**Subject:** AIRPORT PAVEMENT DESIGN AND **Date:** April 30, 2004 **AC
No:** 150/5320-6D

EVALUATION **Initiated by:** AAS-100 **Change:** 3

**1. PURPOSE.** Advisory Circular (AC) 150/5320-6D, Airport Pavement
Design and Evaluation, has been revised to incorporate the contents of
AC 150/5320-16, Airport Pavement Design for the Boeing 777 Airplane, and
to announce design software for Chapters 3 and 4.

**2. PRINCIPAL CHANGES. This document makes three principal changes to
AC 150/5320-6D:**

a.  A new Chapter 7, Layered Elastic Pavement Design, incorporates the
    contents of AC 150/5320-16, which is cancelled. The user's manual
    previously published as an appendix to 150/5320-16 is now available
    as a help file to the LEDFAA design program described in Chapter 7.

b.  The layered elastic design method can now be used as an alternate
    design method to the procedures described in Chapters 3 and 4.
    Layered elastic design procedures were previously reserved for use
    only when the Boeing 777 aircraft was in the anticipated traffic
    mixture.

c.  A new Appendix 5, Airfield Pavement Design Software, announces the
    availability of Microsoft Excel spreadsheets for the design
    procedures described in Chapters 3 and 4. The appendix explains the
    purpose of the spreadsheets and describes how to access both the
    spreadsheets (F806faa.xls and R805faa.xls) and the associated user's
    manuals.

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![](media/image1.png){width="2.7465277777777777in"
height="0.5416666666666666in"}

DAVID L. BENNETT

Director of Airport Safety and Standards

605\. APPLICATION OF RIGID PAVEMENT EVALUATION PROCEDURES 139

606\. USE OF RESULTS 139

607\. REPORTING PAVEMENT STRENGTH 139

**CHAPTER 7. Layered Elastic Pavement Design**

**Section 1. Design Considerations 141**

701\. Purpose 141

702\. Application 141

703\. Background 141

704\. Computer program 141

705\. pavement design considerations 142

706\. FLEXIBLE PAVEMENT dESIGN 142

707\. RIGID PAVEMENT DESIGN 143

708\. Layered Elastic oVERLAY DESIGN 144

TABLE 7-1. RIGID PAVEMENT DISTRESS TYPES USED TO CALCULATE

THE STRUCTURAL CONDITION INDEX, SCI 145

**APPENDIX 1. ECONOMIC ANALYSIS 1**

1\. BACKGROUND 1

2\. ANALYSIS METHOD 1

3\. STEP BY STEP PROCEDURE 1

4\. EXAMPLE PROBLEM - LIGHT-LOAD GENERAL AVIATION AIRPORT 2

TABLE 1. COSTS OF REHABILITATION ACTIVITIES 4

TABLE 3. SUMMARY OF ALTERNATIVES 5

TABLE 4. COMPARATIVE RANKING OF ALTERNATIVES 5

5\. SUMMARY 5

**APPENDIX 2. DEVELOPMENT OF PAVEMENT DESIGN CURVES 1**

1\. BACKGROUND 1

TABLE 1. SINGLE WHEEL ASSEMBLY 1

TABLE 2. DUAL WHEEL ASSEMBLY 1

TABLE 3. DUAL TANDEM ASSEMBLY 1

2\. RIGID PAVEMENTS 1

TABLE 4. PASS-TO-COVERAGE RATIOS FOR RIGID PAVEMENTS 2

FIGURE 1. ASSEMBLY POSITIONS FOR RIGID PAVEMENT ANALYSIS 3

FIGURE 2. PERCENT THICKNESS VS. COVERAGES 4

3\. FLEXIBLE PAVEMENTS 5

TABLE 5. PASS-TO-COVERAGE RATIOS FOR FLEXIBLE PAVEMENTS 5

FIGURE 3. LOAD REPETITION FACTOR VS. COVERAGES 6

**APPENDIX 3. DESIGN OF STRUCTURES FOR HEAVY AIRCRAFT 1**

1\. BACKGROUND 1

2\. RECOMMENDED DESIGN PARAMETERS 1

FIGURE 1. TYPICAL GEAR CONFIGURATIONS FOR DESIGN OF

STRUCTURES 2

**APPENDIX 4. RELATED READING MATERIAL 1**

**APPENDIX 5. AIRFIELD PAVEMENT DESIGN SOFTWARE 1**

## FOREWORD

This AC provides guidance on the structural design and evaluation of
airport pavements.

Although aircraft landing gears play a role in airport pavement design
and evaluation, this AC does not dictate any facet of landing gear
design. In 1958, the FAA adopted a policy of limiting maximum Federal
participation in airport pavements to a pavement section designed to
serve a 350,000-pound (159 000 kg) aircraft with a DC-8-50 series
landing gear configuration. The intent of the policy was to ensure that
future aircraft were equipped with landing gears that would not stress
pavements more than the referenced 350,000-pound (159 000 kg) aircraft.

Throughout the 20th century, aircraft manufacturers accepted and
followed the 1958 policy and designed aircraft landing gears that
conformed to it---even though aircraft gross weights have long exceeded
350,000 pounds (159 000 kg). Despite the greater weights, manufacturers
were able to conform to the policy by increasing the number and spacing
of landing gear wheels. This AC does not affect the 1958 policy with
regard to landing gear design.

The pavement design guidance presented in Chapter 3 is based on methods
of analysis that resulted from experience and past research. The methods
employed in Chapter 3 were adopted in 1978 to exploit advances in
pavement technology and thus provide better performing pavements and
easier-to-use design curves than were previously available. Generally
speaking, the Chapter 3 guidance requires somewhat thicker pavement
sections than were required prior to 1978.

Chapter 6 presents the pavement evaluation portion of this AC. It
relates back to the previous FAA method of design to ensure continuity.
An aircraft operator could be penalized unfairly if an existing facility
was evaluated using a method different from that employed in the
original design. A slight change in pavement thickness can have a
dramatic effect on the payload or range of an aircraft. Since the new
pavement design methodology might produce different pavement
thicknesses, an evaluation of an existing pavement using the new
methodology could result in incompatible results. To avoid this
situation, the evaluation should be based whenever possible on the same
methodology as was used for the design.

Where new aircraft have been added to the traffic mixture at an existing
facility, it may not be possible to evaluate the pavement with the
original design procedure. For example, when a triple dual tandem (TDT)
gear aircraft is added to the traffic mixture at a facility originally
designed in accordance with Chapter 3, it will be impossible to assess
the impact of the new aircraft using the procedures in Chapter 3. In
instances where it is not appropriate to evaluate the pavement with the
original design procedure, the pavement must be evaluated with the newer
design procedures.

The pavement design guidance presented in Chapter 7 implements layered
elastic theory based design procedures. The FAA adopted this methodology
to address the impact of new landing gear configurations such as the TDT
gear, which aircraft manufacturers developed and implemented in the
early 1990s. The TDT gear produces an unprecedented airport pavement
loading configuration, which appears to exceed the capability of the
previous methods of design. Previous methods incorporated some
empiricism and have limited capacity for accommodating new gear and
wheel arrangements.

THIS PAGE INTENTIONALLY BLANK

**CHAPTER 3. PAVEMENT DESIGN**

**SECTION 1. DESIGN CONSIDERATIONS**

**300. SCOPE.** This chapter covers pavement design for airports serving
aircraft with gross weights of 30,000 pounds (13 000 kg) or more.
Chapter 5 discusses the design of pavements serving lighter aircraft
with gross weights under 30,000 pounds (13 000 kg).

**301. DESIGN PHILOSOPHY.** The Foreword of this AC describes the FAA
policy of treating the design of aircraft landing gear and the design
and evaluation of airport pavements as separate entities. The design of
airport pavements is a complex engineering problem that involves a large
number of interacting variables. The design curves presented in this
chapter are based on the California Bearing Ratio (CBR) method of design
for flexible pavements and a jointed edge stress analysis for rigid
pavements. Other design procedures, such as those based on layered
elastic analysis and those developed by the Asphalt Institute and the
Portland Cement Association may be used to determine pavement
thicknesses when approved by the FAA. These procedures will yield
slightly different pavement thicknesses due to different basic design
assumptions.

