined in the decision to use plain or reinforced concrete pavement.
4.4.25.2 Type
and spacing of reinforcement. Reinforcement may be either welded
wire fabric or bar mats installed with end
and side laps to provide complete reinforcement throughout the slab panel. End
laps should be a minimum of
4.4.25.3 Amount
of reinforcement
a) The steel area required for a reinforced
concrete pavement is determined from the subgrade drag formula and the
coefficient of friction formula combined. The resultant formula is expressed as
follows:
where;
As = area of steel per foot of width or
length, square inches
L = length or width of slab, feet
t = thickness of slab, inches
fs = allowable tensile stress in steel, psi
Note.- To determine the area of steel in metric units:
L = should be expressed in metres
t = should be expressed in millimetres
fs =should be expressed in meganewtons per
square metre
The constant 3.7 should be changed to 0.64
As = will then be in terms of square
centimetres per metre
b) In this formula the slab weight is assumed
to be
steel. It is recommended that allowable
tensile stress be taken as two-thirds of the yield strength of the steel. Based
on current specifications the yield strengths and corresponding design stresses
(fs) are as listed in Table 4-13.
Table 4-13. Yield strengths of various grades
of reinforcing steel
3-186
Aerodrome Design Manual
separation course is likely to become
saturated with water and provide rather unpredictable performance. Saturation
of the separation course can be caused by the infiltration of surface water,
ingress of ground or capillary water, or the condensation of water from the
atmosphere. In any event, the water in the separation course usually cannot be
adequately drained and drastically reduces the stability of the overlay.
b)Bituminous overlays for increasing strength
should have a minimum thickness of
4.4.26.3 Bituminous
overlays on existing flexible pavement
a)Use the appropriate basic flexible pavement
curves to determine the thickness requirements for a flexible pavement for the
desired load and number of equivalent design departures. A CBR value is
required for the subgrade material and sub-base. Thicknesses of all pavement
layers must be determined. The thickness of pavement required over the subgrade
and sub-base and the minimum base course requirements must be compared with the
existing pavement to determine the overlay requirements.
b) Adjustments to the various layers of the
existing pavement may be necessary to complete the design. Bituminous surfacing
may have to be converted to base, and base to sub-base conversion may be
required. A high-quality material may be converted to a lowerquality material,
such as surfacing to base. A material may not be converted to a higher quality
material. For example, excess sub-base cannot be converted to base. The
equivalency factors shown in Tables 4-9 and 4-10 may be used as guidance in the
conversion of layers. It must be recognized that the values shown are for new
materials and the assignment of factors for existing pavements must be based on
judgement and experience. Surface cracking, high degree of oxidation, evidence
of low stability, etc., are only a few of the considerations which would tend
to reduce the equivalency factor. Any bituminous layer located between granular
courses in the existing pavement should be evaluated inch for inch as granular
base or sub-base course.
c)To illustrate the procedure of designing a
bituminous overlay, assume an existing taxiway pavement composed of the
following section. The subgrade CBR is 7, the bituminous surface course is 
The total pavement thickness must be
d) The most difficult part of designing
bituminous overlays for flexible pavements is the determination of the CBR
values for the subgrade and sub-base and conversion of layers. Subgrade and
sub-base CBR values can best be determined by conducting field in-place CBR
tests. The subgrade and sub-base must be at the equilibrium moisture content
when field CBR tests are conducted. Normally a pavement which has been in place
for at least 3 years will be in equilibrium. Layer conversions, i.e.,
converting base
to sub-base, etc., are largely a matter of
engineering judgement. When performing the conversions, it is recommended that
any converted thicknesses never be rounded off.
4.4.26.5 Bituminous
overlay on existing rigid pavement. To establish the required thickness of
bituminous overlay for an existing rigid pavement, it is first necessary to
determine the single thickness of rigid pavement required to satisfy the design
conditions. This thickness is then modified by a factor F which controls the
degree of cracking which will occur in the existing rigid pavement. The
effective thickness of the existing rigid pavement is also adjusted by a
condition factor Cb. The F and Cb factors perform two different functions in
the bituminous overlay determination as discussed below:
a) The factor F which controls the degree of
cracking which will occur in the base pavement is a function of the amount of
traffic and the subgrade strength. The F factor selected will dictate the final
condition of the overlay and base pavement. The F factor in effect is
indicating that the entire concrete single slab thickness determined from the
design curves is not needed because a bituminous overlay pavement is allowed to
crack and deflect more than a conventional rigid pavement. More cracking and
deflection is allowable as the bituminous surfacing will not spall and can conform
to greater deflections than a totally rigid pavement. Photographs of various
overlay and
base pavements shown in Figure 4-63
illustrate the meaning of the F factor. Figures 4-63
a), b) and c) show how the overlay and base
pavements fail as more traffic is applied to a bituminous overlay on an
existing rigid pavement. In the design of a bituminous overlay,
the
condition of the overlay and base pavement after the design life should be
close to that shown in Figure 4-63 b). Figure 4-64 is a graph enabling the
designer to select the appropriate F value to yield a final condition close to
that shown in Figure 4-63 b).
b) The condition factor Cb applies to the
existing rigid pavement. The Cb factor is an assessment of the structural
integrity of the existing pavement. The determination of the proper Co, value
is a judgement decision for which only general guidelines can be provided. A Cb
value of 1.0 should be used when the existing slabs contain nominal initial
cracking and 0.75 when the slabs contain multiple cracking. The designer is
cautioned that the range of Cb values used in bituminous overlay designs is
different from the Cr values used in rigid overlay pavement design. The minimum
Cb value is 0.75. A single Cb should be established for an entire area. The Cb
value should not be varied along a pavement feature.
c) After the F factor, condition factor Cb,
and single thickness of rigid pavement have been established, the thickness of
the bituminous overlay is computed from the following formula:
where
t = thickness of bituminous overlay, inches
F = factor which controls the degree of
cracking in the base pavement
h = single thickness of rigid pavement
required for design conditions, inches. Use the exact value of h; do not round
off.
Cb = condition factor for base pavement
ranging from 1.0 to 0.75
he = thickness of existing rigid pavement,
inches
Calculation of bituminous overlay thickness
in metric units should be performed using the formula below:
where
t is in centimetres
h is in centimetres
he is in centimetres
d) The design of a bituminous overlay for a
rigid pavement which has an existing bituminous overlay is slightly different.
The designer should treat the problem as if the existing bituminous overlay
were not present, calculate the overlay thickness required, and then adjust the
calculated thickness to compensate for the existing overlay. If this procedure
is not used, inconsistent results will often be produced.
1) An example of the procedure follows.
Assume an existing pavement consists of a
Cb of 0.9 are appropriate for the existing
conditions.
2) Calculate the required thickness of
bituminous overlay as if the existing
t = 2.5 (0.9 x 14 - 0.9 x 10)
t =
3) An allowance is then made for the existing
bituminous overlay. In this example assume the existing overlay is in such a
condition that its effective thickness is only
e) The formula for calculating the thickness
of bituminous overlays
on rigid pavements is limited in application
to overlay thicknesses which are equal to or less than the thickness of the
base rigid pavement. If the overlay thickness exceeds the thickness of the base
pavement, the designer should consider designing the overlay as a flexible
pavement and treating the existing rigid pavement as a high-quality base
material. This limitation is based on the fact that the formula assumes the
existing rigid pavement will support considerable load by flexural action.
However, the flexural contribution becomes negligible for thick bituminous
overlays.
4.4.26.6 Design
of concrete overlays. Concrete overlays can be constructed on existing
rigid or flexible pavements. The minimum allowable thickness for concrete
overlays is
4.4.26.7 Concrete
overlay on flexible pavement. The design of concrete overlays on existing
flexible pavements is based on the design curves in 4.4.18. The existing
flexible pavement is considered a foundation for the overlay slab.
a) For design of the rigid pavement, the
existing flexible pavement
shall be assigned a k value using Figure 4-35
or 4-55 or by conducting a plate bearing test on the existing flexible
pavement. In either case the k value assigned should not exceed 500.
b) When frost conditions require additional
thickness, the use of non-stabilized material is not allowed as this would
result in a sandwich pavement. The frost protection must be provided by
stabilized material.
4.4.26.8 Concrete
overlay on rigid pavement. The design of concrete overlays on existing
rigid pavements is also predicated on the rigid pavement design curves. The
rigid pavement design curves indicate the thickness of concrete required to
satisfy the design conditions for a single thickness of concrete pavement. Use
of this method requires the designer to assign a k value to the existing
foundation. The k value may be determined by field bearing tests conducted in
test pits cut through the existing rigid pavement, or may be estimated from construction
records for the existing pavement. The design of a concrete overlay on a rigid
pavement requires an assessment of the structural integrity of the existing
rigid pavement. The condition factor should be selected after a pavement
condition survey. The selection of a condition factor is a matter of
engineering judgement. The use of non-destructive testing (NDT) can be of
considerable value in assessing the condition of an existing pavement. NDT can
also be used to determine sites for test pits. In order to provide a more
uniform assessment of condition factors, the following values are defined:
Cr = 1.0 for existing pavement in good
condition - some minor cracking evident but no structural defects.
Cr = 0.75 for existing pavement containing
initial corner cracks due to loading but no progressive cracking or joint
faulting.
Cr = 0.35 for existing pavement in poor
structural condition - badly cracked or crushed and faulted joints.
The three conditions discussed above are used
to illustrate the condition factor rather than establish the only values
available to the designer. Conditions at a particular location may require the
use of an intermediate value of Cr within the recommended range.
a) Concrete overlay without levelling
course. The thickness of the concrete overlay slab applied directly over
the existing rigid pavement is computed by the following formula:
he = required thickness of concrete overlay
h= required single slab thickness determined
from design curves
he = thickness of existing rigid pavement
Cr = condition factor
Due to the inconvenient exponents in the
above formula, graphic displays of the solution of the formula are given in
Figures 4-65 and 4-66. These graphs were prepared for only two different condition
factors, Cr = 1.0 and 0.75. The use of a concrete overlay pavement directly on
an existing rigid pavement with a condition factor of less than 0.75 is not
recommended because of the likelihood of reflection cracking.
b) Concrete overlay with levelling course. In
some instances it may be necessary to apply a levelling course of bituminous
concrete to an existing rigid pavement prior to the application of the concrete
overlay. Under these conditions a different formula for the computation of the
overlay thickness is required. When the existing pavement and overlay pavement
are separated, the slabs act more independently than when the slabs are in
contact with each other. The formula for the thickness of an overlay slab when
a levelling course is used is as follows:
he = required thickness of concrete overlay
h= required single slab thickness determined
from design curves
he = thickness of existing rigid pavement
Cr = condition factor
The levelling course must be constructed of
highly stable bituminous concrete. A granular separation course is not allowed
as this would constitute sandwich construction. Graphic solutions of the above
equation are shown in Figures 4-67 and 4-68. These graphs were prepared for
condition factors of 0.75 and 0.35. Other condition factors between these
values can normally be computed to sufficient accuracy by interpolation.
c) Bonded concrete overlays. Concrete
overlays which are bonded to existing rigid pavements are sometimes used under
certain conditions. By bonding the concrete overlay to the existing rigid
pavement the new section behaves as a monolithic slab. The thickness of bonded
overlay required is computed by subtracting the thickness of the existing
pavement from the thickness of the required slab thickness determined from
design curves.
hc=h-he
where:
he = required thickness of concrete overlay
h = required single slab thickness determined
from design curves
he = thickness of existing rigid pavement
Bonded overlays should be used only when the
existing rigid pavement is in good condition. Defects in the existing pavement
are more likely to reflect through a bonded overlay than other types of
concrete overlays. The major problem likely to be encountered with bonded
concrete overlays is achieving adequate bond. Elaborate surface preparation and
exacting construction techniques are required to ensure bond.
