DOT/FAA/AR-04/46 

Office of Aviation Research 

Washington, D.C. 20591 

Operational Life of Airport 

Pavements 

December 2004 

Final Report 

This document is available to the U.S. public 

through the National Technical Information 

Service (NTIS), Springfield, Virginia 22161. 

U.S. Department of Transportation 

Federal Aviation Administration
NOTICE 

This document is disseminated under the sponsorship of the U.S. 

Department of Transportation in the interest of information exchange. The 

United States Government assumes no liability for the contents or use 

thereof. The United States Government does not endorse products or 

manufacturers. Trade or manufacturer's names appear herein solely 

because they are considered essential to the objective of this report. This 

document does not constitute FAA certification policy. Consult your local 

FAA airports office as to its use. 

This report is available at the Federal Aviation Administration William J. 

Hughes Technical Center's Full-Text Technical Reports page: 

actlibrary.tc.faa.gov in Adobe Acrobat portable document format (PDF). 
 Technical Report Documentation Page 1. Report No.

DOT/FAA/AR-04/46

2. Government Accession No. 3. Recipient's Catalog No.

5. Report Date 

December 2004

 4. Title and Subtitle

OPERATIONAL LIFE OF AIRPORT PAVEMENTS 6. Performing Organization Code

7. Author(s)

Navneet Garg, Edward Guo, and Roy McQueen 

8. Performing Organization Report No. 

10. Work Unit No. (TRAIS) 

 

9. Performing Organization Name and Address

Galaxy Scientific Corporation 

3120 Fire Road

Egg Harbor Township, NJ 08234 11. Contract or Grant No.

 DTFA03-00-D-00011

12. Sponsoring Agency Name and Address

U.S. Department of Transportation 

Federal Aviation Administration

13. Type of Report and Period Covered

 Final Report

Office of Aviation Research

Washington, DC 20591

14. Sponsoring Agency Code

 AAS-100 15. Supplementary Notes 

The FAA William J. Hughes Technical Center’s Technical Officer was Dr. S.K. Agrawal. 16. Abstract 

The objective of the study was to determine whether the Federal Aviation Administration (FAA) standards used to determine the 

appropriate thickness for hot mix asphalt and concrete airfield pavements are in accordance with the FAA standard for a 20-year

life requirement. 

FAA airport pavement design standards, Advisory Circular (AC) 150/5320-6D (1995), including changes 1, 2, and 3 (2004), and

related references, including some unpublished FAA technical reports and full-scale test results, were reviewed. The effects of 

many parameters, directly used in the failure model and indirectly used through the pavement response model, on the pavement

structural life were analyzed. A sensitivity analysis of parameters on pavement structural life was used to quantitatively evaluate 

the effects of the most important parameters in different airport pavement design procedures. 

Some full-scale test results were used to support the findings in the analysis. Much of the airport pavement surveyed data

collected from previous FAA projects, including some unpublished ones, was also reviewed. A portion of that data was used in

this report for the analysis. Based on the surveyed data, it was found that the average Structural Condition Index of both hot mix

asphalt and Portland cement concrete pavements in all age groups is higher than 80. 

Based on the definition adopted in this report, the airport pavements designed following AC 150/5320-6D have sufficient

thickness to provide a 20-year structural life. 

17. Key Words 

FAA standards, Pavement design procedures, 20-year life,

Full-scale test results, Structural Condition Index

18. Distribution Statement 

This document is available to the public through the National

Technical Information Service (NTIS) Springfield, Virginia 

22161. 19. Security Classif. (of this report)

 Unclassified

20. Security Classif. (of this page)

 Unclassified

21. No. of Pages 

 117 

22. Price

Form DOT F1700.7 (8-72) Reproduction of completed page authorized 
ACKNOWLEDGEMENTS 

The work described in this report was performed for the U.S. Department of Transportation, 

Federal Aviation Administration, Airport Technology Research and Development Branch 

(R&D), under Contract DTFA03-00-D-00011. Dr. Satish K. Agrawal is the manager of the 

Airport Technology R&D Branch. 

The contributions of the following individuals are acknowledged: 

Dr. Gordon Hayhoe 

Federal Aviation Administration 

Mr. Chuck Teubert 

Galaxy Scientific Corporation 

Mr. James L. Lafrenz, P.E. 

American Concrete Pavement Association 

Mr. John I. Duval, P.E. 

Asphalt Institute 

David E. Newcomb, P.E., Ph.D. 

National Asphalt Pavement Association 

iii/iv 
TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY xiii 

1. INTRODUCTION 1-1

1.1 Objective 1-1

1.2 Scope 1-1

1.3 Limitations 1-1

1.4 Approach 1-2

1.5 Overview 1-3 

2. DEFINITIONS 2-1

2.1 Pavement Life and Failure 2-1

2.2 Pavement Distresses 2-1

2.3 Flexible Pavement Distresses 2-2

2.3.1 Alligator Cracking 2-2

2.3.2 Bleeding 2-2

2.3.3 Block Cracking 2-3

2.3.4 Corrugation 2-3

2.3.5 Depression 2-4

2.3.6 Jet-Blast Erosion 2-4

2.3.7 Joint Reflection Cracking From PCC 2-4

2.3.8 Longitudinal and Transverse Cracking (Non-PCC Joint Reflective) 2-5

2.3.9 Oil Spillage 2-5

2.3.10 Patching and Utility Cut Patch 2-5

2.3.11 Polished Aggregate 2-6

2.3.12 Raveling and Weathering 2-6

2.3.13 Rutting 2-7

2.3.14 Shoving 2-7

2.3.15 Slippage Cracking 2-8

2.3.16 Swell Distress 2-8

2.4 Rigid Pavement Distresses 2-9

2.4.1 Blowup 2-9

2.4.2 Corner Break 2-9

2.4.3 Longitudinal, Transverse, and Diagonal Cracks 2-10

2.4.4 Durability Cracking 2-10

2.4.5 Joint Seal Damage 2-11

2.4.6 Patch, Small (less than 5 ft2

) 2-11

2.4.7 Patch, Large (over 5 ft2) 2-12

2.4.8 Popouts 2-12

v 
2.4.9 Pumping 2-13

2.4.10 Scaling, Map Cracking, and Crazing 2-13

2.4.11 Settlement or Faulting 2-14

2.4.12 Shattered Slab/Intersecting Cracks 2-14

2.4.13 Shrinkage Cracks 2-15

2.4.14 Spalling (Transverse and Longitudinal) 2-15

2.4.15 Spalling (Corner) 2-15

2.5 Assessing Pavement Failure 2-16

3. PAVEMENT DESIGN PROCEDURES REVIEW AND EFFECTS OF THE

RESPONSE MODEL ON PAVEMENT LIFE 3-1

3.1 Flexible Pavements 3-1

3.1.1 Development of California Bearing Ratio Equation—Pavement as 

Single Layer 3-1

3.1.2 Layered Elastic Design Procedures 3-5

3.2 Rigid Pavements 3-9

3.2.1 Slab Models 3-9

3.2.2 Foundation Models 3-9

3.2.3 Effects of Concrete Elastic Modulus 3-10

3.2.4 Variation of Joint Load Transfer Capabilities 3-12

3.2.5 Effects of Interface Model Between the Slab and the Layer 

Underneath 3-13

3.3 Summary 3-15

4. EFFECT OF FAILURE MODELS ON PAVEMENT LIFE 4-1

4.1 Flexible Pavements 4-1

4.1.1 Failure Model in AC 150/5320-6D 4-1

4.1.2 Failure Model in AC 150/5320-16 (LEDFAA 1.2) 4-4

4.1.3 Failure Model in AC 150/5320-6D Change 3 (LEDFAA 1.3) 4-6

4.2 Rigid Pavements 4-7

4.2.1 Major Changes in Failure Models 4-7

4.2.2 Calculation of Pass-to-Coverage Ratio 4-10

vi 
4.2.3 Uncertainty of the Concrete Flexural Strength 4-10

4.2.4 Control of Top-Down Cracking 4-11

 4.3 Summary 4-13 

5. SENSITIVITY OF INPUTS TO PAVEMENT LIFE 5-1

5.1 Concepts and Formulas 5-1

5.2 Flexible Pavements 5-2

5.2.1 Sensitivity of Pavement Life to HMA Modulus 5-4

5.2.2 Sensitivity of Pavement Life to HMA Thickness 5-4

5.2.3 Sensitivity of Pavement Life to P-209 Base Thickness 5-5

5.2.4 Sensitivity of Pavement Life to Aircraft Gross Weight 5-5

5.3 Rigid Pavements 5-6

5.3.1 Sensitivity of Design Factor R/σ 5-6

5.3.2 Sensitivity to Slab Thickness, Aircraft Gross Weight, Subgrade 

Strength, and Stabilized Base 5-8

 5.4 Summary 5-11 

6. PAVEMENT STRUCTURAL LIFE ANALYSIS 6-1

6.1 Concepts and Assumptions 6-1

6.2 Analysis Procedures and Methods 6-3

6.3 Description of Surveyed HMA Pavements 6-7 

6.3.1 Analysis of Available Data for HMA Pavements 6-8

6.3.2 Distress Analysis 6-11

6.4 Available Data From Published Reports 6-17

6.4.1 Phoenix Sky Harbor International Airport 6-17

6.4.2 John F. Kennedy International Airport 6-18 

6.4.3 Additional Data and Information 6-18

6.5 Description of Surveyed PCC Pavements 6-19

6.6 Analysis of Surveyed PCC Pavements 6-20

 6.7 Summary 6-25 

vii 
7. CONCLUSIONS 7-1

8. REFERENCES 8-1

9. INDUSTRY COMMENTS 9-1

9.1 Comments From the Asphalt Institute 9-1

9.2 Comments From the American Concrete Pavement Association 9-3

9.3 Comments From the National Asphalt Pavement Association 9-5 

LIST OF FIGURES

Figure Page 

2-1 Alligator Fatigue Cracking 2-2 

2-2 Tire Marks Evident in High-Severity Bleeding 2-3 

2-3 High-Severity Block Cracking 2-3 

2-4 Depression in Pavement 2-4 

2-5 Joint Reflection Cracking From PCC 2-4 

2-6 High-Severity Longitudinal and Transverse Cracking 2-5 

2-7 High-Severity Patch 2-6 

2-8 Polished Aggregate 2-6 

2-9 High-Severity Raveling and Weathering 2-7 

2-10 Rutting in a Pavement 2-7 

2-11 Shoving in a Pavement 2-8 

2-12 Severe Slippage Cracking 2-8 

2-13 Blowup in a Pavement 2-9 

2-14 Corner Break 2-9 

2-15 High-Severity Longitudinal Cracking 2-10 

2-16 High-Severity Transverse Cracking 2-10 

viii 
2-17 High-Severity D Cracking 2-11 

2-18 Joint Seal Damage 2-11 

2-19 Small Patch Distress 2-12 

2-20 Large Patch Distress 2-12 

2-21 Popout Distress 2-13 

2-22 Water Bleeding and Pumping 2-13 

2-23 Scaling 2-14 

2-24 Map Cracking 2-14 

2-25 Faulting of Transverse Cracks 2-14 

2-26 Shattered Slab Condition 2-15 

2-27 Joint Spalling at a Transverse Joint 2-15 

2-28 Corner Spalling 2-16 

3-1 LEDFAA Failure Model for all Available Full-Scale Test Data Points With 

Reported Subgrade Shear Failure 3-7 

3-2 Effects of the Concrete Modulus on the Comparison of Measured and 

Calculated Strains 3-11 

3-3 Uncertainty of the Interface Behavior Under a Circular Load at the Slab Center 3-14 

4-1 Comparison of Thickness Factor From Different Relationships 4-2 

4-2 Alpha Factor Versus Coverages 4-4 

4-3 Subgrade Failure Criteria in LEDFAA 1.2 4-5 

4-4 Comparison of Subgrade Failure Models in LEDFAA 4-6 

4-5 Three-Step Adjustment From WES to LEDFAA Failure Models 4-8 

5-1 Pavement Sections for Sensitivity Analysis 5-3 

5-2 Summary of Results From Sensitivity Analyses 5-5 

5-3 The Sensitivities of Design Factor R/σ vs R/σ 5-7

5-4 Failure Models in AC 150/5320-6D and WES 5-8 

ix 
6-1 Effects of Mean and Standard Deviation on Pavement Life—Conceptural Analysis 6-2 

6-2 Average PCI for HMA Runways, Taxiways, and Aprons 6-9 

6-3 Average PCI for HMA Runways 6-9 

6-4 Average PCI for HMA Taxiways 6-10 

6-5 Average PCI for HMA Aprons 6-10 

6-6 Pavement Condition Index Reduction by Distress Groups for HMA Pavements 6-11 

6-7 Pavement Condition Index Reduction Values for Different Functional 

HMA Pavements 6-14 

6-8 Percent of Total Reduction of PCI for Different Functional HMA Pavements 6-14 

6-9 Average SCI for HMA Runways, Taxiways, and Aprons 6-16 

6-10 Surveyed PCC Pavements Grouped by Function 6-20 

6-11 Surveyed PCC Pavements Grouped by Years of Service 6-20 

6-12 Pavement Condition Index of the PCC Pavements by Age 6-21 

6-13 Pavement Condition Index of PCC Pavements by Age and Function 6-22 

6-14 PCIavg of 0- to 30-Year PCC Pavements Grouped by Size 6-22 

6-15 Distribution of PCI for all PCC Pavements Between 16 and 23 Years 6-23 

6-16 Estimated SCI of PCC Pavements by Age and Function 6-24 

6-17 Average SCI for Pavements Older Than 20 Years 6-26 

6-18 Average PCI for Pavements Older Than 20 Years 6-26 

6-19 Difference in Flexible and Rigid PCI and SCI for Pavements Older Than 20 Years 6-27 

x 
LIST OF TABLES 

Table Page 

2-1 Distress Mechanisms 2-17 

3-1 Pass-To-Coverage Ratios for Flexible Pavements 3-5 

3-2 Input Data 3-11 

3-3 Calculated Critical Stress 3-11 

5-1 Aircraft Mix Used for Pavement Design 5-3 

5-2 Results From Sensitivity Analyses 5-4 

5-3 Sensitivity of Different Parameters, Weak Subgrade, Ec = 4500 psi 5-10 

5-4 Sensitivity of Different Parameters—Strong Subgrade, Ec = 30,000 psi 5-10 

6-1 General Information of Surveyed HMA Pavements 6-7 

6-2 General Information on HMA Runways in the U.S. 6-8 

6-3 Pavement Condition Index Reduction by Individual Distresses 6-13 

6-4 Pavement Condition Index Reduction by Six Major Distresses 6-13 

6-5 Structural Condition Index Computation for HMA Airfield Pavements 6-16 

6-6 Pavement Condition Index Summary for HMA Pavements at PHX 6-17 

6-7 Pavement Condition Index Summary for HMA Pavements at JFK 6-18 

6-8 Area-Weighted DSCI by Feature 6-24 

xi 
LIST OF ACRONYMS AND SYMBOLS 

2D Two-dimensional 

2S Dual-tandem wheels 

3D Three-dimensional 

AC Advisory Circular 

ASTM ASTM International 

CBR California Bearing Ratio 

CC1 Construction cycle 1 

CC3 Construction cycle 3 

CDF Cumulative damage factor 

CFR Code of Federal Regulations 

D Durability crack 

d/2 Dual-tandem landing gear geometry factor 

DSCI SCI deduct 

ESWL Equivalent single-wheel load 

FAA Federal Aviation Administration 

FEM Finite element method 

FOD Foreign object debris 

GAO Government Accounting Office 

HMA Hot mix asphalt 

IAD Washington Dulles International Airport 

JFK John F. Kennedy International Airport 

LTD Load transfer efficiency with respect to deflection

MEM Memphis International Airport 

msf Million square feet 

MWHGL Multiple-wheel heavy gear load 

NAPTF National Airport Pavement Test Facility 

NDT Nondestructive test 

PCC Portland cement concrete 

PCI Pavement Condition Index 

PHX Phoenix Sky Harbor International Airport 

PMS Pavement maintenance system

R/σ Design Factor 

RC Compressive strength of concrete 

RF Flexural strength of concrete 

SCI Structural Condition Index 

USACE U.S. Army Corps of Engineers 

WES U.S. Army Waterways Experiment Station 

xii 
EXECUTIVE SUMMARY

The objective of this study was to evaluate whether the Federal Aviation Administration (FAA) 

thickness design standards for flexible and rigid pavements are consistent with the FAA’s 

standard for a 20-year pavement design life requirement using the most up-to-date available 

information on the life of airfield pavements. This report concludes that airport pavements 

designed following FAA Advisory Circular (AC) 150/5320-6D have sufficient thickness to 

provide the required 20-year structural life if other related standards for material preparation, 

construction, and maintenance are appropriately applied. This conclusion was based on 

published reports, papers, and numerical analysis of available data using structural and failure 

models. Comments from representatives of the American Concrete Pavement Association, the 

Asphalt Institute, and the National Asphalt Pavement Association were obtained and are

included in the report. 

The FAA’s requirement for a 20-year pavement design life is contained in AC 150/5320-6D. A 

20-year pavement life represents the total anticipated load applications the pavement will be

subjected to over a 20-year period. For the purpose of thickness design, the total number of load 

applications is the defining parameter. The common terminology that uses years as the measure

of pavement life is simply the practical period of time for which the design assumptions are 

considered valid. The FAA pavement thickness design procedures refer to the determination of 

the thickness of a pavement structure and its components (surface, base, and subbase layers) not 

to the design of the materials in a pavement (e.g., hot mix asphalt or Portland cement concrete 

mixes). The intent is, for flexible pavements, to protect the lower layers, particularly the

subgrade, from shear failure, and for rigid pavements, to protect the Portland cement concrete

top layer from fatigue cracking. 

One of the major problems in pavement evaluation is determining when the pavement has failed. 

A unique definition of pavement overall failure does not exist; rigid and flexible pavements 

exhibit fundamentally different failure modes. The FAA design procedures are calibrated 

against full-scale test results in which the failure criteria are carefully defined. Flexible 

pavement failure is defined as shear failure in the subgrade leading to 1-inch upheaval at the 

pavement surface. Rigid pavement failure is defined as full-depth cracking in 50 percent of the 

slabs. Various aspects of pavement performance, life, and failure for rigid and flexible 

pavements are described. Functional and structural failures are differentiated and discussed. 

Pavement thickness design and resultant pavement life can be significantly affected by the 

choice of design inputs. The results from a sensitivity study show that for flexible pavements, 

pavement life is most sensitive to aircraft gross weight, subgrade strength, and total thickness. 

For rigid pavements, pavement life is most sensitive to slab thickness, flexural strength of the 

concrete, and aircraft gross weight. 

Generally, predicted pavement life is different from actual pavement life because the design 

assumptions can never accurately reflect the operational conditions because of uncertainties in 

material properties, climatic conditions, changes in traffic characteristics and volumes, etc. 

Pavement Condition Index (PCI) is used to evaluate pavement condition and is calculated from a

visual survey of all types of distresses in a pavement that, in combination or alone, can cause the 

xiii 
pavement to reach a condition in which it can no longer perform its intended purpose. The 

distresses are separated into two broad categories: structural and functional. The FAA thickness

design procedure is based on structural failure of pavements observed in full-scale tests in which 

the functional failure-related failure modes are eliminated, or suppressed, as much as possible. 

The functional failure-related distresses that may occur during a full-scale structural failure test 

are ignored in the rating of the structural condition of the test pavement. 

Structural Condition Index (SCI) is defined as the structural component of PCI. In this report, 

PCI data from the field condition surveys were converted into SCI data to evaluate airport

pavement structural life. For rigid pavements, there is a clear definition and procedure for 

calculating SCI. But, currently, there is no definition or established procedure to calculate the

SCI of flexible pavements. This report introduces a procedure to calculate SCI for flexible

pavements using two distresses—alligator cracking and rutting. However, the measured rut 

depth may include a portion of rutting in the surface hot mix asphalt layer. This distress is

related to material and construction defects rather than structural defects. Therefore, the SCI 

values reported for flexible pavements may be lower than they should be (a lower SCI value 

indicates more distress). 

The PCI data from the airport pavement field surveys were used to calculate SCI for flexible and 

rigid pavements at the end of 20 years of use, which is the FAA standard for design. The end of 

the design life of the pavement is also assumed to be indicated by the pavement having reached 

an SCI of 80. Analysis of the survey data showed, on average, that the SCI for both flexible and

rigid pavements, and for all types of pavements (runways, taxiways, and aprons), is higher than 

80 after 20 years. More structural distress is indicated for flexible pavements than for rigid 

pavements over all pavement types, as shown below. The computed PCI values show the same 

trends, except that the differences between flexible and rigid pavements are larger. Flexible 

pavement runway and taxiway PCI values are about the same, but flexible pavement runway and 

taxiway PCI values are about 13 percent lower than the rigid pavement runway and taxiway PCI 

values. This suggests that a discrepancy exists in the relationship between pavement thickness 

design standards and other FAA standards related to the 20-year design life requirement. 

Pavement Type 

SCI of Flexible 

Pavements after 20 Years 

SCI of Rigid 

Pavements after 20 Years 

Percentage 

Difference 

Runways 89.5 92.4 3.1

Taxiways 83.4 89.5 6.8

Aprons 82.4 91.3 9.8

xiv 
1. INTRODUCTION.

1.1 OBJECTIVE. 

The objective of this study was to evaluate whether the Federal Aviation Administration (FAA) 

thickness design standards for flexible and rigid airfield pavements are in accordance with the

FAA standard for 20-year pavement design life requirement using the most up-to-date available 

information on the life of airfield pavements. 

1.2 SCOPE.

The FAA’s requirement for a 20-year pavement design life is contained in FAA Advisory 

Circular (AC) 150/5320-6D. A 20-year pavement design life represents the total anticipated 

number of load applications the pavement will be subjected to over a 20-year period. For the 

purpose of thickness design, the total number of load applications is the defining parameter. 

Consistent with the FAA’s scope for this study, no new data were collected. Rather, currently 

available information from published reports and papers, as well as other available data, was

used for analysis. Definitions of pavement life and pavement failure are provided and discussed 

in this study that include or exclude, as appropriate, factors other than the structural performance

of pavements. The relationship between initial design assumptions and the conditions existing 

during the life of the pavement are also considered. 

1.3 LIMITATIONS.

Constructing and maintaining a structurally and functionally sound pavement requires strict

adherence to FAA standards and practices pertaining to pavement thickness design, material 

selection, construction, inspection, and maintenance. The FAA standards related to various 

aspects of airport pavements are as follows: 

• AC 150/5320-6D—Structural design requirements 

• AC 150/5370-10A—Construction and material requirements 

• AC 150/5320-12C—Friction requirements 

• AC 150/5360-8B—Maintenance requirements 

• Title 14 Code of Federal Regulations (CFR) Part 139—Operational Requirements 

If any of these standards are not followed, a pavement’s full design life will not be realized.

Interaction and proper application of each of these standards can have a significant impact on 

pavement life. The standards recognize that pavements fail for different reasons, e.g., 

deficiencies in performance related to structure, materials, construction, environment, and other. 

These can be broadly classified as structural (e.g., thickness), functional (e.g., skid resistance, 

material durability), and operational (e.g., surface condition) factors. For this study, it was 

assumed that the pavement’s functional and structural requirements were addressed and the 

pavement was properly maintained over its design life. Therefore, this study emphasized a 

review of a pavement’s structural performance with respect to the FAA’s thickness design 

standards. 

1-1 
The FAA’s pavement thickness design procedure refers to the determination of the thickness of

the pavement and its components (e.g., surface, base, and subbase layers) not to the design of the

materials in the pavement (e.g., hot mix asphalt (HMA) or Portland cement concrete (PCC) 

mixes). This study concentrated on thickness and structural design considerations and addressed 

the limitations of readily available data sources. For example, the Pavement Condition Index 

(PCI) number is a numerical indicator that provides a measure of the condition of the pavement 

at the time of inspection. The PCI is based on the distresses observed on the surface of the 

pavement, and it also indicates to some extent the pavement’s structural integrity and surface

operational condition. However, the PCI cannot measure the structural capacity or provide direct 

measurement of skid resistance or roughness, and it does not differentiate between different 

failure modes (i.e., functional or structural). To differentiate between structural or functional 

performance, the basic condition survey data are required, which are often not available. Also, 

information related to material-specific distresses such as alkali silica reactivity in concrete 

pavements and tender mixes in HMA pavements are more often not included or available. 

Satisfying the FAA’s 20-year life requirement also assumes continued, proactive maintenance 

over the life of the pavement. All pavements need to be maintained, even those that are properly 

designed. This also relates to a life cycle cost analysis, which is required during the design stage 

by AC 150/5320-6D. Life cycle costs include initial construction, periodic maintenance, and 

periodic rehabilitation costs, as well as salvage value (which is admittedly difficult to define). 

Due to the many different failure modes, an evaluation of life cycle costs was beyond the scope 

of this study. For example, it is very difficult to get objective maintenance cost data on a facility

basis from airport owners. Also, it is not known if the maintenance was a result of functional or 

structural defects or which standard(s) was not followed properly. The proper identification of 

the defects driving the maintenance is critical in evaluating the efficacy of the various standards.

Therefore, while all these factors are recognized, the limitations of this study prevented an 

objective analysis of maintenance and life cycle costs to be performed. Therefore, this study

concentrated on the structural component (i.e., thickness design) of pavement performance and 

life. 