All manual and electronic pavement designs should be summarized on FAA
Form 5100-1, Airport Pavement Design, which is considered part of the
Engineer's Report. The Engineer's Report should be submitted for FAA
review and approval along with initial plans and specifications.

Because of thickness variations, the evaluation of existing pavements
should be performed using the same method employed for design. Chapter 6
describes in detail procedures to use when evaluating pavements. Details
on the development of the FAA method of design are as follows:

**a. Flexible Pavements.** The flexible pavement design curves presented
in this chapter are based on the CBR method of design. The CBR design
method is basically empirical; however, a great deal of research has
been done with the method, resulting in the development of reliable
correlations. Gear configurations are considered using theoretical
concepts as well as empirically developed data. The design curves
provide the required total thickness of flexible pavement (surface,
base, and subbase) 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 given in a separate
table. A more detailed discussion of CBR design is presented in Appendix
2.

**b. Rigid Pavements.** The rigid pavement design curves in this chapter
are based on the Westergaard analysis of edge loaded slabs. The edge
loading analysis has been modified to simulate a jointed edge condition.
Pavement stresses are higher at the jointed edge than at the slab
interior. Experience shows practically all load-induced cracks develop
at jointed edges and migrate toward the slab interior. Design curves are
furnished for areas where traffic will travel primarily parallel or
perpendicular to joints and where traffic is likely to cross joints at
an acute angle. The thickness of pavement determined from the curves is
for slab thickness only. Subbase thicknesses are determined separately.
A more detailed discussion of the basis for rigid pavement design is
presented in Appendix 2.

**302. BACKGROUND.** An airfield pavement and the aircraft that operate
on it represent an interactive system that must be addressed in the
pavement design process. Design considerations associated with both the
aircraft and the pavement must be recognized in order to produce a
satisfactory design. Producing a pavement that will achieve the intended
design life will require careful construction control and some degree of
maintenance. Pavements are designed to provide a finite life, and
fatigue limits are anticipated. Poor construction and a lack of
preventative maintenance will usually shorten the service life of even
the best-designed pavement.

**a. Variables.** The determination of pavement thickness requirements
is a complex engineering problem. Pavements are subject to a wide
variety of loading and climatic effects. The design process involves a
large number of interacting variables, which are often difficult to
quantify. Despite considerable research on this subject, it has been
impossible to arrive at a direct mathematical solution for thickness
requirements. For this reason, pavement engineers must base pavement
thickness on a 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.
The FAA developed the thickness curves presented in this chapter by
correlating the data obtained from these sources. Pavements designed in
accordance with these standards should have a structural life of 20
years. In addition, as long as there are no major changes in forecast
traffic, the pavements should not require any major maintenance. It is
likely, however, that rehabilitation of surface grades and renewal of
skid-resistant properties will be needed before 20 years because of
destructive climatic effects and the deteriorating effects of normal
usage.

**b. Structural Design.** The structural design of airport pavements
requires determining both the overall pavement thickness and the
thickness of the component parts of the pavement. There are a number of
factors that 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 that make up the pavement structure.

**303. AIRCRAFT CONSIDERATIONS.**

**a. Load.** The pavement design method is based on the gross weight of
the aircraft. The pavement should be designed for the maximum
anticipated takeoff weight of the aircraft. The design procedure assumes
95 percent of the gross weight is carried by the main landing gears and
5 percent is carried by the nose gear. AC 150/5300-13, Airport Design,
lists the weight of many civil aircraft. The FAA recommends using the
maximum anticipated takeoff weight, which provides some degree of
conservatism in the design. This will allow for changes in operational
use and forecast traffic, which is approximate at best. The conservatism
will be offset somewhat by ignoring arriving traffic.

**b. Landing Gear Type and Geometry.** Gear type and configuration
dictate how aircraft weight is distributed to a pavement and how the
pavement will respond to aircraft loadings. Because of this, separate
design curves would be necessary for each type of aircraft unless some
valid assumptions could be made to reduce the number of variables.
However, examination of gear configuration, tire contact areas, and tire
pressure in common use indicate that these factors follow a definite
trend related to aircraft gross weight. Therefore, reasonable
assumptions can be made, the variables reduced, 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 the following design values are
appropriate: a dimension of 20 inches (0.51 m) between the centerline of
the tires for lighter aircraft and a dimension of 34 inches (0.86 m)
between the centerline of the tires for heavier aircraft.

**(3) Dual Tandem Gear Aircraft.** The study indicated the following
design values are appropriate: a dual wheel spacing of 20 inches (0.51m)
and a tandem spacing of 45 inches (1.14 m) for lighter aircraft and a
dual wheel spacing of 30 inches (0.76 m) and a tandem spacing of 55
inches (1.40 m) for heavier aircraft.

**(4) Wide Body Aircraft.** Aircraft such as the B-747, B-767, DC-10,
and L-1011 have large spaced dual tandem gear geometries, which
represent a radical departure from the geometry assumed for dual tandem
aircraft described in paragraph 303b(3) above. Due to the large
differences in gross weights and gear geometries, separate design curves
are provided for these aircraft. The term wide body was originally
applied to these aircraft because of their width compared to other
contemporary aircraft.

**(5) Triple Dual tandem Gear Aircraft.** Aircraft such as the B-777 and
A-380 have landing gears with three rows of dual wheels. Pavement design
requirements for traffic mixtures containing triple dual tandem aircraft
are discussed in Chapter 7.

**c. Tire Pressure.** Tire pressure varies between 75 and 200 psi (515
to 1 380 kPa), 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
(1 380 kPa) may be safely exceeded if other parameters are not exceeded
and a high-stability surface course is used.

**d. 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
National Plan of Integrated Airport Systems, Airport Activity
Statistics, and FAA Air Traffic Activity Reports. Pavement engineers
should consult these publications when developing forecasts of annual
departures by aircraft type.

**304. DETERMINATION OF DESIGN AIRCRAFT.** The forecast of annual
departures by aircraft type will result in a list of several different
aircraft. The required pavement thickness for each aircraft type in the
forecast should be checked using the appropriate design curve and the
forecast number of annual departures for that aircraft. The design
aircraft is the aircraft type that produces the greatest pavement
thickness. It will not necessarily be the heaviest aircraft in the
forecast.

**305.** **DETERMINATION OF EQUIVALENT ANNUAL DEPARTURES BY THE DESIGN
AIRCRAFT.**

**a. Conversions.** Since the traffic forecast is a mixture 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 FAA has established factors to accomplish this
conversion. These factors are constant and apply to both flexible and
rigid pavements. They represent an approximation of the relative fatigue
effects of different gear types. Much more precise and theoretically
rigorous factors could be developed for different types and thicknesses
of pavement. However, at this stage of the design process, such
precision is not warranted and would be impractical for hand calculation
since design changes would require numerous iterations and adjustments.

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               single wheel              1.3

  dual wheel               dual tandem               0.6

  dual tandem              single wheel              2.0

  dual tandem              dual wheel                1.7

  double dual tandem       dual tandem               1.0

  double dual tandem       dual wheel                1.7
  ------------------------ ------------------------- --------------------

After the aircraft have been grouped into the same landing gear
configuration, the following formula should be used to convert to
equivalent annual departures of the design aircraft:

> $$$\text{Log R}_{1} = \text{ Log R}_{2} \times \sqrt{\frac{W_{2}}{W_{1}}}$
>
> where:
>
> R~1~ = equivalent annual departures by the design aircraft
>
> R~2~ = annual departures expressed in design aircraft landing gear
>
> W~1~ = wheel load of the design aircraft
>
> W~2~ = wheel load of the aircraft in question

This computation assumes that 95 percent of the gross weight of the
aircraft is carried by the main landing gears. The procedure discussed
above is a relative rating that compares different aircraft to a common
design aircraft. Since wide body aircraft have significantly different
landing gear assembly spacing than other aircraft, special
considerations are needed to maintain the relative effects. This is done
by treating each wide body as a 300,000-pound (136 100 kg) dual tandem
aircraft when computing equivalent annual departures. Wide body aircraft
should be treated this way in every instance---even when the design
aircraft is a wide body. 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.