4.4.27 Pavement evaluation
4.4.27.1 Purposes
of pavement evaluation
a) Airport pavements are evaluated for
several reasons. Evaluations are needed to establish load carrying capacity for
expected operations, to assess the ability of pavements to support significant
changes from expected volumes or types of traffic, and to determine the
condition of existing pavements for use in the planning or design of improvements
which may be required to upgrade a facility.
b) Evaluation procedures are essentially the
reversal of design procedures. Since the new FAA design methodology described
in this Manual may result in slightly different thicknesses than other design
methods it would be inappropriate to evaluate
existing pavements by the new method unless they had also been designed by that
method. This could reduce allowable loads and penalize aircraft operators. To
avoid this situation, pavements should be evaluated for the various conditions
indicated in the following paragraphs.
4.4.27.2 Evaluations
for expected operations. When airport pavements are subjected to the loads
which were anticipated at the time of design, their evaluation should be based
on that original design method. For example, if a pavement was designed by
method X to serve certain aircraft for a 20-year life and the traffic using the
pavement is essentially the same as was anticipated at the time of design, the
pavement should be evaluated according to method X. The evaluator should
recognize that some deterioration will occur over the 20 year design life. The
load bearing strength of the pavement should not be reduced if the pavement is
providing a safe operational surface. The prior evaluation curves are furnished
in Appendix 4, to facilitate this evaluation policy.
See Figures A4-8 to A4-21.
4.4.27.3 Evaluations
for changing traffic. Evaluations are sometimes required to determine the
ability of an existing pavement to support substantial changes in pavement
loadings. This can be brought on by the introduction of different types of
aircraft or changes in traffic volume. In these instances it is also
recommended that existing pavements be evaluated according to the methods by
which they were designed. The effect of changes in traffic volume are usually
small and will not have a large impact on allowable loads. The effect of
changes in aircraft types depends on the gear weight and gear configuration of
the aircraft. The load carrying capacity of existing bridges, culverts, storm
drains, and other structures should also be considered in these evaluations.
4.4.27.4 Evaluations
for planning and design. Evaluations of existing pavements
to be used in planning or designing
improvements should be based on the method which will be used to design those
improvements. The procedures to be followed in evaluating pavements according
to the design criteria contained in this Manual are as follows:
a) Evaluation steps
1) Site inspection. This may include,
in addition to the examination of the existing drainage conditions and drainage
facilities of the site, consideration of the drainage area, outfall, water
table, area development, etc. Evidence of frost action should be observed.
2) Records research and evaluation.
This step may, at least in part, precede step 1) above. This step is
accomplished by a thorough review of construction data and history, design
considerations, specifications, testing methods and results, as-built drawings,
and maintenance history. Weather records and the most complete traffic history
available are also parts of a usable records file. When soil, moisture, and
weather conditions conducive to detrimental frost action exist, an adjustment
to the evaluation may be required.
3) Sampling and testing. The need for
and scope of physical tests and materials analyses will be based on the
findings made from the site inspection, records research, and type of
evaluation. A complete evaluation for detailed design will require more
sampling and testing than, for example, an evaluation intended for use in a
master plan. Sampling and testing is intended to provide information on the
thickness, quality and general condition of the pavement elements.
4) Evaluation report. Analysis of
steps 1), 2) and 3) should culminate in the assignment of load carrying
capacity to the pavement sections under consideration. The analyses, findings,
and test results should be incorporated in a permanent record for future
reference. While these need not be in any particular form, it is recommended
that a drawing identifying area limits of specific pavement sections be
included.
b) Direct sampling procedures. The
basic evaluation procedure for planning and design will be visual inspection
and reference to the FAA design criteria, supplemented by the additional
sampling, testing, and research which the evaluation processes may warrant. For
relatively new pavement without visible signs of wear or stress, strength may
be based on inspection of the as-constructed sections, with modification for
any material variations or deficiencies of record. Where age or visible
distress indicates the original strength no longer exists, further modification
should be applied on the basis of judgement or a combination of judgement and
supplemental physical testing. For pavements which consist of sections not
readily comparable to FAA design standards, evaluation should be based on FAA
standards after materials comparison and equivalencies have been applied.
1) Flexible pavements. Laboratory or
field CBR tests may be useful in supplementing soil classification tests.
Figure 4-69 shows the approximate relationship between the subgrade
classification formerly used by the FAA and CBR.
Figure 4-69. CBR - FAA subgrade class
comparisons
Conversion of F subgrade classification
factors to CBR is permissible where CBR tests are not feasible. The thickness
of the various layers in the flexible pavement structure must be known in order
to evaluate the pavement. Thickness may be determined from borings or test
pits. As-built drawings and records can also be used to determine thicknesses
if the records are sufficiently complete and accurate.
2) Rigid pavements. The evaluation
requires the determination of the thickness of the component layers, the
flexural strength of the concrete, and the modulus of subgrade reaction.
a) The thickness of the component layers is
usually available from construction records. Where information is not available
or of questionable accuracy, thicknesses may be determined by borings or test
pits in the pavement.
b) The flexural strength of the concrete is
most accurately determined from test beams sawed from the existing pavement and
tested in accordance with ASTM C-78. Sawed beams are expensive to obtain and
costs incurred in obtaining sufficient numbers of beams to establish a
representative sample may
be prohibitive. Construction records may be
used as a source of concrete flexural strength data, if available. The
construction data will probably have to be adjusted for age as concrete
strength increases with time. An approximate relationship between concrete
compressive strength and flexural strength exists and can be computed by the
following formula:
where
R = flexural strength
fc' = compressive strength
Tensile splitting tests (ATSM C-496) can be
used to determine an approximative value of flexural strength. Tensile
splitting strength should be multiplied by about 1.5 to approximate the
flexural strength. It should be pointed out that the relationships between
flexural strength and compressive strength or tensile splitting strength are
approximate and considerable variations are likely.
c) The modulus of subgrade reaction is
determined by plate bearing tests performed on the subgrade. These tests should
be made in accordance with the procedures established in AASHTO T 222. An
important part of the test procedure for determining the subgrade reaction
modulus is the correction for soil saturation which is contained in the
prescribed standard. The normal application utilizes a correction factor
determined by the consolidation testing of samples at in situ and saturated
moisture content. For evaluation of older pavement, where evidence exists that
the subgrade moisture has stabilized or varies through a limited range, the
correction for saturation is not necessary. If a field plate bearing test is
not practical, the modulus of subgradc reaction may be estimated by using Table
4-8.
d) Sub-bases will require an adjustment to
the modulus of subgrade reaction. The thickness of the sub-base is required to
calculate a k value for a sub-base. The sub-base thickness can be determined
from construction records or from borings. The guidance contained in 4.4.19
should be used in assigning a k value to a sub-base.
4.4.27.5 Flexible
pavements. After all of the evaluation parameters of the existing flexible
pavement have been established using the guidance given in the above
paragraphs, the evaluation process is essentially the reverse of the design
procedure. The design curves are used to determine the load carrying capacity
of the existing pavement. Required inputs are subgrade and sub-base CBR values,
thicknesses of surfacing, base
and sub-base courses and an annual departure
level. Several checks must be performed to determine the load carrying capacity
of a flexible pavement. The calculation which yields the lowest allowable load
will control the evaluation.
a) Total pavement thickness. Enter the
lower abscissa of the appropriate design curve with the total pavement
thickness of the existing pavement. Make a vertical projection to the annual
departure level line. At the point of intersection between the vertical
projection and the departure level line make a horizontal projection across the
design curve. Enter the upper abscissa with the CBR value of the subgrade. Make
a vertical projection downward until it intersects the horizontal projection
made previously. The point of intersection of these two projections will be in
the vicinity of the load lines on the design curves. An allowable load is read
by noting where the intersection point falls in relation to be load lines.
b) Thickness of surfacing and base.
The combined thickness of surfacing and base must also be checked to establish
the load carrying capacity of an existing flexible pavement. This calculation
requires the CBR of the sub-base, the combined thickness of surfacing and base
and the annual departure level as inputs. The procedure is the same as that
described in a) above, except that the sub-base CBR and combined thickness of
surfacing and base are used to enter the design curves.
c) Deficiency in base course thickness.
The thickness of the existing base course should be compared with the minimum
base course thicknesses shown in Figure 4-45. Inputs for use of this curve are
total pavement thickness and subgrade CBR. Enter the left ordinate of Figure
4-45 with the total pavement thickness. Make a horizontal projection to the
appropriate subgrade CBR line. At the point of intersection of the horizontal
projection and the subgrade CBR line, make a vertical projection down to the
lower abscissa and read the minimum base course thickness. Notice that the
minimum base course thickness is
d) Deficiency in surfacing thickness.
The thickness of the existing surface course should be compared with that shown
on the appropriate design curve. If the existing surface course is thinner than
that given on the design curve, the pavement should be closely observed for
surface failures. It is recommended that planning to correct the deficiency in
surfacing thickness be considered.
4.4.27.6 Rigid
pavements. The evaluation of rigid pavements for aircraft requires concrete
flexural strength, k value of the foundation, slab thickness, and annual departure
level as inputs. The rigid pavement design curves are used to establish load
carrying capacity. The design curves are entered on the left ordinate with the
flexural strength of the concrete. A horizontal projection is made to the k
value of the foundation. At the point of intersection of the horizontal
projection and the k line, a vertical projection is made into the vicinity of
the load lines. The slab thickness is entered on the appropriate departure
level scale on the right side of the chart.
A horizontal projection is made from the
thickness scale until it intersects the previous vertical projection. The point
of intersection of these projections will be in the vicinity of the load lines.
The load carrying capacity is read by noting where the intersection point falls
in relation to the load lines.
CHAPTER 5. - METHODS FOR IMPROVTNG RUNWAY
SURFACE TEXTURE
5.1 Purpose
5.1.1 Ecar139
requires that the surface of a paved runway be so constructed as to provide
good friction characteristics when the runway is wet. Additional provisions
contain minimum specifications for the configuration of runway surfaces and
recognize in particular the need for some form of special surface treatmernt. The purpose of this chapter is to provide
guidance on proved methods for improving runway surface texture. This includes
essential engineering criteria for the design, construction and treatment of
runway surfaces, the uniform and world-wide application o£ which is considered
important to satisfy the relevant provisions of Ecar139.
5.2 Basic Considerations
5.2.1 Historical
background
5.2.1.1 with
the steady growth of aircraft mass and the associated significant increase in
the take-off and Landing speeds, a number of operational problems have become
apparent with conventional types of runway surfaces. One of the most
significant and potentially dangerous is the aquaplaning phenomenon which has
been held responsible in a number of aircraft incidents and accidents.
5.2.1.2 Efforts
to alleviate the aquarplaning problem have resulted in the development of new
type of runway pavements of particular
surface texture and of improved drainage characteristics. Experience has shown
that these forms of surface finish, apart from successfully minimizing
aquaplaning risks, provide a substantially higher friction level in all degrees
of wetness, i.e. from damp to a flooded surface.
5.2.1.3 It
is now generally agreed that measuring and reporting wet friction conditions is
not required to he done on a daily routine basis. This is the result of the
development of a new philosophy of dealing with the wet runway problem. There
is of course a need for a general improvement of. the friction levels provided
by runway surfaces in "normal" wet conditions and for the elimination
of substandard surfaces in particular.
5.2.1.4 This
has resulted in the definition of minimum acceptable wet friction levels for
new and existing runways. Accordingly runways should be subject to periodic
,evaluation of the friction level by using the techniques identified in
Attachment B of Ecar139 and related documents. This concept favours the
application of the modern technology for the finishing of surfaces which
experience has proved effectively provides the wet friction requirements and
minimizes aquaplaning.
5.2.2 Functional
requirements
5.2.2.1 A
runway pavement, considered as a whole, is supposed to fulfil the following basic Functions:
a) to provide adequate bearing strength;
b) to provide good riding qualities; and
c) to provide good surface friction characteristics.
The first criterion addresses the structure
of the pavement, the second the geometric shape of the top of the pavement and
the third the texture of the actual surface.
5.2.2.2 ALL
three criteria are considered essential to achieve a pavement which will
functionally satisfy the operational requirements. From the operational aspect,
however, the third one is considered the most important because it has a direct
impact on the safety of aircraft operations. Regularity and efficiency may also
be affected. Thus the friction criterion may become a decisive factor for the
selection and the form of the most suitable finish of the pavement surface.