1.4 APPROACH.

The basic approach for this study was as follows: 

• Compile the most up-to-date available data sources on the structural performance of 

airport pavements. 

• Recruit industry experts to review the preliminary work plans and the draft final report. 

• Extract applicable information from the available data sources and reports. 

• Present the data as it becomes available (e.g., PCI, Structural Condition Index (SCI), 

quotes, etc.). 

• Present the results from a study of the sensitivity of pavement life to be changes in the 

design inputs. 

1-2 
1.5 OVERVIEW.

This report summarizes the published reports, papers, and any available data that were collected 

for ongoing FAA projects. Section 2 defines various aspects of pavement performance, life, and 

failure for rigid and flexible pavements and describes the PCI concept. Functional and structural 

failures are differentiated and discussed in section 2, along with examples and photographs. 

Section 3 is an overview of the design procedure development (flexible pavement, rigid 

pavement, and full-scale tests) and design assumptions (with respect to the subgrade support, 

traffic, material strength, and thickness). In section 4, the effect of failure models on pavement

life is presented. Section 5 discusses the sensitivity of inputs to pavement life. Section 6 

summarizes the findings (e.g., quotes, findings, and conclusions) extracted from the published 

and unpublished data sources, reports, and papers. Section 7 presents the summary of findings 

and conclusions from the study. Section 9 summarizes the comments from the industry 

reviewers (the Asphalt Institute and the American Concrete Pavement Association). 

1-3/1-4 
2. DEFINITIONS.

2.1 PAVEMENT LIFE AND FAILURE. 

An airport pavement is a complex engineering structure. Pavement analysis and design involves 

the interaction of four equally important components, which are often difficult to quantify: (1) 

the subgrade (naturally occurring soil), (2) the paving materials (surface layer, base, and 

subbase), (3) the characteristics of applied loads, and (4) climate. 

According to AC 150/5320-6D [1] paragraph 302 a., “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 due to

destructive climatic effects and deteriorating effects of normal usage.” The FAA pavement

design procedure refers to the determination of the thickness of pavement and its components

(surface, base, and subbase layers) not to the design of the materials in pavements (e.g., HMA or 

PCC mixes). The material and construction requirements are specified in AC 150/5370-10A [2]. 

Failure in pavements is not a phenomenon of chance, but a phenomenon that has a definite 

mechanical cause. When the pavement is incapable of performing the task it was designed for, it 

has failed. Failure could be structural (deep structure rutting, alligator cracking, longitudinal or

transverse cracks in slabs, etc.) or functional (surface rutting, roughness, loss of skid resistance,

etc.). A unique definition of pavement failure does not exist. ASTM International D 5340 [3] 

uses PCI to rate a pavement. A pavement with a PCI value less than 10 is defined as failed. The 

current FAA design specification, AC 150/5320-6D Change 3, 2004, paragraph 708 b., states, 

“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.” It is also important to point out that the decision to take a pavement

out of service (or end of pavement life) depends not only on pavement condition, but also on the 

pavement functional type (runway, taxiway, or apron) and other subjective parameters. 

Since the objective of this report was to evaluate the FAA design specification, the pavement

structural life was defined as the period from when the pavement starts its service (SCI = 100) to

the time when SCI = 80. The use of SCI = 80 is understood as the definition of the end of 

structural life for PCC pavements; however, the use of SCI = 80 to define the end of structural 

life of flexible pavement is a new concept that is not well established. Alligator cracking and 

rutting (load-related distresses) were used in this report to calculate the SCI of flexible

pavements. 

2.2 PAVEMENT DISTRESSES.

Distresses in a pavement structure leads to all types of pavement failure. Pavement distresses are 

external indicators of pavement deterioration caused by loading, environmental factors, 

construction deficiencies, or combinations thereof. Pavement failure could be structural or 

functional. Structural failure requires careful and detailed examination of the failure mechanism 

and the pavement layer contributing to the failure. Repairs are generally very expensive and the 

2-1 
pavement may need reconstruction. Functional failures are generally easier and less expensive to 

fix. The type of distress in the pavement gives an insight to the type of failure, either structural 

or functional. In some cases, the two types of distresses interact with each other. For example, if 

untreated, functional failures may lead to or accelerate the structural failure of the pavement. 

The following sections describe the different types of distresses for HMA (flexible pavements)

and PCC (rigid) pavements, respectively, as considered in an airfield pavement evaluation per 

ASTM D 5340-03 [3]. The definitions of distresses in the following sections were quoted from 

reference 3 and the photographs were taken from the Strategic Highway Research Program 

Distress Identification Manual [4].

2.3 FLEXIBLE PAVEMENT DISTRESSES.

Sixteen types of distresses are considered in flexible airport pavements. These distresses could

be caused by material, environmental, construction, structural, or operational factors, and are 

described in the following sections. 

2.3.1 Alligator Cracking.

A series of interconnecting cracks caused by fatigue failure of the HMA surface under repeated 

traffic loading is called alligator cracking, as shown in figure 2-1. High tensile strains at the 

bottom of the HMA layer under a wheel load initiate fatigue cracks. The crack propagates to the 

surface as a series of parallel cracks, which get interconnected with repeated loading. Alligator

cracking, which is predominant in the wheel paths, is considered to be a major structural distress 

that can lead to structural failure of the pavement. 

FIGURE 2-1. ALLIGATOR FATIGUE CRACKING 

2.3.2 Bleeding.

Bleeding is a film of bituminous material on the pavement surface that usually becomes quite 

sticky. It is generally caused by excessive amounts of HMA or tars in the mix or low air void 

content, or both. It occurs when asphalt fills the voids of the mix during hot weather, then 

expands out onto the surface. It is caused by poor mix design and is a material-related distress.

Bleeding leads to functional failure. Figure 2-2 shows high-severity bleeding. 

2-2 
FIGURE 2-2. TIRE MARKS EVIDENT IN HIGH-SEVERITY BLEEDING 

2.3.3 Block Cracking.

Block cracks are interconnected cracks dividing the pavement surface into approximately 

rectangular pieces (1 by 1 ft to 10 by 10 ft). It is generally caused by the shrinkage of the HMA 

and daily temperature cycling. It is not load-related and would classify as a distress caused by 

material and environmental factors. Block cracking normally occurs over a large portion of 

pavement area and indicates that the asphalt has hardened significantly. Block cracking causes 

functional failure of the pavement structure. Figure 2-3 shows high-severity block cracking. 

FIGURE 2-3. HIGH-SEVERITY BLOCK CRACKING 

2.3.4 Corrugation. 

Corrugation is a series of closely spaced ridges and valleys (ripples) occurring at fairly regular 

intervals (usually less than 5 ft) along the pavement perpendicular to the direction of traffic. It is

generally caused by the combined action of traffic and unstable pavement surface or base. 

Corrugation leads to functional failure of the pavement. 

2-3 
2.3.5 Depression. 

Depressions are localized pavement surface areas having elevations slightly lower than those of 

the surrounding pavement. Depressions can be caused by settlement of the foundation soil, or it 

can be built during construction. Depressions could cause hydroplaning of the aircraft and leads 

to functional failure. Figure 2-4 shows a depression in a highway pavement. 

FIGURE 2-4. DEPRESSION IN PAVEMENT 

2.3.6 Jet-Blast Erosion.

Jet-blast erosion causes darkened areas on the pavement surface where the bituminous binder has

been burned or carbonized. This type of distress is caused by operational factors. 

2.3.7 Joint Reflection Cracking From PCC.

This type of distress occurs on pavements having an asphalt overlay on PCC slabs and is caused

by the movement of the PCC slab beneath the HMA surface because of thermal and moisture 

changes, and in response to loading. It is therefore partially load-related, but it does not 

necessarily lead to further structural deterioration of the underlying structure and is considered to 

be a functional distress. Figure 2-5 shows high-severity joint reflection cracking. 

FIGURE 2-5. JOINT REFLECTION CRACKING FROM PCC 

2-4 
2.3.8 Longitudinal and Transverse Cracking (Non-PCC Joint Reflective).

The longitudinal cracks are parallel to the pavement centerline or lay down direction. They may 

be caused by (1) a poorly constructed paving lane joint, (2) shrinkage of the HMA surface due to 

low temperatures or the hardening of the asphalt, or (3) a reflective crack caused by cracks

beneath the surface course, including cracks in the slabs (but not at the joints). Transverse cracks

extend across the pavement approximately at right angles to the pavement’s centerline or 

direction of lay down. They may be caused by (2) or (3) above. These types of cracks are not 

associated with loading. This type of distress is mainly caused by construction, material, and 

environmental factors. This type of distress leads to the functional failure of pavement. Figure 

2-6 shows longitudinal and transverse cracking. 

(a) Longitudinal Cracking (b) Transverse Cracking 

FIGURE 2-6. HIGH-SEVERITY LONGITUDINAL AND TRANSVERSE CRACKING 

2.3.9 Oil Spillage.

Deterioration or softening of the pavement surface caused by spilled oil, fuel, or other solvents is 

called oil spillage. Oil spillage is caused by operational factors and leads to functional failure. 

2.3.10 Patching and Utility Cut Patch.

A patch is an area where the original pavement has been removed and replaced with either

similar or different material. A patch is considered a defect, no matter how well it performs. 

Traffic load, material, and poor construction practices can cause patch deterioration. A patch is 

considered to be a function distress. Figure 2-7 shows a high-severity patch. 

2-5 
FIGURE 2-7. HIGH-SEVERITY PATCH 

2.3.11 Polished Aggregate.

Aggregate polishing is caused by repeated traffic applications. The surface binder is worn away

to expose coarse aggregate. Skid resistance is reduced and leads to functional failure. Figure 2-8 

shows polished aggregate in a pavement surface. 

FIGURE 2-8. POLISHED AGGREGATE 

2.3.12 Raveling and Weathering.

Wearing away of the pavement surface caused by dislodging of aggregate particles and loss of 

asphalt binder leads to raveling and weathering. It indicates significant hardening of the asphalt 

binder and can cause severe foreign object debris (FOD) problems and leads to functional 

failure. Figure 2-9 shows raveling and weathering of a pavement surface. 

2-6 
FIGURE 2-9. HIGH-SEVERITY RAVELING AND WEATHERING 

2.3.13 Rutting.

Rutting is a longitudinal surface depression in the wheel path. Pavement uplift may occur along 

the sides of the rut. Rutting is caused by the permanent deformation occurring in any of the 

pavement layers or subgrade, which is usually caused by consolidation or lateral movement of 

the materials due to traffic loads. Significant rutting can lead to major structural failure of the 

pavement. A rut depth of 1 inch is considered to indicate functional failure due to the ponding it 

can cause. Figure 2-10 shows rutting in a pavement. 

FIGURE 2-10. RUTTING IN A PAVEMENT 

2.3.14 Shoving .

Shoving is the longitudinal displacement of a localized area of the pavement surface, as shown in 

figure 2-11. Braking and turning aircraft are generally the cause. Also, PCC pavements

occasionally increase in length where they adjoin flexible pavements. This growth shoves the 

asphalt pavement, causing it to swell and crack. The PCC slab growth is caused by a gradual 

opening of the joints, as they are filled with incompressible materials that prevent them from 

closing. Shoving is a materials-related distress, and a functional failure that can lead to structural 

failure. 

2-7 
FIGURE 2-11. SHOVING IN A PAVEMENT

2.3.15 Slippage Cracking.

Slippage cracking occurs when braking or turning wheels cause the pavement surface to slide

and deform. The slippage cracks, shown in figure 2-12, are crescent- or half-moon-shaped 

cracks having two pointed ends away from the direction of traffic. It generally occurs when 

there is a low-strength surface mix or a poor bond between the surface and the next layer of 

pavement structure. This is classified as a functional distress that can lead to structural failure if

not corrected. 

FIGURE 2-12. SEVERE SLIPPAGE CRACKING 

2.3.16 Swell Distress.

An upward bulge in the pavement’s surface characterizes swell distress. It may occur sharply

over a small area or as a longer, gradual wave. Either type of swell can be accompanied by 

surface cracking. It is generally caused by frost action in the subgrade or by swelling soil, but a 

small swell can occur on the surface of an asphalt overlay over PCC as a result of a blowup 

(refer to section 2.4.1) in the PCC slab. A swell is classified as a functional distress. 

2-8 
2.4 RIGID PAVEMENT DISTRESSES.

Common types of distress in concrete pavement are pumping, faulting, cracking, and joint 

deterioration. ASTM D 5340-03 [3] lists 15 distress types for jointed concrete pavements, which 

are described in the following sections. The distress definitions are the same for both plain and 

reinforced jointed concrete pavements, with the exception of linear cracking, which is defined 

separately for plain and reinforced jointed concrete pavements. 

2.4.1 Blowup.

Blowups, as shown in figure 2-13, occur in hot weather, usually at transverse cracks or joints that 

are not wide enough to permit expansion of the concrete slabs. The infiltration of the 

incompressible materials into the joint space leads to insufficient width. When expansion cannot 

relieve enough pressure, a localized upward movement of the slab edges or shattering occurs in 

the vicinity of the joint. Blowups can also occur at utility cuts and drainage inlets. Blowups 

increase FOD potential and lead to functional failure. 

FIGURE 2-13. BLOWUP IN A PAVEMENT 

2.4.2 Corner Break.

A corner break is a crack that intersects the joint at a distance less than or equal to one-half of the 

slab length on both sides, measured from the corner of the slab. Factors causing corner breaks 

are load repetitions combined with loss of support and curling stresses. Figure 2-14 shows a 

corner break. 

FIGURE 2-14. CORNER BREAK 

2-9 
2.4.3 Longitudinal, Transverse, and Diagonal Cracks.

Longitudinal, transverse, and diagonal cracks divide the slab into two or three pieces and are 

generally caused by a combination of load repetitions, curling stresses, and shrinkage stresses. 

Low-severity cracks are usually warp- or friction-related and are not considered major structural 

distresses. Medium- or high-severity cracks are usually working cracks and are considered 

major structural distresses. Figures 2-15 and 2-16 show high-severity longitudinal and 

transverse cracking respectively. Low-severity cracks have little or minor spalling (no FOD 

potential). If nonfilled, they have a mean width less than approximately 1/8 inch. A filled crack

can be of any width but the filler material must be in satisfactory condition. For mediumseverity cracks, one of the following conditions must exist: (1) a filled or nonfilled crack is 

moderately spalled (some FOD potential); (2) a nonfilled crack has a mean width between 1/8 

and 1 inch; (3) a filled crack is not spalled or lightly spalled, but the filler is in unsatisfactory 

condition; or (4) the slab is divided into three pieces by two or more cracks, one of which is at 

least medium severity. For high-severity cracks, one of the following conditions must occur: (1) 

a filled or nonfilled crack is severely spalled (definite FOD potential); (2) a nonfilled crack has a

mean width greater than approximately 1 inch, creating potential tire damage; or (3) the slab is

divided into three pieces by two or more cracks, one of which is at least high severity. 

FIGURE 2-15. HIGH-SEVERITY FIGURE 2-16. HIGH-SEVERITY 

LONGITUDINAL CRACKING TRANSVERSE CRACKING 

2.4.4 Durability Cracking.

Durability (D) cracking is caused by the concrete’s inability to withstand environmental factors 

such as freeze-thaw cycles. It appears as a pattern of cracks running parallel to a joint or a crack. 

This type of cracking can eventually lead to disintegration of the concrete within 1 to 2 feet of 

the joint or crack and is classified as a function distress. Figure 2-17 shows high-severity D 

cracking. 

2-10 
FIGURE 2-17. HIGH-SEVERITY D CRACKING 

2.4.5 Joint Seal Damage.

Joint seal damage is any condition that enables soil or rocks to accumulate in the joints or allows

a significant amount of water infiltration. Accumulation of incompressible materials prevents 

the slabs from expanding and may result in buckling, shattering, or spalling. Typical joint seal 

damage are (1) stripping of joint sealant, (2) extrusion of joint sealant, (3) weed growth, (4)

hardening of filler, (5) loss of bond to the slab edges, and (6) lack or absence of sealant in the 

joint. Joint seal damage is classified as a functional distress. Figure 2-18 shows joint seal

damage. 

FIGURE 2-18. JOINT SEAL DAMAGE 

2.4.6 Patch, Small (less than 5 ft2

).

A patch is an area where the original pavement has been removed or replaced by a filler material. 

Poor construction of the patch, loss of support, heavy load repetitions, moisture, and thermal 

gradients can all cause distress. A patch is classified as a functional distress. Figure 2-19 shows 

a small patch distress. 

2-11 
FIGURE 2-19. SMALL PATCH DISTRESS 

 

2.4.7 Patch, Large (over 5 ft

2

).

An example of a large patch could be a patch that has replaced the original pavement because of 

the placement of underground utilities and is classified as a functional distress. Figure 2-20 

shows a large patch. 

FIGURE 2-20. LARGE PATCH DISTRESS 

2.4.8 Popouts.

A popout is a small piece of pavement that breaks loose from the surface due to a freeze-thaw 

action in combination with expansive aggregates, see figure 2-21. Popouts usually range from 

approximately 1 to 4 inches in diameter and from 1/2 to 2 inches in depth. This type of distress 

is generally caused by material and environmental factors. It is a functional distress (FOD) 

rather than a structural distress. 

2-12 
FIGURE 2-21. POPOUT DISTRESS 

2.4.9 Pumping.

Pumping is the ejection of material by water through joints or cracks caused by the deflection of

the slab under moving loads. As water is ejected, it carries particles of gravel, sand, clay, or silt,

resulting in loss of pavement support. Pumping near joints indicates poor joint sealer and loss of 

support that will lead to cracking under repeated loads. Pumping can occur at cracks as well as

joints and is classified as a functional distress. Figure 2-22 shows water bleeding and pumping. 

FIGURE 2-22. WATER BLEEDING AND PUMPING 

2.4.10 Scaling, Map Cracking, and Crazing.

This distress refers to a network of shallow, fine, or hair-like cracks that extend only through the 

upper surface of the concrete. The cracks intersect at angles of 120°. It is generally caused by 

overfinishing the concrete and may lead to surface scaling. Breakdown of the slab surface

occurs to a depth of 1/4 to 1/2 inch. Deicing salts, improper construction, freeze-thaw cycles,

2-13 
and poor aggregate may also cause scaling. These distresses are classified as a functional

distress and are construction-related. Figures 2-23 and 2-24 show scaling and map cracking, 

respectively.

FIGURE 2-23. SCALING FIGURE 2-24. MAP CRACKING 

2.4.11 Settlement or Faulting.

Difference of elevation at a joint or crack, caused by upheaval, consolidation, or a buildup of 

loose materials under the trailing slabs, is called settlement or faulting and is classified as a 

functional distress. Figure 2-25 shows faulting of transverse cracks. 

FIGURE 2-25. FAULTING OF TRANSVERSE CRACKS 

2.4.12 Shattered Slab/Intersecting Cracks.

Cracks that break the slab into four or more pieces due to overloading or inadequate support or 

both are called intersecting cracks, whereas a high-severity level of this distress is referred to as a 

shattered slab and is classified as structural failure. Figure 2-26 shows a shattered slab condition. 

2-14 
FIGURE 2-26. SHATTERED SLAB CONDITION 

2.4.13 Shrinkage Cracks.

Shrinkage cracks are hairline cracks that are usually only a few feet long and do not extend 

across the entire slab. They are formed during the setting and curing of concrete and usually 

does not extend through the depth of the slab and is classified as a functional distress. However, 

shrinkage crack is also the term used to describe a load-induced crack that extends only part of 

the way across a slab. In this case, it is classified as a structural distress (see change 3 of

reference 1).

2.4.14 Spalling (Transverse and Longitudinal).

Joint spalling is the breakdown of slab edges within 2 feet of the side of the joint. A joint spall 

does not extend vertically through the slab but intersects the joint at an angle. Spalling is caused 

by excessive stresses at the joint or crack caused by infiltration of incompressible materials or

traffic loads or weak concrete at the joints (caused by overworking) combined with traffic loads 

and is classified as a structural distress. Figure 2-27 shows spalling at a transverse joint. 

FIGURE 2-27. JOINT SPALLING AT A TRANSVERSE JOINT 

2.4.15 Spalling (Corner).

Corner spalling is the raveling or breakdown of the slab within 2 feet of the corner. A corner 

spall differs from a corner break in that the spall usually angles across to intersect the joint, while 

2-15 
a break extends vertically through the slab. A corner spall is classified as a structural distress. 

Figure 2-28 shows a corner spall. 

FIGURE 2-28. CORNER SPALLING 

2.5 ASSESSING PAVEMENT FAILURE.

One of the major problems in pavement evaluation is determining when the pavement has failed. 

According to the ASTM Standard Test Method for Airport Pavement Condition Surveys [3], a 

PCI value is computed based on the amount and severity of distresses (described in sections 2.3 

and 2.4). The PCI value is a numerical rating of the pavement condition that ranges from 0 to

100, with 0 being the worst possible condition and 100 being the best possible condition. A 

pavement is defined as failed when the PCI value drops below 10. However, the reduction in the 

PCI value could be caused by distresses that lead to structural failure or distresses that lead to 

functional failure, or both, and airport pavements are typically considered to be unusable well 

before the PCI has reached a value of 10. 

The FAA design procedure is based on the structural failure of pavements observed in full-scale 

tests [5]. For rigid pavements, the failure criterion is initial crack, which is defined as the 

condition when at least 50 percent of the slabs contain one or more cracks due to loading of the 

slab (see references 13 and 48). This corresponds to an SCI of 80, where the SCI is defined as 

the structural component of the PCI. The failure criterion for flexible pavements is based on 1 

inch of upheaval (shear failure in the subgrade) outside the traffic path or significant cracking 

(cracking in HMA layer has occurred to such an extent that the pavement is no longer

waterproof).

Since the FAA thickness design procedure is based on structural failure (rather than functional 

failure), only those factors or distresses that lead to structural failure were included in the 

evaluation of pavement life in subsequent sections of this report. 

Table 2-1 summarizes the distresses in flexible and rigid pavements with the typical failure

mechanism for each distress. 

2-16 
TABLE 2-1. DISTRESS MECHANISMS 

Pavement Distress Mechanism Failure Mode

Alligator Cracking Load Structural 

Bleeding Materials Functional 

Block Cracking Climate Functional 

Corrugation Materials Functional

Depression Construction Functional

Jet Blast Operational Functional

Joint Reflection Cracking Climate Functional

Longitudinal/Transverse Cracking Climate Functional

Oil Spillage Operational Functional

Patching Other Functional

Polished Aggregate Materials Functional

Weathering and Raveling Climate Functional 

Rutting Load Structural 

Shoving Materials Functional/Structural

Slippage Cracking Construction Functional

HMA 

Swell Other Functional

Blow-up/Buckling Climate Functional 

Corner Break Load Structural

Linear Cracking Load Structural

D Cracking Materials Functional

Joint Seal Damage Climate Functional

Patch, Small Other Functional

Patch, Large/Utility Cut Other Functional

Popouts Materials Functional

Pumping Other Functional

Scaling/Map Cracking/Crazing Construction Functional

Faulting Other Functional

Shattered Slab Load Structural 

Shrinkage Cracking Load/Materials Structural/Functional

Joint Spalling Construction/Load Structural 

PCC 

Corner Spalling Construction/Load Structural 

2-17 
Of these, the following distresses are characterized as structural (i.e., load) related for the

purpose of defining pavement life for consideration in the FAA thickness design standard: 

Flexible Pavement Rigid Pavement 

 Alligator cracking Corner break 

 Rutting* Linear Cracking 

 Shattered Slab 

 Shrinkage Cracking* 

 Joint Spalling* 

 Corner Spalling* 

* These distresses could also be caused by nonstructural mechanisms. 

Based on the above, this report used a procedure to convert the surveyed PCI data into SCI data 

for evaluation of airport pavement life. The details will be discussed in section 6. The following 

section discusses the factors affecting the structural life of airport pavements, overview of 

development of design procedures, and design assumptions. 

2-18 
3. PAVEMENT DESIGN PROCEDURES REVIEW AND EFFECTS OF THE RESPONSE

MODEL ON PAVEMENT LIFE.

This section presents a review of conventional and mechanistic procedures used by the FAA for

the design of flexible and rigid pavements. An understanding of the design theory is important 

in order to properly define and understand pavement structural life. It is also important to note

that the failure mechanisms in flexible and rigid pavements are fundamentally different. Flexible 

pavement failure is typically defined as shear failure in the subgrade, leading to rutting at the 

pavement surface. Rigid pavement failure is typically defined as a function of cracking of the

slabs. The failure criteria for each were developed from full-scale pavement tests dating from 

the 1940s to the present time. 

3.1 FLEXIBLE PAVEMENTS.

3.1.1 Development of California Bearing Ratio Equation—Pavement as Single Layer. 

The need for pavement design for heavier aircraft started in the 1940s. Richard Ahlvin 

chronicles the development of airport pavement design procedures in his report “Origin of 

Developments for Structural Design of Pavements” [6]. The earliest design method was based 

on an established empirical highway design method based on California Bearing Ratio (CBR).

The CBR method was selected for the following reasons: 

• It was correlated with service behavior and construction methods (1928-1942). 

• It could be quickly and easily adapted to airfield pavement design. 

• It was thought to be reasonable. 

• It had been successfully used in states other than California. 

• The subgrade strength (CBR) could be assessed using simple portable test equipment in 

the laboratory or in the field. 

• Testing could be performed on soil samples representing field conditions. 