**b. Example.** Assume an airport pavement is to be designed for the
following forecast traffic.

  ------------ ---------------- ------------- --------------- -------------
  Aircraft     Gear Type        Average       Maximum Takeoff Maximum
                                Annual        Weight (lbs.)   Takeoff
                                Departures                    Weight (kg)

  727-100      dual             3,760         160,000         72 580

  727-200      dual             9,080         190,500         86 410

  707-320B     dual tandem      3,050         327,000         148 330

  DC-9-30      dual             5,800         108,000         49 000

  CV-880       dual tandem      400           184,500         83 690

  737-200      dual             2,650         115,500         52 390

  L-1011-100   dual tandem      1,710         450,000         204 120

  747-100      double dual      85            700,000         317 520
               tandem                                         
  ------------ ---------------- ------------- --------------- -------------

**(1)** **Determine Design Aircraft.** A pavement thickness is
determined for each aircraft in the forecast with 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
thus is the design aircraft.

**(2)** **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.

**(3)** **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.

  ------------ ---------- --------------- -------- ----------- ----------- --------------
  Aircraft     Equiv.     Wheel Load:     Wheel    Wheel Load  Wheel Load  Equiv. Annual
               Dual Gear  (lbs)           Load:    of Design   of Design   Departures
               Departs.                   (kg)     Aircraft:   Aircraft:   Design
                                                   (lbs)       (kg)        Aircraft

  727-100      3,760      38,000          17 240   45,240      20 520      1,891

  727-200      9,080      45,240          20 520   45,240      20 520      9,080

  707-320B     5,185      38,830          17 610   45,240      20 520      2,764

  DC-9-30      5,800      25,650          11 630   45,240      20 520      682

  CV-880       680        21,910          9 940    45,240      20 520      94

  737-200      2,650      27,430          12 440   45,240      20 520      463

  L-1011-100   2,907      35,625**^1^**   16 160   45,240      20 520      1,184

  747-100      145        35,625**^1^**   16 160   45,240      20 520      83
  ------------ ---------- --------------- -------- ----------- ----------- --------------

Total = 16,241

> **^1^** Wheel loads for wide body aircraft are taken as the wheel load
> for a 300,000-pound (136 100 kg) dual tandem aircraft for equivalent
> annual departure calculations.

**(4)** **Final Result.** In this example, the pavement would be
designed for 16,241 annual departures of a dual wheel aircraft weighing
190,500 pounds (86 410 kg). The design, however, should provide for the
heaviest aircraft in the traffic mixture, the B747-100, when considering
depth of compaction, thickness of asphalt surface, drainage structures,
etc.

**c.** **Other Methods.** More refined methods of considering mixed
traffic are possible. These refined methods might consider variations in
material properties due to climatic effects, takeoff versus landing
loads, aircraft tread dimensions, etc. The FAA allows the use of these
refined methods under the conditions given in paragraph 301.

**306.** **TRAFFIC DISTRIBUTION.** Research studies have shown that
aircraft traffic is distributed laterally across runways and taxiways
according to statistically normal (bell-shaped) distribution. FAA Report
No. FAA-RD-36, Field Survey and Analysis of Aircraft Distribution on
Airport Pavements, dated February 1975, contains research information on
traffic distribution. The design procedures presented in this AC
incorporate the statistically normal distribution in the departure
levels. In addition to the lateral distribution of traffic across
pavements, it also considers traffic distribution and the nature of
loadings for aprons and high-speed turnoffs.

**SECTION 3. RIGID PAVEMENT DESIGN**

**324. GENERAL.** Rigid pavements for airports are composed of Portland
cement concrete placed on a granular or treated subbase course that is
supported on a compacted subgrade. Under certain conditions, a subbase
is not required (see paragraph 326).

**325. CONCRETE PAVEMENT.** The concrete surface must provide a nonskid
surface, prevent the infiltration of surface water into the subgrade,
and provide structural support to the aircraft. The quality of the
concrete, acceptance and control tests, methods of construction and
handling, and quality of workmanship are covered in Item P-501, Portland
Cement Concrete Pavement.

**326. SUBBASE.** The purpose of a subbase under a rigid pavement is to
provide uniform stable support for the pavement slabs. A minimum
thickness of 4 inches (100 mm) of subbase is required under all rigid
pavements, except as shown in Table 3-10 below:

**TABLE 3-10. CONDITIONS WHERE NO SUBBASE IS REQUIRED**

  ---------------- -------------- -------------- -------------- --------------
  Soil             Good Drainage: Good Drainage: Poor Drainage: Poor Drainage:
  Classification   No Frost       Frost          No Frost       Frost

  GW               **X**          **X**          **X**          **X**

  GP               **X**          **X**          **X**          

  GM               **X**                                        

  GC               **X**                                        

  SW               **X**                                        
  ---------------- -------------- -------------- -------------- --------------

Note: X indicates conditions where no subbase is required.

**327. SUBBASE QUALITY.** The standard FAA subbase for rigid pavements
is 4 inches (100 mm) of Item P-154, Subbase Course. In some instances,
it may be desirable to use higher-quality materials or thicknesses of
P-154 greater than 4 inches (100 mm). The following materials are
acceptable for use as subbase under rigid pavements:

Item P-154 -- Subbase Course

Item P-208 -- Aggregate Base Course

Item P-209 -- Crushed Aggregate Base Course

Item P-211 -- Lime Rock Base Course

Item P-304 -- Cement Treated Base Course

Item P-306 -- Econocrete Subbase Course

Item P-401 -- Plant Mix Bituminous Pavements

Materials of higher quality than P-154 and/or greater thicknesses of
subbase are considered in the design process through the foundation
modulus (k value). The costs of providing the additional thickness or
higher-quality subbase should be weighed against the savings in concrete
thickness.

**328. STABILIZED SUBBASE.** Stabilized subbase is required for all new
rigid pavements designed to accommodate aircraft weighing 100,000 pounds
(45 400 kg) or more. Stabilized subbases are as follows:

> Item P-304 -- Cement Treated Base Course
>
> Item P-306 -- Econocrete Subbase Course
>
> Item P-401 -- Plant Mix Bituminous Pavements

The structural benefit imparted to a pavement section by a stabilized
subbase is reflected in the modulus of subgrade reaction assigned to the
foundation. Exceptions to the policy of using stabilized subbase are the
same as those given in paragraph 320.

**329. SUBGRADE.** As with a flexible pavement, the subgrade materials
under a rigid pavement should be compacted to provide adequate stability
and uniform support; however, the compaction requirements for rigid
pavements are not as stringent as for flexible pavement because of the
relatively lower subgrade stress. For cohesive soils used in fill
sections, the top 6 inches (150 mm) must be compacted to 90 percent
maximum density. Fill depths greater than 6 inches (150 mm) must be
compacted to 90 percent maximum density or meet the requirements of
Table 3-2. For cohesive soils in cut sections, the top 6 inches (150 mm)
of the subgrade must be compacted to 90 percent maximum density. For
noncohesive soils used in fill sections, the top 6 inches (150 mm) of
fill must be compacted to 100 percent maximum density, and the remainder
of the fill must be compacted to 95 percent maximum density or meet the
requirements of Table 3-2. For cut sections in noncohesive soils, the
top 6 inches (150 mm) of subgrade must be compacted to 100 percent
maximum density and the next 18 inches (460 mm) of subgrade must be
compacted to 95 percent maximum density. Swelling soils require special
considerations. Paragraph 314 contains guidance on the identification
and treatment of swelling soils.