5.2.3 Problem
identification
5.2.3.1 when
in a dry and clean state, individual runways generally provide comparable
friction characteristics with operationally insignificant differences in
friction levels, regardless of the type of pavement (asphalt/cement concrete)
and the configuration of the surface. Moreover, the Friction level available is
relatively unaffected by the speed of the aircraft. Hence, the operation on dry
runway surfaces is satisfactorily consistent and no particular engineering
criteria for surface friction are, needed for this case.
5.2.3.2 In
contrast, when the runway surface is affected by water to any degree of wetness
(i.e. from a damp to a flooded state), the situation is entirely different. For
this condition, the friction levels provided by individual _runways drop
significantly from the dry value and there is considerable disparity in the
resulting friction level between different surfaces. This variance is due to
differences in the type of pavement, the form of surface finish (texture) and
the drainage characteristics (shape). Degradation of available friction (which
is particularly evident when aircraft operate at high speeds) can have serious
implications on safety, regularity or efficiency of operations. the extent will
depend on the friction actually required versus the friction provided.
5.2.3.3 The
typical reduction of friction when a surface is wet and the reduction of
friction as aircraft speed increases are
explained by the combined effect of viscous and dynamic water pressures to
which the tire/surface is subjected. This pressure causes a partial loss of
"dry" contact the extent of which tends to increase with speed. there
are conditions where the loss is practically total and the friction drops to
negligible values. This is identified
a
viscous, dynamic or rubber-reverted aquaplaning. The manner in which these
phenomena affect different areas of the tire/surface interface and how they
change in size with speed is illustrated in figure. 5-1.
Figure
5-1. Areas of tire/surface interface
5.2.3.4 In
the light of these considerations, it may be said that the wet runway case
appears as a significant hazard and a potential threat to flight operations.
Efforts to achieve a general improvement of the situation are, therefore, well
justified. As mentioned earlier, the application of modern runway surface
treatment is considered the most practical and effective technique to improve
the friction characteristics of a wet runway.
5.2.4 Design
objectives
5.2.4.1 In
the light of the foregoing considerations, the objectives for runway pavement
design, which are similarly applicable for maintenance, can be formulated as
follows:
A runway pavement should be so designed and
maintained as to provide a runway surface which meets adequately all functional
requirements at all times throughout the anticipated lifetime of the pavement,
in particular:
a) to provide in all anticipated conditions
of wetness, high friction levels and uniform friction characteristics; and
b) to minimize the potential risk of all
forms of aquaplaning, i.e. viscous, dynamic and rubber-reverted aquaplaning. Information on these types of
aquaplaning is contained in the EAC 139-19 .
5.2.4.2 As
is outlined below, the provision of adequate wet runway friction is closely
related to the drainage characteristics of the runway surface. The drainage
demand in turn is determined by local precipitation rates. Drainage demand,
therefore, is a local variable which will essentially determine the engineering
efforts and associated investments/costs required to achieve the objective. In
general, the higher the drainage demand, the more stringent the interpretation
and application of the relevant engineering criteria will become.
5.2.5 Physical
design criteria
5.2.5.1 General.
The problem of friction on runway surfaces affected by water can in the light
of the latest state-of-the-art be interpreted as a generalized drainage problem
consisting of three distinct criteria:
a) surface drainage (surface shape);
b) tire/surface interface drainage
(macrotexture); and
c) penetration drainage (microtexture).
The three criteria can significantly be
influenced by engineering measures and it is important to note that all of them
must be satisfied to achieve adequate friction in all possible conditions of
wetness, i.e.. from a damp to a flooded surface.
5.2.5.2 Surface
drainage. Surface drainage is a basic requirement of utmost
importance. It serves to minimize water depth
on the surface, in particular in the area of the wheel path. The objective is
to drain water off the runway in the shortest path possible and particularly
out of the area of the wheel path. Adequate surface drainage is provided
primarily by an appropriately sloped surface (in both the longitudinal and
transverse directions) and surface evenness. Drainage capability can, in
addition, be enhanced by special surface treatments such as providing closely spaced
transverse grooves or by draining water initially through the voids of a
specially treated wearing course (porous friction course). The effectiveness of
the drainage capability of modern types of surfaces is evident in that the
surfaces when subjected to even high rainfall rates retain a rather damp
appearance. It should be clearly understood, however, that special surface
treatment is not a substitute for poor runway shape, be it due to inadequate
slopes or lack of surface evenness. This may be an important consideration when
deciding on the most effective method for improving the wet friction
characteristics of an existing runway surface.
5.2.5.3 Tire/surface
interface drainage (macrotexture). The purpose of interface drainage (under
a moving tire is twofold:
a) to prevent as far as feasible residual
surface bulkwater from intruding into the forward area of the interface; and
b) to drain intruding water to the outside of
the interface.
The objective is to achieve high water
discharge rates from under the tire with a minimum of dynamic pressure
build-up. It has been established that
this can only be achieved by providing a surface with an open macrotexture.
5.2.5.4 Interface
drainage is actually a dynamic process, i.e., is highly susceptible to the square
of speed. Macrotexture is therefore particularly important for the provision of
adequate friction in the high speed range. From the operational aspect, this is
most significant because it is in this speed range where lack of adequate
friction is most critical with respect to stopping distance and directional
control capability.
5.2.5.5 In
this context it is worth while to make a comparison between the textures
applied in road construction and runways. The smoother textures provided by
road surfaces can achieve adequate drainage of the footprint of an automobile
tire because of the patterned tire treads which significantly contribute to
interface drainage. Aircraft tires, however, cannot be produced with similar
patterned treads and have only a number of circumferential grooves which
contribute substantially less to interface drainage. Their effectiveness
diminishes relatively quickly with tire wear. The more vital factor, however,
which dictates the macrotexture requirement is the substantially higher speed
range in which aircraft operate. This may explain why some conventional runway
surfaces which were built to specifications similar to road surfaces
(relatively closed-textured) show a marked drop in wet friction with increasing
speed and often a susceptibility to dynamic aquaplaning at comparatively small
water depths.
5.2.5.6 Adequate
macrotexture can be provided by either asphalt or cement concrete surfaces,
though not with equal effort, stability or effectiveness. With cement concrete pavement
surfaces, the required macrotexture may be achieved with transverse wire comb
texturing when the surface is in the plastic stage or with closely spaced
transverse grooves. With asphalt surfaces, the provision of macrotexture may be
achieved by providing open graded surfaces.
5.2.5.7 A
further design criteria calls for best possible uniformity of surface texture.
This requirement is important to avoid undue fluctuations in available friction
since these fluctuations would degrade antiskid braking efficiency or may cause
tire damage.
5.2.5.8 The
surface finish considered most effective from the standpoint of wet friction is
grooving in the case of Portland cement concrete and the porous friction course
in the case of asphalt. Their effectiveness can be explained by the fact that
they not only provide good interface drainage, but also contribute
significantly to bulkwater drainage.
5.2.5.9 Penetration
drainage (microtexture). The purpose of penetration drainage is to
establish "dry" contact between the asperities of the surface and the
tire tread in the presence of a thin viscous water film. The viscous pressures
which increase with speed tend to prevent direct contact except at those
locations of the surface where asperities prevail, penetrating the viscous
film. This kind of roughness is defined as microtexture.
5.2.5.10 Microtexture
refers to the fine-scale roughness of the individual aggregate of the surface
and is hardly detectable by the eye, however, assessable by the touch.
Accordingly, adequate microtexture can be provided by the appropriate selection
of aggregates known to have a harsh surface. This excludes in particular all
polishable aggregates.
5.2.5.11 Macro-
and microtexture are both vital constituents for wet surface friction, i.e.
both must adequately be provided to achieve acceptable friction characteristics
in all different conditions of wetness. The combined effect of micro- and
macrotexture of a surface on the resulting wet friction versus speed is
illustrated in Figure 5-2 indicating also that the design objective formulated
in 5.2.4 can be achieved by engineering means.
5.2.5.12 A
major problem with microtexture is that it can change within short time periods
(unlike macro texture), without being easily detected. A typical example of
this is the accumulation of rubber deposits in the touchdown area which will
largely mask microtexture without necessarily reducing macrotexture. The result
can be a considerable decrease in the wet friction level. This problem is
catered for by periodic friction measurements which provide a measure of
existing microtexture. If it is determined that low wet friction is caused by
degraded surface microtexture, there are methods available to effectively
restore adequate microtexture for existing runway surfaces (see 5.3).
5.2.6 Minimum
specifications
5.2.6.1 The
basic engineering specifications for the geometrical shape (longitudinal
slope/transverse slope/surface evenness) and for the texture (microtexture) of
a runway surface are contained in Ecar139.
5.2.6.2 Slopes. All new runways should be designed with
uniform transverse profile in accordance with the value of transverse slope
recommended in Ecar139 and with a longitudinal profile as nearly level as
possible. A cambered transverse section from a centre crown is preferable but
if for any reason this cannot be provided then the single runway crossfall
should be carefully related to prevailing wet winds to ensure that surface
water drainage is not impeded by the wind blowing up the transverse slope. (In
the case of single crossfalls it may be necessary at certain sites to provide
cutoff drainage along the higher edge to prevent water from the shoulder
spilling over the runway surface.) Particular attention should be paid to the
need for good drainage in the touchdown zone since aquaplaning induced at this
early stage of the landing, once started, can be sustained by considerably
shallower water deposits further along the runway.
5.2.6.3 If
these ideal shape criteria are met, aquaplaning incidents will be reduced to a
minimum, but departures from these ideals will result in an increase of
aquaplaning probability, no matter how good the friction characteristic of the
runway surface may be. These comments hold true for major reconstruction
projects and, in addition, when old runways become due for resurfacing the
opportunity should be taken, wherever possible, to improve the levels to assist
surface drainage. Every improvement in shape helps, no matter how small.
5.2.6.4 Surface
evenness. This is a constituent of runway shape which requires equally
careful attention. Surface evenness is also important for the riding quality of
high speed jet aircraft.
5.2.6.5 Requirements
for surface evenness are described in Ecar139, Attachment A, 5, and reflect
good engineering practices. Failure to meet these minimum requirements can
seriously degrade surface water drainage and lead to ponding. This can be the
case with aging runways as a result of differential settlement and permanent
deformation of the pavement surface. Evenness requirements apply not only for
the construction of a new pavement but throughout the life of the pavement. The
maximum tolerable deformation of the surface should be specified as a vital
design criterion. This may have a significant impact on the determination of
the most appropriate type of construction and type of pavement.
5.2.6.6 With
respect to susceptibility to ponding when surface irregularities develop,
runway shapes with maximum permissible transverse slopes are considerably less
affected than those with marginal transverse slopes. Runways exhibiting ponding
will normally require a resurfacing and reshaping to effectively alleviate the
problem.
5.2.6.7
Surface texture. Surface microtexture requirements are specified in
Ecar139 in terms of average surface texture depth, which should not be less
than
interface drainage. Higher values of average
texture depth may be required where rainfall rates and intensities are a
critical factor to satisfy interface drainage demand. Surfaces which fall short
of the minimum requirement for average surface texture depth will show poor wet
friction characteristics, particularly if the runway is used by aircraft with
high landing speeds. Remedial action is, therefore, imperative. Methods for
improving the wet friction characteristics of runways are described in 5.3.
5.2.6.8 As
outlined earlier, uniformity of the texture is also an important criterion. In
this respect, there are several specific types of surfaces which meet this
requirement (see 5.3). These surfaces will normally achieve average texture
depths higher than
5.2.6.9 The
microtexture of a surface does not normally change considerably with time,
except for the touchdown area as a result of rubber deposits. Therefore,
periodic control of available average surface texture depth on the
uncontaminated portion of the runway surface will only be required at long
intervals.
5.2.6.10 With
respect to microtexture there is no direct measure available to define the
required fine scale roughness of the individual aggregate in engineering terms.