3.1.1.1 Previous Full-Scale Tests and Studies Related to Flexible Pavement Design.

Design curves, using the single-wheel criterion, were developed for 7,000- and 12,000-lb wheel 

loads in 1942. Later on, using extrapolation, design curves were provided for 25,000-, 40,000-, 

and 70,000-lb wheel loads. At that time, only single-wheel aircraft existed, with B-17 and B-24 

military bombers being the heaviest aircraft. The mathematical models for stress, strain, and 

deflection extended only to a single uniform layer (homogenous, isotropic, half-space) and only

permitted special case (such as under the load center) determinations. A set of CBR versus

thickness curves for single-wheel aircraft for wheel loads from 4,000 to 70,000 lb was derived. 

These design curves were verified and modified by extensive accelerated traffic tests on existing

pavements of known composition and specially constructed full-scale test sections. Some minor

adjustments were made in the high CBR range curves. 

3-1 
Design curves for 150,000- to 200,000-lb single-wheel loads were developed and verified using 

the results from Stockton No. 2 tests [7]. 

In the 1940s, single-wheel load design criteria were used to provide criteria for dual and dualtandem landing gear loads by replacing the total strut load of a multiple-wheel gear with an

equivalent single-wheel load (ESWL). Studies on pavement responses (stress and deflection) 

showed that at, and near, the pavement surface individual wheels act independently. At very

large depths, the overlapping of wheel-load effects resulted in a similar stress and deflection, as 

would be induced by the total strut load acting on a single wheel. The depth at which interaction

effects start to become significant was established to be half the spacing between the edges of the 

tire prints of duals or the dual half of dual tandems and given the designation d/2. The depth at 

which the total strut load acts as if it were applied on a single wheel was established to be twice

the center-to-center distance between duals or twice the center-to-center distance of diagonally 

opposite wheels of dual tandems and given the designation duel-tandem wheel (2S). Between 

depths d/2 and 2S, the ESWL was represented a by log-log plot of load versus depth by a straight

line relationship between the load on one wheel at a depth of d/2 and the total strut load at a 

depth of 2S. The stresses and deflections were determined using Newmark’s charts. 

Tests conducted in 1952 [8] led to the development of an improved method in which the ESWL

is defined as the single-wheel load that causes the same deflection at some specified depth in the 

pavement as all wheels in the landing gear acting together. Deflections are calculated using 

Boussinesq’s equation. The specified depth is usually the depth of the top of the layer to be 

protected from shear failure (subgrade, subbase, or base). This method, commonly referred to as 

the CBR method, was adopted by the U.S. Army Waterways Experiment Station (WES) and the 

FAA to replace the d/2-2S method for ESWL determination, but the d/2-2S method is still used

by some organizations and countries. 

In the 1940s, design curves were developed for 100-psi tire pressure using the d/2-2S method. 

With increasing aircraft weights and tire pressures, design curves were developed for tire 

pressures of 200 and 300 psi using the CBR method. Accelerated pavement testing confirmed the 

validity of these extrapolations [9 and 10]. 

The single-wheel curves were depicted as a set of curves by using the early CBR equation: 

t = k P (3-1) 

where 

t = thickness of structure 

P = wheel load 

k = a constant for a particular CBR and tire pressure 

The constant k varies inversely with CBR and increases somewhat with tire pressure. Also, it 

was realized that the interaction between load magnitude and repetitions is a significant factor for 

the purpose of pavement design and evaluation. The multiple-wheel tests of 1949-1950 [8] 

3-2 
found the design methods based on d/2 and 2S to be reasonably satisfactory but a bit 

unconservative. 

In 1956, the CBR equation (equation 3-1) was modified to include tire pressure and CBR 

directly, as shown in equation 3-2 [11 and 12]. 

 













 = − CBR pπ

t P 1

8.1

1 (3-2) 

or 

 π

A

CBR

P = − 8.1

t (3-3) 

where 

t = thickness of structure, in. 

P = wheel load or ESWL, lb 

CBR = the strength test rating value 

A = tire contact area, in2

Equations 3-2 and 3-3 represent the basic form of the CBR equation without the provision for 

variation in stress repetitions (coverages). According to reference 6, no specific date can be

related to the acceptance of coverages as an element of design and evaluation applicable to the 

critical or design load. A 1949 study showed the trend of required thickness versus coverages.

In the following years, the design criteria were accepted as representing a 5000 coverages useful

life (nominally 20 years). The mathematical relationship relating percent of design thickness to 

coverages of critical load with 5000 coverages at 100 percent design thickness is shown in 

equation 3-4. 

0.23* 0.15

100

1 % = + 



 





t LogC (3-4) 

(%t = 100 at C = 5000) 

where C = coverages and log is the common or base 10 log. Equation 3-5 gives a more general

form of the CBR equation: 















 = + − CBR pπ

t LogC P 1

8.1

1 (0.23* 0.15) (3-5) 

With the advent of new aircraft such as the Boeing 747 and C-5, multiple-wheel heavy gear load 

(MWHGL) tests were carried out at WES to verify and modify greatly extrapolated design and 

evaluation criteria. The failure criteria for the pavements were excessive cracking in the HMA

layer or 1-inch upheaval outside the traffic lane. The upheaval is indicative of shear failure in 

the subgrade or any one or more of the other layers in the pavement that are composed of 

3-3 
unbound materials. The results of these tests [5] were used to modify equation 3-5 (for C =

5000), and a third degree form of the CBR equation was developed as follows: 

2 3

0.0481 1.1562 0.6414 0.4730 



 



  −



 



  −



 



 = − − p

CBR Log

p

CBR Log

p

CBR Log At (3-6) 

The α-factor was used to adjust thickness for other than 5,000 coverages. 

 





















 



  −



 



  −



 



 = − −

2 3

0.0481 1.1562 0.6414 0.4730

p

CBR Log

p

CBR Log

p

CBR Log A

t

α (3-7) 

3.1.1.2 Advisory Circular 150/5320-6D for Flexible Pavement Design.

The FAA design procedures (AC 150/5320-6D [1]) are based on the WES criteria (CBR 

method), as described above. Full-scale test data and observations of in-service pavements were 

used to determine pavement thicknesses necessary to protect subgrades with various CBR values 

from shear failure. These thicknesses were developed for single-wheel loadings. For gear

assemblies other than single wheel, the ESWL for the assembly was computed based on

deflection. Once the ESWL was established, the pavement section thickness was determined 

from relationships given in equations 3-6 and 3-7. The load repetitions are represented on the 

design curves in terms of annual departures, which are assumed to occur over a 20-year life. The 

departures are then converted to coverages. Each pass (departure) of an aircraft is converted to 

coverages using a single pass-to-coverage ratio, which is developed assuming a normal

distribution and applying standard statistical techniques. (A full description of the pass-tocoverage concept is given in reference 5, together with examples of calculating numerical 

values.) The pass-to-coverage ratios used in developing the flexible pavement design charts in 

AC 150/5320-6D are listed in table 3-1. Annual departures are converted to coverages by 

multiplying by 20 and dividing the product by the pass-to-coverage ratios listed in table 3-1. The 

pavement section thickness determined from a design chart is multiplied by the appropriate load

repetition factor (α-factor) to give the final pavement thickness required for various traffic 

levels. 

The design procedure is based on a design aircraft. In a mix of aircraft (for which the pavement 

is being designed), the design aircraft is the aircraft requiring the greatest pavement thickness. 

The design aircraft is not necessarily the heaviest aircraft in the mix. 

In the CBR method, the calculation of ESWL assumes that pavement layers all have the same

stiffness. Equivalency factors are used to convert the thickness of one type of material to 

another. With the advent of the tridem landing gear configuration (B-777), it was found that, due

to the high degree of interaction among individual wheel deflections, the CBR-based design 

procedure gives very conservative thickness designs. An alternative design procedure, based on 

a layered elastic analysis model, was developed. 

3-4 
TABLE 3-1. PASS-TO-COVERAGE RATIOS FOR FLEXIBLE PAVEMENTS 

Design Curve Pass-to-Coverage Ratio 

Single Wheel 5.18 

Dual Wheel 3.48 

Dual Tandem 1.84 

A300 Model B2 1.76 

A300 Model B4 1.73 

B-747 1.85

B-757 1.94

B-767 1.95

C-130 2.07

DC 10-10 1.82 

DC 10-30 1.69 

L-1011 1.81

3.1.2 Layered Elastic Design Procedures.

In 1995, the FAA published AC 150/5320-16 [13] for the thickness design of airport pavements

serving the B-777 airplane. In 2004, AC 150/5320-16 was cancelled and the layered elastic

design procedures were included in chapter 7 of AC 150/5320-6D. The use of the LEDFAA 

design procedures is now approved for all aircraft types and traffic mixes containing aircraft with 

gross weights in excess of 30,000 lb. Thickness design for pavements not subject to B-777 

aircraft can also be determined using the design charts in chapter 3 of AC 150/5320-6D as 

previously discussed. The layered elastic failure criterion was revised based on recent full-scale 

tests at the National Airport Pavement Test Facility (NAPTF). 

3.1.2.1 Full-Scale Tests at the NAPTF.

The design of the NAPTF was primarily determined by the need to develop pavement design 

procedures for the new generation of large civil transport aircraft, including the B-777, B-747 

stretch, and the Airbus A380 class aircraft. Current design procedures predict a significant 

amount of interaction between the loads from multiple wheels and closely spaced trucks that will 

be used on these aircraft, particularly within the subgrade in flexible pavements. But the true 

degree of interaction is not known, and measurements from full-scale tests are required to 

determine how closely wheels and trucks can be spaced without significant load interaction. The 

NAPTF is located at the FAA William J. Hughes Technical Center, Atlantic City International 

Airport, New Jersey. A 1.2-million-lb pavement test vehicle spans two sets of railroad tracks 

that are 76 feet apart. The vehicle is equipped with six adjustable dual-wheel loading modules. 

A hydraulic system applies the load to the wheels of the modules. The major specifications for 

the test pavement area are as follows: 

• Test pavement is 900 feet long by 60 feet wide. 

3-5 
• Nine independent test items (six flexible and three rigid) along the length of the track. 

The pavement cross-sectional details can be found on the FAA Airport Technology 

Research and Development Branch web site: www.airporttech.tc.faa.gov. 

• Twelve test wheels capable of being configured to represent two complete landing gear 

trucks having from two to six wheels per truck and adjustable up to 20 feet forwards and 

sideways. 

• Wheel loads adjustable to a maximum of 75,000 lb per wheel. 

The primary objectives of the testing were to 

• provide additional traffic data for incorporation in new thickness design procedures under 

development by the FAA. 

• provide full-scale pavement response and failure information for use in airplane landing 

gear design and configuration studies. 

• re-examine the CBR method of design for flexible pavements. 

All three of these objectives were established with particular reference to the level of pavement

damage expected from the six-wheel landing gear on the B-777 airplane relative to the dual and 

dual-tandem landing gears used on aircraft in the remainder of the fleet. 

The failure criteria for the flexible test items are the same as those defined in reference 5 for the 

U.S. Army Corps of Engineers (USACE) MWHGL test series. The pavement was defined as

failed when either of the following conditions occurred: 

• One-inch upheaval outside the traffic lane, signifying structural shear failure in the 

subgrade or other supporting layers 

• Surface cracking to the point that the pavement is no longer waterproof, signifying 

complete structural failure of the surface layer 

The results from the full-scale flexible pavement testing at the NAPTF are summarized in 

reference 14. The construction cycle one (CC1) flexible pavement test items were constructed 

with both conventional crushed stone base (P-209) and asphalt stabilized base (P-401 Base). The 

conventional structures had base courses consisting of 8 inches of FAA specification P-209,

crushed aggregate base course, and the stabilized base structures had base courses consisting of 5 

inches of FAA specification P-401, plant mix bituminous pavement. Full structural failure did 

not occur in the low-strength CC1 test items, probably because the subgrade material contained a 

significant amount of silt and the upper layers of the subgrade dried somewhat over the long 

period of time between construction and the start of traffic testing. Subgrade strengths of 

approximately 5 CBR were measured after testing was completed instead of the 3 CBR design 

target. Therefore, results for only the medium-strength test items, which did show the expected 

subgrade shear failures, were used in the modification of the failure model. During construction 

3-6 
cycle 3 (CC3), four new flexible pavements with 5-inch P-401, 8-inch P-209 crushed stone base, 

and P-154 subbase (16, 24, 34, and 43 inches for LFC-1, LFC-2, LFC-3, and LFC-4 

respectively) were constructed. Test items LFC-1, LFC-2, and LFC-3 failed during traffic 

testing, and the data from these test items were used for the modification of the failure model. 

For updating the flexible pavement failure model in the LEDFAA computer program for airport 

pavement thickness design, a database of existing full-scale test data for heavy airplane wheel

loads was assembled, and the CC1 and CC3 test results were added. The vertical strain at the top

of the subgrade was then calculated for each data point using the LEDFAA structural model. 

Material properties were based on best estimates of the data provided in the reports of the fullscale tests. The pass-to-coverage ratio is calculated in LEDFAA for an equivalent wheel width 

projected onto the top of the subgrade; the same procedure is used for constructing the LEDFAA 

failure model. Consequently, coverages to failure in LEDFAA are not the same as coverages to

failure in the CBR method, as discussed below. 

Figure 3-1 shows the results of vertical strain plotted against coverages to failure on a log-log 

scale. The NAPTF results match well with the existing data, and the six-wheel data points do 

not show significant deviation from the rest of the data points (ideally, whatever the gear

geometry, wheel load, and structural properties, all the data points should fall on a single 

describing curve). The data points also indicate a change in characteristic slope at high coverage 

levels. 

0.0001

0.001

0.01

1 10 100 1,000 10,000 100,000 1,000,000

No. of Coverages to Failure

Vertical Subgrade Strain, inch/inch

? Structural Layers Study - 6

? NAPTF - 9

CC1-MFC-4W

CC1-MFC-6W

CC3-LFC1-6W

CC3-LFC1-4W

CC3-LFC2-6W

CC3-LFC2-4W

CC3-LFC3-6W

CC3-LFC3-4W

CC1-MFS-4W

CC1-MFS-6W

LEDFAA 1.3 Failure Model

 ? Stockton - 8

? MWHGL - 7

FIGURE 3-1. LEDFAA FAILURE MODEL FOR ALL AVAILABLE FULL-SCALE TEST 

DATA POINTS WITH REPORTED SUBGRADE SHEAR FAILURE 

3-7 
The dark line passing through the middle of the data points in figure 3-1 is the least squares 

curve fit to the data. The pair of straight lines marked LEDFAA 1.3 Failure Model is the failure 

model implemented in LEDFAA 1.3, which provides thickness designs with the LEDFAA 

structural and traffic models that are compatible with the thickness designs determined according

to the conventional CBR procedures. The second straight line in the LEDFAA failure model was 

introduced to maintain compatibility with the CBR method and to make allowance for the 

leveling off of the data points indicated in figure 3-1. Further adjustment of this failure model is 

expected before the follow-on computer program FEDFAA is finalized, but not to the extent that

thickness designs will be significantly changed. 

3.1.2.2 Advisory Circular 150/5320-6D, Chapter 7.

The layered elastic design procedures included in chapter 7 of AC 150/5320-6D are 

computerized with the LEDFAA program. LEDFAA implements layered elastic theory-based 

design procedures developed under the sponsorship of the FAA for new and overlay design of 

flexible and rigid pavements. The major changes in the response calculation from AC 150/5320-

6D to LEDFAA are as follows: 

• The critical response in LEDFAA is the vertical strain at the top of the subgrade and the 

horizontal strain at the bottom of the HMA layer. Deflection was used in AC 150/5320-

6D. 

• The pavement model used in computing critical response was changed from a single 

layer elastic semispace to a multiple-layer elastic system. 

• The response calculation changed from constant tire contact area to constant tire pressure. 

Design charts (nomographs) were replaced with a computer program. The design aircraft 

concept was replaced by design for fatigue failure expressed in terms of a cumulative damage 

factor (CDF) using Miner’s rule. Also, the major material property of the pavement layers is 

now uniformly expressed as an elastic modulus instead of the previous CBR for flexible 

pavements. The design process considers two modes of failure for flexible pavement: 

• Vertical strain in the subgrade to limit subgrade rutting. 

• Horizontal strain at the bottom of the HMA layer to limit fatigue failure. 

The major assumptions are as follows: 

• Materials are homogenous, linear elastic, and isotropic. 

• Since the solution is performed in radial space, only circular loads can be analyzed. 

Other load shapes can only be approximated as sums of superimposed circular loads. 

• Loads must be vertical. 

• All the layers are of uniform thickness and infinite in horizontal extent. 

3-8 
3.2 RIGID PAVEMENTS.

The first step in predicting pavement life is to calculate the critical stresses of the pavement 

based on a pavement structural model with well-defined boundary conditions, material

properties, and traffic loads. Then the critical stress is adjusted to obtain the working stress that

will be used in the failure model for design. Therefore, the calculated stresses have significant 

effects on the predicted pavement life. The major effects on the pavement life through the 

calculated stresses are analyzed in this section. 

3.2.1 Slab Models.

For more than a half century, an infinitely large, elastic, and thin plate was used to model the 

PCC pavement slabs for calculating the critical stress [15 and 16]. The same model was used in

pavement thickness design by most design procedures [17, 1, 18, and 19]. The application of 

this model assumes that the behavior of a PCC slab under a load perpendicular to the slab surface 

is similar to a thin plate under the same load. Therefore, the model simplifies a threedimensional (3D) problem into a two-dimensional (2D) one, and it makes all tools that were

developed for thin plate analysis applicable for calculating the pavement slab critical stress.

These tools include close-form equations [15 and 16], an integration solution by computer

program H51 [20], and 2D finite element solutions [21-24]. 

The use of the thin plate model has been supported by long-time theoretical analysis: 3D solids 

can be modeled by a 2D plate if the length of one dimension is smaller than one-third of the 

other two dimensions. For an 18-inch-thick, 15- by 15-foot slab, the ratio is 1/10; much less than

1/3. Two models other than the thin plate are often used. One models the PCC slab by an 

infinitely large elastic layer with the same thickness as the slab [25 and 26]. The other models

the slab with 3D finite elements [27 and 28]. 

Although there are pros and cons associated with each model and the simplified models were

effectively used in the past, the advent of more complex landing gears as well as the need to model 

all layers of the support system has led to a preference for 3D finite element methods (FEM) slab

models. 

3.2.2 Foundation Models.

The modeling of the pavement foundation led to more uncertainties. A real PCC pavement 

consists of slabs on several layers of different materials. The multiple-layer foundation supports

the slabs on top through its ability to resist compression, shear, and bending forces (and 

moments). The foundation is a very complicated system. First, it is not a linear elastic system. 

The modulus of the materials is functions of stress states. Second, the interface behaviors 

between two adjacent layers, especially the two top layers, are very complicated. The friction

forces vary along with the load magnitude, direction, gear configuration, and repetition number 

of the loads. Before 3D finite element techniques were used in pavement engineering, it was 

almost impossible to model the foundation behaviors in detail. 

3-9 
A very simple model—dense liquid foundation—has been used for more than a half century [15]. 

Only one parameter, foundation modulus k, is used to describe the foundation’s resistance to 

compression. The major differences between the dense liquid foundation model and the multiple 

layer foundation are as follows: 

• The dense liquid foundation model does not consider shear deformation. 

• A pavement with multiple layers needs many parameters, thickness, modulus, and 

Poisson’s ratio of each layer to define its characteristics. The dense liquid foundation is 

defined by only one parameter, the foundation modulus k. An empirical procedure [17 

and 29] was used to calculate the effective k at the top of a layered support system as an 

expedient. However, direct characterization of all supporting layers, such as used in a 3D 

FEM model, is preferable. 

Currently, all foundation models are still used in various design methodologies but the variation 

of the parameters used to define the pavement foundation can lead to variations in the calculated

critical stresses, which further lead to variations in the predicted pavement life of the design. 

3.2.3 Effects of Concrete Elastic Modulus.

As previously mentioned, the working stress (based on the calculated critical stress under a load) 

is defined as the damage indicator of the pavement. Therefore, the pavement life is very 

sensitive to the calculated critical stress, and it is important to verify the correctness and accuracy

of the calculated stress of different models. Unfortunately, it is difficult to do so by testing, and 

the problem remains unsolved. Determination of the concrete elastic modulus Ec for design and

analysis is still a pending problem. The critical stress in the slabs, and hence the pavement life

determination, is influenced by the uncertainty in the concrete modulus. 

Westergaard [16] proposed a closed-form solution (see equation 12 in reference 16) for

calculating the critical edge stress of a slab when a circular load is applied tangential to the free 

edge of an infinitely large slab. 

(1 ) 1.18 (1 2 ) } 2

1

3

4 ) 1.84

100 { ( (3 )

3(1 )

4

3

2 l

a

ka

E h Ln

h

P c

E μ μ μ

μ π

μ σ + − + − + × +

×

+

+ = (3-8) 

where 

P = Load magnitude in lbs

μ = Poisson’s ratio of concrete 

h = Thickness of the slab, in. 

Ec = Elastic Modulus of the concrete, psi 

k = Foundation modulus in PCI 

a = Radius of the tire foot print, in. 

l = Relative radius of stiffness of the pavement = (Ec×h3

/[12(1-μ

2

)k])0.25

3-10 
For comparison, a 30- by 30-ft slab was used to calculate the critical edge stress from the 

Westergaard solution with a 2D finite element model [30]. Both cases used the same tire 

pressure, load magnitude, and load center location. Six- by six-inch elements were used in the 

2D model. All other input data are listed in table 3-2. The critical stresses calculated by 

equation 3-8 and the 2D program using two values of concrete modulus are presented in table 

3-3. 

TABLE 3-2. INPUT DATA 

Thickness Slab Size Poisson’s Ratio 

17 inches 30 by 30 ft 0.15 

Load P Tire pressure Foundation modulus k

50,000 lbs 180 psi 200 psi 

TABLE 3-3. CALCULATED CRITICAL STRESS 

Ec

Equation 8 of 

reference 16 Jslab2004 

4,000,000 432.3 429

6,000,000 455.2 452

Difference in Percent 50 5.3 5.4

The results presented in table 3-3 suggest that the calculated critical stress is not sensitive to the

elastic modulus Ec of the concrete. Two calculations using Ec = 6,000,000 and 4,000,000 psi 

have only about a 5 percent difference. 

Figure 3-2 indicates that a significant discrepancy exists when Ec = 4,000,000 psi is used to 

compare the calculated and measured strains at the NAPTF. However, when Ec = 5,500,000 psi 

(obtained from tests per ASTM C 215) is used, the discrepancy becomes significantly less. 

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Micro Strain

CSG3-Upper, Measured

CSG3-Lower, Measured

CSG3-Lower, Calculated, Ec=5,500,000psi

CSG3-Upper, Calculated, Ec=5,500,000psi

CSG3-Lower, Calculated, Ec=4,000,000psi

CSG3-Upper, Calculated, Ec=4,000,000psi

CSG3

FIGURE 3-2. EFFECTS OF THE CONCRETE MODULUS ON THE COMPARISON OF 

MEASURED AND CALCULATED STRAINS 

3-11 
The above analysis indicates that though the accuracy of Ec is important in strain comparison, it 

is much less important in stress comparison. Therefore, the FAA has been using Ec = 4,000,000 

psi in the design standards without change. 

When concrete modulus of elasticity is obtained by the static test method, according to ASTM C 

469, it should be noted that the standard states, “The modulus of elasticity and Poisson’s ratio 

values are applicable with a customary working stress range (0% to 40% of the ultimate concrete

strength).” The document also states, “The modulus of elasticity values obtained will usually be 

less than moduli derived under rapid load applications (dynamic and seismic rates, for example), 

and will usually be greater than values under slow load application or extended load duration, 

other test conditions being the same.” 

3.2.4 Variation of Joint Load Transfer Capabilities.

Load transfer in rigid pavements needs to be appropriately considered for PCC pavement

analysis and evaluation. Therefore, the FAA supported several projects [31-34] to improve the 

understanding of the joint characteristics. Variation in load transfer efficiency that differs from 

design assumptions can affect calculated slab stresses and resultant pavement life. 

Doweled joints have been verified to provide more uniform load transfer year round. WES 

surveyed test pavements at the Denver Airport for several years. Dong [33] reported that the 

load transfer capability of the joints defined by the ratio of falling weight deflectometer 

deflections on the two sides of the joint load transfer efficiency with respect to deflection (LTD) 

varied very little for the hinged joints. The average LTDs varied from 88.7% to 84.3% between 

summer and winter. The average LTDs varied from 81.3% in summer down to 28.2% in the 

winter for dummy joints because the width of the joint opening increases in the winter and the 

load transfer capability through the joint interlock action becomes much less. However, though 

the joint opening width is also increased in the winter for the doweled joints, the dowels still 

keep the joint load transfer capability at an acceptable level. The average LTDs measured at the 

Denver airport varied from 75% in the summer to 66% in the winter. Based on surveyed data 

from five airports (900,000 linear feet of joints total), reference 31 reports dummy and key joints 

received much higher joint-related distresses (spalling and cracks) than doweled and hinged 

joints when different types of joints are located in the same area. Though the tie bars in the

hinged joints can always tightly hold the two slabs together and provide the maximum load 

transfer capability, the hinged joints cannot be used for all joints since they cannot allow the slab 

free movement when the slabs need to move under temperature or moisture variations.