**a. Contamination.** In rigid pavement systems, repeated loading might
cause intermixing of soft subgrade soils and aggregate base or subbase.
This mixing can create voids below the pavement in which moisture can
accumulate, causing pumping to occur. Chemical and mechanical
stabilization of the subbase or subgrade can effectively reduce
aggregate contamination (see paragraph 207). Geotextiles have been found
to be effective at providing separation between fine-grained subgrade
soils and pavement aggregates (FHWA-HI-90-001 Geotextile Design and
Construction Guidelines). Geotextiles should be considered for
separation between fine-grained soils and overlying pavement aggregates.
In this application, the geotextile is not considered to act as a
structural element within the pavement. Therefore, the modulus of the
base or subbase is not increased when a geotextile is used for
stabilization. For separation applications, the geotextile is designed
based on survivability properties. FHWA-HI-90-001 contains additional
information about design and construction using separation geotextiles.

**330. DETERMINATION OF FOUNDATION MODULUS (k VALUE) FOR RIGID
PAVEMENT.** In addition to the soils survey and analysis and
classification of subgrade conditions, rigid pavement design also
requires the determination of the foundation modulus. The k value should
be assigned to the material directly beneath the concrete pavement.
However, the FAA recommends that a k value be established for the
subgrade and then corrected to account for the effects of the subbase.

**a. Determination of k Value for Subgrade.** The preferred method of
determining the subgrade modulus is by testing a limited section of
embankment that has been constructed to the required specifications. The
plate bearing test procedures are given in AASHTO T 222, Nonrepetitive
Static Plate Load Test of Soils and Flexible Pavement Components for Use
in Evaluation and Design of Airport and Highway Pavements. If the
construction and testing of a test section of embankment is impractical,
the values listed in Table 2-3 may be used. The values in Table 2-3,
however, are approximate, and engineering judgment should be used when
selecting a design value. Fortunately, rigid pavement is not overly
sensitive to k value, and an error in estimating k will not have a large
impact on rigid pavement thickness.

**b. Determination of k Value for Granular Subbase.** It is usually not
practical to determine a foundation modulus on top of a subbase by
testing, at least in the design phase. Usually, the embankment and
subbase will not be in place in time to perform any field tests, so the
k value will have to be assigned without the benefit of testing. The
probable increase in k value associated with various thicknesses of
different subbase materials is shown in Figure 2-4. The upper graph in
Figure 2-4 should be used when the subbase is composed of well-graded
crushed aggregate, such as P-209. The lower graph in Figure 2-4 applies
to bank-run sand and gravel, such as P-154. Both curves in Figure 2-4
apply to unstabilized granular materials. Values shown in Figure 2-4 are
guides and can be tempered by local experience.

**c. Determination of k Value for Stabilized Subbase.** As with granular
subbase, the effect of stabilized subbase is reflected in the foundation
modulus. Figure 3-16 shows the probable increase in k value with various
thicknesses of stabilized subbase located on subgrades of varying
moduli. Figure 3-16 is applicable to cement stabilized (P-304),
Econocrete (P-306), and bituminous stabilized (P-401) layers. Figure
3-16 assumes a stabilized layer is twice as effective as well-graded
crushed aggregate in increasing the subgrade modulus. Stabilized layers
of lesser quality than P-304, P-306, or P-401 should be assigned
somewhat lower k values. After a k value is assigned to the stabilized
subbase, the concrete slab thickness design procedure is the same as
that described in paragraph 331.

  --------- ------- --------- ------- -------- ------- -------- ------- -------- ------- -------- ------- -------- ------- -------- -------- -------- -------- -------- -------- -------- -------- -------- ----
  **CBR**   **3**             **4**            **5**            **6**            **7**            **8**            **9**            **11**            **13**            **16**            **20**            

                                                                                                                                                                                                            

                                                                                                                                                                                                            

  **FAA**           **F10**           **F9**           **F8**           **F7**           **F6**           **F5**           **F4**            **F3**            **F2**            **F1**            **Fa**   
  --------- ------- --------- ------- -------- ------- -------- ------- -------- ------- -------- ------- -------- ------- -------- -------- -------- -------- -------- -------- -------- -------- -------- ----

SUBGRADE CLASS

**FIGURE 6-1. CBR--FAA SUBGRADE CLASS COMPARISONS**

**(1) Subbase and Base Equivalencies.** For evaluation purposes, the FAA
recommends the equivalency factor ranges shown in Tables 3-6 through 3-9
for subbase and base. The actual value selected will depend on the
composition, quality, and condition of the layer. If experience or
physical test results show that other values are valid, they may be used
in lieu of the values recommended here. Subbase or base courses should
not be assigned a higher equivalency factor than any layer above it in
the pavement structure. The FAA does not permit the conversion of
material to a higher classification, such as subbase to base, except
where excess stabilized base course (P-401 or P-304) exists immediately
under a flexible surface, in which case the stabilized material may be
counted as an equal thickness of surface.

**(2) Surfacing.** Broken hot mix asphalt surface course (shrinkage
cracks due to age and weathering, without evidence of base failure) must
be evaluated as an equal thickness of nonstabilized base. A hot mix
asphalt surface, well maintained and with limited cracking, might
justify use of an equivalency between the limits noted.

**603. APPLICATION OF FLEXIBLE PAVEMENT EVALUATION PROCEDURES.** 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 presented in Chapters 3 or 5 are used to determine the
load carrying capacity of the existing pavement. Required inputs are
subgrade and subbase CBR values, thicknesses of surfacing, base and
subbase courses, and an annual departure level. Several checks must be
performed to determine the load carrying capacity of a flexible
pavement. The calculation that yields the lowest allowable load will
control the evaluation.

**a. Total Pavement Thickness.** Enter the lower abscissa of the
appropriate design curve in Chapters 3 or 5 with the total pavement
thickness of the existing pavement. Make a vertical projection to the
annual departure level line. For light load pavements, described in
Chapter 5, a single pivot line is used. At the point of intersection
between the vertical projection and the departure level line, or single
pivot line in the case of light load pavements, 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 the 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 subbase, the combined thickness of surfacing and base, and
the annual departure level as inputs. The procedure is the same as that
described in subparagraph a above, except that the subbase CBR and
combined thickness of surfacing and base are used to enter the design
curves.

**c. Minimum Base Course Thickness.** The thickness of the existing base
course should be compared with the minimum base course thicknesses in
Table 3-4 or Figure 5-2. Notice that the minimum base course thickness
is 4 inches (100 mm) for heavy load pavements and 3 inches (75 mm) for
light load pavements. If there is a deficiency in the thickness of the
existing base course, the pavement should be closely monitored for signs
of distress. The FAA recommends that overlaying the pavement to correct
the deficiency be considered.

**d. Minimum Surface 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. The FAA recommends that correction of the deficiency in
surfacing thickness be considered.

**604. RIGID PAVEMENTS.** Evaluation of rigid pavements requires, at a
minimum, the determination of the thickness of the component layers, the
flexural strength of the concrete, and the modulus of subgrade reaction.

**a. Layer Thicknesses.** The thickness of the component layers is
sometimes 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. Concrete Flexural Strength.** 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. However, this method
is often impractical, as sawed beams are expensive to obtain and costs
incurred in obtaining sufficient numbers of beams to establish a
representative sample are prohibitive. Construction records, if
available, may be used as a source of concrete flexural strength data,
but the construction data will probably have to be adjusted for age, as
concrete strength increases with time.

**(1) Correlations with Other Strength Tests.** Correlations between
concrete flexural strength and other concrete strength tests are
available, but the correlations between flexural strength and other
strength tests are approximate, and considerable variations are likely.

**(i) Tensile Split Strength.** An approximate relationship between
concrete flexural strength and tensile splitting strength (ASTM C 496)
exists and can be computed with the following formula:

R = 1.02( T ) + 117

where:

> R = flexural strength, psi
>
> T = tensile split strength, psi
>
> Note: For conversions in metric units, the above formula remains the
> same---except the 117 psi constant should be changed to 0.81 MPa.