Accordingly, there are no relevant specifications in Ecar139. However, from
experience it is known that good aggregate must have a harsh surface and sharp
edges to provide good water film penetration properties. It is also important that
the aggregate be actually exposed to the surface and not coated entirely by a
smooth material. Since microtexture is a vital constituent of wet friction
regardless of speed, the adequacy of microtexture provided by a particular
surface can be assessed generally by friction measurements.
Lack of microtexture will result in a
considerable drop in friction levels throughout the whole speed range. This
will occur even with minor degrees of surface wetness (e.g., damp). This rather
qualitative method may be adequate for detecting lack of microtexture in
obvious cases
5.2.6.11 Degradation
of microtexture caused by traffic and weathering may occur, in contrast to
microtexture, within comparatively short time periods and can also change with
the operational state of the surface. Accordingly, short-termed periodic checks
by friction measurements are necessary, in particular with respect to the
touchdown areas where rubber deposits quickly mask microtexture.
5.2.6.12 Runway
surface friction calibration. Ecar139 requires runway surfaces to
be calibrated periodically to verify their
friction characteristics when wet. These friction characteristics must not fall
below levels specified by the State for new construction (minimum design
objective) and for maintenance. Wet friction levels, reflecting minimum
acceptable limits for new construction and maintenance, which are in
use in some States are given in Attachment B,
7 of Ecar139.
5.2.6.13 For
the design of a new runway, the optimum application of the basic engineering
criteria for runway shape and texture will normally provide a fair guarantee of
achieving levels well in excess of the applicable specified minimum wet
friction level. When large deviations from the basic specifications for shape
or texture are planned, it will then be advisable to conduct wet friction
measurements on different test surfaces in order to assess the relative
influence of each parameter on wet friction, prior to deciding on the final
design. Similar considerations apply for surface texture treatment of existing
runways.
5.3 Surface treatment of runways
5.3.1 General
5.3.1.1 The
methods described in this section are based on the experiences of several
States. It is important that a full engineering appreciation of the existing
pavement be made at each site before any particular method is considered, and
that, once selected, the method is suitable for the types of aircraft
operating. It should be noted that with respect to the improvement of the
friction characteristics of existing runway pavements, a reshaping of the
pavement may be required in certain cases prior to the application of special
surface treatment in order to be effective.
5.3.2 Surface
dressing of asphalt
5.3.2.1 Operational
considerations. Aircraft with dual tandem undercarriage at tire pressure
1930 kPa and all-up masses exceeding
5.3.2.2 Consideration
of existing pavement. The over-all shape and profile of the existing runway
is not as important as it is with other treatments and, where a number of
transverse and longitudinal slope changes occur in the runway length, surface
dressing is probably the only suitable method short of expensive reshaping. In
spite of the fact that the over-all shape need not be ideal, nevertheless, for
a successful application of this treatment, the compacting equipment must be
capable of following the minor surface irregularities to ensure a uniform
adhesion of the chippings. Where this condition cannot be ensured, a new
asphalt wearing course may be necessary before applying the surface dressing.
5.3.2.3 Effectiveness
of treatment. A satisfactory surface dressing will initially raise the
friction coefficient of the surface to a high value which, thereafter,
depending on the intensity of traffic, will slowly decrease. Normally an
effective life of up to five years can be expected.
5.3.2.4 Runway
ends. Runway ends used for the start of take-off should not be treated.
Aircraft will scuff in turning, both fuel spillage and heat will soften the
binder, and blast will tend to loosen chippings.
Part 3.- Pavements 3-213
Figure 5-3. Surface dressing of asphalt
5.3.2.5 Chippings. The chippings may be from one of the
following groups: Basalt, Gabbro, Granite, Gritstone, Hornfels, Porphyry or
Quartzite.
5.3.2.6
Mechanical gritter. The chippings are distributed by a mechanical
gritter of approved type incorporating a mechanical feed capable of ensuring
that the selected rate of spread is rigidly maintained throughout the work.
5.3.2.7 Restrictions
during bad weather. Work
must not be carried out during periods of rain, snow or sleet or on frozen
surfaces or on those on which water is lying. When weather conditions dictate,
suitable protection must be afforded to the chippings during delivery.
5.3.2.8 Existing
pit covers, gully gratings and aerodrome markings. These must be protected
by masking, and the surface dressing finished neatly around them. When masking
of the aerodrome markings is not indicated, they may be obliterated.
5.3.2.9 Preparation
of the existing surfacing. Immediately before spraying the binder, the existing
surfaces must be thoroughly cleaned by mechanical brooms, supplemented by hand
brooming if necessary. All vegetation, loose materials, dust and debris, etc.,
must be removed as indicated.
5.3.2.10 Application
of surface binder. The binder must be applied at the selected rate without
variation and so that a film of uniform thickness results. Particular care must be taken to
avoid dripping, spilling and creating areas of excessive thickness.
5.3.2.11 Application
of coated chippings. The temperature of the chippings when applied to the
sprayed surface binder must be not less than
binder and
5.3.2.12 Rolling.
The coated chippings must be rolled immediately after spreading and before loss
of heat.
5.3.2.13 Final
sweeping and rolling. Within three days of the gritting operation all loose
chippings must be swept from the surface with hand-brooms, loaded onto trucks
and removed as directed. Then the entire surface must again be thoroughly
rolled at least three more times. All chippings must adhere firmly to the
finished surface which should be of uniform texture and colour. The surface
must be entirely free of irregularities due to scabbing, scraping, dragging, droppings,
excessive overlapping, faulty lane or transverse junctions, or other defects,
and it must be left clean and tidy. Under no circumstances should swept up
chippings be re-used.
5.3.3 Grooving_
of pavements
5.3.3.1 Operational
considerations. There are no operational objections to the grooving of existing
surfaces. Experience of operating all types of aircraft from grooved surfaces
over a number of years indicates that there is no limit within the foreseeable
future to the aircraft size, loading or type for which such surfaces will be
satisfactory. There is inconclusive evidence of a slightly greater rate of tire
wear under some operational conditions.
5.3.3.2 Methods
of grooving include the sawing of grooves in existing or properly cured asphalt
(Figure 5-4) or Portland cement concrete pavements, and the grooving or wire
combing of Portland cement concrete while it is in the plastic condition. Based
on current techniques, sawed grooves provide a more uniform width, depth, and
alignment. This method is the most effective means of removing water from the
pavement/tire interface and improves the pavement skid resistance. However, plastic grooving and wire
combing are also effective in improving drainage and friction characteristics
of pavement surfaces. They are cheaper to construct than the sawed grooves,
particularly where very hard aggregates are used in pavements. Therefore the
cost-benefit relationship should be considered in deciding which grooving
technique should be used for a particular runway.
Figure 5-4. Grooving of asphalt surface
(Note.- Scale shows
5.3.3.3
Factors to be considered. The following factors should be considered in
justifying grooving of runways:
a) historical review of aircraft
accidents/incidents related to aquaplaning at airport facility;
b) wetness frequency (review of annual
rainfall rate and intensity);
c) transverse and longitudinal slopes, flat
areas, depressions, mounds, or any other abnormalities that may affect water
run-off;
d) surface texture quality as to slipperiness
under dry or wet conditions. Polishing of aggregate, improper seal coating,
inadequate microtexture/macrotexture, and contaminant buildup are some examples
of conditions which may affect the loss of surface friction;
e) terrain limitations such as drop-offs at
the ends of runway end safety areas;
f) adequacy of number and length of available
runways;
g) crosswind effects, particularly when low
friction factors prevail; and
h) the strength and condition of existing
runway pavements.
5.3.3.4
Evaluation of existing pavement. Asphalt surfaces must be examined to
determine that the existing wearing course is dense, stable and
well-compacted. If the surface exhibits
fretting or where large particle fractions of coarse aggregate are exposed on
the surface itself, then other methods will need to be considered, or
resurfacing will have to be undertaken before grooving is put in hand. Rigid pavement must be examined to ensure
that the existing surface is sound, free of scaling or extensive spalls, or
"working cracks". Apart from the condition of the surface itself, the
ratio between transverse and longitudinal slopes becomes important. If the
longitudinal slopes are such that the water run-off is directed along the
runway instead of clearing quickly to the runway side drains, then a condition
could arise when the grooves would fill with free water, fail to drain quickly
and possibly encourage aquaplaning. For the same reason, surfaces with
depressed areas should be repaired or replaced before grooving.
5.3.3.5
Effectiveness of treatment. Transverse grooving will always result in a
measurable increase of the friction coefficient, though the extent of the
improvement will be related to the quality of the existing surface. The
duration of this improvement will depend on the properties of the asphalt
wearing course, the climate and traffic. Experience has shown that grooving
does not result in an increase of the rate of deterioration of the asphalt. The
improvement also applies to rigid pavement surfaces as they are not adversely
affected by the grooving. No grooves becoming clogged with dust, industrial
waste, or other. contaminants have been found although some minor rubber deposits
have been observed.
5.3.3.6 Technique.
The surface is to be grooved across the runway at right angles to the runway
edges or parallel to non-perpendicular transverse joints, where applicable,
with grooves which follow across the runway in a continuous line without break.
The machine for grooving will incorporate disc flails (Figure 5-5) or flail
cutters or a sawing machine (Figure 5-6) incorporating a minimum of 12 blades. Sawing machines include water tanks and
pressure sprays. Commonly used groove configurations are
5.3.3.7 The
grooves may be terminated within
specify the contractor's liability for damage
to light fixtures and cable. Clean up is extremely important and should be continuous
throughout the grooving operation. The waste material collected during the
grooving operation must be disposed of by flushing with water, sweeping, or
vacuuming. If waste material is
flushed, the specifications should state whether the airport owner or
contractor is responsible for furnishing water for cleanup operations. Waste
material collected during the grooving operation must not be allowed to enter
the airport storm or sanitary sewer, as the material will eventually clog the
system. Failure to remove the material can create conditions that will be
hazardous to aircraft operations.
5.3.3.8 Plastic
grooves and wire comb. Grooves can be constructed in new Portland cement
concrete pavements while in the plastic condition. The "plastic
grooving" or wire comb (see Figure 5-7) technique can be included as an
integral part of the paving train operation. A test section should be
constructed to demonstrate the performance of the plastic grooving or wire
combing equipment and set a standard for acceptance of the complete product.
5.3.3.9 Technique.
Tolerances for plastic grooving should be established to define groove
alignment, depth, width, and spacing. Suggested tolerances are ±
5.3.3.10 The junction of groove face and
pavement surface should be squared or rounded or slightly chamfered.
Hand-finishing tools, shaped to match the grooved surface, should be provided.
The contractor should furnish a "bridge" for workmen to work from to
repair any imperfect areas. The equipment should be designed and constructed so
that it can be controlled to grade and be capable of producing the finish
required. If pavement grinding is used to meet specified surface tolerances, it
should be accomplished in a direction parallel to the formed grooves.
Grooving runway intersections
5.3.3.11 General. Runway intersections
require a decision as to which runway's continuous grooving is to be applied.
The selection of the preferred runway will normally be dictated by surface
drainage aspects, except that if this criterion does not favour either runway,
consideration will be given to other relevant criteria.
5.3.3.12 Criteria. The main physical
criterion is surface drainage. Where drainage characteristics are similar for
the grooving pattern of either runway, consideration should be given to the
following operational criteria:
- aircraft ground speed regime;
- touchdown area; and
- risk assessment.
5.3.3.13 Surface drainage. The primary
purpose of grooving a runway surface is to enhance surface drainage. Hence, the
preferred runway is the one on which grooves are aligned closest to the
direction of the major downslope within the intersection area. The major
downslope can be determined from a grade contour map.
5.3.3.14 The above aspect is essential
because intersection areas involve, by design, rather flat grades (to satisfy
the requirement to provide smooth transition to aircraft travelling at high
speeds) and, therefore, are susceptible to water ponding.
5.3.3.15 Where appropriate, consideration may
be given to additional drainage channels across the secondary runway where the
groove pattern terminates in order to prevent water from this origin from
affecting the intersection area.