Therefore, doweled joints are needed when the load transfer capability must be kept at an

acceptable level, and the dowel modeling becomes necessary in pavement design and analysis. 

Brill [32] found that two simplified equations were used in engineering practice to calculate the 

joint stiffness that makes the doweled joint functionally equivalent to an interlocked joint. The

joint stiffness calculated by one equation is approximately twice that calculated by another that 

was proposed by Huang [22]. Brill proved that the equation proposed by Huang was correct. 

Based on the correct equation, the equivalent joint stiffness of a doweled joint using 1.25-inch 

dowels with 15-inch spacing or 1.5-inch dowels with 18-inch spacing should be 47,000 psi and 

55,000 psi respectively. However, a joint stiffness value of 100,000 psi has been continuously 

3-12 
used in PCC pavement design and analysis for many years. The assumptions governing load 

transfer may contribute to the uncertainty of the critical stress computations. Additionally, the 

following factors may also increase the uncertainty of the slab critical stress. 

• In FAA design specifications, 25% of the load is assumed to be transferred from one side 

to the other when the load is located at the joint. The same values are used year around. 

However, most joints in service are dummy joints [31] and provide very different load 

transfer capabilities between summer and winter. Reference 34 (see figures 5.20, 5.21, 

and 5.22 of reference 34) reports that the measured opening widths of doweled and 

dummy joints are significantly higher than the hinged joint when the measured average 

temperature of the pavement is low. The dowel bars do not reduce the variation of joint 

opening. Therefore, this variation increases the variation of the slab critical stress. 

• The load transfer capability of dowel bars also reduces along with the number of 

repetitive heavy loads. Experimental research [35 and 36] indicates that gaps defined as

dowel looseness are developed between the dowels and the surrounding concrete. Five 

percent of the total number of repetitive loads produced about 50% of the dowel 

looseness at the end of pavement life. That is, the dowel bars mostly work under a

certain looseness and reduce the load transfer capability. This looseness has not been

considered in airport pavement design specifications, leading to the increase of critical 

stresses in the slab. 

3.2.5 Effects of Interface Model Between the Slab and the Layer Underneath.

For new PCC pavement design, the FAA specification assumes that the interface between the 

concrete slab and the layer underneath is frictionless or fully unbonded. The assumptions used 

for bonding at layer interfaces can have an effect on the computation of the critical stresses with

a resultant impact on pavement life. 

The data obtained from the test pavement at the Denver Airport [33] showed that the interface is

partially bonded rather than fully unbonded. To further investigate the interface behavior, the 

FAA conducted several tests. The results obtained from a single slab test showed the uncertainty 

of the interface behavior. The detailed pavement structure, materials, and testing procedures are 

located in reference 37. 

When the test was first conducted (October 20, 2003) using an 18-inch plate, the slab had 

reached its highest curling. The friction force in the interface shifted the neutral axis of the cross

section from the middle of the slab thickness to approximately 1.5 inches from the slab top 

surface. The strains in one of the embedded strain gages (CSG2A), 1.5 inches from the slab top, 

held at almost zero when the load increased from 0 to 20,000 lb (figure 3-3). 

3-13 
Measured Strains at CSG2,

Center Load, Different Load Size and ETGs

-25

-20

-15

-10

-5

0

5

10

15

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

Loads (Lbs)

CSG2 (micro-strains)

Upper,6" Plate, 01/09/2004 Middle,6" Plate, 01/09/2004 Lower,6" Plate, 01/09/2004

Upper, 18" Plate, 12/05/2003 Middle, 18" Plate, 12/05/2003 Lower, 18" Plate, 12/05/2003

Upper,18" Plate, 10/20/2003 Middle,18" Plate, 10/20/2003 Lower,18" Plate, 10/20/2003

 CSG-2

Load Location

FIGURE 3-3. UNCERTAINTY OF THE INTERFACE BEHAVIOR UNDER A CIRCULAR

LOAD AT THE SLAB CENTER

A second static test with an 18-inch plate was conducted (December 5, 2003) after the slab 

curling reduced to its lowest curling state (the four-corner curling displacements dropped from 

an average of 195 down to 79 mils after watering the surface). The load was applied at the slab

center, again using an 18-inch plate. Figure 3-3 shows that the upper microstrain was -12.5 and 

the lower microstrain was about 9. The sensor at the middle of the slab thickness obtained a 

microstrain = -1.5, which indicates the interface friction had less effect than it had on October 

20, 2003. 

On January 9, 2004, the test was repeated by using a 6-inch plate. The results are also shown in 

figure 3-3. The maximum difference in microstrains between the measured upper and lower 

strains under a 6-inch plate (20 + 6 = 26) was slightly higher than under an 18-inch plate (12.5 +

9 = 21.5). However, the measured middle strain under a 6-inch plate was -7, which indicated 

stronger friction effects. 

Figure 3-3 shows the difference of the magnitudes of peak strains measured at the upper and 

lower strain gages. The existence of friction reduces the magnitude of the measured lower strain. 

This indicates that the fully unbonded interface model used by the FAA is more or less overly 

conservative in estimating the maximum stress at the slab bottom.

The recent tests conducted at the NAPTF verify what had been observed in the test pavement at

the Denver Airport: the interface is partially bonded and the bonded degree varies all the time,

which increases the variation of the critical stress at the bottom of the PCC slabs. The variations

in the critical stresses will impact pavement life. 

3-14 
3.3 SUMMARY.

The evolution of the FAA design standards over time has been discussed in this section. An 

understanding of the design theory and the fundamental difference in the failure mechanism 

between rigid and flexible pavements is important in evaluating how the structural lives of rigid 

and flexible pavements are influenced by the design procedures and governing assumptions. 

For flexible pavements, the CBR-based design procedure, alpha factor, and the current layered 

elastic theory-based design procedures were discussed. The failure criteria for both conventional 

and mechanistic design methods were based on the results of full-scale tests at WES and the 

NAPTF. The results of the NAPTF tests were recently used to modify the flexible pavement

failure criteria. It is believed that these revised criteria will result in more economical and

reliable design procedures. 

For rigid pavements, different structural models used over time were discussed. Different 

factors, such as load transfer at joints and layer interface effects on critical stresses, affect rigid

pavement life. These factors and their relationship to the design method were summarized. 

3-15/3-16 
4. EFFECT OF FAILURE MODELS ON PAVEMENT LIFE.

As discussed in section 3, rigid and flexible pavements exhibit fundamentally different failure 

modes. For rigid pavements, failure is defined as a function of slab cracking; for flexible 

pavements, failure is defined with respect to permanent shear deformation in the subgrade, 

resulting in surface rutting. Each failure mode is reflected in the failure models that are 

imbedded in the design procedures. The models are generally described in terms of the number

of load repetitions to failure, where failure is defined as an unacceptable degree of slab cracking 

and subgrade deformation for rigid and flexible pavements respectively. 

This section reviews the failure models used in the FAA’s conventional and mechanistic design 

of flexible and rigid pavement. The failure model algorithms described herein can be used to 

compute pavement life. Therefore, an understanding of the pavement models and their effect on 

pavement life is important. 

4.1 FLEXIBLE PAVEMENTS.

As previously discussed, the results of full-scale pavement tests were used to develop failure 

models used in the FAA’s conventional (CBR) and mechanistic (layered elastic) design methods. 

Details of the development and description of the flexible pavement design models are described 

in the following sections. 

4.1.1 Failure Model in AC 150/5320-6D.

The design procedures in AC 150/5320-6D are based on the concept of ESWL. The pavement

response related to failure is deflection. ESWL is the wheel load that yields the same maximum 

deflection as a multiple-wheel load. The contact area of ESWL is equal to the contact area of 

one of the wheels of the multiple-wheel assembly. In the CBR-based design procedure, the

thickness factor (η) is a function of CBR/p. The earliest form of the relationship between η and

CBR/p is derived from equation 3-2 in section 3 and is as follows: 

π

η

1

8.1* = − CBR

p (4-1) 

(for 0.22 > CBR/p) 

where 

CBR = the strength test rating value 

p = the contact pressure of ESWL

Based on the test results from the MWHGL study, the CBR equation was modified (equation 3-6 

in section 3) and the relationship between η and CBR/p is shown below: 

2 3

0.0481 1.1562 0.6414 0.4730 



 



  −



 



  −



 



 = − − p

CBR Log

p

CBR Log

p

CBR

η Log (4-2) 

4-1 
In AC 150/5320-6D and the computer program F806, equation 4-1 is used when CBR/p is less 

than 0.22. The following relationship is used for 1 ≥ CBR/p ≥ 0.22: 

2

0.05 0.35187 0.51492 



 



  +



 



 = − p

CBR Log

p

CBR

η Log (4-3) 

For CBR/p > 1, equation 4-4 is used: 





 



 = − p

CBR

η 0.05 0.005 Log (4-4) 

Figure 4-1 shows the relationship between the thickness factor η and CBR/p from different

relationships. 

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 0.1 0.2 0.3 0.4 0.5

CBR/p

Thickness Factor

Equation 4-1 (earliest equation)

Equation 4-2 (Post-MWHGL Tests)

FAA AC 150/5320-6C & 6D, Program F806

APPLICABLE RANGE OF 

EARLIEST EQUATION

POSSIBLE RANGE OF 

CBR/p IN PRACTICE

FIGURE 4-1. COMPARISON OF THICKNESS FACTOR FROM DIFFERENT 

RELATIONSHIPS 

Values of CBR/p for design are typically in the ranges of 0.02 to 0.1 for protection of subgrades, 

0.05 to 0.2 for protection of subbases, and 0.2 to 0.5 for protection of bases. 

4-2 
The number of allowable coverages is calculated by reorganizing the CBR equation as follows: 

 

0.65

0.23* * 0.0481 1.1562 0.6414 0.4730

2 3 −



































































 − 













 −













 − −

=

p

CBR Log

p

CBR Log

p

CBR A Log

t LogC

 (4-5) 

where 

C = number of coverages 

A = contact area 

t = pavement thickness 

CBR = strength test rating value 

p = contact pressure of ESWL 

Equation 4-5 provides the empirical link between predicted pavement life (coverages) and 

pavement response (p = ESWL/contact area of one wheel on the landing gear). ESWL is a true

representation of pavement response because it is calculated as the single-wheel load that causes 

the same vertical deflection in the structure as the landing gear of interest. Both deflections are

computed at depth “t” below the top of the pavement. In the normal nomenclature of pavement 

design, the CBR method for airport pavements is a true mechanistic-empirical model. 

The MWHGL [5] tests indicated that the above equation does not apply for aircraft gears with 

multiple wheels. Based on the test results, an alpha factor or the load traffic factor was 

introduced in the MWHGL report. Equation 4-6 shows the relationship between alpha factor, 

pavement thickness, subgrade CBR, and contact pressure of ESWL. 

 





















 



  −



 



  −



 



 = − −

2 3

0.0481 1.1562 0.6414 0.4730

p

CBR Log

p

CBR Log

p

CBR Log A

t

α (4-6) 

where 

 

α = load traffic factor 

A = contact area 

t = pavement thickness 

CBR = strength test rating value 

p = contact pressure of ESWL 

A standard chart of alpha factors with coverages for different numbers of wheels on the landing 

gear is shown in figure 4-2. 

4-3 
FIGURE 4-2. ALPHA FACTOR VERSUS COVERAGES 

(From the COMFAA program) 

With the advent of the Boeing-777 (six-wheel landing gear, 540,000 to 580,000 lbs projected to 

over 750,000 pounds), emphasis again shifted onto alpha factor because preliminary analyses 

indicated that aircraft with six-wheel landing gears would require significantly thicker flexible

pavements than the existing fleet. The value of alpha factor at 10,000 coverages from inception 

to 1995 was 0.788. In 1995, as a result of reanalysis of the MWHGL data by the USACE, the 

International Civil Aviation Organization assigned an interim alpha factor value of 0.72 at 

10,000 coverages for six-wheel gears. In all the analysis of data for the development of the alpha 

factor chart, alpha factor does not vary with CBR for a given gear configuration. Further 

information on these developments can be found in reference 38. 

Recent analysis of combined MWHGL [5] and NAPTF [14] data showed that a least squares 

quadratic curve fit gives an alpha factor at 10,000 coverages for six-wheel gears of 0.718 to 

0.729. The alpha factor should vary with CBR and gear geometry—the CBR design model, 

however, holds it constant. During the tests conducted at the NAPTF, the six-wheel gear test 

results did not show any unexpected levels of pavement damage compared to the four-wheel 

gears. Also, the FAA does not need alpha factors for landing gears with more than six wheels 

because of the transition to the layered elastic system. 

4.1.2 Failure Model in AC 150/5320-16 (LEDFAA 1.2).

The FAA’s LEDFAA design procedure based on layered elastic analysis is more mechanistic in 

nature. Critical pavement responses, such as the maximum tensile strain at the bottom of the 

HMA layer and maximum vertical strain at the top of the subgrade, are related to pavement life.

JULEA (developed by the USACE) is the layered elastic computational program in LEDFAA1.2 used for layered elastic analysis. The maximum horizontal tensile strain at the bottom of the

HMA layer is computed and related to the number of coverages (life of HMA surface) 

4-4 
considering asphalt fatigue failure. The Heukelom and Klomp model has been adopted and is as 

follows: 

Log10C Log10 H 665* Log10EA = 2.68 − 5* ε − 2. (4-7) 

where 

C = number of coverages to failure 

EA = elastic modulus of asphalt concrete, psi 

εH = maximum horizontal tensile strain at the bottom of the HMA layer 

The concept of coverages as used in the FAA’s mechanistic design procedures is discussed in 

more detail in section 4.2.2. The maximum vertical strain at the top of the subgrade is used to 

control subgrade rutting. The following equation is used to predict the number of coverages to 

failure and is based on the analysis of full-scale test data: 

0.559 0.0658*

000245*

10 0.000247 0. 10,000 *

ESG

V

Log ESG C 



 



 + = ε (4-8) 

where 

ESG = elastic modulus of subgrade soil, psi 

εV = maximum vertical strain at the top of the subgrade 

From equation 4-8, the number of coverages to failure depends on the subgrade elastic modulus

and maximum vertical strain at the top of the subgrade. Figure 4-3 shows the subgrade failure 

criteria. 

0.0001

0.001

0.01

1 10 100 1,000 10,000 100,000 1,000,000

Coverages

Subgrade Vertical Strain

LEDFAA-1.2 (E=5K)

LEDFAA-1.2 (E=10K)

LEDFAA-1.2 (E=15K)

FIGURE 4-3. SUBGRADE FAILURE CRITERIA IN LEDFAA-1.2 

4-5 
4.1.3 Failure Model in AC 150/5320-6D Change 3 (LEDFAA 1.3).

As per AC 150/5320-6D Change 3, (issued in April 2004), LEDFAA 1.3 became an official 

FAA design standard and can be used in lieu of AC 150/5320-6D design charts for any aircraft 

mix. In the LEDFAA 1.3 computer program, LEDNEW was replaced by LEAF—a layered 

elastic computational program developed by the FAA. As far as the failure models are 

concerned, the failure model for HMA fatigue is the same as that in LEDFAA 1.2 (equation 4-7). 

The failure model for subgrade rutting was modified based on the availability of additional fullscale test data from the NAPTF and the reanalysis of all the previous full-scale test data. The

dependency on the number of coverages to failure on subgrade modulus has been eliminated. 

The failure model is shown in equations (4-9a) and (4-9b). 

when 12,100 0.004 8.1

 ≤



 



 C = C

V ε (4-9a) 

when 12,100 0.002428 14.21

 >



 



 C = C

V ε (4-9b) 

Figure 4-4 shows a comparison of failure models in LEDFAA 1.2 and LEDFAA 1.3.

0.0001

0.001

0.01

1 10 100 1,000 10,000 100,000 1,000,000

Coverages

Subgrade Vertical Strain

LEDFAA-1.3 (NEW)

LEDFAA-1.2 (E=5K)

LEDFAA-1.2 (E=10K)

LEDFAA-1.2 (E=15K)

FIGURE 4-4. COMPARISON OF SUBGRADE FAILURE MODELS IN LEDFAA 

4-6 
4.2 RIGID PAVEMENTS.

4.2.1 Major Changes in Failure Models. 

In AC 150/5320-6D [1], the following failure model equations are used: 

5000 10 ( 5000)

1) / 0.15603

1.3 (

= × > − × COV Coverage

RF

σ (4-10a) 

5000 10 ( 5000)

1) / 0.07058

1.3 (

= × ≤ − × COV Coverage

RF

σ (4-10b) 

where 

σ = the working stress in the design 

R = flexural strength 

COV = coverage of the load 

A table of the pass-to-coverage ratios used for rigid pavement design is provided in AC 

150/5320-6D. 

Pavement life, defined in section 2, can be expressed by the number of coverages. Therefore, in 

the failure model in AC150/5320-6D, the pavement life is a function of R/σ—the ratio between 

concrete flexural strength and the working stress used in design. 

AC 150/5320-6D was recently revised to incorporate the contents of AC 150/5320-16, Airport 

Pavement Design for the Boeing 777 Airplane. The elastic multiple-layer model is used instead

of the semi-infinite thin plate on the dense liquid foundation model. The failure model adopted 

was based on the following model developed by Rollings [39]. 

0.002269

0.2967 (0.3881 0.000039 ) SCI Log10COV R

SCI

F − − + × × = σ (4-11) 

Where, RF is still defined the same as in equation 4-10. σ is also the working stress in a design

that equals the maximum interior stress calculated by using a multiple elastic layer model. 

Equation 4-11 has been replaced by equation 4-12 in LEDFAA (AC 150/5320-16 and AC 

150/5320-6D, Change 3, 2004): 

0.002269

0.2967 (0.3881 0.000039 ) F F SCI Log10COV R

SCI

S SC

F − − × + × × × = σ (4-12a) 

S

S

SC F

F

F

×

− × = 0.0039

0.392 0.3881 (4-12b) 

4-7 
Where Fs and Fsc are two factors introduced to provide compensation for the new model. Fsc is 

defined by equation 4-12b to maintain the number of coverages for SCI = 100 using equations 4-

11 and 4-12 for any given value of R/σ where σ is the working stress (rather than the critical 

stress directly calculated by the structural model). 

Two major differences exist between equations 4-10 and 4-12: 

• A different structural model is used. The elastic multiple layer system is used in 

calculating the critical interior stress in equation 4-12 rather than in a thin semi-infinite 

large plate on the dense liquid model in equation 4-10. 

• The new failure model is adjusted by adjusting the working stress. The adjustments are 

normally implemented in three steps. The first step is to appropriately consider the

contribution of the subbase layer under the PCC slab. A modification will be made if one 

of the following conditions is true: (1) a stabilized subbase with thickness thicker than 4 

inches or with modulus other than 500,000 psi or (2) a crushed aggregate subbase with a 

thickness other than 8 inches. The second step is to consider the difference between edge 

and interior stress. The third step is for comprehensively compensating the different 

formats of failure models. This three-step adjustment is presented in figure 4-5. 

50

60

70

80

90

100

110

10 100 1000 10000 100000 1000000

Log N

SCI

Modification Step I

stabilized 

compensation

Modification Step III:

failure model calibration

Modification Step II: Final Model in LEDFAA

from interior to

equivalent edge stress

Original WES Model:

interior stress

FIGURE 4-5. THREE-STEP ADJUSTMENT FROM WES TO LEDFAA FAILURE MODELS

One additional parameter, SCI, is introduced into the new model to define the pavement

structural condition during its service. SCI is assumed to remain at 100 from a new pavement 

until the first full-depth crack is observed. After that, SCI is assumed to continuously decrease 

4-8 
with six observed distresses. The six distresses for calculating the reduction of SCI proposed by 

Rollings [39], page 30, are 

• Corner break 

• Longitudinal, transverse, and diagonal cracking 

• Shattered slabs 

• Shrinkage cracks (cracking partial width of the slab) 

• Spalling along the joints

• Spalling at the corner 

Many distresses due to nonload factors, such as D cracking, joint seal damage, patching, scaling, 

crazing, and map crazing, are not counted in calculating the SCI reduction. When equations 4-11 

and 4-12 are used for pavement thickness design, SCI = 80 is defined as the end of pavement life

and corresponds to 50% of the slabs having been cracked. However, the comprehensive PCI 

would be more or less lower than 80 since the PCI of 100 would subtract the reduction values 

contributed from the listed six distresses, and from other distresses due to nonload factors (see 

ASTM D 5340). 

It should be noted that when equation 4-10 was used (1978), the quantitative definition to define 

the end of pavement life was not clear. The new FAA design specifications (after 1995) 

introduced a quantitative definition of the end of pavement life with SCI = 80. The advantages 

of introducing SCI in the design procedure include 

• The use of SCI differentiates the two types of distresses in design: one is structuralrelated due to loads, and the other is non-structural-related due to nonload effects. The 

main objective of the design specification is to determine the pavement structural

components under the traffic loads. 

• FAA design specifications provide design procedures for new PCC pavement and for

overlays on an existing PCC pavement. Conceptually, the condition of the existing 

pavement prior to being overlaid must be defined before an overlay thickness is 

determined. AC 150/5320-6D uses an empirical method to define the existing pavement

condition factors Cr and Cb. Experienced pavement engineers who observe the pavement

surface distresses determine the values of these factors. Although the method is rough, 

more than 20 years of experience was accumulated. However, PCI rather than SCI was 

used to collect survey data for many years. Therefore, when the SCI began to be used in 

the LEDFAA program, formulae to determine SCI was recommended. 

• The use of SCI possesses the potential to improve the design procedure in the future. 

This is particularly true when more SCI data becomes available from pavement surveys. 

It has been recognized for a long time that the same value of PCI from two deteriorated 

PCC pavements, one receiving much more structural-related distresses than the other,

needs significantly different procedures to repair. It has also been understood that there 

is an optimal period for repairing a deteriorated pavement. The use of SCI in the design

model will be helpful to better evaluate the pavement condition and get a cost-effective 

rehabilitation. 

4-9 
4.2.2 Calculation of Pass-to-Coverage Ratio.

The design procedure in AC 150/5320-16 [13] was adopted in the revised version of AC

150/5320-6D (April 30, 2004) for a traffic mix with and without B-777 aircraft. A pavement 

cross section is divided into 81 sections, each 10 inches wide. This procedure is also used for 

computing flexible pavement CDF. CDF is calculated for the center of each section using the 

following equation:

Coverage to failure

Applied Coverages CDF = (4-13)

The value of coverage to failure is obtained by using the failure model equation 4-12, and the 

value of applied coverages is the number of applied load repetitions divided by the pass-tocoverage ratio. 

Pass-to-coverage ratio is found by exactly the same procedure as was used to calculate the values 

in the table published in AC 150/5320-6D. The definition of coverage is: one coverage occurs 

at a point on the pavement when that point is covered by any tire print of the aircraft gear. Based

on the definition, the following equation is used to calculate the pass-to-coverage ratio: 

) 2 2/ 1 / Prob ( , ,

1

,

i

j k i

i

k i

m

k

i j i

W X X W P C = ∑ X − < < + =

 (4-14) 

where 

Pi = Pass number of the ith type of aircraft 

Cj,i = Coverage of the ith type aircraft at the jth strip 

Wi = Tire width of the ith type aircraft 

Xj = Distance between the centerline of the jth strip and the centerline of the pavement 

Xk, i = Distance between the centerline of the kth tire of the ith type aircraft

Prob = Calculation symbol for probability of 

m = Number of tires of the ith type aircraft

Equation 4-14 indicates that each strip of the pavement (perpendicular to the transverse line) 

receives different amounts of the maximum stress, or different amounts of coverages, Cj,i. 

Therefore, pass-to-coverage varies along the transverse surface line of the pavement and leads to 

different CDF values at each strip. The failure model in AC 150/5320-6D has only one pass-tocoverage number for each type of aircraft. 

These and other assumptions must be used to calculate the pass-to-coverage ratio. These also 

introduce uncertainties in pavement life prediction. 

4.2.3 Uncertainty of the Concrete Flexural Strength.

Differences in the flexural strength of concrete (RF) used for design and the flexural strength of 

the in-place concrete can be a significant factor in differences between the predicted life of a 

4-10 
pavement and the observed life of the pavement. Possible sources of such differences are 

differences between design requirements and in-place acceptance requirements, allowances for 

statistical variations in the strength of in-place acceptance samples, environmental conditions at 

placement and during curing, and variations in construction practices. 

AC 150/5320-6D requires that “Concrete flexural strength should be determined by ASTM C 78 

Test Method. The design flexural strength of the concrete should be based on the age and 

strength the concrete will be required to have when it is scheduled to be opened to traffic.” In 

contrast, AC 150/5370-10A requires that P-501 specification concrete be designed to achieve a 

28-day flexural strength for acceptance. The engineer has the option to designate the time period 

if the specified strength is required earlier than 28 days. The specifications, therefore, require 

that the pavement thickness design engineer make allowances for differences between the 

strength of the concrete at the time of acceptance testing and the length of time that passes after 

placement before the pavement is put into service. It is not known if this typically leads to overor underconservatism in the selection of the design strength of the concrete. 

AC 150/5370-10A also states “Higher flexural strength can be specified when local materials 

make this economically feasible. However, it must be recognized that due to variations in 

materials, operations, and testing, the average strength of concrete furnished by a supplier must 

be higher than the specified strength to insure a good statistical chance of meeting the acceptance 

criteria throughout the duration of the job.” This typically leads to a conservative selection of the 

design strength of the concrete. 