This equation can be used with 85 percent confidence that the estimated
flexural strength is at least as strong as the strength derived from the
original regression analysis detailed in Concrete Strength
Relationships, Miscellaneous Report Number S-74-30, published by the US
Army Engineer Waterways Experiment Station, December 1974.

**(ii) Compressive Strength.** Flexural strength can be estimated from
compressive strength (ASTM C 39) using the formula below:

$$R = 9\sqrt{f_{c}^{'}}$$

where:

> R = flexural strength
>
> $f_{c}^{'}$= compressive strength

**c.** **Modulus of Subgrade Reaction.** 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 that
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 subgrade reaction may be determined by nondestructive testing or
estimated by using Table 2-3 in Chapter 2 of this AC. Fortunately,
pavement evaluation is not too sensitive to the modulus of subgrade
reaction.

**(1)** **Adjustment for Subbase.** An adjustment to the modulus of
subgrade reaction will be required if a subbase exists beneath the
existing pavement. The thickness of the subbase is required to calculate
an adjusted k value. The subbase thickness can be determined from
construction records or from borings. The guidance contained in Chapter
3, Section 3, should be used in assigning a k value to a subbase.

**605. APPLICATION OF RIGID PAVEMENT EVALUATION PROCEDURES.** The
evaluation of rigid pavements for aircraft weighing more than 30,000
pounds (13 600 kg) requires concrete flexural strength, k value of the
foundation, slab thickness, and annual departure level as inputs. The
rigid pavement design curves in Chapter 3 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.

**606. USE OF RESULTS.** If the evaluation is being used for planning
purposes and the existing pavement is found to be deficient in
accordance with the design standards given in Chapters 3 or 5, the
sponsor should be notified as to the deficiency and should consider
corrective action. If the evaluation is being used as part of the design
for a project to reconstruct or upgrade the facility, the procedures
given in Chapters 3, 4, or 5 should be used to design the reconstruction
or overlay project. In the latter case, the main concern is not the load
carrying capacity, but the difference between the existing pavement
structure and the section that is needed to support forecast traffic.

**607. REPORTING PAVEMENT STRENGTH.** The International Civil Aviation
Organization (ICAO) developed a standardized method of reporting airport
pavement strength known as the Aircraft Classification Number/Pavement
Classification Number (ACN/PCN). This method is based on the concept of
reporting strength in terms of a standardized equivalent single wheel
load. This method of reporting pavement strength is discussed in FAA AC
150/5335-5, Standardized Method of Reporting Airport Pavement
Strength--PCN.

**608. EVALUATIONS OF EXISTING PAVEMENT WITH LAYERED ELASTIC
PROCEDURES.** When an existing pavement must be evaluated for use by new
aircraft not covered in the nomographs provided in Chapter 3, the
layered elastic design procedures described in Chapter 7 may be used.
However, an evaluation with these procedures might present unique
difficulties when trying to correlate results to original thickness
requirements. In many cases, the original traffic mixture might not be
known and only the design aircraft gross weight and annual departures be
available. Since the evaluation is for current usage, the existing and
forecasted traffic mixture should be used to evaluate the pavement
requirements.

Existing pavements may also be evaluated for use by new aircraft with
the ACN/PCN system described in AC 150/5335-5. The ACN value of new
aircraft can be compared to the existing PCN value of the pavement to
determine whether restricted or unrestricted operations of the new
aircraft should be permitted.

THIS PAGE INTENTIONALLY BLANK

# CHAPTER 7. LAYERED ELASTIC PAVEMENT DESIGN

**701. PURPOSE.** The design procedure presented in this chapter
provides a method of design based on layered elastic analysis developed
to calculate design thicknesses for airfield pavements. Layered elastic
design theory was adopted to address the impact of new gear and wheel
arrangements such as the triple dual tandem (TDT) main gear. The Boeing
777 and the Airbus A-380 are examples of aircraft that utilize this gear
geometry. The TDT gear produces an airport pavement loading
configuration that appears to exceed the capability of the previous
methods of design, which incorporate some empiricism and have limited
capacity for accommodating new gear and wheel arrangements. This design
method is computationally intense and is thus in the form of a computer
program called LEDFAA.

**702. APPLICATION.** The procedures and design software identified in
this chapter are primarily intended to provide pavement thickness design
standards for airfield pavement intended to serve aircraft traffic
mixtures that include aircraft utilizing TDT or other complex main gear
configurations. The pavement design procedures presented in this chapter
may also be used as a design alternate for the design procedures
presented in Chapters 3 and 4. In instances where newer aircraft gear
configuration, tire pressures, and/or wheel loads are not appropriate
for the nomographs presented in Chapters 3 and 4, the FAA recommends
using the layered elastic design procedures. Pavement designs for
traffic mixes that do not include aircraft with the TDT main gear must
conform to the minimum pavement sections defined in Chapters 3, and 4. A
companion design prepared in accordance with Chapters 3 and 4 must be
submitted with the layered elastic design for FAA approval. To aid in
the design review, the summary information from the design software
should be printed and included with the pavement design submittal.

The LEDFAA program may not be used to compare individual aircraft
pavement thickness requirements for pavement designs, in accordance with
Chapters 3 and 4, to individual aircraft thickness requirements for
pavement designs based on the layered elastic design methodology. The
program may not be used to evaluate existing pavement structures
designed in accordance with Chapters 3 and 4 against single aircraft
thickness requirements using the layered elastic design methodology. Any
comparison between the design methodology of LEDFAA and Chapters 3 and 4
must be performed utilizing the entire traffic mixture (see paragraph
704e).

**703. BACKGROUND.** The TDT main landing gear is unique in that it has
six wheels arranged as three pairs of wheels in a row. When the TDT main
gear assembly is analyzed using the conventional FAA design methodology,
the pavement thickness requirements are considered to be unduly
conservative. This is particularly noticeable for flexible pavements. In
1995, the FAA adopted a layered elastic airport pavement design
methodology for the Boeing 777 to reduce some of the conservatism
experienced with the methods presented in Chapters 3 and 4 and to phase
in a more mechanistic approach. The U.S. Army Corps of Engineers,
Waterways Experiment Station, Vicksburg, MS, originally developed the
layered elastic method of design for flexible and rigid pavements and
overlays of rigid pavements. Verification of the new flexible pavement
design procedure was performed by full-scale pavement testing at the
National Airfield Pavement Test Facility, William J. Hughes Technical
Center, Atlantic City, NJ.

**704. COMPUTER PROGRAM.** The design method is computer-based and is
called LEDFAA. The core program is LEAF, a layered elastic computational
program implemented as a Microsoft Windows[^TM^]{.smallcaps} ActiveX
dynamic link library written in Visual Basic^TM^ 6.0. The remainder of
the program is written in Visual Basic^TM^ and operates under Microsoft
Windows^TM^.

**a. Aircraft Considerations.** A wide variety of aircraft with
pertinent pavement design characteristics are stored in the program
library. The designer has considerable latitude in selecting and
adjusting aircraft parameters.

**b. Metric Units.** The program may be operated with U.S. customary or
metric dimensions.

**c. Availability.** LEDFAA can be downloaded from the Office of Airport
Safety and Standards website (http://www.faa.gov/arp/).

**d. Related Reference Material.** The internal help file for LEDFAA
contains a user's manual, which provides detailed information on proper
execution of the program. The manual also contains additional technical
references for specific details of the LEDFAA design procedure.