5.3.3.16 Aircraft speed. Since grooving is
particularly effective regarding wet surface friction characteristics in the
high ground speed regime, preference should be given to that runway on which
the higher ground speeds are frequently attained at the intersection.
5.3.3.17 Touchdown area. Provided the speed
criterion does not apply, the runway on which the intersection forms part of
the touchdown area should be preferred because grooving will provide rapid
wheel spin-up on touchdown in particular when the surface is wet.
5.3.3.18 Risk assessments. Eventually, the
selection of the primary runway can be based on an operational judgement of
risks for overruns (rejected take off or landing) taking into account:
runway use (take off/landing);
- runway lengths;
- available runway end safety areas;
- movement rates; and
-
particular operating conditions.
5.3.4 Scoring
of cement concrete
5.3.4.1 operational
considerations. There do not appear to be any operational objections to the
scoring of existing
5.3.4.2 Consideration
of existing pavement. It will be understood that it would be difficult to score
uniformly concrete surfaces which are "rough". Pavements with damaged
or poorly formed joints, or on which laitance has led to extensive &palling
of the surface, would be equally difficult to score. If the existing surface is
reasonably free of these defects, there are no other engineering limitations to
scoring.
5.3.4.3 Effectiveness
of treatment. Transverse scoring of concrete improves considerably the friction
characteristics of pavements initially textured at the time of Construction
with bolts, burlap or brooms. The useful life of the treatment depends on the
frequency of traffic but in general the scoring remains effective for the life
of the concrete.
5.3.4.4 Runway ends. Runway ends should be left
unscored to make it easier to wash down and clean off fuel and oil droppings.
Moreover, engine blast can be more damaging on a scored than on an untexured
surface. The directional control of an aircraft moving from the
taxiway on to the runway can become reduced,
presumably because of a tendency of the tires to track in the scores. In
addition, a possibility of an increase in tire wear in turning cannot be
totally discounted.
5.3.4.5 Technique. An acceptable "trial" area
should be available for inspection and it is recommended that this be provided
at the aerodrome to determine a precise texture depth requirement, as this will
tend to vary with the quality of the concrete. The runway is to be scored transversely
by a single pass of a cutting drum (Figure 5-9) incorporating not less than 50
circular segmented diamond saw blades per
5.3.5 Reflex
percussive technique
5.3.5.1 The
reflex percussive technique is predominantly applied for grooving of existing
runway surfaces and represents a cost-effective alternative to saw-cut grooving
techniques. It has been successfully applied on various types of runway
surfaces to provide adequate grooving. The technique can also effectively be
used for other purposes, such as removal of rubber deposits in touchdown zone
areas or for the restoration of micro/macrotexture of a degraded existing
runway surface.
5.3.5.2 The
reflex percussive technique uses star-shaped or pentagonal disk flails. The
specification of the cross section and spacing of the grooves will be dictated
primarily by the drainage requirements determined from local precipitation
conditions and the slopes of the runway surface. For cement concrete surfaces,
the pitch ranges normally from
Portland cement Width/depth/pitch 10/3/27 mm,
concrete: edges and trough rounded (see Figure 5-10)
Asphalt surface: Width/depth/pitch 9/3/58 mm,
edges and trough rounded (see Figure 5-11).
5.3.5.3 The surface of the Portland cement concrete or
asphalt surface is to be grooved perpendicular to the runway centre line or
parallel to non-perpendicular transverse joints, where applicable, in
continuous uninterrupted lines terminating approximately
5.3.6 Porous friction course
5.3.6.1 The porous friction course consists of an
open-graded, bituminous surface course composed of mineral aggregate and
bituminous material, mixed in a central mixing plant, and placed on a prepared
surface (Figure 5-12). This
friction course is deliberately designed not only to improve the
skid-resistance but to reduce aquaplaning incidence by providing a
"honeycomb" material to ensure a quick drainage of water from the
pavement surface direct to the underlying impervious asphalt. The porous
friction course is able, because of its porosity and durability, to maintain
over a long period a constant and relatively high wet friction value.
5.3.6.2 Limitations of porous friction course.
Friction courses of this kind should only be laid on new runways of good shape,
or on reshaped runways approaching the criteria expected for new runways. They
must always be over densely graded impervious asphalt wearing courses of high
stability. Both of these requirements are necessary to ensure a quick flow of
the water below the friction course and over the impervious asphalt to the
runway drainage channels.
5.3.6.3 Runway
ends. The porous friction course is not recommended at the runway ends. Oil
and fuel droppings would clog the interstices and soften the bitumen binder,
and jet engine heat would soften the material which blast would then erode.
Erosion would tend to be deeper than on a normal dense asphalt and the possibility
of engine damage through ingestion of particles of runway material should not
be discounted. Scuffing might occur in turning movements during the first few
weeks after laying. For these reasons, it is recommended that runway ends be
constructed of brushed or grooved concrete, or of a dense asphalt.
5.3.6.4 Aggregate. The aggregate consists of
crushed stone, crushed gravel, or crushed slag with or without other inert
finely divided mineral aggregate. The aggregate is composed of clean, sound,
tough, durable particles, free from clay balls, organic matter, and other
deleterious substances. The type and grade of bituminous material is to be
based on geographical location and climatic conditions. The maximum mixing
temperature and controlling specification is also to be specified.
5.3.6.5 Weather and seasonal limitations. The
porous friction course is to be constructed only on a dry surface when the
atmospheric temperature is 10° C and rising (at calm wind conditions) and when
the weather is not foggy or rainy.
5.3.6.6 Preparation of existing surfaces.
Rehabilitation of an existing pavement for the placement of a porous friction
course includes: construction of bituminous overlay, joint sealing, crack
repair, reconstruction of failed pavement and cleaning of grease, oil, and fuel
spills. Immediately before placing the porous friction course, the underlying
course is to be cleared of all loose or deleterious material with power lowers,
power brooms, or hand brooms as directed. A tack coat is to be placed on those
existing surfaces where a tack coat is necessary for bonding the porous
friction course to the existing surface. If emulsified asphalt is used,
placement of the porous friction course can be applied immediately. However, if
cutback asphalt is used, placement of porous friction course must be delayed
until the tack coat has properly aired.
5.3.7
Emulsified asphalt slurry seal
5.3.7.1 The
emulsified asphalt slurry seal course consists of a mixture of emulsified
asphalt, mineral aggregate, and water, properly proportioned, mixed, and spread
evenly on a prepared underlying course of existing wearing course. The
aggregate consists of sound and durable natural or manufactured sand, slag,
crusher fines, crushed stone, or crushed stone and rock dust, or a combination
thereof. The aggregate is to be clean and free from vegetable matter, dirt,
dust, and other deleterious substances. The aggregate is to have a gradation
within the limits shown below.
5.3.7.2 The
Type I gradation is used for maximum crack penetration and is usually used in
low density traffic areas where the primary objective is sealing. The Type II
gradation is used to seal and improve skid resistance. The Type III gradation
is used to correct surface conditions and provide skid resistance.
5..3.7.3 Mineral filler is only used if needed to improve the workability of the
mix or to improve the gradation of the aggregate. The filler is considered as
part of the blended aggregate.
5.3.7.4 Tack
coat. The tack coat is a diluted asphalt emulsion of the same type
specified for the slurry mix. The ratio of asphalt emulsion to water should be
1 to 3.
5.3.7.5 Weather
limitations. The slurry seal is not applied if either the pavement or the
air temperature is 13° C or below or when rain is imminent. Slurry placed at lower
temperatures usually will not cure properly due to poor dehydration and poor
asphalt coalescence.
5.3.7.6 Cleaning
existing surface. Prior to placing the tack coat and slurry seal coat,
unsatisfactory areas are to be repaired and the surface cleaned of dust, dirt,
or other loose foreign matter, grease, oil, or any type of objectionable
surface film. Any standard cleaning method is acceptable except that water
flushing is permitted in areas where considerable cracks are present in the
pavement surface. Any painted stripes or marking on the surface to be treated
are to be removed before applying the tack coat. When the surface of the
existing pavement or base is irregular or broken, it must be repaired or
brought to uniform grade and cross section. Cracks
wider than
5.3.7.7 Application
of bituminous tack coat. Following the preparation for sealing, application
of the diluted emulsion tack coat is done by means of a pressure distributor in
amounts between 0,23 to 0.68 L/m2. The tack coat is to be applied at least two
hours before the slurry seal, but within the same day.
5.3.7.8 The main items of design in emulsified asphalt
slurry seals are aggregate gradation, emulsified asphalt content, and
consistency of the mixture. The aggregates, emulsified asphalt, and water
should form a creamy-textured slurry that, when spread, will flow in a wave
ahead of the strike-off squeegee. This will allow the slurry to flow down into
the cracks in the pavement and fill them before the strike-off passes over. The
cured slurry is to have a homogeneous appearance, fill all cracks, adhere
firmly to the surface, and have skid resistant texture.
CHAPTER 6. - PROTECTION OF ASPHALT PAVEMENTS
6.1
The problem
6.1.1 Since
petroleum-base fuels and lubricants contain solvents for asphalt, their
spillage on asphaltic pavements creates problems. Severity of problems is
related to the degree of exposure to the penetrating solvents.
6.1.2 The
highly volatile gasolines and high octane fuels of the past have been less of a
problem since they quickly evaporated when spillage occurred and systems using
these fuels have provided good containment. Massive and frequently repeated
spillage can be a problem, of course, since such fuels are excellent solvents.
Fuel spillage surfaced as a particular problem with the advent of turbine and
jet engines.The kerosene and light oil jet fuels involved do not readily
evaporate and early engine systems routinely spilled quantities of fuel on
engine shutdown. Hydraulic fluids and lubricating oils, which evaporate or
"cure out" even less rapidly than jet fuels, can also cause or
contribute to problems.
6.1.3 Since
the severity of adverse effects of spillage on asphalt pavements is related to
exposure, concern must be for the number of times spillage is repeated in one
location, the length of time the spilled fuel or oil remains on (or in) the
pavement, and the location and extent of spillage on the pavement. It has been
found that a single spillage of jet fuel, and even several spillages in the
same location when there is time for evaporation and curing between spillages,
do not normally have a significant adverse effect on the pavement. However,
some staining and a tender pavement are to be expected during the curing
period.
6.1.4 Spillages
can result from routine operations such as engine shutdown, fuel tank sediment
draining, consistent use of solvents for cleaning of engine or hydraulic system
elements, etc. More commonly
spillage is the result of fuel handling operations, of spilled oil or hydraulic
fluid, or accumulated drippings from engine oil leakage or mishandling.
6.1.5 Thus
locations of concern on pavements are those where aircraft are regularly
fuelled, parked, or serviced. The broad areas of landing and taxiing operations
will not be of concern, since even spillages attendant to aircraft accidents
will be minimized by clean-up and represent only a single spillage which will
cure without permanent damage. Even fuel burned on the asphalt surface will
normally only leave a surface scar of no structural significance.
6.1.6 In
areas where spillage occurs repeatedly or spilled fuel or oil remains for long
periods on the pavement the solvent action softens the asphalt and reduces
adhesion to the surface aggregate. While heat from the sun or warm air
conditions help evaporate solvents and re-cure the asphalt, the elevated
temperatures contribute to the asphalt softening. The result of the spillage,
aggravated by heat, can be shoving of the asphalt mix, tire tread printing,
tracking of asphalt to adjacent areas or production of loose material, and
pavement abrasion also producing loose material on the pavement surface. In maintenance and work areas asphalt and
grit picked up by tools, shoes, and clothing can be transferred to mechanical
systems.
6.1.7 The
surface texture and condition of pavements have a bearing on the severity of
the problem. Open or porous pavements will be more readily penetrated by fuel
or oil and will slow the evaporation and re-cure process. It has been found
that rubber tire traffic, whether from rolling or traffic tends to close the
surface and retard fuel penetration. Cracks and joints, not well sealed, are a
particular source of trouble. These provide access for fuel to deeper zones
within the pavement, provide greater surface areas for fuel intake, and retain
fuel much longer thereby retarding evaporation and cure. Low areas which will
retain or pond fluids, whether adjacent to cracks or joints or in central areas
of pavement, will prolong exposure to spilled fuel.