With respect to differences between the strengths of laboratory samples and field samples or the 

in-place concrete, several projects conducted by the FAA at the NAPTF verify that the values of 

compressive strength (RC) and RF measured in the laboratory under the standard conditions 

required by the appropriate ASTM test methods are significantly different than those measured 

with field-cured specimens. The variations of the compressive and flexural strengths of the 

field-cured specimens are much larger than the variations of the compressive and flexural 

strengths of the specimens cured in the laboratory. 

The findings from the FAA projects indicate that the strength of the in-place concrete in the field 

could be significantly different than the flexural strength used for thickness design. All the 

equations in section 4.2.1 show that the number of coverages to failure is very sensitive to the

ratio of RF and σ. Therefore, variations in pavement life must be influenced by the variation of

the concrete strength in the field as well as by differences between the thickness design strength

and the concrete mix design strength. 

4.2.4 Control of Top-Down Cracking.

When the critical stress in a pavement is calculated for design, only the maximum stress at the 

bottom of the slab is considered in current FAA design specifications. However, unexpected 

top-down cracking has a significant effect on the pavement life. Corner cracking is initialized

from the top of the slab due to a gear or tire load located at a corner of the slab, and if there were 

no restrictions on the joint spacing, top-down cracks would become the earliest major distresses 

observed under heavy aircraft traffic. Therefore, both AC 150/5320-6C [17] and -6D [1] set 

requirements for joint spacing to minimize the occurrence of top-down cracking. 

4-11 
A rule of thumb for joint spacing given by the Portland Cement Association was adopted in AC 

150/5320-6C [17]. As a rough guide, the joint spacing (in feet) should not exceed twice the slab

thickness (in inches). The ratio of slab length to slab width should not exceed 1.25 in unreinforced pavements. All PCC slabs were divided into three thickness groups: less than 9 

inches, between 9 to 12 inches, and thicker than 12 inches. The maximum joint spacing for 

transverse and longitudinal joints were recommended for each group (Table 3-7, AC 150/5320-

6C, [17]). 

The risk of top-down cracking was more noticeable by analyzing surveyed data [37]. It had also 

been noticed that smaller slabs reduced the risk of top-down cracking. However, large slabs (25 

by 25 ft) were favored by construction techniques (requiring lower maintenance costs for 

repairing the joint-related distress). Therefore, a balance between the risk of top-down cracking

and favorable slab size from a construction and maintenance viewpoint is necessary. It was also 

recognized that PCC pavements supported by a stabilized subbase are subject to higher warping 

and curling stresses than those supported on unstabilized foundations. Therefore, smaller slabs 

were recommended for pavements supported on a stabilized subbase. 

Two minor modifications to AC 150/5320-6C were effected in AC 150/5320-6D. One more

group was added to Table 3-7, requiring the use of smaller slabs (12.5 by 12.5 ft) for very thin 

slabs (6 inches). The requirements of Table 3-7 in AC 150/5320-6C were only recommended for 

PCC pavements without a stabilized subbase. For pavements with a stabilized subbase, it is 

required that the ratio of the joint spacing to the radius of relative stiffness be between 4 and 6.

This was modified in Change 2 of AC 150/5320-6D to require that the ratio of the joint spacing 

to the radius of relative stiffness be 5.0 or less. The radius of relative stiffness (l) is calculated by 

the following equation. 

4

2

3

12 (1 ) k

E h l

× − ×

× = μ

 (4-15) 

where 

E = modulus of elasticity of the concrete, usually 4 million psi 

h = slab thickness, inch

μ = Poisson’s ratio of concrete, usually 0.15 

k = modulus of subgrade reaction, psi 

These requirements are the only provisions in the rigid pavement design procedure by which the 

problem of top-down cracking (such as corner cracking) caused by combined loading and curling

is addressed, since the thickness design is based only on load-induced stresses calculated at the 

bottom of an assumed flat slab. 

 

4-12 
4.3 SUMMARY.

The failure model is an integral part of any pavement design procedure. The pavement responses 

are related to the allowable number of load repetitions using statistical and empirical transfer 

functions. Different failure models and their development over the years have been discussed. 

Generally, the predicted and actual pavement performance have not always compared favorably 

in the past. The full-scale test results and improvement in structural models (used for predicting 

critical pavement responses) are used for improving the failure models. The shift factors are 

used in various transfer functions to adjust the structural model calculated response to more 

realistically reflect field-observed pavement distress and performance. The pavement responses 

used in the failure models are for a given set of conditions (climate, pavement structure, material 

properties, aircraft loading, etc.). Pavement responses will vary as any of the above-mentioned

conditions vary. The mechanistic design procedures must account for the effect of varying 

inputs to the structural model. 

4-13/4-14 
5. SENSITIVITY OF INPUTS TO PAVEMENT LIFE.

Pavement design is a stochastic process. The thickness design and resultant pavement life can be 

significantly affected by the choice of the design inputs. Since it is impossible to precisely 

define the absolute value of the primary design variables, there is always inherent uncertainty

concerning the robustness of the design. This section discusses the sensitivity of the design 

inputs to computed pavement thickness and resultant pavement life for existing design models

for airport pavements contained in AC 150/5320-6D and LEDFAA. 

5.1 CONCEPTS AND FORMULAS.

A sensitivity study is a powerful tool to clarify, verify, quantitatively understand, and compare

existing airport pavement design specifications. This tool was used in developing the computer

program LEDFAA [29]. In 1995, a sensitivity study [41] was conducted to determine the input 

data for a model developed by WES, Corps of Engineers [42, 43, and 39] and made the design 

thickness comparable to those obtained by the FAA design procedure AC 150/5320-6C [17]. Two 

other FAA reports [44 and 45] have been drafted but not yet published. In these two reports, the

concepts, derivation, and numerical results of the sensitivity for selected parameters in the FAA 

airport pavement design models up to 1994 were presented. 

Each pavement design needs many input parameters. When a design model is determined, different

input parameters have different influences on the pavement life. Some parameters are insensitive to 

the effects on pavement life; however, for some parameters; a small change will lead to a significant 

change in pavement life. The effects of many parameters on pavement life were qualitatively

discussed in sections 3 and 4. 

The sensitivity of pavement life, L, to any variable x is defined as 

L

x

x

L Sx L ∂∂ , = (5-1) 

This equation is used for both HMA and PCC pavement sensitivity analysis. In cases where a 

pavement is subjected to only one type of aircraft, the pavement life is directly related to the 

number of passes of that aircraft. Therefore, L in equation 5-1 is related to the number of passes 

of aircraft N. However, for a pavement under a mixed traffic, the explicit relationship between 

the pavement life and the number of passes of an aircraft does not exist. Numerical analysis

becomes necessary to calculate the sensitivity of x. In this case, the following approximate 

equation should be used: 

2 [ ]

[ ] [ ] , L x

x

x

L x x L x x

L

x

x

L Sx L ×

× ∆

+ ∆ − − ∆

≅

∂

∂ = (5-2)

5-1 
where 

x = value of the variable 

∆x = A very small quantity that is significantly smaller than the

value of x

L(x+∆x), L(x,) and L(x-∆x) = Value of L at x = x+∆x, x and x-∆x, respectively

After values of x and ∆x are selected for a parameter, L(x+∆x), L(x), and L(x-∆x) can be 

calculated. Then the sensitivity can be calculated by substituting the results into equation 5-2. 

A positive value of Sx,L indicates that the pavement life increases when x increases, and a 

negative value indicates that the pavement life decreases when x increases. A high magnitude of 

Sx,L indicates that x is sensitive to L, no matter if it is positive or negative. The above definition 

of the sensitivity is a nondimensional quantity; therefore, it is independent of the unit of any 

variable and makes sensitivity of different variables comparable to each other. 

From equation 5-1, the sensitivity is not only a function of the derivative of pavement life to the

variable it is also a function of the values of the pavement life and the value of the variable. That 

is, the sensitivity varies with different pavement structures under different aircraft even if the

pavement is designed using the same failure model. 

5.2 FLEXIBLE PAVEMENTS.

Flexible pavement design is based on the vertical strain at the top of the subgrade. The number 

of coverages to failure is related to the subgrade strain, as follows: 

when 12,100 0.004 8.1≤ 













 C = C

v ε

when 12,100 0.002428 14.21> 













 C = C

v ε

where 

C = the number of coverages to failure 

ε v = the vertical strain at the top of the subgrade 

Subgrade vertical strain is computed using elastic layer theory and is affected by the stiffness 

(modulus) and thickness of layers above the subgrade. Equation 5-2 is used to compute the

sensitivity of input parameters like P-401 HMA modulus, P-401 HMA thickness, P-209 base 

thickness, and the gross aircraft weight. The sensitivity of these parameters was studied for three

pavement structures designed using LEDFAA for three types of subgrades—low strength

(CBR-3), medium strength (CBR-8), and high strength (CBR-15). The aircraft mix (anticipated

at John F. Kennedy International Airport (JFK)) used for the pavement design is shown in table

5-1. 

5-2 
TABLE 5-1. AIRCRAFT MIX USED FOR PAVEMENT DESIGN 

No. Name

Gross Weight 

(lb) Annual Departures

Percent Annual 

Growth 

1 A300-600 375,900 3,838 0

2 A320 162,000 15,101 0

3 A330 507,000 1,015 0

4 B-757 270,000 7,544 0

5 B-737-800 174,200 1,561 0

6 B-747-200 833,000 2,207 0

7 B-747-400 873,000 8,519 0

8 B-767-200 335,000 6,178 0

9 B-767-300ER 409,000 9,635 0

10 B-777-200 632,500 3,111 0

11 Concorde 410,000 406 0

12 Fokker F100 100,000 12,117 0

13 DC-9-32 121,000 569 0

14 DC-9-51 121,000 488 0

15 A340-500/600 750,000 2,441 0

16 A340-500/600 Belly 750,000 2,441 0

17 A380-800 1,340,000 5,475 0

18 B-747-SP 696,000 3 0

19 DC-8 358,000 504 0

20 MD-11 621,000 3,315 0

21 MD-11 Belly 621,000 3,315 0

Figure 5-1 shows the pavement designs for the three subgrade types. Table 5-2 and figure 5-2 

summarize the results from the sensitivity analyses. 

FIGURE 5-1. PAVEMENT SECTIONS FOR SENSITIVITY ANALYSIS 

5-3 
TABLE 5-2. RESULTS FROM SENSITIVITY ANALYSES

CBR x ∆x x-∆x x+∆x L(x) L(x+∆x) L(x-∆x) Sx,L

Sensitivity to HMA Modulus (ksi) 

3 200 20 180 220 20 20.9 19.1 0.4500

8 200 20 180 220 20 22.3 17.8 1.1250

15 200 20 180 220 20 24.1 16.3 1.9500

Sensitivity to HMA Thickness (in.) 

3 5 0.5 4.5 5.5 20 22.0 18.2 0.9500

8 5 0.5 4.5 5.5 20 25.2 15.9 2.3250

15 5 0.5 4.5 5.5 20 31.7 12.4 4.8250

Sensitivity to P-209 Thickness (in.) 

3 63 1.26 61.74 64.26 17.9 23.2 13.9 12.9888

8 17 0.34 16.66 17.34 17.3 19.2 15.6 5.2023

15 13 0.26 12.74 13.26 16.1 19.2 13.5 8.8509

Sensitivity to B-737 Gross Weight (lb) 

3 173,000 17,300 1,55,700 190,300 20 7.5 88 -20.1250

8 173,000 17,300 1,55,700 190,300 20 8.1 83.5 -18.8500

15 173,000 17,300 1,55,700 190,300 20 9.0 74.9 -16.4750

Sensitivity to A380 Gross Weight (lb) 

3 1,300,000 130,000 1,170,000 1,430,000 30 8.4 124.4 -19.3333

8 1,300,000 130,000 1,170,000 1,430,000 30.2 8.3 127.9 -19.8013

15 1,300,000 130,000 1,170,000 1,430,000 28.9 9.1 105.7 -16.7128

5.2.1 Sensitivity of Pavement Life to HMA Modulus.

Of all the parameters studied in sensitivity analysis, the HMA modulus had the lowest 

sensitivity, as shown in figure 5-2. The sensitivity of pavement life to HMA modulus increases

with the increase in subgrade strength (CBR). 

5.2.2 Sensitivity of Pavement Life to HMA Thickness.

Figure 5-2 shows that the sensitivity of pavement life to HMA thickness is higher than the

sensitivity to HMA modulus, but comparatively lower when compared to other parameters

studied. For the pavements on high-strength subgrade, the sensitivity of pavement life to HMA 

thickness was higher when compared to the low-strength subgrade. 

5-4 
0.45 1.13 1.95 0.95

2.33

4.83

12.99

5.20

8.85

-20.13

-18.85

-16.48

-19.33 -19.80

-16.71

-25

-20

-15

-10

-5

0

5

10

15

20

3 8 15

Subgrade CBR of Pavements

Sensitivity Index

AC Modulus AC Thickness P-209 Thickness

B-737 Gross Weight A-380 Gross Weight

FIGURE 5-2. SUMMARY OF RESULTS FROM SENSITIVITY ANALYSES 

5.2.3 Sensitivity of Pavement Life to P-209 Base Thickness.

Pavement life is more sensitive to P-209 base thickness when compared to the HMA modulus

and HMA thickness. The sensitivity decreases with the increase in subgrade strength. Figure 5-

2 shows that the sensitivity is higher for high-strength subgrade compared to medium-strength

subgrade. The P-401 stabilized base thickness for the pavements on low- and medium-strength 

subgrade is 12 inches (thickness is based on minimum thickness requirements, as per AC 

150/5320-6D). However, for the pavement on high-strength subgrade, the P-401 stabilized base 

thickness is 6 inches because of minimum thickness requirements for P-209 subbase. This is the 

reason for greater P-209 thickness sensitivity on high-strength pavement compared to the lowstrength pavement. 

5.2.4 Sensitivity of Pavement Life to Aircraft Gross Weight.

Two aircraft, one heavy A380 (gross weight 1,300,000 lb) and one light B-737 (gross weight 

173,000 lb) were considered for the sensitivity analysis. Three pavements (one each on low-, 

medium-, and high-strength subgrade) were designed using LEDFAA 1.3 for each of the two

aircraft, and the sensitivity of gross weight on pavement life was studied. As shown in figure 5-

2, aircraft gross weight has the highest sensitivity on pavement life. Also, since the sensitivity is

negative, it means that pavement life reduces with increase in aircraft gross weight. 

 

5-5 
5.3 RIGID PAVEMENTS.

Two types of sensitivities are of most interest in the sensitivity analysis for PCC pavement 

design. The first type uses the failure model in which the explicit relation between the pavement 

life indicator (coverage) and the pavement failure indicator (design factor R/σ) exists. Therefore, 

the closed-form solution of the sensitivity can be obtained. Since the failure model was 

developed based on full-scale tests plus engineer’s experience, it is generally an empirical 

equation. Therefore, the sensitivity has to be affected by the format and values of parameters 

used in the model. 

The second type of sensitivity was analyzed through the critical stress calculated using a specific 

response model (i.e., a mechanistic-based procedure). The sensitivity results will be affected by 

both failure and response models. Since the critical response of a pavement is calculated with a 

computer program, the sensitivity is also calculated with the design program, and only numerical 

results are available. 

Though equations 4-12a and 4-12b are failure models, the parameters FS and FSC are functions of

the material properties and thickness of the layer under the slab. They are also functions of the

subgrade modulus. The sensitivity of design factor R/σ cannot be directly and simply derived by 

taking a derivative for equation 4-12. Therefore, the closed-form solutions of the sensitivity of 

R/σ based on equation 4-10 (model in AC 150/5320-6D) and equation 3-2 (the FAA mechanistic

failure model) and the numerical results for equation 4-11 are presented separately. 

 

5.3.1 Sensitivity of Design Factor R/σ.

Since the closed-form equation exists between the design factor and the number of coverages, 

the closed-form solution of sensitivities of the design factors R/σ on the coverage may be derived 

by using equation 5-1 for the design procedure in AC 150/5320-6D. 

5000

1.3 2 0.15063

(10)

/ , × >

× × = for COV Ln R S R COV

σ σ (5-3a) 

5000

1.3 2 0.07058

(10)

/ , × ≤

× × = for COV Ln R S R COV

σ σ (5-3b) 

Similarly, the closed-form solution of sensitivity based on the design procedure is 

σ σ

R

SCI

Ln S R COV ×

+ × = 0.3881 0.000039

(10)

/ , (5-4) 

Figure 5-3 illustrates the relationship between the sensitivity and the design factor where SCI = 

80 has been used to indicate the end of pavement structural life. 

5-6 
0

4

8

12

16

20

0 0.4 0.8 1.2 1.6 2 2.4

R/Sigma

6D

WES

FIGURE 5-3. THE SENSITIVITIES OF DESIGN FACTOR R/σ VS R/σ

The two sensitivity curves of R/σ (calculated from equations 5-3a, 5-3b, and 5-4) are presented

in figure 5-3; however, they are not completely comparable. The flexural strength R used in the

two models follows the same standard test method ASTM C 78, the critical stress σ calculated

by using two different models: plate on dense liquid foundation in AC 150/5320-6D and the

elastic multiple-layer system for the WES models. The more important difference is that

calculated critical stress is at different locations: free edge for AC 150/5320-6D and interior for

the WES models. 

The two failure models are presented in figure 5-4. When the failure model of AC 150/5320-6D 

was developed, only four full test points were available for coverages > 5000. Unfortunately, for 

two of them the test pavement never failed. Therefore, 5000 coverages were selected as a point 

where two failure curves were recommended. One is for coverages lower than 5000, the other is 

for coverage higher than 5000. The second curve was probably selected because insufficient

full-scale data were available at the time. Though two curves are used, the failure model still is a 

continuous function at the point coverage = 5000. However, figure 5-3 shows that the sensitivity 

of the design factor R/σ has a significant jump at coverage = 5000. 

It can be seen in figures 5-3 and 5-4 that the failure model and sensitivity curve in the design

procedure, based on elastic multiple-layer theory, are continuous function (WES model [43 and 

39]). 

5-7 
0

0.5

1

1.5

2

2.5

1 10 100 1000 10000 100000 1000000

Coverages in Log

R/Sig

ma

6D

WES

FIGURE 5-4. FAILURE MODELS IN AC 150/5320-6D AND WES 

5.3.2 Sensitivity to Slab Thickness, Aircraft Gross Weight, Subgrade Strength, and 

Stabilized Base.

The sensitivities of the following six parameters are analyzed following equation 5-2. 

• R = Concrete flexural strength 

• Hc = Thickness of PCC slab

• Esubg = Modulus of subgrade 

• HStab = Thickness of the stabilized subbase layer 

• HSubb = Thickness of the subbase layer 

• QG = Gross weight of the aircraft 

The analysis was conducted for PCC pavements built on two types of subgrade, with a modulus 

of 4500 psi, representing a very weak subgrade, and the other with a modulus of 30,000 psi, 

representing a very strong subgrade. The analysis was also conducted for three types of aircraft 

loads: a single tire with a 55,000-lb load, a B-727 aircraft with a gross weight of 200,000 lb, and 

a B-747 aircraft with a gross weight of 800,000 lb. The annual departures were set to 1200 for

all. The LEDFAA program was used to calculate the slab thickness for 20 departure years for 

each aircraft. A rounded thickness value close to the required thickness was selected for the 

sensitivity analysis. 

After a few numerical trials, it was found that pavement design life is very sensitive to concrete 

flexural strength, slab thickness, and aircraft gross weight; therefore, the values of ∆x were 

5-8 
selected as 1% to 1.5% of the values of the parameters. If the same level of ∆x is used for

analyzing the sensitivity of the thickness of stabilized subbase, the thickness of granular subbase,

and the subgrade modulus, the difference between L(x+∆x) and L(x-∆x) will be too small to

assure the accuracy of the calculation. Therefore, higher values are used for those three 

parameters. The detailed information on the input and output data is shown in table 5-3 for the 

weak subgrade and in table 5-4 for the strong subgrade. 

The findings are summarized below. 

• The numerical analysis of sensitivity had to be done case by case. The selected six cases 

covered subgrade strengths from weak to strong and covered the single, dual, and 

dual-tandem gears. The characteristics of each parameter’s sensitivity were clearly seen 

in the numerical analysis. 

• The three parameters with the highest sensitivity (the magnitudes of the calculated 

sensitivity) in each group were slab thickness, flexural strength, and aircraft gross weight. 

• The sensitivity of subgrade strength of the weak subgrade was higher than the strong 

subgrade. 

• The sensitivity of stabilized base thickness is always higher than granular base thickness. 

The difference between the two sensitivities for a pavement built on high-strength 

subgrade was more than for a pavement built on a weak subgrade. 

5-9 
TABLE 5-3. SENSITIVITY OF DIFFERENT PARAMETERS, WEAK SUBGRADE, 

EC = 4500 psi 

Single-wheel load 55,000 lb, 1200 annual passes, design PCC thickness = 14.09′′

Parameters x ∆x L(x-∆x) L(x) L(x-∆x) Sx, L

R 700 7 19.9 17.7 15.8 11.6

Hc 14 0.2 23.2 17.7 13.6 19.0

Esubg 4,500 225 18.5 17.7 16.9 0.9

HStab 6 0.5 18.9 17.7 16.6 0.8

HSubb 10 1 18.5 17.7 17 0.4

QG 55,000 550 15.8 17.7 19.6 -10.7

B-727, 200,000 lb, 1200 annual passes, design PCC thickness = 16.46′′

Parameters x ∆x L(x-∆x) L(x) L(x-∆x) Sx, L

R 700 7 43.6 38.3 33.6 13.1

Hc 17 0.2 46.9 38.3 29.5 19.3

Esubg 4,500 225 39.7 38.3 36 1.0

HStab 6 0.5 41.2 38.3 35.3 0.9

HSubb 10 1 39 38.3 36.6 0.3

QG 200,000 2,000 33.9 38.3 43.4 -12.4

B-747, 800,000 lb, 1200 annual passes, design PCC thickness = 15.34′′

Parameters x ∆x L(x-∆x) L(x) L(x-∆x) Sx, L

R 700 7 16.2 14.4 12.8 11.8

Hc 15 0.2 17.6 14.4 11.8 15.1

Esubg 4,500 225 15.9 14.4 13 2.0

HStab 6 0.5 15.3 14.4 13.6 0.7

HSubb 10 1 15.1 14.4 13.8 0.5

QG 800,000 8,000 12.9 14.4 16.2 -11.5

TABLE 5-4. SENSITIVITY OF DIFFERENT PARAMETERS—STRONG SUBGRADE, 

EC = 30,000 psi 

Single-wheel load 55,000 lb, 1200 annual passes, design PCC thickness = 13.48′′

Parameters x ∆x L(x+∆x) L(x) L(x-∆x) Sx, L 

R 700 7 12.4 11.1 10 10.8

Hc 13 0.2 14.2 11.1 8.7 16.1

Esubg 3,0000 6,000 11.3 11.1 11 0.1

HStab 6 0.5 11.7 11.1 10.6 0.6

HSubb 10 1 11.2 11.1 11 0.1

QG 55,000 550 10.2 11.1 12.1 -8.6

B-727, 200,000 lb, 1200 annual passes, design PCC thickness = 15.36′′

Parameters x ∆x L(x+∆x) L(x) L(x-∆x) Sx, L 

R 700 7 15.4 13.7 12.2 11.7

Hc 15 0.2 16.9 13.7 11.2 15.6

Esubg 3,0000 3,000 14 13.7 13.5 0.2

HStab 6 0.5 14.3 13.7 13.2 0.5

HSubb 10 1 13.9 13.7 13.6 0.1

QG 200,000 2,000 12.4 13.7 15.3 -10.6

5-10 
TABLE 5-4. SENSITIVITY OF DIFFERENT PARAMETERS—STRONG SUBGRADE, 

EC = 30,000 psi (Continued) 

B-747, 800,000 lb, 1200 annual passes, design PCC thickness = 12.67′′

Parameters x ∆x L(x+∆x) L(x) L(x-∆x) Sx, L 

R 700 7 31.6 28 24.9 12.0

Hc 13 0.2 34.3 28 22.9 13.2

Esubg 30,000 3,000 29.5 28 26.5 0.5

HStab 6 0.5 29.6 28 26.7 0.6

HSubb 10 1 28.4 28 27.6 0.1

QG 800,000 8,000 25.3 28 31.1 -10.4

5.4 SUMMARY.

Designing the thickness of a pavement needs many input parameters and different input 

parameters have different influences on pavement life. Pavement life is insensitive to some

parameters, i.e., a small change in the parameter has only limited effects on pavement life. 

However, for other parameters, pavement life is very sensitive to changes in their values, i.e., a 

small change will lead to a significant change in pavement life. The following can be 

summarized from the results of the parametric sensitivity analysis that was presented in this

section. 

For flexible pavements, pavement life is most sensitive to aircraft gross weight and subgrade 

strength. Pavement life is more sensitive to P-209 crushed stone base thickness when compared 

to HMA modulus and HMA thickness. For rigid pavements, pavement life is most sensitive to 

slab thickness, flexural strength of the concrete, and aircraft gross weight. To compare the 

sensitivities to thickness and stiffness effects between flexible and rigid pavements, the two 

pavement types (rigid and flexible) were evaluated using the aircraft mix provided in table 5-1. 