**e. Aircraft Traffic Mixture.** LEDFAA was developed and calibrated
specifically to produce pavement thickness designs consistent with
previous methods based on a mixture of different aircraft rather than
individual aircraft. If a single aircraft is used for design, a warning
will appear in the Aircraft Window indicating a non-standard aircraft
list is used in the design. This warning is intended to alert the user
that the program was intended for use with a mixture of different
aircraft types. Nearly any traffic mix can be developed from the
aircraft in the program library. Solution times are a function of the
number of aircraft in the mix. The LEDFAA design procedure deals with
mixed traffic differently than did previous design methods.
Determination of a design aircraft is not required to operate LEDFAA.
Instead, the program calculates the damaging effects of each aircraft in
the traffic mix. The damaging effects of all aircraft are summed in
accordance with Miner\'s Law. When the cumulative damage factor (CDF)
sums to a value of 1.0, the design conditions have been satisfied.

**705. PAVEMENT DESIGN CONSIDERATIONS.** There are distinct differences
between the design methodology presented in Chapters 3 and 4 and the
methodology contained in LEDFAA. These differences, along with some
common design assumptions between the two methods, are discussed below.

**a. Design Life.** The FAA design standard for pavements is based on a
20-year design life. The computer program is capable of considering
other design life time frames, but the use of a design life other than
20 years constitutes a deviation from FAA standards.

**b. Traffic Mix.** As noted in paragraph 704e, traffic mix
considerations are handled differently by the layered elastic design
method. The procedures described in Chapters 3 and 4 require the traffic
mixture to be converted into a single design aircraft and all annual
departures converted to equivalent annual departures of the design
aircraft. The design aircraft is determined by selecting the most
damaging aircraft based on the anticipated gross weight and the number
of departures for each aircraft. The layered elastic design program does
not convert the traffic mixture; instead, it analyzes the damage to the
pavement section for each aircraft and determines a final thickness for
the total cumulative damage. LEDFAA considers the placement of each
aircraft's main gear in relationship to the pavement centerline. It also
allows the pavement damage associated with a particular aircraft to be
completely isolated from one or more of the other aircraft in the
traffic mixture.

**c. Design Reliability.** The reliability of the layered elastic method
of design should be approximately the same as the reliability of the CBR
method of pavement design.

**d. Materials.** In the layered elastic design procedure, pavement
materials are characterized by thickness, elastic moduli, and Poisson\'s
ratio. Layer thicknesses can be varied, except where minimum thicknesses
are required. Elastic moduli are either fixed or variable, depending on
the material. The permissible range of variability for elastic moduli is
fixed to ensure reasonable values. Poisson\'s ratio for all material is
fixed. Materials are identified with their corresponding FAA
specification designations; for example, crushed stone base course is
identified as Item P-209. The list of materials contains an undefined
layer with variable properties. If an undefined layer is used, a warning
will appear in the Structure Window stating that a non-standard material
has been selected and its use in the structure will require FAA
approval.

**e. Minimum Layer Thickness.** When the layered elastic design
procedure is used in lieu of the design procedures in Chapters 3 and 4,
LEDFAA will not automatically establish the minimum layer thickness for
each layer, as required. The user must consult paragraphs 706, 707, and
708 below to assure the minimum thickness requirements are obtained.

**706. FLEXIBLE PAVEMENT DESIGN.** The design process considers two
modes of failure for flexible pavement: vertical strain in the subgrade
and horizontal strain in the asphalt layer. Limiting vertical strain in
the subgrade is intended to preclude failure by subgrade rutting.
Limiting horizontal strain at the bottom of the asphalt surfacing layer
guards against pavement failure initiated by cracking of the asphalt
surface layer.

**a. Hot Mix Asphalt Surfacing.** Hot mix asphalt surfacing should meet
the requirements of FAA Item P-401. A minimum thickness of 5 inches (127
mm) of hot mix surfacing is required for traffic mixes that include
aircraft with the TDT gear. The minimum thickness of hot mix surfacing,
as shown in Figures 3-2 through 3-15, is required for traffic mixes that
do not include aircraft with the TDT gear. A fixed modulus value for hot
mix surfacing is set in the program at 200,000 psi (1 380 MPa). This
modulus value was chosen to produce results that closely matched
thickness requirements for pavements designed with the CBR methodology.

**b. Base Course.** A minimum 5-inch-thick (127 mm) stabilized base
course is required for pavements serving aircraft with the TDT gear.
LEDFAA includes two stabilized flexible base options, designated P-401
and Variable. The word flexible is used to indicate that these bases
have a higher Poisson\'s ratio (0.35), act as flexible layers as opposed
to rigid layers, and are not likely to crack. P-401 is the standard FAA
Item P-401 bituminous base, which has a fixed modulus of 400,000 psi (2
760 MPa). The variable stabilized flexible base can be used to
characterize a bound base, which does not conform to the properties of
P-401. It has a variable modulus ranging from 150,000 to 400,000 psi (1
035 to 2 760 MPa). This modulus range was selected to closely duplicate
results obtained from using an equivalency factor range of 1.2 to 1.6 in
the CBR method of design. Stabilized (rigid) bases, P-304, and P-306 may
also be used as base course; although, they are subject to cracking and
can induce reflection cracking in the hot mix asphalt surfacing. Item
P-301, Soil Cement Base, is not acceptable for use as a base course for
these pavements.

When the layered elastic design procedure is used in lieu of the design
procedures in Chapters 3 and 4, the minimum base thickness will be
determined in a manner similar to the procedure described in paragraph
319b. The minimum base thickness must be determined by increasing the
subgrade CBR value to 20 and removing all subbase layers. The minimum
thickness required will be the greater of the resulting base thickness
from the LEDFAA program or the minimum base thickness in Table 3-4.

For traffic mixtures with aircraft exceeding 100,000 lbs (45 350 kg) but
not containing a TDT gear aircraft, a stabilized base course is
required, as noted in paragraph 320. When using the LEDFAA design
procedure, the minimum stabilized base thickness is 6 inches (150 mm).

**c. Subbase Course.** Subbases may be aggregate or bound materials. The
minimum thickness of subbase for structural purposes is 3 inches (76
mm). Additional thickness might be required for practical construction
limitations. Acceptable aggregate materials are P-209, Crushed Aggregate
Base Course; P-208 Aggregate Base Course; or P-154, Subbase Course.
Acceptable bound materials are P-401, P-304, and P-306. Use of Item
P-301 is limited to locations not subject to freeze-thaw cycles. More
than one layer of subbase material may be used, i.e., P-209 over a layer
of P-154. Layering must be done so as not to produce a sandwich
(granular layer between two bound layers) section and to assure that
material quality increases toward the top of the pavement section.

For traffic mixtures with aircraft exceeding 100,000 lbs (45 350 kg), a
stabilized subbase is required, as noted in paragraph 320. Acceptable
materials are specified in Tables 3-6 and 3-7.

**d. Subgrade.** The subgrade is assumed to be infinite in thickness and
is characterized by either a modulus or CBR value. LEDFAA converts CBR
to modulus by multiplying it by 1,500. Subgrade compaction and
embankment construction should be in accordance with Table 3-2.

**e. Seasonal Frost and Permafrost.** Seasonal frost and permafrost
effects should be considered by applying the techniques in Chapter 2.

**707. RIGID PAVEMENT DESIGN.** The design process considers one mode of
failure for rigid pavement, cracking of the concrete slab. Failure of
subbase and subgrade layers is not considered. Limiting the horizontal
stress at the bottom surface of the concrete surfacing layer guards
against failure by cracking of the surface layer. LEDFAA iterates on the
concrete layer thickness until the CDF reaches a value of 1.0. Once a
CDF of 1.0 is achieved, the section satisfies the design conditions.

**a. Concrete Pavement Surfacing.** Concrete pavement surfacing should
meet the requirements of Item P-501. The minimum concrete surfacing
thickness is 6 inches (152 mm).

**b. Subbase Course.** When the layered elastic design procedure is used
in lieu of the design procedures in Chapters 3 and 4, the subbase must
meet the requirement of paragraphs 326, 327, and 328.

**c. Stabilized Subbase Course.** Bound materials are required for
subbase under rigid pavements serving aircraft with the TDT gear.
Acceptable bound materials are P-304, P-306, and P-401, variable
stabilized rigid and variable stabilized flexible. The minimum thickness
of subbase is 4 inches (102 mm). More than one layer of subbase may be
used, i.e., P-306 over a layer of P-209. Layering must be done so as not
to produce a sandwich (granular layer between two bound layers) section.