6.2 Treatment of the problem
6.2.1 The
best treatment is avoidance of spillage and this may be possible in many cases
of operational spillage and some accidental spillage. Fuel tank sediment
drainage can be caught and need not be allowed on the pavement. Drip pans can
be used for oil drip locations and for bleeding or servicing of hydraulic
systems. Trays may be practical to catch engine shut-down spillage or small
quantities of refuelling spillage.
6.2.2 Removal
of the spilled fuel or oil and reduction of exposure through clean up is the
next aspect of treatment. Spilled fuel or oil can be flushed off the pavement
with water. Addition of detergents assists the process of separating the fuel
and especially oil from the asphalt pavement. While this has been a common
treatment there are beginning to be environmental complaints from effects of
the run-off. A vacuuming process, with suitable equipment, can be used to
remove spilled fuel and some fuel recovery is possible. Absorbent materials can
also be used for fuel and oil pickup with suitable arrangement for disposal.
Rolls, pads, and granular materials are all used and in some cases wringers are
used for fuel recovery. There is another aspect of absorption by granular
materials in spillage areas to consider. Accumulations of dust and sand, either
blown or man placed, will absorb small spillages, oil drippings, etc., and form
a mat which contains the spilled material and reduces its availability for
soiling of personnel and equipment. While this temporarily facilitates movement
of personnel it can greatly increase exposure of the pavement to effects of the
fuel and oil.
6.2.3 Since
problems are aggravated by repeated exposure to spillage, it is sometimes
possible to relocate aircraft parking, fuelling, or servicing positions to
ameliorate the deterioration.
6.2.4 Spillage
problems cannot develop if spilled fuel or oil is not allowed to come in
contact with the asphalt pavement. Protective coatings have accordingly been
developed to provide a barrier between the fuel or oil and the pavement, which
is then not affected by the spilled fuel or oil.
6.3 Protective coatings
6.3.1 Protective coating materials are
generally liquids, some heated to become liquid, which when spread on the
pavement cure or set to become a protective coating. These are commonly
referred to as seal coats when common spray application and bituminous
materials are involved. Most of the liquid materials can be applied in any of
several ways including spraying using hand sprays or asphalt distributor
equipment, pouring on the surface and spreading using squeezes, rolling onto
the surface with paint rollers, and application or spreading using brushes.
Single and multiple applications are variously employed, and fine aggregate may
be spread and embedded in the coating before setting or curing to improve wear
or skid resistance
6.3.2 Coating
materials in emulsion form can be extended and premixed with fine aggregate to
form a slurry and applied as a slurry seal.* Single or multiple applications
can be used here also. Two layer applications are common.
6.3.3 Thin
overlays of materials not affected by spillage can be applied to protect
asphalt pavements. Conventional construction methods are applicable unless some
very unconventional materials are employed.
6.4 Materials for protective coatings
6.4.1 Coal-tar
pitch is only slightly soluble to insoluble in the light petroleum fractions
(napthas) which are solvents for asphalts and can be employed, in much the same
way as is asphalt, in pavement applications. Also, in many places, depending
upon relative availability and economic circumstances, tar has been cost
competitive with asphalt for spray applications and as a binder for pavements.
Thus coal-tar pitch is used as a protective sealer**and is the basic ingredient
in various commercially offered sealers for protective coating applications.
6.4.2
Because tar is more temperaute sensitive than asphalt means of adjusting
the temperature response to one similar to asphalt were studied. addition of
latex rubber would accomplish this purpose and it was subequently found that
the rubberized tar (commonly called tar-rubber) gave a somewhat better
performance than unmodified tar. For these reasons the most favoured and some
of the best performing protective coatings are rubberized coal-tar pitch
emulsions. The United States FAA Engineering Brief No. 22, "Asphalt Rubber
and Rubberized Coal Tar Pitch Emulsion," presents comments and a guide
specification for "Rubberized Coal Tar Pitch Emulsion Seal Coat (For
Bituminous Pavements)" which is representative of material quantities and
characteristics as well as application methods which apply. In the
6.4.3 Sealing
materials are offered which employ epoxies and polymers of various types either
alone or in a bituminous base, which can be tar or asphalt. While these have
attributes which should make them effective, experience with their application
in the field is limited. Therefore trial test applications are recommended to
help assess effectiveness before broad applications are undertaken. These
materials range in price in the
6.4.4 Tar-rubber
binder materials and, in at least one instance, epoxy-asphalt binder of a type
used for bridge deck protection, have been placed as overlays of asphalt
pavements to provide protection from fuel spillage along with structural
upgrading. These are effective so long as cracking can be controlled (prevented
or cracks kept sealed). Cost of the tar-rubber binder is perhaps twice the cost
of asphalt mix while the epoxy-asphalt may run to five times the cost of
asphalt mix but can be placed as a very thin (
* ASTM D-3910 Standard Practice of Design,
Testing, and Construction of Slurry Seal.
** ASTM D-3423 Standard Practice for
Application of Emulsified Coal-Tar Pitch (Mineral Colloid Type).
6.5 Application
6.5.1 Surfaces
to receive protective coatings must be thoroughly cleaned. Any surface films of
oil need to be carefully removed. Areas of pavement which have become affected
by prior fuel spillage and any badly cracked areas must be removed and replaced
with sound pavement and these patches should be thoroughly cured (2 to 4 weeks)
prior to the sealing. All but very narrow cracks must be cleaned and filled
with crack filler.
6.5.2 Methods
of application should follow standard practice as recommended by airfield or
highway authorities, trade associations, or the product manufacturer. Seal coat
guidance can be found in ASTM D-3423 or the United States FAA Engineering Brief
No. 22, Appendix B. Slurry seal guidance will be found in ASTM D-3410.
6.5.3 Commonly,
single applications of seal or slurry seal are such as to provide 0.3 to 0.5
kg/m2 of residual bitumen. Two and even three applications are usual. Surfaces
should be moist but not wet for emulsion applications and temperatures should
be favourable both for application and subsequent cure -
6.6 Protection gained
6.6.1 Durability
and wear can vary with the materials and applications, the surface cleaning and
preparation, maintenance of the protective coating, and of Course exposure to
spillage and traffic. Testing and experience have shown that good coatings,
well applied to clean well prepared surfaces and properly maintained, will
provide satisfactory protection in most cases. In areas of very severe
exposure, as at central fuelling points, no protective coatings have been found
to be entirely satisfactory.
6.6.2 In
other than the most severe spillage locations unsatisfactory behaviour can be
experienced when elements of good practice are ignored. Some material
formulations and application methods, either individually or in concert, can
result in imperfect coverage by the seal coating. Bubbles can exist at
application (sometimes called fish eyes) and leave holes in the coating or
bubbles can form beneath a coating after cure and on breaking leave holes, and
coatings can shrink and crack. Improper surface cleaning can result in a poor
bond and peeling of the coating. And cracks in the coated pavement will tend to
come through the protective surface coating.
6.6.3 When
fuel can gain access through holes or cracks in the seal coat, through peeled
areas, or through cracks reflected from the lower pavement, or when fuel
saturated pavement has not been removed and is covered by the seal coat,
conditions are worsened rather than improved by the seal since, in addition to
not preventing access of the spilled fuel or oil to the asphalt, the seal coat
greatly inhibits the evaporation and cure-out of the spillage.
6.6.4 Overlays
of tar-rubber binder give spillage protection and are not subject to bubble
holes, peeling, or wear through. Tar-rubber overlays are subject to shrinkage,
cracking and to crack reflection from underlying pavements. They must be properly compacted since
pavements having voids of as much as 6 per cent will be porous enough to permit
penetration of jet fuel.
6.7 Maintenance consideration
6.7.1 Maintenance
includes clean-up of spills as discussed earlier under "treatment of the
problem". Ponding must be prevented to avoid extending exposure from
spillage. Other maintenance is concerned with maintaining integrity of the
protective coating. Cracks must be
kept sealed with a fuel resistant sealer. Retreatment
must be employed when deterioration, wear through, or peeling leads to openings
in the coating. Accidental scars must be closed. If asphalt patching is required
then the surface, after suitable cure, needs to be coated against spillage
effects.
6.8 Some related concerns
6.8.1 Some
seal coats provide reduced skid resistance, and while fuel resistant coatings
are not commonly employed on aerodromes in areas of severe skidding potential,
the problem, should it intrude, can be treated through embedment of sand size
aggregate in the seal coat before final cure.
6.8.2 As
earlier mentioned there is developing concern for the flushing of spilled fuel
and oil, and of chemicals employed to assist the removal of oils, into adjacent
drains. Catchments and acceptable disposal practices may be required.
6.8.3 Spilled
fuel which finds its way into subsurface drains and culverts can be a safety
hazard. Such spillage can develop explosive fuel-air mixtures in the confined
drains and a spark ignition will result in an explosion. The risk to life and property can be real and consequential.
6.8.4 There
can be a question as to the desirability of rolling seal coats. Rolling can improve
film adhesion, and, as earlier mentioned, close surface pores and reduce fuel
penetration. Generally, therefore, rolling of bituminous seals using flat (no
tread) rubber tire rollers should be beneficial, but whether the resulting
improvement warrants the rolling effort has not been established.Steel wheel
rolling would not be of benefit and may damage the coating. Any rolling of
polymeric seals might be undesirable, and supplier recommendations should be
followed.
CHAPTER 7. - STRUCTURAL CONCLRNS
FOR CULVERTS AND BRIDGES
7.1 Problem description
7.1.1 Subsurface
structures for drainage or access must commonly be crossed by pavements which
support aircraft. Such facilities are subject to the added loading imposed by
the aircraft sometimes directly as in the case of bridges, subsurface terminal
facilities, and the like, but more often indirectly as loading transmitted to
buried pipes and culverts through the soil layer beneath the pavement.
7.1.2 These
subsurface structures must be considered in connexion with evaluation of
pavement strength. The patterns of stresses induced by surface wheel loads as
they are transmitted downward are not the same on the subsurface structures as
on the subgrade. This is not only because these structures are not at subgrade
level but also because the presence of the structure distorts the patterns.
Thus the considerations which permit use of the ACN-PCN method to limit
pavement overloading are not necessarily adequate to protect subsurface
structures. In some cases the subsurface structure can be the critical or
limiting element thereby necessitating the reporting of a lower PCN for the
pavement.
7.1.3 In
the design of new facilities care must be given to the structural adequacy of
pipes, culverts, and bridged crossings, not only for the contemplated design
loadings but for possible future loadings to avoid a need for very costly
corrective treatments made necessary by a growth in aircraft loadings.
7.2 Types of substructures
7.2.1 Probably
the most common and least apparent buried structures at aerodromes are pipes
facilitating drainage of surface or subsurface water. These can range in
diameter from
7.2.2 Box
culverts which are either square or rectangular in shape are commonly used for
stream crossings beneath pavements. They are designed for the hydraulic flow
and the loads to be supported. They are usually of cast in situ reinforced
cement concrete. Span between side
walls can vary from about 1 to
7.2.3 Arches
of structural metal plates, of the type used for constructing large diameter
pipes are sometimes used in preference to short bridges to span stream or
pavement crossings. In such cases, soil is placed beside and above the arch up
to subgrade level and the pavement constructed thereon. In rare cases tunnels
may pass beneath aerodrome pavements.
7.2.4 Bridges
are used in a number of cases for highways to pass beneath taxiways and runways
and, increasingly, subsurface terminal facilities are placed beneath aprons and
taxiways. These are designed to support the using aircraft and structure dead
loads. Also runway extensions over water are sometimes placed on bridges
supported on piles and these must be designed to accommodate aircraft loads in
addition to their dead weight.