The flexible pavement cross section (from LEDFAA 1.3) is shown in figure 5-1. Increasing the

P-401 surface thickness from 5 to 6 inches increased the predicted pavement life from 20 to 24.2 

years. Increasing the P-401 modulus from 200,000 to 300,000 psi (50 percent increase in 

modulus) at 5 inches thick also increased pavement life from 20 to 24.2 years. 

The rigid pavement cross section (from LEDFAA 1.3) is 21.37 inches of P-501 PCC surface, 6 

inches of P-306 econocrete base, and 10 inches of P-209 subbase. Rounding off the P-501 

thickness from the 20-year design thickness of 21.37 inches to 22 inches (an increase of 0.63 

inches) increased the predicted pavement life from 20 to 32.7 years. Increasing the flexural 

strength of the P-501 from 700 to 735 psi (an increase of 5 percent) increased the pavement’s 

predicted life from 20 to 40.2 years. This shows that a slight increase in thickness and flexural 

strength of P-501 has a much larger effect on pavement life compared to the changes in P-401 

thickness and stiffness. 

5-11 
The last three sections presented the factors affecting pavement life from theoretical concepts 

(structural models, failure models, and changes in input parameters to structural models). 

Generally, the predicted pavement life is different from actual pavement life because of the 

design assumptions (uncertainties in material properties, climatic conditions, changes in load

characteristics, etc.). In section 6, the field performance data from different airports is analyzed 

to study the conditions of the pavements and the distresses as the pavements approach the end of 

their design lives. 

5-12 
6. PAVEMENT STRUCTURAL LIFE ANALYSIS.

The evaluation of FAA thickness design procedures was conducted in three major steps. 

Step one was based on the models used for the thickness design. The effects of different

parameters on the pavement operational life due to the response and failure models were 

discussed in sections 3, 4, and 5. 

Step 2 focuses on the available pavement surveyed data published in technical reports and 

papers. The major sources of the published data were obtained from references 6, 37, and 46-49. 

Step 3 takes advantage of unpublished data collected from pavement condition surveys of airport 

pavements. In the past 10 years, the FAA has supported projects for collecting and analyzing 

field-surveyed data for special purposes. The major sources of the unpublished data were 

obtained from references 50 through 57. 

The analysis of steps 2 and 3 are presented in this section. 

6.1 CONCEPTS AND ASSUMPTIONS.

After reviewing the FAA design specifications in sections 3, 4, and 5, it was found that although 

a failure model can quantitatively predict the pavement structural life, many factors can affect 

pavement life. Theoretically, pavement life is a random variable rather than a deterministic

unknown that can be solved by a deterministic procedure. Therefore, an appropriate evaluation 

of the suitability and reliability of a design procedure to predict pavement life can be conducted 

based on a statistical analysis. A brief discussion follows of the two most important statistical 

parameters, the mean and the standard deviations, that influence the reliability and suitability of a 

design procedure. 

Since pavement life is a random variable, the probability density function of pavement life can 

be used to conduct the analysis. For simplification, pavement life is assumed to be subject to 

Gaussian (or normal) distribution, and its probability density function is defined in equation 6-1. 

2

2

2

( )

2

1 ( ) σ

π σ

×

− −

×

× =

L m

p L e (6-1)

where 

L = pavement life 

m = mean (average) of the pavement life 

σ = standard deviation of the pavement life. Conceptually, the value of σ indicates 

the degree of variation of pavement life. 

e = 2.71828 

p(L) = Probability density function of pavement life 

6-1 
To illustrate the effects of the parameters, three groups were selected: 

• Group I: m = 20 years and σ = 5 years 

• Group II: m = 20 years and σ = 2.5 years 

• Group III: m = 25 years and σ = 5 years 

The probability density functions are presented in figure 6-1(a). 

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 5 10 15 20 25 30 35 40

Pavement Life in Year

Probability Density

Mean=20 years, Std=5 years

Mean=20 years, Std=2.5 years

Mean=25 years, Std=5 year

Approximate 16% of

pavement life would be 

shorter than 15 years, 

and 2.3% of pavement

life would be shorter

than 10 years

Approximate 2.3% of

pavement life would be 

shorter than 15 years

Approximate 2.3% of 

pavement life would be

shorter than 15 years

(a) Probability Density Function of Pavement Life 

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35 40

Pavement Life in Year

Probability

Mean=25 years, Std=5 years

Mean=20 years, Std=5 years

Mean=20 years, Std=2.5 years

50% 

16%

16% 2.3% 

(b) Probability Function of Pavement Life 

FIGURE 6-1. EFFECTS OF MEAN AND STANDARD DEVIATION ON PAVEMENT 

LIFE—CONCEPTUAL ANALYSIS 

6-2 
The probability of pavement life being shorter than Le (end of life) years can be calculated by

following equation:

(6-2) ∫

−∝

< =

Le

prob(L Le) p(L)dL

Substituting equation 6-1 into 6-2, one can calculate the percentage of pavements shorter than

Le. The probability functions for the above three groups are presented in figure 6-1(b). 

The curve of pavement life from groups I and II have the same mean, 20 years, but different 

standard deviations. First, 50% of the pavements in these two groups will fail equal to or earlier 

than they are expected. Second, figure 6-1(b) indicates that about 16% of the pavements in 

group I will fail within 15 years and about 2.3% of the pavements will fail within 10 years.

However, only 2.3% of the pavements in group II will fail within 15 years. Figure 6-1(b) also 

indicates that if the average of pavement life increases from 20 to 25 years and the standard 

deviation remains the same, the structurally failed pavements will drop from 50% to 16% at the 

end of 20 years. Two findings may be summarized: 

• When the means of the two groups of pavements are the same, the pavements with 

smaller standard deviations (pavements in group II) are more reliable than those with 

larger standard deviations (in group I). 

• When the standard deviations are the same, such as the pavements in groups I and III, the 

pavements with higher mean will be more reliable. 

In general, the mean value of pavement life is controlled by the requirements of AC 150/5320-

6D for design and by the materials preparation, construction, and maintenance quality control 

requirements of AC 150/5370-10A, AC 150/5320-12, AC 5360-8, and 14 CFR 134. 

As discussed in sections 3, 4, and 5, pavement life is a random variable that has a relatively large 

standard deviation. Therefore, one should not expect that a single population containing 

pavements built in different environmental and geographic regions would have a very low 

standard deviation. The curve drawn from the pavement lives in group III shows that when the 

mean of pavement life increases to 25 years, only about 2.3% of the pavements fail within 15 

years and 16% of the pavements fail within 20 years. 

This is reasonable if the intent of the design procedure is to ensure that airport pavements 

substantially comply with the 20-year life requirement; the average life must be greater than 20 

years. An increase in the average above 20 years will depend on the magnitude of the standard

deviation (i.e., variability) of observed pavement life. 

6.2 ANALYSIS PROCEDURES AND METHODS.

The evaluation of the suitability and reliability of pavement life determined by FAA design 

specifications cannot be conducted with theoretical analysis. Instead, a statistical analysis of 

field data must be used since 

6-3 
• many assumptions must be used in theoretical analysis, bringing unexpected errors into 

the analytical results. Statistical analysis does not use assumptions. 

• factors which can neither completely nor appropriately be considered in a theoretical

analysis can be considered in a statistical analysis because the data are collected from 

operational pavements experiencing the effects of the factors.

• statistical errors may be reduced to a low level if the number of samples is sufficiently

large. 

In 1990, 7964 airports in the United States had paved runways [52]. For such a large number of 

samples, one can estimate the probabilistic properties by analyzing the data in a selected sample 

group. Two parameters are often used to define the statistical properties of the data are 

mean, expressed as 

n

X

n

i

i

X

∑= = 1 μ (6-3)

where 

xi = values of samples selected 

μx = sample mean 

η = selected number of samples 

and standard deviation, expressed as 

σ

μ

X

i

i

n

X

n =

−

−

=

∑( ) 2

1

1 (6-4)

In the above two equations, n is the sample size selected from the population. The mean of the

data indicates the average value of the data, and the standard deviation of the data indicates how

the data are distributed around the mean. 

If the total number of samples, N, in the population is greater than the selected number of 

samples n for analysis (if N > n), namely, only a portion of the pavement data are used, the mean 

of the selected samples will also be a random variable, Y (reference 58, section 10.1.3). The 

mean of Y is expressed as 

μ Y = μ X (6-5)

where 

N = total number of samples 

Y = random variable 

6-4 
This indicates that the mean of the selected data equals the mean of all the data. 

The standard deviation of Y is expressed as 

σ σ Y X

N n

n N

= × −

× − ( )1 (6-6)

Equation 6-6 indicates that the more data used, the less variation and more reliable the statistical 

analysis will be. 

As discussed in section 2, PCI is an index used to describe the pavement condition as affected by 

traffic loads, environment, and other sources. The PCI is calculated by equation 6-7 (reference 

39, page 27): 

) (6-7) 

)

100 ( , , 1 1

j i j

m

i

n

j

PCI a f Ti S D i

∑∑= =

= − ×

where 

a = adjustment factor (see ASTM D 5340) 

m = total number of distress type 

ni = total number of severity levels for the ith distress 

f (Ti, Sj, Dij) = deduct value for distress type Ti, at the severity level Sj, existing at 

density Dij

Another index, SCI, can be calculated by equation 6-8: 

100 ( , , (6-8) 

1 1

j i j

m

i

n

j

SCI a f Ti S D S i

∑∑= =

= − ×

The value of m in equation 6-7 is always equal to or greater than the value of mS in equation 6-8 

since mS is the types of distresses related to the pavement structural failure and m is the number

of all distress types. Therefore 

(6-9)

DE DM DC DO all other deducts

PCI SCI all other deducts

= + + +

= −

where 

DE = deducts due to environment-related distresses such as raveling and weathering in 

HMA pavements 

DM = deducts due to material-related distresses such as popouts in PCC pavements 

6-5 
DC = deducts due to construction-related distress such as bleeding in HMA pavements 

DO = deducts due to operations- and maintenance-related distresses such as patching or 

utility cuts in PCC pavement 

The above statements may conceptually be used for both HMA and PCC pavements. The use of 

SCI = 80 to define the end of structural life of rigid pavements is well established. However, for

flexible pavements, it is a new concept and has not been presented before. ASTM D 5340 uses 

16 distresses for calculating the PCI value for HMA and 15 distresses for calculating the PCI 

value for PCC pavements. Most of the distresses are not solely structurally related, and they are 

contributed by multiple sources or by the interaction among two or more sources. Therefore,

there is no unique definition of the distresses for SCI calculation. 

In this study, the following six distresses used by Rollings [39] are used to calculate the deducts

of SCI for PCC pavements. 

• Corner break 

• Longitudinal, transverse, and diagonal cracks 

• Shattered slabs 

• Shrinkage cracks 

• Spalling along joints

• Spalling corners 

Two distresses, alligator cracking and rutting, were used to calculate the SCI deducts for HMA 

pavements. 

If an adequate quantity of surveyed pavements with detailed information on the above distresses 

is available and SCI = 80 is defined as the end of pavement structural life, curves similar to the 

curves in figures 6-1(a) and 6-1(b) can be generated. 

Because of the time limitation for preparing this report, only distress survey data readily 

available to the authors was used. All of this data was collected before 1996, and although all of 

the distress survey reports contained PCI information, only a portion of them had detailed 

information on the types of distresses, with densities, suitable for calculating SCI values. 

Therefore, PCI had to be used for pavement operational life analysis in the following sections.

However, equation 6-9 indicates that SCI is always greater than or equal to PCI. For example, if

a group of pavements aged from 17 to 23 years has a mean PCI value of 80, one may easily 

conclude that the mean SCI value of the pavements must be greater than 80. Although the exact 

mean of pavement life based on SCI = 80 cannot be obtained using the existing data, the 

conclusion can still be made that the mean of pavement life is higher than 20 years for this 

example. Furthermore, by establishing a relationship between SCI and PCI from those survey 

reports containing sufficient information to compute SCI values, it is possible to make a better 

estimate of the mean pavement life for SCI = 80 than simply saying that it is higher than that 

given by PCI alone. 

6-6 
Theoretically, pavement structural life is defined as a pavement service period in years from the

time the pavement is put into service until the time the value of SCI = 80 is reached. However, 

in practice, the end of pavement life is determined by the airport operator, or by other external 

factors, and may occur when SCI is significantly lower than 80, or when PCI has reached a 

threshold value requiring remediation even though the SCI is still significantly above 80. In 

addition, threshold values are different for pavements with different functional purposes 

(runway, taxiway, or apron) and when the pavements are located in different environmental or 

geographical regions. The threshold value also depends on the airport size: large, medium, or 

small hub, or general aviation airport. 

6.3 DESCRIPTION OF SURVEYED HMA PAVEMENTS.

Eckrose/Green Associates provided all the field data used in this section of the report. The 

detailed procedures for surveying pavements may be found in reference 59. The data represent 

30 airports from 10 states. There are a total of 2423 features with a combined area of 161 

million square feet (msf), which is equivalent to the area of 107 standard runways (10,000 feet

by 150 feet = 1.5 msf). Table 6-1 shows the distribution of surveyed pavements by state. 

TABLE 6-1. GENERAL INFORMATION OF SURVEYED HMA PAVEMENTS

State 

Airport 

ID 

Number of 

Features 

Airport 

Size 

No. of 

HMA 

Runways 

Total Area 

(msf) 

Area of MHA 

Runways 

(msf) 

MA BOS 294 Large 5 18.94 5.52

TX ELP 136 Medium 3 10.82 3.42

TX HOU 54 Medium 1 2.38 1.14

TX IAH 70 Large 3 4.98 3.51

FL DAB 112 Small 3 6.4 2.79

FL FLL 179 Medium 3 10.79 2.92

FL GNV 76 Commuter 2 4.14 1.54

FL JAX 8 Medium 1 0.3 1.5

FL MLB 91 Small 3 6.3 2.55

FL PBI 245 Medium 3 25.68 2.47

FL PFN 88 Commuter 2 3.96 1.68

FL PIE 114 Small 4 5.19 3.17

FL PNS 80 Small 2 4.0 1.76

FL RSW 35 Medium 1 4.35 1.80

FL SRQ 63 Small 2 4.21 1.8 

FL TLH 71 Small 2 5.77 2.11

FL TPA 63 Medium 3 4.4 2.3

IN EVV 39 Commuter 3 2.73 2.32

IN FWA 52 Small 2 3.04 3.08

MN DLH 46 Commuter 1 2.66 0.85

TN BNA 97 Medium 1 6.34 1.65

ND BIS 49 Commuter 3 4.2 2.1

6-7 
TABLE 6-1. GENERAL INFORMATION OF SURVEYED HMA PAVEMENTS (Continued) 

State 

Airport 

ID 

Number of 

Features 

Airport 

Size 

No. of 

HMA 

Runways 

Total Area 

(msf) 

Area of MHA 

Runways 

(msf) 

ND FAR 13 Commuter 1 0.57 0.63

SD FSD 26 Small 1 2.15 1.0

SD RAP 64 Commuter 2 3.11 1.58

OK TUL 59 Medium 1 2.77 0.92

WI GRB 20 Small 1 0.83 0.24

WI LSE 78 Commuter 3 5.18 2.87

WI MKE 62 Medium 3 2.89 3.46

WI MSN 39 Small 2 2.86 0.76

TOTAL 2423 67 161.94 63.44

Table 6-2 categorizes the surveyed data by airport size and area in relation to the total area in the

United States. 

TABLE 6-2. GENERAL INFORMATION ON HMA RUNWAYS IN THE U.S. 

Airport Size 

Total Area in U.S. 

(msf) 

Surveyed Area 

(msf) Percentage 

Large Hub 88.7 9.03 10.2 

Medium Hub 91.6 21.6 23.6 

Small Hub 164.4 19.3 11.7 

Total Hubs 344.7 49.93 14.5 

Commuter 405.9 13.6 3.4

6.3.1 Analysis of Available Data for HMA Pavements.

6.3.1.1 General Information. 

Figure 6-2 shows the average PCI for runways, taxiways, and aprons without differentiating the

overlaid and non-overlaid pavements. 

Figure 6-2 shows that a higher PCI is observed for younger pavements. The average PCI of 

pavements less than or equal to 15 years old are similar in all three groups. However, for the 

pavements older than 15 years, runways had the highest PCI and aprons had the lowest PCI. 

If the pavements are separated into pavements with no overlay and pavements with an overlay, 

the average PCI seems to be independent of the number of overlays for the same functional 

pavement type (runway, taxiway, or apron) in different age groups. The results are shown in 

figures 6-3, 6-4, and 6-5 for runways, taxiways, and aprons, respectively, and are reported from

the original (new pavement) construction date or from the date of last overlay (if the pavement 

was overlaid). 

6-8 
FIGURE 6-2. AVERAGE PCI FOR HMA RUNWAYS, TAXIWAYS, AND APRONS 

FIGURE 6-3. AVERAGE PCI FOR HMA RUNWAYS 

6-9 
FIGURE 6-4. AVERAGE PCI FOR HMA TAXIWAYS 

FIGURE 6-5. AVERAGE PCI FOR HMA APRONS 

6-10 
6.3.2 Distress Analysis.

6.3.2.1 Distress Groups. 

AC 150/5380-6A [60] requires that PCI be determined based on the guidelines and procedures in 

ASTM D 5340 [3] for both PCC and HMA pavements. Sixteen distresses are identified for 

HMA pavements (discussed in section 2). For the purpose of this report, the distresses were 

divided into five groups as follows: 

• Group-1: Cracking—Includes longitudinal and transverse cracks, alligator (or fatigue)

cracking, block cracking, slippage cracking, and reflection cracking. 

• Group-2: Disintegration—Includes raveling and weathering. 

• Group-3: Distortion—Includes rutting, corrugation, shoving, depression, and swelling. 

• Group-4: Loss of Skid Resistance— Includes bleeding, polished aggregate, and fuel 

spillage. 

• Group-5: Others—Includes jet blast and patching distresses. 

The accumulated deduct values due to distresses in a group are defined as the reduction of PCI 

for the group. Figure 6-6 shows the importance of different distress groups in HMA pavements. 

For example, about 82 percent of the group PCI reduction for runway pavements were

contributed by distress group 1 (cracking), and 8.1 percent of it was contributed by distress group 

3 (distortion). 

FIGURE 6-6. PAVEMENT CONDITION INDEX REDUCTION BY DISTRESS GROUPS 

FOR HMA PAVEMENTS 

6-11 
Figure 6-6 shows that 

• Group 1 distresses (cracking) are the main source of PCI reduction for all three types of 

flexible pavements (runways-82.2%, taxiways-71.8%, and aprons-72%). 

• Group 3 distresses (distortion) are the second most important reason for PCI reduction. 

The PCI reduction for runways, taxiways, and aprons are 8.1, 17.1, and 16.9 percent, 

respectively, or one-tenth to one-fifth of group 1 distresses. 

• The runways have more cracks than taxiways and aprons, whereas taxiways and aprons

have more distortion distresses, such as rutting. 

• The loss of skid resistance (group 4) causes PCI reductions of 1.6, 3.1, and 1.9 percent 

for the runways, taxiways, and aprons, respectively. 

• Disintegration distresses (group 2) PCI reductions for runways, taxiways, and aprons are 

6.6, 5.4, and 6.6 percent, respectively. 

• The PCI values reduced by jet blast and patching (group 5) distresses for the three types

of pavements are 1.5, 2.6, and 2.6 percent, respectively. 

6.3.2.2 Individual Distresses.

Table 6-3 shows the PCI reduction from the total deduct PCI value by the 16 distresses. Only 5 

distresses among the 16 caused PCI reductions higher than 2. Each of the five distresses cause 

the PCI deduct values to be higher than 5 or 10 percent or more of the total surveyed area. They 

are longitudinal and transverse cracking, alligator cracking, rutting, block cracking, and raveling. 

The deduct value used to calculate the PCI of pavements not only depends on the sum of the

deduct values of all distresses observed, but also on the number of distresses that cause deduct 

values of PCI greater than 5. Therefore, the above five distresses may be defined as the most 

often observed and influential distresses among the sixteen. The total percent of PCI reduction 

attributed to these five distresses is 88.4 percent for all pavement types (runways, taxiways, and 

aprons). 

Table 6-4 shows the effect of six major distresses on different pavement groups (runways, 

taxiways, and aprons). 

6-12 
TABLE 6-3. PAVEMENT CONDITION INDEX REDUCTION BY INDIVIDUAL

DISTRESSES 

Distress 

PCI 

Deduct 

PCI Deduct by Percent 

of Total PCI 

Percent of Area 

with PCI 

Deduct > 5 

Alligator Cracking 12.27 29.69 43.8 

Bleeding 0.81 1.96 3.3

Block Cracking 2.91 7.04 10.6 

Corrugation 0.01 0.02 0.1

Depression 0.81 1.95 5.1

Jet Blast 0.01 0.02 0.0 

Joint Reflection Cracking 1.61 3.89 6.8 

Longitudinal and Transverse Cracking 14.16 34.25 62.1 

Oil Spillage 0.13 0.32 0.2 

Patching 0.93 2.26 5.2

Polished Aggregate 0.01 0.03 0.2 

Raveling and Weathering 2.52 6.08 11.5 

Rutting 4.62 11.17 20.7

Shoving from PCC 0.0 0.0 0.0 

Slippage Cracking 0.05 0.12 0.4 

Swell 0.49 1.18 3.2

TABLE 6-4. PAVEMENT CONDITION INDEX REDUCTION BY SIX 

MAJOR DISTRESSES

PCI Reduction

Long./

Tran. 

Cracks 

Alligator

Cracking 

Rutting Block 

Cracking 

Raveling 

Joint 

Reflection 

Cracks 

Area

(msf) 

Ave.

PCI 

Ave.

Age 

All Value 15.5 8.6 2.0 2.0 2.2 1.3

Runway Percent 46.4 25.8 5.9 6.0 6.6 3.9

54.1 83.2 10.7

All Value 12.5 13.0 6.0 2.4 2.2 1.2

Taxiway Percent 30.9 32.0 14.7 5.9 5.4 2.9

63.0 81.3 11.0

All Value 15.3 17.7 6.7 5.8 3.8 3.1

Apron Percent 26.2 30.4 11.5 10.0 6.6 5.4

28.3 76.0 14.1

6-13 
Figures 6-7 and 6-8 show the effects of six distresses on different functional pavements. 

FIGURE 6-7. PAVEMENT CONDITION INDEX REDUCTION VALUES FOR DIFFERENT 

FUNCTIONAL HMA PAVEMENTS 

FIGURE 6-8. PERCENT OF TOTAL REDUCTION OF PCI FOR DIFFERENT 

FUNCTIONAL HMA PAVEMENTS 

6-14 
Figures 6-7 and 6-8 show that alligator cracking (load-induced distress) caused the highest 

reduction of PCI for aprons, followed by taxiways and runways. In other words, aircraft loadinduced problems are more serious on aprons and taxiways compared to runways. Longitudinal 

and transverse cracking is the major source of PCI reduction on runways. This type of distress is 

mainly caused by construction and environmental factors. Less rutting was observed on runways 

compared to taxiways and aprons. 

Further analyses of the data showed that non-overlaid pavements experienced more rutting and

alligator cracking compared to overlaid pavements. The longitudinal and transverse cracking 

and reflection cracking cause more PCI reduction on overlaid pavements than on non-overlaid 

pavements. 

As far as pavement age is concerned, longitudinal and transverse cracking is the number one 

source of PCI reduction for younger pavements (1 to 8 years), while alligator cracking becomes

more serious for older pavements (17 to 23 years). The importance of rutting on PCI reduction 

decreases when the pavement gets older. 

6.3.2.3 Structural Condition Index.

SCI is defined as the structural component of the PCI. For rigid pavements, SCI is clearly 

defined, and there is a procedure to calculate it [39]. Currently, for flexible pavement, there is no 

definition for SCI or a procedure to calculate it. This report used two distresses, alligator 

cracking and rutting, to calculate deducts of SCI for flexible pavements. Alligator cracking is

load-related and is a structural problem. Rutting is a structural problem but could be caused by 

non-load-related mechanisms (such as poor asphalt mix design, inadequate compaction in the

asphalt, base, or subbase layer). The procedure developed to calculate SCI for flexible 

pavements is described below. 

Figure 6-2 shows the PCI values for runways, taxiways, and aprons. Table 6-4 lists the PCI 

reduction by six major distresses for all types of pavements (runways, taxiways, and aprons). It 

is assumed that the PCI deduct for a given pavement type (runway, taxiway, or apron) is the 

same for the pavement of all ages. For runway pavements, from table 6-4, the SCI deduct 

(DSCI) is calculated as the sum of PCI reduction due to alligator cracking and rutting (25.8+5.9

= 31.7 percent). For taxiways and aprons, the DSCI is calculated as 46.7 percent (32+14.7) and 

41.9 percent (30.4+11.5) respectively. 

The SCIs were calculated for flexible pavements using equation 6-10. 

100

(%) 100 (100 ) DSCI SCI = − − PCI × (6-10) 

Where DSCI(%) is the area-weighted distress deducts due to the load related distresses. 

The results are shown in table 6-5 and figure 6-9. 