**d. Subgrade.** The subgrade is assumed to be infinite in thickness and
is characterized by either a modulus or k value. The computer converts k
to modulus by using the logarithmic relationship log E = 1.415 + 1.284
log k. Subgrade compaction requirements and embankment construction
should be in accordance with Chapter 3 and AC 150/5370-10.

**e. Seasonal Frost and Permafrost.** Seasonal frost and permafrost
effects should be considered by applying the techniques in Chapter 2.

**f. Jointing Details.** Jointing details for rigid pavements are
presented in Chapter 3. The limitations on jointing of rigid pavements
for wide body aircraft also apply to pavements designed to serve
aircraft with the TDT gear.

**708. LAYERED ELASTIC OVERLAY DESIGN.** Layered system design permits a
direct design approach for overlays. The overlay design method in
Chapter 4 relies on an empirically based thickness deficiency approach.
The layered system design calculates the thickness of overlay required
to provide a 20-year life, which satisfies the layered elastic failure
criteria for limiting stress or strain. The 20-year life thickness is
defined as the design thickness. Dr. R. S. Rollings designed the design
method for overlays of rigid pavement through an FAA-funded research
effort, as listed in Appendix 4. Overlay pavements are grouped into four
different types as follows:

-   Hot Mix Asphalt Overlay of Existing Flexible Pavement

-   Concrete Overlay of Existing Flexible Pavement

-   Hot Mix Asphalt Overlay of Existing Rigid Pavement

-   Concrete Overlay of Existing Rigid Pavement

**a. Overlays of Existing Flexible Pavements.** The design of an overlay
for an existing flexible pavement is essentially the same as designing a
new pavement. The existing flexible pavement is characterized by
assigning the appropriate thicknesses and moduli of the existing layers.
A qualified engineer should be consulted to characterize the existing
pavement layers.

**(1) Hot Mix Overlay of an Existing Flexible Pavement.** A trial
thickness of overlay is selected, and the program iterates until a CDF
of 1.0 is reached. The overlay thickness required to achieve a CDF of
1.0 is the design thickness.

**(2) Concrete Overlay of an Existing Flexible Pavement.** The design of
a concrete overlay on an existing flexible pavement is essentially the
same as designing a new rigid pavement. The existing flexible pavement
is characterized by assigning the appropriate thicknesses and moduli of
the existing layers. A trial thickness of overlay is selected, and the
program iterates until a CDF of 1.0 is reached. The overlay thickness
required to achieve a CDF of 1.0 is the design thickness. The design
process is relatively simple; however, the characterization of the
existing pavement layers requires judgment by a qualified engineering.
The program assumes the interface between the concrete overlay and the
existing flexible surface is frictionless.

**b. Overlays of Existing Rigid Pavements.** The design of overlays for
an existing rigid pavement is complex because deterioration of the
underlying pavement as well as deterioration of the overlay must be
considered. The condition of the existing rigid pavement prior to
overlay is important and is expressed in terms of the structural
condition index (SCI). The SCI is derived from the pavement condition
index (PCI). (Additional guidance on deriving an SCI is provided in the
LEDFAA user's manual.) The PCI is a numerical rating indicating the
operational condition of an airport pavement based on visual survey. The
scale ranges from a high of 100 to a low of 0, with 100 representing a
pavement in excellent condition. The PCI is measured using ASTM standard
test method D 5340, Standard Test Method for Airport Pavement Condition
Index Survey. For rigid pavements, 15 different types of distress are
considered in measuring the PCI. These distress types all reduce the PCI
of a pavement, depending on their severity and relative effect on
performance. Not all distress types are indicative of structural
distress. Rollings has identified 6 distress types that are indicative
of the structural condition of the pavement. Table 7-1 lists these 6
distress types.

+-----------------------------------------------------------------------+
| **TABLE 7-1. RIGID PAVEMENT DISTRESS TYPES USED TO CALCULATE THE      |
| STRUCTURAL CONDITION INDEX, SCI**                                     |
+-----------------------------------------------------------------------+
| > Corner Break                                                        |
+-----------------------------------------------------------------------+
| > Longitudinal/Transverse/Diagonal Cracking                           |
+-----------------------------------------------------------------------+
| > Shattered Slab                                                      |
+-----------------------------------------------------------------------+
| > Shrinkage Cracks^a^ (cracking partial width of slab)                |
+-----------------------------------------------------------------------+
| > Spalling--Joint                                                     |
+-----------------------------------------------------------------------+
| > Spalling--Corner                                                    |
+-----------------------------------------------------------------------+

> ^a^ Used only to describe a load-induced crack that extends only part
> of the way across a slab. The SCI does not include conventional
> shrinkage cracks due to curing problems.

An SCI of 80 is consistent with the current FAA definition of initial
failure of a rigid pavement, i.e., 50 percent of the slabs in the
traffic area exhibit initial structural cracking. The SCI allows a more
precise and reproducible rating of a pavement's condition than previous
FAA condition factor ratings, Cb and Cr.

**(1) Hot Mix Asphalt Overlays of Existing Rigid Pavements.** The design
process for hot mix overlays of rigid pavements considers two conditions
for the existing rigid pavement to be overlaid: a SCI of the existing
pavement that is equal to or less than 100.

**(i) Structural Condition Index Less Than 100.** The most likely
situation is one in which the existing pavement is exhibiting some
structural distress, i.e., the SCI is less than 100. If the SCI is less
than 100, the overlay and base pavement deteriorate at a given rate
until failure is reached. LEDFAA assumes an overlay thickness and
iterates on the thickness of overlay until a 20-year life is predicted.
A 20-year predicted life satisfies the design requirements.

**(ii) Structural Condition Index Equal to 100.** An existing pavement
with an SCI of 100 might require an overlay to strengthen the pavement
in order to accept heavier aircraft. If the SCI of the base pavement is
equal to 100, an additional input is required---the CDFU, cumulative
damage factor used, which estimates the amount of pavement life used up
prior to overlay. LEDFAA assumes the base pavement will deteriorate at
one rate while the SCI is equal to 100 and at a different rate after the
SCI drops below 100. As with an SCI less than 100, a trial overlay
thickness is input, and the program iterates on that thickness until a
20-year life is predicted. The design thickness is the thickness that
provides a 20-year predicted life.

**(2) Concrete Overlays of Existing Concrete Pavements.** The design of
a concrete overlay of an existing rigid pavement is the most complex
type of overlay to be designed. Deterioration of the concrete overlay
and existing rigid pavement must be considered as well as the degree of
bond between the overlay and existing pavement. LEDFAA considers two
degrees of bond and addressed each separately for thickness design.

**(i) Fully Unbonded Concrete Overlay.** An unbonded concrete overlay of
an existing rigid pavement is one in which steps are taken to
intentionally eliminate bonding between the overlay and existing
pavement. Commonly, the bond is broken by applying a thin hot mix layer
to the existing rigid pavement. The interface friction coefficient
between the overlay and existing pavement is set to reflect an unbonded
condition. The interface coefficient is fixed and cannot be changed by
the user. As with hot mix asphalt overlays, an SCI is required to
describe the condition of the existing pavement. A trial overlay
thickness is input, and LEDFAA iterates until a 20-year service life is
predicted. The thickness that yields a 20-year service life is the
design thickness.

**(ii) Partially Bonded Concrete Overlay.** A partially bonded overlay
is one in which no particular effort is made to either eliminate or
achieve bond between the concrete overlay and the existing rigid
pavement. Such overlays are normally appropriate for existing rigid
pavements when the SCI is 77 or greater. The interface coefficient is
set to reflect a small degree of friction between the overlay and base
pavement. This coefficient is fixed and cannot be changed by the user.
An SCI for the existing pavement is required. A trial overlay thickness
is input, and LEDFAA iterates until a 20-year service life is predicted.
The thickness that yields a 20-year service life is the design
thickness.