7.3 Some guiding concepts
7.3.1 The
discussion in Chapter 3, 3.2.4, on Aircraft Loading is pertinent to concepts of
distribution of stresses from surface loads within embankments beneath
pavements. High stress surface loads are distributed by the pavement structure
and as the loads extend downward they are further distributed over wider areas
with consequent reduction in stress magnitudes. As the pattern of stress goes
deeper and extends over wider areas, the effects of adjacent wheels overlap
leading to doubling or even greater multiplying of the stress induced by one
wheel. The deeper the pattern extends, the farther apart individual wheels can
be and still have interacting effects. These are the patterns of stresses
introduced by the live loads (aircraft) into the ground beneath pavements, and
along with the mass of the soil and pavement, represent the magnitudes of
stresses or loading delivered to buried structures.
7.3.2 The
presence of a buried structure (which does not act in the same manner as the
soil it displaces) has a significant impact on the pattern of live and dead
load stresses (ambient stresses) induced by the surface loads, pavement and
backfill material. A concrete pipe, for instance, is much stiffer in the
vertical direction than is the adjacent soil. Thus compression (vertical
deflexion) of the soil under aircraft loading results in a relative upward
thrust of the rigid pipe into the soil with a consequent accumulation of
greater than ambient stress and loading. This is why some deeply buried rigid
pipes are protected by soft (baled straw, loose soil, etc.) zones above the
pipe. In such cases, the vertical stiffness of the pipe and soft zone is less
than the stiffness of soil beside the pipe and stresses are accumulated more by
the adjacent soil. This is also why the character and condition of bedding and
backfill are very important.
7.3.3 Box
culverts accumulate stresses in the same way as rigid pipes but the impact on
the structure is not the same. The vertical sidewalls of box culverts while
much stiffer than the soil are far stronger than necessary to sustain the
accumulated stresses or loading, and the span between sidewalls is less stiff
than the sidewalls and subject to reduced stress. It should be noted that these
reductions are small, however, and are reduced from the higher stresses
accumulated on the stiff box culvert.
7.3.4 Metal
and other flexible pipes are generally less stiff vertically than adjacent soil
and not subject to stress accumulations in the manner of rigid pipes However,
metal pipes are very . stiff in circumference and some larger diameter pipes
with deep corrugations and located near the surface can accumulate more than
ambient loading Large metal arches with fixed. footings can also be relatively
stiff structures.
7.4 Evaluation of subsurface structures
7.4.1 General
7.4.1.1 Every
subsurface structure beneath a pavement must be considered in connexion with
evaluation of the pavement. And while specific determinations would in each
case require careful structural analysis, the likelihood that a particular
structure would prove more critical than the pavement aircraft loads depends
greatly on the type, size, and location of the structure.Accordingly, certain
guidance, can be suggested to assist in determining which structures can at
small risk considered not to be limiting, be in limiting , which ones are
marginal and need to be carefully considered , and which require study and
analysis to define load limitations or needed strengthening
7.4.2
Deeply buried structures
7.4.2.1 The
live load on deeply buried structures tends to be only a small fraction of the
dead load so that pipes or culverts of moderate size and smaller, which do not
accumulate an undue share of the live load, will not limit surface loadings.
This will include pipe diameters or structure spans up to about one-third of
the protective cover (distance between pavement surface and top of pipe or
culvert). Table 7-1 indicates the thickness of protective cover of soil and
pavement structure above drainage structures of not too large span which will
spread the load sufficiently, considering combining of effects from adjacent
wheels, to reduce the pressure induced on the structure by aircraft (live)
loads to less than 10 per cent of the earth (dead) load it is not likely chat
an added 10 per cent of pressure will exceed the structural capacity of
in-service pipes or culverts. Where aircraft to be supported have tire loads
greater than 200 kN somewhat greater cover depths may be needed to attain the
10 per cent limitation on increased (live load) pressure.
Table 7-1. Protective cover needed over
structures beneath aerodrome pavements
Number of wheels* Cover depth in metres
1 4
2 5
4 6
8 7.5
16, 9.5
Pipes and culverts of the sizes indicated
(about one-third of the depth of cover) and depths equal to or greater than
that shown in Table 7-1 should not require a separate load limitation of the
overlying pavement.
7.4.2.2 Structures
at shallower depths need more detailed examination. Whether load limitations
beyond those for protection of the pavement may be needed will depend on
rigidity of the pipe or culvert, bedding and backfill, pavement stucture, and
conservatism of the original design. Sufficient analysis should be made either
to confirm that the buried structure does not require a more critical load
limitation than the pavement or to establish appropriate load limitations.
7.4.2.3 Wide
span structures; i.e., very large pipes, arches, and wide box culverts, even
with substantial cover will tend to accumulate stress from surface loads (by
soil arching) and may have to support virtually all of the aircraft (live) load
as well as the earth (dead) load.Thus any structure whose span exceeds about
one-third of the cover depth should be carefully analysed to establish surface
load limits or possible need for strengthening.
Consider all wheels within or touching a
circle whose diameter equals the depth of protective cover over the structure.
7.4.3 Shallow
pipes, conduits, subdrains, and culverts
7.4.3.1 The
ACN-PCN method limits aircraft mass to prevent over-stress of the pavement
subgrade and overlying layers. These same limits tend to protect shallow buried
structures from over-stress, except for quite large (over 3 or
7.4.3.2 Shallow
structures of substantial span (over 3 or
7.4.4
At surface drains, conduits, and the like
7.4.4.1 Collector
drains, box conduits (for lighting, wiring, fuel lines, etc.), and any similar
pavement crossing installations, are sometimes placed directly at the pavement
surface. These would rarely be so large that more than a single wheel would
need to be supported by the installation at any time. Consequently, only single
wheel loadings need be of concern for the design as well as evaluation.
7.4.5
Bridges supporting aerodrome pavements
7.4.5.1 Need
for passage of highway and rail traffic beneath aerodrome pavements and the
placement of terminal connexions and facilities beneath taxiway and apron
pavements has required the use of bridges to support the pavements and using
aircraft. Such structures receive little if any protection from pavement load
limitations and must be separately considered in establishing safe loadings.
The original design analyses will have established the type and magnitude of
loads for which the bridges are adequate. If the intended usage has changed and
pavements are likely to be used by markedly heavier aircraft or aircraft with
different undercarriage configuration than considered in design, a new analysis
will be needed to establish the suitability of the structure for such usage.
7.4.6 Pile
supported structures
7.4.6.1 Sometimes
runways and taxiways extend over water and these are placed on pile supported
structures. These, as for bridges, will have been subject to design analyses to
provide for the contemplated loads. Here again there will be a need for
re-analysis if operations by heavier aircraft or aircraft with substantially
different undercarriage layout are contemplated.
7.4.7
Tunnels under pavements
7.4.7.1 Tunnels
behave in a manner similar to large diameter pipes and can be considered to
respond in much the same manner. Thus shallower tunnels would require careful
analysis of expected increased aircraft loads on overlying pavements. Deeply
buried tunnels might require only casual examination if cover depths were
sufficient to minimize induced live loads
7.4.8
Treatment of severely limiting cases
7.4.8.1 Where
structures beneath pavements limit aircraft loads beyond the PCN (which is
assessed to protect the pavement) these limitations will need to be reported in
terms of specific aircraft type and load (mass) as exceptions. Where multiple
taxiways permit avoidance of the critical structures the problem can be handled
by local routing of aircraft. If, however, all aircraft must cross the critical
structure the limitation must be emphasized when reporting pavement strengths.
Only very shallow structures and extreme overloading - except for bridges or
pile supported pavements - represent some hazard to aircraft, and aircraft
safety will rarely if ever be compromised by overload of buried (earth covered)
structures. Bridges and pile supported pavements receive the loading directly
and must be structurally capable of supporting the imposed loadings.
7.4.8.2 Load
limitations on critical structures can be eliminated either by special analyses
which establish that larger than intended design loadings can be sustained, or
by strengthening. Commonly, design conservatism, better-than-minimum
installation, larger-than-needed safety factors and more searching design type
analyses may result in larger allowable loadings. These can range from a simple
restudy of the design data to extensive field study of the installation
including study of surrounding backfill or measurement of strain or deflexion
response of the structure under load. An example of such a study can be found
in the April 1973 issue of Airport World under the title, "
7.4.8.3 The
strengthening of a substructure can be accomplished using internal bands,
struts, or liners to strengthen or reduce span in pipes, culverts, arches,
etc., but these reduce the designed drainage capacity. Sometimes structures can
be stiffened by grouting surrounding soil from the surface or from inside the
structure. It may be possible to introduce compressible zones of soil or other
material above pipes or culverts and reduce the transmission of pavement loads
to the buried structure. Also, provision of load distributing pavement
structures (buried slabs for instance) may reduce loads on pipes, culverts or
drains. Of course, re-design and reconstruction is the obvious ultimate solution.
Some bridges or pile-supported pavements may be strengthened by adding elements
(beams, etc.) to the existing structure.
7.5 Considerations in design of new
facilities
7.5.1 Structural
concerns for drainage and similar structures in relation to the evaluation of
pavements for load support capacity have been discussed earlier in this
chapter. Patterns of behaviour in connexion with size, flexibility, live and
dead loads, deep and shallow cover have been indicated, and these apply also to
design considerations where new facilities are planned. This section will
amplify some of the earlier discussions and treat aspects of structural
behaviour of somewhat more direct concern for design.
7.5.2 Loads.
Loads which must be considered in design of buried structures are those
resulting from the weight of overlying soil and pavement structure (overburden)
plus those induced by aircraft or other
vehicles on the pavement above. Heavy construction loads passing over pipe
before it has its full protective cover may also need to be considered. These
loads produce the patterns of ambient stress present in embankments where they
are not disrupted by the presence of pipe or other structures or by the pockets
of loose, dense or other types of soil introduced by the installation of pipes,
culverts, etc. It is the distortion of the ambient stress patterns by the
character of the pipe or structure,
the nature of the pipe bedding, any trench
used during installation, and the type and larger or smaller than complicates
the design only nominal guidance. compacted density of the backfill around the
pipe which leads to ambient stress loads on the buried structures. This too is
what complicatees the design problem and
leads to established design methods which provide only nominal guidance .
7.5.3 Ambient
overburden stresses are the result of the mass of overlying soil and pavement
structure and can be directly determined. Stresses induced by aircraft tire
loads can be calculated using the theory for a uniformly distributed circular
load on the surface of a continuum. The theory for an elastic layered
continuum, with suitable elastic constants (E, u), should be preferred, but the
theory for a single layer system (Boussinesq) will provide reasonable stress
determinations for flexible pavements and deeper installations beneath rigid
pavements. Plots or tabulations of single layer stresses can be found in
references such as: the 1954 Highway Research Board Proceedings, HRB Bulletin
342 of 1962, Yoder's textbook on "Principles of Pavement Design"
(United States), Croney's text "The Design and Performance of Road
Pavements" TRRL (United Kingdom). Stresses for the combined effects of
several wheels can be determined by superposition of the single wheel stresses
at pertinent lateral spacings. Because of the time rate of response of soil to
rapid loading it is not necessary to consider any added dynamic effects of the
aircraft loading.
7.5.4 The
ambient stresses which obtain at the various depths beneath the pavement are
thus a combination of the overburden (dead load) stresses and the aircraft
landing gear load (live load) stresses. It is these stresses modified by the
existence and behaviour of a pipe or other buried structure and any distortions
due to its installation that determine the loads which must be supported by the
pipe or structure. In general, hard (stiff) elements or zones will accumulate
stress from the adjacent embankment soil while soft elements or zones will shed
stress to the adjacent soil. Thus the more rigid structures, such as box
culverts, concrete pipe, and the like, will tend to be subject to greater
stress and load than that implied by the ambient stress, while more flexible
structures, such as steel, aluminium, and plastic pipe or rigid structures
provided with an overlying zone of loose soil, straw, sawdust, etc. will tend
to be subject to less than the ambient stress.
7.5.5 A
most important consideration in the determination of loadings for design of
buried structures is in providing for future upgrading of pavement facilities
and growth in aircraft masses supported. Where upgrading is likely in the
future the design of buried structures beneath pavements for the heavier
loadings expected will commonly be far less costly during the original design
and construction than when left for subsequent modification.
7.5.6 Pipes.
Pipes are described generally in 7.2.1 and most types are covered by ASTM
standards for the pipe characteristics and tests to determine pipe strength.