6-15 
TABLE 6-5. STRUCTURAL CONDITION INDEX COMPUTATION FOR HMA 

AIRFIELD PAVEMENTS 

Pavement Age (years) PCI DSCI (%) SCI 

0-5 95.6 98.6

6-10 85.8 95.5

11-15 77.3 92.8

16-20 78.6 93.2

Runway 

> 20 66.8 

31.7 

89.5 

0-5 95.9 98.1

6-10 83.1 92.1

11-15 77.1 89.3

16-20 69.9 85.9

Taxiway 

> 20 64.5 

46.7 

83.4 

0-5 94.5 97.7

6-10 86.6 94.4

11-15 78.6 91.0

16-20 63.7 84.8

Apron 

> 20 58.1 

41.9 

82.4 

FIGURE 6-9. AVERAGE SCI FOR HMA RUNWAYS, TAXIWAYS, AND APRONS 

6-16 
Only load-related distress deducts were considered for computing the SCI. The load-related

distresses for flexible pavements are alligator cracking and rutting (table 2-1). Figure 6-9 shows 

that the average SCI for pavements older that 20 years is higher than 80. 

The average SCI for age > 20 years for runways is approximately 90. This is higher than the 

taxiways (≈ 83) and aprons (≈ 82). Therefore, in general, the SCI of runways is higher than the 

SCI of aprons and taxiways. The slow speed of aircraft on taxiways and aprons results in lower 

HMA stiffness (since HMA is a visco-elastic material) and longer load durations. Both these 

factors are related to HMA fatigue (alligator cracking) and pavement rutting. 

6.4 AVAILABLE DATA FROM PUBLISHED REPORTS.

Kohn [46] evaluated the FAA design procedures for high-traffic volume pavements. The 

airports studied, with flexible pavements, were Phoenix Sky Harbor International Airport (PHX) 

in Arizona and JFK in New York. The information collected from these sites was construction 

data (materials, thickness, strength, and physical properties), PCI, traffic history, and 

nondestructive test (NDT) data (deflection basins, dynamic stiffness modulus, etc.). 

6.4.1 Phoenix Sky Harbor International Airport.

All the pavement sections at PHX were conventional flexible pavements varying in age from 2 to 

16 years (at the time of the study) with a mean age of 8.5 years. A pavement section consisted of 

asphalt surface, aggregate base course, select material subbase, and natural subgrade. One 

section was constructed with a soil-cement subbase. Table 6-6 summarizes the PCI data. 

TABLE 6-6. PAVEMENT CONDITION INDEX SUMMARY FOR HMA PAVEMENTS 

AT PHX 

Percent Deduct Values Based on Distress 

Section Mechanism

No. 

Age

(years) 

Total No. of 

Sample Units 

Average

PCI 

Standard

Deviation Load Environment Other

1 7 16 29 7.9 55.2 1.4 43.2

2 16 17 54 10.4 72.1 17.6 10.4

3 2 19 65 11.5 45.9 0.0 54.1

4 14 13 94 8.4 59.2 40.8 0.0

5 4 17 43 4.3 24.0 3.5 72.5

6 4 8 48 1.7 0.0 6.2 93.8

7 5 9 70 24.1 52.3 2.8 44.9

8 15 19 40 14.1 91.1 8.9 0.0

The condition of the pavements ranged from poor to excellent with an average PCI of 55.4 and a 

standard deviation of 20.5. The large range was due to the age of the pavements (2 to 16 years). 

Two types of problems were reported in the pavements: (1) fatigue or alligator cracking of the 

asphalt surface in the older pavements (load-related) and (2) excessive bleeding in the newer 

pavements (construction- or material-related). FAA thickness designs were reported in reference 

46 using NDT-determined material properties and design values. The designs based on NDT 

properties were very close to existing designs (except for pavement sections 2 and 5, where the 

6-17 
FAA required granular material thicknesses based on NDT properties were larger than the 

existing thicknesses). The existing surface thickness was equal to or greater (1 to 2 inches) than 

the required thickness. Using the design values, the thicknesses obtained from the FAA design 

procedures were generally less than the existing pavement thicknesses (except for pavement 

section 3, which required an additional 6 inches of granular material to match the FAA design

based on NDT properties). 

The pavement condition was assessed using USACE failure criteria (rutting, upheaval, and 

cracking), and none of the pavement sections were declared failed. Considering the age of the 

pavements, it was reported that the design procedure was adequate for rutting criteria; however, a 

criterion was needed for asphalt surface fatigue cracking. The report revealed a limitation to the 

construction specifications used in conjunction with the design method. It recommended 

inclusion of a method of using local material specifications when environmental conditions may 

have a significant effect on the pavement’s performance. 

6.4.2 John F. Kennedy International Airport (identified as KIA in reference 46).

There were three flexible pavement sections at JFK. Two of the sections had an HMA surface 

and a bituminous macadam base course. The third section consisted of an HMA surface over a 

lime-cement-fly ash base course. Table 6-7 summarizes the PCI data. 

TABLE 6-7. PAVEMENT CONDITION INDEX SUMMARY FOR HMA 

PAVEMENTS AT JFK 

Percent Deduct Values Based 

Section on Distress Mechanism

No. 

Age

(years) 

Total No. 

of Sample

Units 

Average

PCI 

Standard

Deviation Load Environment Other

1 5 11 92 7.1 67.5 14.4 18.1

2 20 25 50 9.9 69.4 27.6 3.0

3 13 12 76 3.2 0.0 97.9 2.1

The average PCI for the three flexible pavement sections was 72.6 (very good) with a standard 

deviation of 21.9. The primary distress observed in the flexible sections was random cracking 

and some small areas of fatigue cracking. No rutting caused by shearing of subgrade was 

observed. The actual pavement thicknesses compared well with the design thicknesses using 

NDT data, except for pavement section 3 where the thickness difference was 24 inches (actual 

thickness provided was more than the design thickness). 

The general conclusion stated in the report was that the FAA design procedure was adequate for 

design against subgrade rutting but needs a method to control fatigue cracking. 

6.4.3 Additional Data and Information.

A 1982 study by the General Accounting Office (GAO) [61] looked at the condition of runways 

at 46 small airports. The study concluded that the condition of the runways at these airports was 

deteriorating because of deferred maintenance. The report does not go into detail about the type

of distresses observed. It does mention some surface deficiencies such as cracks. Any

6-18 
functional distresses in a pavement surface, if left unrepaired, may lead to the structural failure of

the pavement. The report highlights the need of timely repair to prevent rapid deterioration of

runways. 

In 1998, the GAO performed a study on 1154 National Airspace System Airports [62] and 

analyzed the PCI data. The study reported that the runways were in good condition. Seventyseven percent of the runways in the GAO’s runway pavement database were ranked in the three 

highest categories—excellent, very good, and good. Eleven percent were rated fair, and twelve 

percent were ranked poor, very poor, or failed. The report does not specify whether the 

pavements in poor condition were due to load-related distresses or functional distresses. The

PCI data were not available for further analysis in this study. 

Grogan [47] undertook a study to evaluate the performance of stabilized base courses. All the 

pavement sections studied with asphalt-stabilized base had an HMA surface. They reported that 

all the pavements except one had major rehabilitation or reconstruction in the past 10 years (from 

date of study). Most of the rehabilitation work was limited to the surface course, such as 

grinding off and replacing with a like or greater thickness of HMA. Distresses observed were all 

mix design (materials problem) related, such as bleeding and rutting of HMA rather than 

structural distresses. 

6.5 DESCRIPTION OF SURVEYED PCC PAVEMENTS.

An FAA project was conducted in 1996 to evaluate the influence of slab size on the performance

of PCC pavements by analyzing the airport survey data in conjunction with theoretical analysis.

A technical report [63] was published in April 2000. The data from this report included 288.4 

msf of pavement data from 2820 features (for a definition see AC 150/5380-6 [60], the same as 

pavement section defined in ASTM D 5340) of 174 airports. These airports are located in 23 

states (six FAA regions and Hawaii) and one in Japan, three of which are military airports: 

Marine Corps Air Stations Futenma, Okinawa, Japan; Kaneohe Bay, Hawaii; and El Toro, 

California. The number of features and corresponding areas of the pavements in each state and 

FAA region are listed in table 10 of reference 63. 

The percentages of pavements grouped by function are shown in figure 6-10. The percentage of 

pavements surveyed from runway, taxiway, and apron are almost the same. The percentages of 

pavements grouped by the years having been serviced are presented in figure 6-11. About 38% 

of all surveyed PCC pavements older than 20 years were still in service during the survey. It

should be noticed that the real lives of the pavements in this group are longer than 20 years. Of 

the pavements younger than 20 years, about one-third are between 16 to 20 years. The surveyed 

pavements with age between 11-15 years are less than all other groups (only 10% of total 

pavements). The data for preparing the FAA report [63] were also used for the analysis of this 

report. 

6-19 
33%

34%

33% Apron 97.4 msf

Runway 95.3 msf

Taxiway 95.7 msf

FIGURE 6-10. SURVEYED PCC PAVEMENTS GROUPED BY FUNCTION 

15%

10%

20%

18%

20%

17%

0-5 yr

6-10 yr

11-15 yr

16-20 yr

21-30 yr

> 30 yr

FIGURE 6-11. SURVEYED PCC PAVEMENTS GROUPED BY YEARS OF SERVICE 

6.6 ANALYSIS OF SURVEYED PCC PAVEMENTS.

The average PCI values of all surveyed pavements grouped by age are shown in figure 6-12.

The average PCI decreases when the age increases. The average PCI in the pavement group 

between 21 to 30 years old is 79. As discussed previously, the SCI of a pavement is always 

equal to or greater than the PCI of the pavement since the deducts for PCI are attributed to all 15

types of distresses and the deducts for SCI are only attributed to 6 distresses. Therefore, the 

average SCI of the pavements in this group should be greater than 80. Though the data are 

sufficient to show the average pavement life, figure 6-12 still shows that a large amount of 

pavements (57.6 msf among the 288 msf, about 20% of the total surveyed pavement) still had an 

SCI greater than 80 after 21 to 30 years of service. This implicitly indicates that the average 

structural life of PCC pavements is longer than 20 years. 

6-20 
40

45

50

55

60

65

70

75

80

85

90

95

100

0-5 6-10 11-15 16-20 21-30 > 30 

Pavement Age in years

Surveyed Average PCI

FIGURE 6-12. PAVEMENT CONDITION INDEX OF THE PCC PAVEMENTS BY AGE 

Figure 6-13 presents more detailed PCI values for different functional pavements: runways, 

taxiways, and aprons. The PCI values of surveyed apron pavements are the lowest in each age 

group. This may be attributed to different maintenance procedures used for runways, taxiways, 

and aprons. This may also be caused by the loading position of the aircraft on the aprons. One 

wheel at a slab corner is frequently observed only on pavement aprons and is most critical to the 

development of top-down corner cracks. 

As concluded in reference 63, slab size is a sensitive factor to the PCI value. Figure 6-14 clearly

verifies that a large slab size leads to quicker deterioration. 

The PCI distributions for three functional pavements from 16 to 23 years old are presented in 

figure 6-15. About 80% of the runway pavements in that age group had a PCI ≥ 80. However,

only 45% of the apron pavements in that age group had a PCI ≥ 80. Different functional

pavements would have different requirements for operation. Therefore, different threshold 

values are needed for determining the need for major rehabilitation or reconstruction of a PCC 

pavement. 

6-21 
40

45

50

55

60

65

70

75

80

85

90

95

100

0-5 6-10 11-15 16-20 >20

Pavement Age in years

Surveyed Average PCI

Apron

Taxiway

Runway

FIGURE 6-13. PAVEMENT CONDITION INDEX OF PCC PAVEMENTS BY AGE 

AND FUNCTION 

4 0

4 5

5 0

5 5

6 0

6 5

7 0

7 5

8 0

8 5

9 0

9 5

< 50 0 s f 5 0 0 to 6 2 5 sf > 6 2 5 sf

P a v e m e n t A g e in ye a rs

Surveyed Average P

A p r o n

T a x iw a y

R u n w a y

FIGURE 6-14. PCIAVG OF 0- TO 30-YEAR PCC PAVEMENTS GROUPED BY SIZE 

6-22 
0

10

20

30

40

50

60

70

80

90

100

50 55 60 65 70 75 80 85 90 95 100

PCI

Percent of Pavements

Apron

Taxiway

Runway

FIGURE 6-15. DISTRIBUTION OF PCI FOR ALL PCC PAVEMENTS BETWEEN 

16 AND 23 YEARS 

A rough procedure was used in this report to approximate the SCIs from the surveyed PCI 

values. Pavement survey data, where the load-related and non-load-related distresses were

available were analyzed. Equation 6-10 was used to estimate the SCI value from the surveyed 

PCI values. 

Two additional assumptions were also used: 

• The DSCI(%) depends on the function of the pavements and is different for runways, 

taxiways, and aprons. 

• The DSCI(%) is the same for pavements in different age groups so the SCI of pavements 

in different age groups may be estimated by using the same DSCI(%) if the pavement

types are the same. 

R.D. McQueen and Associates collected the data for the projects at the Memphis International 

Airport (MEM) and Washington Dulles International Airport (IAD). All distresses are divided 

into load-related (six distresses) and non-load-related (all other) distresses. The reorganized data

are listed in table 6-8. By using the DSCI(%) in table 6-5 and combining the last two groups 

(age between 21 to 30 and > 30) into one group (age > 20), the SCI of runways, taxiways, and 

aprons for all age groups are plotted in figure 6-16. 

6-23 
TABLE 6-8. AREA-WEIGHTED DSCI BY FEATURE 

Area 

(sf)

Age 

(years)

DSCI in 

Airport Section PCI Percent 

Runway: The area-weighted DSCI = 35.9 

MEM RW18R-01 466,875 N/A 70 41

MEM RW18R-01A 934,375 N/A 90 50

IAD RW1R-01 170,000 44 90 6

IAD RW1R-1L 170,000 44 75 22

IAD RW1R-1R 170,000 44 69 45

IAD RW1R-02 235,000 44 83 10

IAD RW1R-2L 235,000 44 74 44

IAD RW1R-2R 235,000 44 70 43

IAD RW1R-03 171,000 44 86 8

IAD RW1R-3L 171,000 44 69 31

IAD RW1R-3R 44 75

Taxiway: The area-weighted DSCI = 38.5 

TWM-02 66,250 64 56.68

MEM TWM-03 225,000 N/A 68 26.98

171,000 27

MEM N/A

MEM TWM-04 57,0000 N/A 44 28.13

IAD TLD-03 457,500 21 37

TLD-04 67,500 4

25

IAD 90 70

IAD TLE-01 168,777 14 52 63

IAD TLE-02 55 62

145,716 14 60

IAD TWA-02 27,580

90,437 14

IAD TLE-03 46

15 63 44

IAD TWB-02 27,232 15 78 40

IAD TWE-02 82,180 9 96 17

Apron: The area-weighted DSCI = 20.3 

IAD APRBR-01 357,500 9 65 8

IAD APRBR-02 385,000 9 74 11

IAD APRBR-03 319,688 9 73 42

IAD APRBR-04 649,300 7 73 22

40

45

50

55

60

65

70

75

80

85

90

95

100

105

0-5 6-10 11-15 16-20 >20

Pavement Age in years

Estimated SCI

Apron Taxiway Runway

FIGURE 6-16. ESTIMATED SCI OF PCC PAVEMENTS BY AGE AND FUNCTION 

6-24 
The findings are summarized below. 

• The average structural life of the surveyed PCC pavements are higher than 80 for the 

pavements in all age groups. 

• Since the PCC airport pavements were designed for SCI = 80 at 20 years service, it can

be concluded that the average PCC pavement structural life is significantly longer than 20 

years. 

• Comparison of figures 6-13 and 6-16 indicates that the differences of PCIs are higher 

than the differences of SCIs in each age group. The differences are higher in the older 

age group than in the younger age group. This comparison shows that the variations of 

pavement conditions are mainly caused by the non-load-related effects. 

6.7 SUMMARY.

This section summarizes the findings (e.g., quotes, findings, and conclusions) extracted from 

published and unpublished data sources, reports and papers. Generally, PCI data is available 

from airfield pavement surveys. An attempt was made to separate non-load-related distresses 

from load-related distresses. The structural condition for PCC pavements is clearly defined by 

SCI. In this section, SCI was also defined for flexible pavements. It is a new concept that has 

not been used before but something that warrants further study and evaluation. The PCI 

deduction due to load-related distresses (alligator cracking and rutting) was considered for

computing the SCI for flexible pavements. Figure 6-17 shows that the SCI near the pavement 

design life (20 years old) for both flexible and rigid pavements and for all types of pavements

(runways, taxiways, and aprons) is higher than 80. Slightly more structural damage for flexible 

taxiways and aprons is observed compared to rigid taxiways and aprons. The flexible pavement 

SCI includes rutting, a portion of which was probably contributed by rutting in the surface HMA 

layer. This would represent material and construction defects rather than structural defects. 

HMA rutting is difficult to extract from the total rutting obtained from routine condition survey

data. Consequently, the SCI values for flexible pavements are probably understated. Figure 

6-18 shows the comparison between flexible and rigid pavement PCIs for pavements older than 

20 years. 

Differences between flexible and rigid SCI and PCI for pavements older than 20 years are shown 

in figure 6-19. 

As shown in figure 6-19, the difference between flexible and rigid pavement PCIs is higher than 

between flexible and rigid pavement SCIs. Generally, flexible PCIs are lower than rigid PCIs. 

This difference indicates that factors other than structural are more prevalent in the deterioration 

of flexible pavements. This could argue for the review of material and construction standards 

and maintenance practices for flexible pavements. 

6-25 
FIGURE 6-17. AVERAGE SCI FOR PAVEMENTS OLDER THAN 20 YEARS

FIGURE 6-18. AVERAGE PCI FOR PAVEMENTS OLDER THAN 20 YEARS

6-26 
FIGURE 6-19. DIFFERENCE IN FLEXIBLE AND RIGID PCI AND SCI FOR PAVEMENTS 

OLDER THAN 20 YEARS 

The findings of previously published reports on flexible pavement performance are quoted 

below: 

• Kohn [46]: 

“The analysis of the data collected during this phase of the study did tend to indicate the 

general adequacy of the current design procedures in terms of obtaining a design 

thickness. Factors were discovered which indicate improvements are needed in areas

other than the determination of thickness. In general, it can be said that the FAA 6C 

design method is adequate from the standpoint of thickness design.” 

• Kohn [46]: 

“The flexible pavement sections surveyed were found to be much closer to design 

thickness when the NDT properties were used in the design procedure. None of the 

sections was found to be failed using the rutting criteria. Some of the sections were 

failed under the cracking criteria. This fact presents a problem in the design method 

since the cracking encountered was alligator- or fatigue-type cracking. The current 

method specifies minimum thicknesses for critical pavement sections, but no method is 

present to check the fatigue life of the surface for a given set of conditions.”

6-27 
• Grogan [47]: 

As previously identified in section 6.4.3 the study to evaluate the performance of 

stabilized base courses concluded for cement-stabilized bases that: “The overall 

performance of cement-stabilized bases can be considered good. All of the pavements

containing cement-stabilized bases have at minimum, performed for their design life. 

Some of the more heavily used pavements have required replacement after more than 20 

years in service. However, these pavements have likely supported a great deal of more

traffic than they were originally designed for. The good performance of these pavements 

even with the probable large increase in operations over original design, indicates that the 

pavements were over designed, that the stabilized base helps provide good performance, 

or that, as is known, the loads are more critical than the actual number of operations and 

that as long as the load magnitude designed for is accurate, the number of operations only

needs to be within an order of magnitude. Usually the pavements are designed to carry 

the maximum gross weight of the design aircraft and most aircraft rarely if ever reach

their maximum gross allowable weight during normal daily operations.” This is

applicable for rigid pavement performance also. 

• GAO [61]: 

The report states that: “(1) runways that were constructed at small airports with ADAP 

funds are deteriorating faster than necessary because airport owners are deferring critical 

pavement maintenance; (2) in addition to shortening a runway’s useful life, deferring

maintenance generally results in serious damage to the basic runway structure, thus

increasing the cost to rehabilitate the runway.” 

“The airports in the national system have runways that are in generally good condition. 

Seventy-seven percent of the runways in GAO’s runway pavement database were ranked 

in the three highest categories – excellent, very good, and good. Eleven percent were 

rated fair; the remainder were rated in the three lowest categories – poor, very poor, and 

failed.” 

• GAO [62]: 

The findings of previously published reports on rigid pavement performance are quoted below. 

• Kohn [46]: 

“In summary, a majority of the rigid pavement sections are as thick as or thicker than the

current FAA procedure would require to carry the current traffic. None of the sections was

found to be in any condition close to the failure condition used in the design procedure.

............. Thus, the design procedure appears to be adequate from a thickness point of 

view.” 

6-28 
• Grogan [47]: 

“The overall results of this study indicate that Portland cement concrete pavements with 

cement stabilized layers are performing well and are supporting aircraft at a level of 

performance at or above the design level for the life of the pavements.” 

• Ahlvin [6]:

“The survey program led conclusively to the finding that rigid pavements are performing 

much better than design criteria would indicate.” 

The current research study and the review of previously published studies indicate and 

demonstrate that the FAA thickness design procedures for airport pavements (which are based on 

the structural failure) are adequate for the 20-year design life requirements. 

6-29/6-30 
7. CONCLUSIONS.

The objective of this study was to evaluate whether the Federal Aviation Administration (FAA) 

thickness design standards for flexible and rigid pavements are consistent with the FAA’s 

standard for 20-year pavement design life requirement. Numerical analysis using structural and 

failure models and the analysis of surveyed pavement data (available from published and 

unpublished resources) was performed to achieve the objective. Based on the analysis of the 

surveyed data of airport pavements and using a Structural Condition Index (SCI) value of 80 as

the minimum acceptable index for structurally sound pavement, it was concluded that the FAA 

thickness design standards for flexible and rigid pavements are adequate and are consistent with 

the 20-year design life requirements, if other related standards for material preparation, 

construction, and maintenance are appropriately applied. 

Available data from airport pavement condition surveys indicated that the average SCIs of both

hot mix asphalt (HMA) and Portland cement concrete (PCC) pavements in all age groups were 

higher than 80. Based on the definition adopted in this report, the average pavement structural 

lives for runways, taxiways, and aprons were greater than 20 years. In other words, airport 

pavements designed following Advisory Circular (AC) 150/5320-6D have sufficient thickness to 

provide the required 20-year structural life. 

It is necessary to differentiate the concepts and effects of the Pavement Condition Index (PCI) 

and SCI indexes. Sixteen and fifteen distresses are currently used to define the service condition 

of HMA and PCC pavements, respectively, and to calculate the values of PCI. Two pavements 

surveyed with the same PCI value may have very different service conditions and may need 

different maintenance and rehabilitation procedures for repair. Therefore, the value of PCI alone 

should not be used to evaluate the standards for pavement structural (or thickness) design. The 

structural, or load-related, SCI is also necessary to establish pavement rehabilitation 

requirements. 

In addition to the SCI data, the PCI data for flexible and rigid pavements were also compared. 

Although the SCIs for flexible and rigid runways were nearly the same, the differences between 

the PCI values for rigid and flexible pavements were larger, as well as those for aprons and 

taxiways. The PCIs for flexible pavements over 20 years of age were generally lower than the 

corresponding PCIs for rigid pavements. This indicated that, while the structural performance of 

flexible and rigid pavements were comparable, a difference in functional (e.g., materials and 

construction) performance was noted. Therefore, while the data evaluated during the study 

confirmed the adequacy of the FAA’s structural design procedures, the data indicated that

improvements in the construction and material standards for flexible pavements should be 

considered. 

Since the importance of each functional pavement type (runway, taxiway, and apron) is different 

for each airport, different criteria may be applied for their maintenance and rehabilitation.

Therefore, different thresholds of PCI may be used by an airport authority for defining the end of 

The surface layer thickness and stiffness effects were examined. A slight increase in thickness

and flexural strength of P-501 has a much larger impact on pavement life compared to the 

changes in P-401 thickness and stiffness. 

7-1 
service life of runway, taxiway, and apron pavements. These differences do not necessarily 

directly relate to differences in structural performance. 

The development of FAA design procedures and models has kept pace with technology. The

procedures have evolved to become more mechanistic in nature. The design model now 

considers the contribution of all layers above the subgrade for HMA pavements and beneath the 

surface layer for PCC pavements to separately evaluate the contribution of each layer. The new 

physical model is closer to the real pavement. The traffic model for both the HMA and the PCC 

pavements has also changed from empirical to more mechanistic. The latest model determines 

the pavement thickness using a cumulative damage factor concept. The damaging potential of 

each aircraft is computed and added using Miner’s hypothesis. This concept simulates traffic 

loading in a more appropriate way. 

The concept of SCI for flexible pavements was introduced in this study. Two distresses, 

alligator cracking and rutting, are recommended in this report to calculate SCI for HMA 

pavement and to evaluate the FAA design standards for HMA pavement thickness determination. 

The flexible pavement SCI includes rutting. Rutting may include a portion that was probably 

contributed by rutting in the surface HMA layer. This distress was related to material and 

construction defects rather than structural defects. HMA rutting was difficult to extract from the 

total rutting obtained from routine condition survey data. Consequently, the SCI values for 

flexible pavements were probably understated. SCI provides the potential to further improve

airport evaluation in the future, and further study and evaluation of the concept is recommended. 

The condition SCI = 80 is defined as the end of pavement structural life. The point at which the

SCI values drops below 100 is defined as the appearance of the first full-depth cracks in rigid 

pavements. 

7-2 
8. REFERENCES.

1. FAA, Office of Airport Safety and Standards, “Standards for Airport Pavement Design 

and Evaluation,” AC 150/5320-6D, U.S. Department of Transportation, 1995. 