THIS PAGE INTENTIONALLY BLANK

**APPENDIX 4. RELATED READING MATERIAL**

**1.** Electronic copies of the latest versions of the following FAA
publications are available on the FAA website at
**http://www.faa.gov/**. Printed copies can be requested from the
Department of Transportation, Subsequent Distribution Office, Ardmore
East Business Center, 3341 Q 75th Ave, Landover, MD 20785. The
Department of Transportation, however, will charge a fee for some of
these documents. Advisory Circular 00-2, Advisory Circular Checklist,
provides a list of all current ACs.

**a.** AC 00-2, Advisory Circular Checklist and Status of Other FAA
Publications.

b.  AC 150/5300-13, Airport Design.

**c.** AC 150/5320-5, Airport Drainage.

**d.** AC 150/5320-12, Measurement, Construction, and Maintenance of
Skid Resistant Airport Pavement Surfaces.

**e.** AC 150/5300-9, Predesign, Prebid, and Preconstruction Conferences
for Airport Grant Projects.

**f.** AC 150/5335-5, Standardized Method of Reporting Airport Pavement
Strength--PCN.

**g.** AC 150/5370-10, Standards for Specifying Construction of
Airports.

**h.** AC 150/5370-11, Use of Nondestructive Testing Devices in the
Evaluation of Airport Pavements.

**i.** AC 150/5380-6, Guidelines and Procedures for Maintenance of
Airport Pavements.

**2.** Copies of the following reports can be obtained from the National
Technical Information Service, 5285 Port Royal Road, Springfield, VA
22161 or at **http://www.ntis.gov/**.

**a.** FAA-RD-73-169, Review of Soil Classification Systems Applicable
to Airport Pavement Design, May 1974, by Yoder; AD-783-190.

**b.** FAA-RD-74-30, Design of Civil Airfield Pavement for Seasonal
Frost and Permafrost Conditions, October 1974, by Berg; ADA-006-284.

**c.** FAA-RD-74-36, Field Survey and Analysis of Aircraft Distribution
on Airport Pavements, February 1975, by Ho Sang; ADA-011-488.

**d.** FAA-RD-76-66, Design and Construction of Airport Pavements on
Expansive Soils, June 1976, by McKeen; ADA-28-094.

**e.** FAA-RD-73-198-1, Design and Construction and Behavior Under
Traffic of Pavement Test Sections, June 1974, by Burns, Rone, Brabston,
Ulery; AD-785-024.

**f.** FAA-RD-74-33, III, Design Manual for Continuously Reinforced
Concrete Pavements, May 1974, by Treybig, McCullough, Hudson;
AD-780-512.

**g.** FAA-RD-75-110-II, Methodology for Determining, Isolating and
Correcting Runway Roughness, June 1977, by Seeman and Nielsen;
ADA-044-378.

**h.** FAA-RD-73-198-111, Design and Construction of MESL, December
1974, by Hammitt; AD-005-893.

**i.** FAA-RD-76-179, Structural Design of Pavements for Light Aircraft,
December 1976, by Ladd, Parker, Pereira; ADA-04 l-300.

**j.** FAA-RD-74-39, Pavement Response to Aircraft Dynamic Loads, Volume
II--Presentation and Analysis of Data, 1974, by Ledbetter; ADA-022-806.

**k.** DOT/FAA/RD-74/199, Development of a Structural Design Procedure
for Flexible Airport Pavements, November 1974, by Barker and Brabston,
ADA-019-205.

**l.** DOT/FAA/RD-77/81, Development of a Structural Design Procedure
for Rigid Airport Pavements, April 1979, by Parker, Barker, Gunkel, and
Odom, ADA-069-548.

**m.** FAA-RD-81-78, Economic Analysis of Airport Pavement
Rehabilitation Alternatives, October 1981, by Epps and Wootan,
ADA-112-550

**n.** DOT/FAA/PM-87/19, Design of Overlays for Rigid Airport Pavements,
April 1988, by R. S. Rollings, ADA-194-331.

**3.** Copies of ASTM standards can be obtained from the ASTM
International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken,
PA 19428 or at **http://www.astm.org/**.

**4.** Copies of AASHTO standards can be obtained from the American
Association of State Highway and Transportation Officials, 444 North
Capitol Street NW, Suite 249, Washington, DC 20001 or at
**http://www.transportation.org/aashto/home.nsf/FrontPage/**.

**5.** Copies of UFC 3-260-02, Pavement Design for Airfields, Department
of the Army, Unified Facility Criteria (UFC), June, 2001 can be obtained
from **http://65.204.17.188//report/doc_ufc.html**

The Unified Facility Criteria supersedes the following technical manuals
previously noted in this AC:

TM5-824-2, Flexible Airfield Pavements, Department of the Army Technical
Manual

> TM5-824-3, Rigid Pavements for Airfields Other than Army, Departments
> of the Army and the Air Force,

TM5-818-2, Pavement Design for Frost Conditions, Department of the Army

**6.** Copies of MS-11, Full Depth Asphalt Pavements for Air Carrier
Airports, January 1973; IS-154, Full Depth Asphalt Pavements for General
Aviation, January 1973; and MS-10, Soils Manual, Fourth Edition (1986),
can be obtained from the Asphalt Institute, Research Park Drive, PO Box
14052, Lexington, KY 40512 or at **http://www.asphaltinstitute.org/**.

**7.** Copies of Special Technical Publication M-5, The Estimation of
Concrete Flexural Strength from Other Types of Strength Tests, DATE, by
W. Charles Greer, can be obtained from MACTEC Inc., Director of
Publications, 1105 Sanctuary Parkway, Suite 300 Alpharetta, Georgia.
30004.

**8.** Copies of FHWA-HI-90-001, Geotextile Design and Construction
Guidelines, October 1989, can be obtained from the Department of
Transportation, Federal Highway Administration, Turner Fairbanks Highway
Research Center, 6300 Georgetown Pike, McLean, VA 22101.

**APPENDIX 5. AIRFIELD PAVEMENT DESIGN SOFTWARE**

**1. BACKGROUND.** This appendix announces software to aid with the
design of airfield pavements in accordance with the methods presented in
Chapters 3 and 4 of this AC. The software presented in this appendix
uses Microsoft Excel as a platform with Visual Basic for Applications
(VBA) Macros to facilitate the design process.

**2. AVAILABLE SOFTWARE AND SUPPORT MATERIAL.** Two programs
(spreadsheets) are available to determine pavement thickness
requirements in accordance with this AC. Program **F805FAA.XLS**
determines pavement thickness requirements for flexible pavement
sections and bituminous overlays on existing flexible pavement sections.
Program **R805FAA.XLS** determines pavement thickness requirements for
rigid pavement sections and bituminous or Portland cement concrete
overlays on existing rigid or flexible pavement sections.

Reference manuals, which guide users through each step, are available
for both programs. The manuals assume users are familiar with the design
requirements of the AC.

Pavement designs developed using the Frost Design feature of the
spreadsheets are consistent with the Reduced Subgrade Strength method
described in Chapter 3.

The spreadsheets will produce thickness designs consistent with the
nomographs provided in this AC. Small variations should be expected due
to difficulties with visual interpretation of the nomographs.

**3. ACCESS TO SOFTWARE.** Design software and user manuals may be
downloaded directly from the FAA Office of Airport Safety and Standards
website (**http://www.faa.gov/arp/**). Software links are located on the
\"resources\" and \"engineering\" pages of this site. Updates or
additions to the design software and manuals will be posted online, as
well.

**4. USE OF SOFTWARE.** Numerical results from the programs may be used
to complete FAA Form 5100-1, Airport Pavement Design. When used to
develop the pavement design, the printed results of the software should
be attached to Form 5100-1. Results from the program design summary and
the aircraft mixture data provide sufficient information to reproduce
and review the pavement thickness design. Additional design information
is required to complete Form 5100-1.