Concrete, clay, asbestos-cement, solid wall plastic, and other geometrically
similar types of pipe are made in a variety of wall thicknesses and/or
reinforcements, as well as diameters to provide an array of strengths for use
in design of installations.
Steel, aluminium, and some plastic pipes are
made in a variety of gauges (thicknesses of material) and corrugation
configurations to provide an array of pipe stiffnesses and side-wall strengths
for installation design purposes. While round pipes are most common there are
elliptical pipes - used vertically for increased strength or horizontally for
low head - and pipe arches having rounded crown and flattened invert for
special application as access ways, utility ducts, etc.
7.5.7 Design
limitations for rigid pipe are commonly established to control the progression
of cracking at the crown and invert. Prevention of cracks wider than
7.5.8 Installation
conditions. Bedding, backfill, and trench conditions of pipe installation
can have significant effect on performance. Pipe
can be placed on flat compacted earth, on a 60°, 90°, or 120" shaped bed,
on a sand or fine gravel cushion, in a lean or competent concrete cradle, etc. Pipe can be placed in a narrow or wide
trench, shallow or deep trench, vertical or sloping sidewall trench, or no
trench. Backfill can be poorly compacted beneath (haunches) or beside the pipe
and can be the same as adjacent embankment material or a select sand, gravel,
or other superior material, or it can be a stabilized (cement or lime) soil.
Rigid pipe can be insulated from its normal accumulation of greater than
ambient stress by placing a soft zone of loose soil, straw, foamed plastic,
leaves, or similar material above the pipe. All of these many variables can
have an impact on the design loads to be considered.
7.5.9 Design.
Because of the many variables in loading, pipe characteristics, and
installation conditions design concepts, methods, and supporting methods for
characterizing behaviour of materials are beyond what can be presented here.
Design details can be found in some geotechnical textbooks, such as "Soil
Mechanics" by Krynine (United States), "Soil Engineering" by
Spangler (United States) and in trade literature, such as "Concrete Pipe
Design Manual" of the American Concrete Pipe Association (United States
Library of Congress Catalog No. 78-58624), "Handbook of Steel Drainage and
Highway Construction Products" of the American Iron and Steel Institute
(United States Library of Congress Catalog No. 78-174344) and in the many references
to technical literature contained in these documents. Some specific design
guidance for minimum protective cover beneath flexible or rigid pavement for
several types of pipe precomputed based on selected (common) installation
conditions can be found in the United States FAA manual on "Airport
Drainage" AC 150/5320-5B, as well as in the two trade literature manuals
referenced above.
7.5.10 Other
structures. Design of bridges and pile supported extensions over water, which
support aircraft loads directly, must follow accepted structural design
practice. It will be most important to anticipate future aircraft growth loads
to avoid very costly subsequent strengthening. Box culverts will be subject to
the ambient stresses (7.5.3) increased by the upthrust of such stiff structures
into the overlying embankment (7.5.4). The resulting load should be determined
by careful analysis, but should fall between about 130 per cent and 170 per
cent of the load due only to ambient stress depending upon span of the structure,
magnitude and extent of surface load, protective cover depth, and soil
stiffness adjacent to the culvert. Any large corrugated metal arches (over
CHAPTER 8. - CONSTRUCTION OF ASPHALTIC
OVERLAYS
8.1 Introductio8.1.1 The
volume and frequency of operations at many airports makes it virtually
mandatory to overlay (resurface) runways portion by portion so that they may be
returned to operational status during peak hours. The purpose of this chapter
is to detail the procedures to be used by those associated with such
overlaying, viz. the airport manager, project manager, designer and contractors
to ensure that the work is carried out most efficiently and without loss of
revenues, inconvenience to passengers or delays to the air traffic systems. A
unique feature of such off-peak construction is that a temporary ramp (a transition
surface between the overlay and the existing pavement) must be constructed at
the end of each work session so that the runway can be used for aircraft
operations once the work force clears the area. This chapter includes guidance
on the design of such temporary ramps, however, it is not the intent of this
chapter to deal with the design of overlays per se. For guidance on the latter
subject, the reader should refer to Chapter 4.
8.2 Airport authority's role
8.2.1
Project co-ordination
8.2.1.1 Off-peak
construction is, by its very nature, a highly visible project requiring close
co-ordination with all elements of the airport during planning and design and
virtually daily during construction. Once a runway paving project has been
identified by the airport, it is important that the nominees of the airport
authority, users and the Civil Aviation Authority of the State meet to discuss
the manner in which construction is to be implemented. The following key
personnel should be in attendance at all planning meetings: from the airport
authority - the project manager, the operations, planning, engineering and
maintenance directors; from the airlines - local station managers and head
office representatives where appropriate; from the civil aviation authority -
representatives from Air Traffic Services and Aeronautical Information
Services. The agenda should include:
a)
determination of working hours. Since time is of the essence in
off-peak construction, the contractor should be given as much time as possible
to overlay the pavement each work period. A minimum period of 81/2 hours is
recommended. Work should be scheduled for a time period that will displace the
least amount of scheduled flights. The selection of a specific time period
should be developed and co-ordinated with airline and other representatives
during the initial planning meetings. Early identification of the hours will
allow the airlines to adjust future schedules, as needed, to meet construction
demands. It is essential that the runway be opened and closed at the designated
time without exception, as airline flight schedules, as well as the
contractor's schedules, will be predicated on the availability of the runway at
the designated time;
b) identification of operational factors
during construction and establishment of acceptable criteria include:
1) designation of work areas;
2) aircraft operations;
3) affected navigation aids (visual and
non-visual aids);
4) security requirements and truck haul
routes;
5) inspection and requirements to open the
area for operational use;
6) placement and removal of construction
barricades;
7) temporary aerodrome pavement marking and
signing;
8) anticipated days of the week that
construction will take place; and
9) issuance of NOTAM and advisories;
c) lines of communication and co-ordination
elements. It is essential that the project manager be the only person to
conduct co-ordination of the pavement project. The methods and lines of
communication should be discussed for determining the availability of the
runway at the start of each work period and the condition of the runway prior
to opening it for operations
d) special aspects of construction including
temporary ramps and other details as described herein; and
e) contingency plan in case of abnormal
failure or an unexpected disaster.
8.2.2 Role
of project manager
8.2.2.1 Project
manager. It is essential that the airport authority select a qualified
project manager to oversee all phases of the project, from planning through
final inspection of the completed work. This individual should be experienced
in design and management of aerodrome pavement construction projects and be
familiar with the operation of the airport. The project manager should be the
final authority on all technical aspects of the project and be responsible for its
co-ordination with airport operations. All contact with any element of the
airport authority should be made only by the project manager so as to ensure
continuity and proper co-ordination with all elements of aerodrome operations.
Responsibilities should include:
a)
planning and design:
1) establishment of clear and concise lines
of communications;
2) participation as a member of the design
engineer's selection team; 3) co-ordination of project design to meet
applicable budget constraints;
4) co-ordination of airport and airlines with
regards to design review, including designated working hours, aircraft
operational requirements, technical review and establishment of procedures for
co-ordinating all work; and
5) chairmanship of all meetings pertaining to
the project; and
b)
construction:
1) complete management of construction with
adequate number of inspectors to observe and document work by the contractor;
2) checking with the weather bureau, airport
operations and air traffic control prior to starting construction and
confirming with the contractor's superintendent to verify if weather and air
traffic conditions will allow work to proceed as scheduled;
3) conferring with the contractor's project
superintendent daily and agreeing on how much work to attempt, to ensure the
opening of the runway promptly at the specified time each morning. This is
especially applicable in areas where pavement repair and replacement are to
take place; and
4) conducting an inspection with airport
operations of the work area before opening it to aircraft traffic to ensure
that all pavement surfaces have been swept clean, temporary ramps are properly
constructed and marking is available for aircraft to operate safely
8.2.2.2 Resident
engineer. The designation of a resident engineer, preferably a civil engineer,
will be of great benefit to the project, and of great assistance to the project
manager. Duties of the resident engineer should include:
a) preparation of documentation on the work
executed during each work period;
b) ensuring all tests are performed and
results obtained from each work period;
c) scheduling of inspection to occur each
work period;
d) observing contract specifications
compliance and reporting of any discrepancies to the project manager and the
contractor; and
e) maintaining a construction diary.
8.2.3 Testing
requirements
8.2.3.1 There
is no requirement for additional tests for off-peak construction versus
conventional construction. The only difference with off-peak construction is
that it requires acceptance testing to be performed at the completion of each
work period and prior to opening to operations and results reviewed before
beginning work again. These procedures normally will require additional
personnel to ensure that tests are performed correctly and on time.
8.2.4
Inspection requirements
8.2.4.1 One
of the most important aspects of successful completion of any kind of paving
project is the amount and quality of inspection performed. Since the airport
accepts beneficial occupancy each time the runway is open to traffic,
acceptance testing must take place each work period. In addition to the project
manager and resident engineer, the following personnel are recommended as a
minimum to observe compliance with specifications:
a) Asphalt plant inspector. A plant
inspector with a helper whose primary duty it will be to perform quality
control tests, including aggregate gradation, hot bin samples and
b) Paving inspectors. There should be
two paving inspectors with each paving machine. Their duties should include
collection of delivery tickets, checking temperatures of delivered material,
inspection of grade control methods, and inspection of asphalt lay-down
techniques and joint construction smoothness
C) Compaction inspector. The
compaction inspector should be responsible for observing proper sequencing of
rollers and for working with a field density meter to provide the contractor
with optimum compaction information.
d) Survey crew Finished grade information from each work
period is essential to ensuring a quality job. An independent registered
surveyor and crew should record levels of the completed pavement at intervals
of at least
e) Pavement repair inspector. Shall be
responsible for inspection of all pavement repairs and surface preparation
prior to paving.
f) Electrical inspector. Ensures
compliance with specifications.
8.3
Design considerations
8.3.1 General.
Plans and specifications for pavement repair and overlay during off-peak
periods should be presented in such detail as to allow ready determination of
the limits of pavement repair, finish grades and depths of overlay. Plans and
specifications are to be used for each work period by the contractor and
inspection personnel, and should be clear and precise in every detail
8.3.2 Pavement
survey
8.3.2.1 A
complete system of bench marks should be set on the side of the runway or
taxiway to permit a ready reference during cross-sectioning operations. The
bench marks should be set at approximately
8.3.2.2 After
finish grades and transverse slope of the runway are determined, a tabulation
of grades should be included in the plans for the contractor to use in bidding
the project and for establishment of erected stringline. The tabulation of
grades should include a column showing existing runway elevation, a column
showing finish overlay grade, and a column showing depth of overlay. Grades
should be shown longitudinally every
8.3.3
Special details
8.3.3.1
Details pertaining to the following items should be included in the
plans:
a) Temporary ramps. At the end of each
hot mix asphalt concrete overlay work period, it will be necessary to construct
a ramp to provide a transition from the new course of overlay to the existing
pavement. The only exception to construction of a ramp is when the depth of the
overlay is
exceed 2 per cent. A temporary ramp may be
constructed in two ways, depending upon the type of equipment that is
available. The most efficient way is to utilize a cold planing machine to
heel-cut the pavement at the beginning and at the end of the work period
overlay (refer to Figure 8-1). If cold planing equipment is not available, then
a temporary ramp should be constructed as shown in Figure 8-
b) In-pavement lighting. Details
depicting the removal and re-installation of iu-pavement lighting are to be
included on the plans where applicable. The details should depict the removal
of the light fixture and extension ring, placement of a target plate over the
light base, filling the hole with hot mix dense graded asphalt until overlay
operations are complete, accurate survey location information, core drilling
with a
c) Runway markings. During the course
of off-peak construction of a runway overlay, it has been found acceptable, if
properly covered by a NOTAM, to mark only the centre line stripes and the
runway designation numbers on the new pavement until the final asphalt lift has
been completed and final striping can then be performed. In some cases where
cold planing of the surface or multiple lift overlays are used, as many as
three consecutive centre line stripes may be omitted to enhance the bond
between layers.