2. FAA, Office of Airport Safety and Standards, “Standards for Specifying Construction of 

Airports,” AC 150/5370-10A, U.S. Department of Transportation, 1989. 

3. ASTM D 5340-98 “Standard Test Method for Airport Pavement Condition Surveys,” 

Annual Book of ASTM Standards, Volume 04.03, ASTM International, Pennsylvania. 

4. Distress Identification Manual for the Long-Term Pavement Performance Project, 

Strategic Highway Research Program, SHRP-P-338, 1993. 

5. Ahlvin, R.G., et al., U.S. Army Waterways Experiment Station, “Multiple-Wheel Heavy 

Gear Load Pavement Tests,” Volume I, Technical Report S-71-17, Vicksburg, 

Mississippi, 1971. 

6. Richard G. Ahlvin, U.S. Army Waterways Experiment Station, “Origin of Developments

for Structural Design of Pavements,” Technical Report GL-91-26, Vicksburg, 

Mississippi, 1991. 

8. U.S. Army Waterways Experiment Station, “Design of Flexible Airfield Pavements for 

Multiple-Wheel Landing Gear Assemblies, Report No. 1, Test Sections With Lean Clay 

Subgrade,” Technical Manual TM-3-349, Vicksburg, Mississippi, September 1952. 

9. U.S. Army Waterways Experiment Station, “Investigation of Effects of Traffic With 

High-Pressure Tires on Asphalt Pavements,” Technical Manual TM-3-312, Vicksburg, 

Mississippi, May 1950. 

11. U.S. Army Waterways Experiment Station, “Design of Flexible Airfield Pavements for 

Multiple-Wheel Landing Gear Assemblies, Report No. 2, Analysis of Existing Data,” 

Technical Manual TM-3-441, Vicksburg, Mississippi, November 1956. 

7. U.S. Army Corp of Engineers Sacramento District, “Accelerated Traffic Tests at 

Stockton Airfield, Stockton, California (Stockton Test No. 2),” Prepared by O.J. Porter & 

Co., Consulting Engineers, Sacramento, CA, May 1948. 

10. U.S. Army Waterways Experiment Station, “Effects of Traffic With Small High-Pressure 

Tires on Asphalt Pavements,” Technical Manual TM-3-314, Vicksburg, Mississippi, June 

1950. 

12. U.S. Army Waterways Experiment Station, “Mathematical Expression of the CBR 

Relations,” Technical Report 3-441, Vicksburg, Mississippi, November 1956. 

13. FAA, Office of Airport Safety and Standards, “Airport Pavement Design for the Boeing 

777 Airplane,” AC 150/5320-16, U.S. Department of Transportation, 1995. 

8-1 
14. Hayhoe, Gordon, “Traffic Testing Results From the FAA’s National Airport Pavement

Test Facility,” 2nd International Conference on Accelerated Pavement Testing, 

University of Minnesota, Minneapolis, Minnesota, September 26-29, 2004. 

15. Westergaard, J.M., “Stresses in Concrete Pavements Computed by Theoretical Analysis,”

Public Roads, Vol. 7, 1926. 

16. Westergaard, H.M., “New Formulas for Stresses in Concrete Pavements of Airfields,” 

Transactions, American Society of Civil Engineers, Vol. 113, 1948, pp. 425-439. 

17. FAA, Office of Airport Safety and Standards, “Standards for Airport Pavement Design 

and Evaluation,” AC 150/5320-6C, U.S. Department of Transportation, 1978. 

18. Packard, R.G., “Design of Concrete Airport Pavements,” Engineering Bulletin Portland

Cement Association, Skokie, Illinois, 1973. 

19. ACPA, “Airpave 2000 User’s Manual,” Skokie, Illinois, 2001. 

20. Kreger, W.C., “Computerized Aircraft Ground Flotation Analysis—Edge-Loaded Rigid 

Pavement,” Research Report ERR-FW-572, Research and Engineering Departments, 

General Dynamics, supported by General Dynamics Research Program, Task RDP 414-

61-506, January 1967. 

21. Iaonnides, A.M., Donnelly, J., Thompson, M.R., and Berenberg, E.J., “Analysis of Slabson-Grade for a Variety of Loading and Support Conditions,” Report AFOSR-83-0143, 

submitted to Air Force Office of Scientific Research, Air Force Systems Command, 

United States Air Force, Bolling AFB, DC 20332, December 1984. 

23. Khazanovich, L., Yu, H.T., and Beckemeyer, C., “Application of ISLAB2000 for

Forensic Studies,” Proceeding of the Second International Symposium on 3D Finite 

Element for Pavement Analysis, Design and Research, Charleston, West Virginia,

October 11-13, 2000. 

25. Shell Oil Company, “BISAR: Bitumen Structures Analysis in Roads, User’s Manual,” 

Koninklijke/Shell – Labratorium, Shell Research, N.V. Amsterdam, 1978. 

22. Huang, Y.H., “Pavement Analysis and Design,” Prentice Hall, 1993. 

24. Guo, E.H., Sherwood, J.M., and Snyder, M.B., “Component Dowel-Bar Model for LoadTransfer Systems in PCC Pavements,” ASCE Journal of Transportation Engineering, 

Vol. 121, No. 3, May/June 1995. 

26. Hayhoe, G., “Leaf - A New Layered Elastic Computational Program,” Proceedings of the 

2002 Federal Aviation Administration Technology Transfer Conference, Atlantic City, 

NJ, 2002. 

8-2 
27. Maker, B.M., “NIKE3D – A Nonlinear, Implicit, Three-Dimensional Finite Element 

Code for Solid and Structural Mechanics – User’s Manual,” Report UCRL-MA-105268, 

Rev. 1, Lawrence Livermore National Laboratory, Livermore, California, 1995. 

28. Brill, R.D., “Development of Advanced Computational Models for Airport Pavement 

Design,” FAA report DOT/FAA/AR-97/47, August 1998. 

29. Federal Aviation Administration, Office of Airport Safety and Standards, “LEDFAA 

User’s Manual,” 1995. 

30. Guo, E.H. and Dong, M., “Jslab2004, User’s Manual Upgrade Supplement,” 2004. 

32. Brill, R.D. and Guo, E.H., “Load Transfer in Rigid Airport Pavement Joints,” Proceeding 

of the 26th International Air Transportation Conference, June 18-21, 2000, pp.13-24. 

31. Guo, E., “Performance of Different Types of Joints in Airport PCC Pavements,” FAA 

internal draft report under Contract DTFA03-92-00051, 1996. 

33. Dong, M. and Hayhoe, G.F., “Analysis of Falling Weight Deflectometer Tests at Denver 

International Airport,” The 2002 Federal Aviation Administration Technical Transfer 

Conference, Atlantic City, NJ, 2002. 

34. Rufino, D., “Mechanistic Analysis of Pavement Response From Denver International 

Airport,” technical report of research under FAA grant DOT 95-C-001, 2004. 

35. Teller, L.W., “Performance of Dowels Under Repetitive Loading,” Public Roads, Vol. 

30, No. 1, April 1958. 

36. Snyder, M.B., “Dowel Load Transfer Systems for Full-Depth Repairs of Jointed Portland 

Cement Concrete Pavements,” PhD Thesis, University of Illinois, 1989. 

37. Guo, E., Dong, M., Daiutolo, H., and Ricalde, L., “Analysis of the Observed and 

Predicted Responses of a Curled Single Slab,” FAA Worldwide Airport Technology 

Transfer Conference, Atlantic City, April 2004. 

38. Gervais, E., Hayhoe, G., and Garg, N., “Towards a Permanent ACN Solution for 6-Wheel 

Landing Gear Aircraft,” Proceedings, ASCE 2003 Airfield Pavements Specialty 

Conference, Las Vegas, NV, September 2003. 

39. Rollings, R.S., “Design of Overlays for Rigid Airport Pavements,” FAA report 

DOT/FAA/PM-87/19, April 1988. 

40. ASTM C 78, “Standard Test Method for Flexural Strength of Concrete (Using Simple

Beam With Third-Point Loading,” Annual Book of ASTM Standards, Vol. 04.02. 

8-3 
41. McQueen, R.D., Hayhoe, G., Guo, E., Rice, J., and Lee, X., “A Sensitivity Study of 

Layer Elastic Theory for Airport Pavement Design,” Proceeding of the Second

International Conference on Road and Airfield Pavement Technology, Singapore, 

September 22-29, 1995. 

42. Barker, W.R. and Brabston, W., “Development of a Structural Design Procedure for 

Flexural Airport Pavements,” FAA report FAA-RD-74-199. 

43. Parker, F., Barker, W., Gunkel, R., and Odom, E., “Development of a Structural Design 

Procedure for Rigid Airport Pavements,” FAA report FAA-RD-77-81. 

44. Hayhoe, G. and Guo, E., “Parametric Sensitivity Study,” FAA report (draft) under 

Contract DTFA03-93-C-00021, June 1994. 

45. Hayhoe, G. and Guo, E., “Sensitivity Analysis for PCC Airport Pavements,” FAA 

technical report (draft) under Contract DTFA03-93-C-00021, September 1994 

46. Kohn, S., “Evaluation of the FAA Design Procedures for High Traffic Volume

Pavements,” U.S. Army Waterways Experiment Station, FAA report DOT/FAA/PM84/14, Vicksburg, Mississippi, October 1985. 

47. Grogan, W.,P., Weiss, P., Charles, Jr., A., and Rollings, R.S., “Stabilized Base Courses 

for Advanced Pavement Design: Report 1: Literature Review and Field Performance 

Data,” U.S. Army Waterways Experiment Station, Vicksburg, Mississippi, FAA report 

DOT/FAA/AR-97/65, September 1999. 

48. Smith, K.D., Roesler, J.R., and Houghton, J.E., “Review of Fatigue Models for Concrete 

Airfield Pavement Design,” American Concrete Pavement Association, Skokie, Illinois, 

September 2002. 

50. EGA (Eckrose/Green Associates Inc.), “Airport Pavement Evaluation.” General Edward 

Lawrence Logan International Airport, Boston, MA, 1992. 

51. HLA, PCI and SCI, “Dallas/Fort Worth International Airport Pavement Management

Consulting Services, Final Report’, March 1992. 

49. Thuma, R.G. and Lafrenz, J.L., “A Comparison of Rigid Airfield Pavement Design 

Methods,” 2002. 

52. McQueen, R.D., “Evaluation of Runway 10C-28C and Taxiway W Pavements at Greater 

Pittsburgh International Airport, PA,” engineering report, October 1991. 

53. McQueen, R.D., “Evaluation of Airfield Pavements at Cincinnati/Northern Kentucky 

International Airport,” engineering report for Survey Results, February 1993. 

8-4 
54. McQueen, R.D., “Evaluation of Runway and Taxiway Pavements at Atlantic City 

International Airport, Atlantic City, NJ,” June 1995. 

55. McQueen, R.D., “Evaluation of Shoulder Pavements for A-380 Operation at Memphis 

International Airport, TN,” 2002. 

56. ERES Consultants, Inc., “Evaluation of the Airside Pavement at Raleigh-Durham 

International Airport, Morrisville, North Carolina,” March 1995. 

57. Guo, E, “Statistical Performance Analysis of Airport Asphalt Pavements,” unpublished 

draft report for the FAA under Contract DTFA03-97-P-0236. 

58. Kurtz, M. “Handbook of Applied Mathematics for Engineers and Scientists,” McGraw 

Hill, 1991. 

59. Green, W.H. and Eckrose, R.A., “Airport Pavement Inspection by PCI,” Text of Training 

Program for Pavement Condition Surveyors, 1996. 

60. Federal Aviation Administration, Office of Airport Safety and Standards, “Guidelines

and Procedures for Maintenance of Airport Pavements,” AC 150/5380-6A, July 2003. 

61. Comptroller General, United States General Accounting Office, “Runways at Small 

Airports are Deteriorating Because of Deferred Maintenance: Action Needed by FAA 

and the Congress,” GAO/CED-82-104, Report to the Congress, September 1982. 

62. United States General Accounting Office, “Airfield Pavements—Keeping Nation’s 

Runways in Good Condition Could Require Substantially Higher Spending,” 

GAO/RCED-98-226, Report to the Chairman, Committee on Commerce, Science, and 

Transportation, U.S. Senate, July 1998. 

63. Guo, E.H., “Effects of Slab Size on Airport Pavement Performance,” FAA report, 

DOT/FAA/AR-99/83, April 2000. 

8-5/8-6
9. INDUSTRY COMMENTS.

For the preparation of this report, qualified industry experts from the Asphalt Institute and the 

American Concrete Pavement Association were recruited to review the work plan and the final 

draft. Although opportunities to review the work in progress did not become available, 

comments from the representatives of the respective organizations on the final draft report 

follow. 

9.1 COMMENTS FROM THE ASPHALT INSTITUTE.

The conclusion that the FAA thickness design procedures adequately provide for a 20-year 

operational life appears to be valid based on the methodology in this report. While we generally 

concur with the conclusions, we recognize that the methodology of analysis to reach its stated 

conclusions, specifically, the use of structural condition index (SCI) to define failure in hot mix 

asphalt (HMA) pavements is a new concept that may or may not be valid. Therefore, this 

concept and methodology of analysis needs to be investigated further. 

With regard to the general topic of FAA thickness design, the consensus throughout the 

HMA industry is that the FAA procedure for flexible pavements could be substantially 

improved in the following important ways: 

 

a) : in the current procedure in FAA AC 

150/5320-6D using LEDFAA, the modulus for P-401 surface layers is fixed at 200,000 

psi and the modulus of P-401 base layers is fixed at 400,000 psi. This is a shortcoming 

that conflicts with the fact that HMA stiffness is dependent on temperature. All else 

being the same, we should expect that a P-401 mix in Miami will perform differently than 

a P-401 mix in Bangor simply due to the difference in climate. The current FAA method 

does not allow us to account for this important temperature dependency. 

HMA Modulus—Temperature Dependence

b) : Another fundamental issue with the FAA design 

method is that by establishing fixed values for stiffness, the FAA does not take complete 

advantage of high quality materials available for HMA mixes, such as improved asphalt 

binders that provide improved high temperature stiffness to resist rutting while 

maintaining low temperature flexibility to resist cracking. 

HMA Modulus—Material Properties

c) : The current FAA procedure sums the

accumulated horizontal strains at the bottom of the P-401 surface layer. While this is the 

correct procedure for traditional flexible pavement designs using HMA over aggregate 

base, the FAA method does not adjust to account for the presence of an asphalt stabilized 

base P-401 layer, which is required for airport pavements that support heavy aircraft 

(greater than 100,000 lb.). When one is present, horizontal strains should be calculated at 

the bottom of the P-401 base in order to accurately analyze the effect of fatigue in deep 

HMA structures. 

Calculation of Horizontal Strains/Stabilized Bases

d) Full-Depth Asphalt: If the improvements listed above were to be enacted, the FAA would 

be in a position to evaluate full-depth asphalt pavements in which standard P-401 mixes 

are used exclusively above a prepared subgrade. Full-depth asphalt pavements have been 

9-1
in use for more than 30 years in the highway industry. Furthermore, this strategy fits well 

with the FAA stabilized base requirement for heavy aircraft operations, and takes it a step 

further by using P-401 in place of all aggregate layers. By analyzing temperaturedependency in HMA, allowing for improved HMA material properties in the design, and 

accurately analyzing the fatigue failure in the pavement section, the FAA would be able 

to assess whether full-depth P-401 pavements are a viable alternative for commercial 

airport applications. 

e) : The most significant new development in structural design of 

HMA pavements is the perpetual pavement concept. This design concept is essentially a 

full-depth asphalt pavement that uses non-standard HMA mixes at particular layers in the 

pavement structure. From the bottom up, a typical perpetual pavement structure consists

of a well-prepared subgrade to provide firm foundation support, a flexible base layer to 

resist fatigue cracking, a rut-resistant intermediate layer, and a renewable, rut-resistant 

surface course that can be milled and replaced periodically. The general concept is that 

distresses in a perpetual pavement are driven to the surface, which can be renewed as 

needed, say every 15-20 years. The firm foundation support, appropriate material 

selection, and adequate thickness design prevent the structure from failure due to 

deformation at the top of the subgrade. The flexible base layer resists the accumulation 

of fatigue damage. The renewable surface layer allows for the management of top-down 

cracking. Perpetual pavements are the future of the HMA industry and the FAA should 

begin to study their effectiveness for airport applications. 

Perpetual Pavements

The HMA industry concurs with the conclusion that “the data indicate that improvements in the 

construction and material standards for flexible pavements should be considered.” The 

following recommendations are offered as areas of further study: 

 

a) : the FAA may want to evaluate the use of the Superpave mix 

design technology as an improved method to build rut-resistant mixes for airfield 

pavements. 

Superpave Mix Design

 

b) : SMA has been used successfully in the construction of rutresistant mixes for heavy highway applications and should be considered for use on 

commercial airports. 

Stone Matrix Asphalt (SMA)

 

c) : As is demonstrated in your report, 

construction and material deficiencies contribute to the overall functional performance of 

airfield pavements. These issues can range from the longitudinal joint construction and 

obtaining adequate mat density to material segregation (mechanical and thermal), 

moisture susceptibility and other issues that apply to the entire HMA industry, not just 

airports. We share the FAA’s concern about construction and material quality and we

would be happy to discuss ways in which the FAA and the HMA industry could approach 

research into these issues. 

Materials Standards and Construction Practices

9-2
9.2 COMMENTS FROM THE AMERICAN CONCRETE PAVEMENT ASSOCIATION.

Section 705 of Vision 100 – Century of Aviation Reauthorization Act provides the Federal

Aviation Administration (FAA) with an opportunity that is un-matched by any recent event in

the history of airfield pavements. Specifically, the provision requires FAA to review the

standards used to determine the appropriate thickness for airfield pavements are in accordance 

with the standard 20 – year-life requirement and make appropriate adjustments if the 

Administrator determines that such standards are not in accordance with that requirement. The 

industry advocates that adjustments to the standards are necessary, to advance to the state of the

art. 

This report does not answer the mandate of Section 705. It does, however, support industry’s

concern that “[G]enerally the predicted and actual pavement performance have not always 

compared favorably.” We appreciate that the FAA now has additional and recent support that 

clearly shows the problem with its many pavement standards. But, we still must address the 

mandate of Section 705. 

The discrepancy in the relationship between pavement design thickness and other related FAA 

standards, as related to the 20-year design life is evident, for example, in two documents. 

 

• Advisory Circular 150/5320-6D provides for designing pavement thickness based upon 

the number of aircraft departures, weight of the aircraft, design strength of concrete and 

the value of sub-base support. 

These FAA documents provide one example and show that identification of the problem is

relatively easy and direct. And as this report states “[I]ncreasing the flexural strength of P-501 

from 700 to 735 psi (an increase of 5-percent) increases the pavement predicted life from 20 to 

40.2 years.” (“Increasing the flexural strength of P-501 from 700 to 735 psi (an increase of 5 

percent) increased the pavement’s predicted life from 20 to 40.2 years” after editing.)

This report provides three substantiating reasons why the FAA design, materials and 

construction standards must be reviewed and adjustments made. 

This report presents the failure criteria for pavement in terms of several different 

variables and takes a position “[W]hen the pavement is incapable of performing the 

task it was designed for, it has failed.” Subsequently, it states however that “[A] 

• Advisory Circular 150/5370-10 (Item P-501) requires that the constructed strength 

exceed the design strength. 

Determining a solution to the interrelated FAA standards problem is a more complex matter, but 

we believe answers can be obtained through proper research and consultation with the industry. 

1. This report substantiates that the FAA definition of pavement failure is unreasonable. 

However, the introduction of an arbitrary standard, as this report suggests, does not solve 

the problem. We reference the following to illustrate the conflicts in the draft report on 

this issue. 

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unique definition of pavement failure does not exist.” The report then proceeds to 

create a new definition of failure for AC pavements based upon the Structural 

Condition Index (SCI). The stated relationship between SCI and pavement failure is 

“an SCI of 80 is consistent with the current FAA definition of initial failure of a rigid

pavement” (50% of slabs in traffic area exhibit initial cracking). This definition is the 

basic problem inherent in the FAA standards. If 50% of the slabs in a pavement 

feature have initial cracking (low intensity cracking) then the correlation between SCI 

and failure definition is consistent. Beyond that a correlation is not expected. The 

SCI is a design variable used for determining the thickness for rigid pavement overlay 

calculations. Additionally, and more important, creating an SCI for asphalt 

pavements goes beyond current practice. I am not aware of any situation or practice 

where SCI is associated with flexible pavement performance. 

3. This report identifies that possibly the FAA materials and/or construction standards could 

be improved, but does not define what adjustments to the standards should be considered. 

This report implies that the standards for concrete pavement may be conservative but 

could not quantify this issue. The draft report also clearly indicates a need to improve 

standards for flexible pavements. There is a strong recommendation for a definition of 

pavement failure consistent with field performance. 

While each of these three major points in this report shows that possibly some of the FAA 

standards should be adjusted, it does not recommend doing the necessary research to accomplish 

the tasks that are necessary to answer the questions raised. And, the desktop computer model 

assessment used to support the report does not close the widening gap between “predicted and 

actual pavement performance.” Additional specific comments are provided as separate 

correspondence. 

2. This report concludes that “airport pavements designed following AC 150/5320 6C and 

6D have sufficient thickness to provide the required 20-year structural life.” (“airport

pavements designed following Advisory Circular (AC) 150/5320-6D have sufficient 

thickness to provide the required 20-year structural life” after editing.) This statement, 

however, does not address the intent of the question posed by Section 705 and it conflicts 

with another statement in the report. Specifically, the report “confirm(s) the adequacy of 

the FAA structural design procedures” (“confirm(s) the adequacy of the FAA structural 

design procedures” after editing) but then states that “the data indicate that improvements

in the construction and material standards (not the design standards) for flexible

pavements should be considered” (“the data indicated that improvements in the 

construction and material standards for flexible pavements should be considered” after 

editing). 

We present the following research plan for concrete pavement that the FAA should adopt as a 

roadmap to answer the questions that the report raises. With this research, we can obtain a valid

and technically correct answer to the Section 705 mandate. 

The following research projects are required. The most up-to-date information on the life of 

airfield pavements is not adequate to answer the outstanding questions related to design 

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assumptions and pavement performance. These projects will be accomplished and the results

used to (1) develop a reasonable definition of pavement failure, and (2) examine the variables

inherent in the design assumptions that have the most significant influence on pavement life. 

Project 1 – Define Pavement Failure in Terms of Performance. The project will involve doing a 

survey of domestic service airports in each of the four levels of size and making a determination

about the condition of rigid airfield pavement when owner decisions are made regarding

replacement. Estimated time to complete is 18 months. Estimated cost is $450,000. 

Project 2 – Evaluate the Range of the Concrete Modulus (E ) for Typical Concrete Placements 

and Impact on Pavement Performance. Typically, the value of the concrete modulus is assumed 

to be 4,000,000 psi. However, higher strength concretes have higher values of modulus.

Investigate the variability and determine the impact on pavement performance. Estimated time

to complete is 24 months. Estimated cost is $250,000. 

c

Project 3 – Investigate the Load Transfer Efficiency of Joints on Both Aggregate and Stabilized

Subbase. Load transfer efficiency is dependent upon ambient environment and subbase support.

Stabilized subbase is required for those airfield pavements that are used to support airplanes 

weighing in excess of 100,000 pounds. Accomplish studies that determine the joint load transfer

efficiencies for both subbase types on different subgrades. Investigate the impact of load transfer 

efficiency on pavement performance. Estimated time to complete is 18-months. Estimated cost 

is $800,000. 

Project 4 – Investigate Subbase Restraint on the Performance of Concrete Pavements.

Mechanistic design procedures have allowed for the modeling of the different layers in a 

pavement system and the interaction of those layers in supporting loads. Assumptions have been 

made about the amount of friction developed between the subbase materials and the concrete for 

rigid pavements. A sensitivity analysis suggests that the character of the friction can impact 

pavement performance. The impact has not been characterized. Estimated time to complete is 

12 months. Estimated cost is $250,000. 

Project 5 – Evaluate the Accuracy of Predicted Pavement Stress versus Actual Pavement Stress. 

Pavement models assume that stress concentrations occur at various locations within a pavement 

structure depending upon the location and distribution of loads. Accomplish full scale testing

that will validate the assumptions related to Cumulative Damage Factor (CDF) and the accuracy 

of models. Estimated time to complete is 18-months. Estimated cost is $900,000. 

9.3

Given the restraints of the study, the National Asphalt Pavement Association (NAPA) believes 

the FAA did a good job putting the report together. The Asphalt Institute responses are 

constructive and reflect our views. In addition, NAPA has the following two items for

consideration: 

a) Any change to structural design procedures or assumptions regarding pavement structural 

life needs to be based on a study that is specifically designed to address the issue. In 

other words, it is acceptable to review the structural performance of pavements based 

COMMENTS FROM THE NATIONAL ASPHALT PAVEMENT ASSOCIATION.

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upon existing pavements for exploratory purposes, but substantive changes will require 

more careful investigation and documentation. 

b) Section 5 of the report seemed to be based entirely upon an analytical study of the effects 

of changes in input to changes in performance. This requires experimental validation 

before it is put forward. If one changes the thickness or modulus of HMA or PCC, the 

resulting change in pavement response will be more or less dramatic, depending upon the 

“baseline” values that are assumed. 

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