U.S. Department 

of Transportation 

Federal Aviation 

Administration

Advisory

Circular

Subject: USE OF NONDESTRUCTIVE 

TESTING IN THE EVALUATION OF 

AIRPORT PAVEMENTS

Date: 12-29-04 

Initiated by: AAS-100

AC No.: 150/5370-11A 

Change: 

1. PURPOSE OF THIS ADVISORY 

CIRCULAR. This advisory circular (AC) focuses 

on nondestructive testing (NDT) equipment that 

measures pavement surface deflections after applying 

a static or dynamic load to the pavement. It also 

briefly introduces other types of nondestructive 

measuring equipment to illustrate how supplementing 

NDT data with other test data may improve the 

quality and reliability of the pavement evaluation. 

2. APPLICATION OF THIS AC. This AC 

provides guidance and recommendations on datacollection equipment and methods of data analysis 

that are used to conduct NDT; however, other 

methods, techniques, and variations of those outlined 

here may be used provided the appropriate local 

Federal Aviation Administration (FAA) Airports 

Office approves them. 

3 CANCELLATION. AC 150/5370-11 Use of 

Nondestructive Testing Devices in the Evaluation of 

Airport Pavements, dated June 5, 1976, is cancelled. 

4. USE OF METRICS. To promote an orderly 

transition to metric units, this AC contains both 

English and metric dimensions. 

5. COPIES OF THIS AC. The FAA is in the 

process of making all ACs available to the public 

through the Internet. These ACs may be found by 

selecting the Regulations and Policies link on the 

FAA home page (www.faa.gov). You may also 

request a printed copy of this and other ACs from the 

United States (U.S.) Department of Transportation, 

Subsequent Business Office, Annmore East Business 

Center, 3341 Q 75th Avenue, Landover, MD, 20785. 

David L. Bennett 

Director, Office of Airport Safety and Standards 
AC 150/5370-11A 12/29/2004 

Page i 

CONTENTS 

CHAPTER 1 – INTRODUCTION.............................................................................................................................1 

1. GENERAL. .........................................................................................................................................1 

2. BACKGROUND.................................................................................................................................1 

3. LIMITATIONS TO NDT...................................................................................................................2 

4. RELATED ADVISORY CIRCULARS. ...........................................................................................2 

5. ORGANIZATION OF THIS AC. .....................................................................................................2 

CHAPTER 2 – DESCRIPTION OF NDT PROCESS..............................................................................................3 

6. GENERAL. .........................................................................................................................................3 

7. PAVEMENT STIFFNESS AND SENSOR RESPONSE.................................................................3 

8. DEFLECTION BASIN.......................................................................................................................3 

9. USE OF NDT DATA. .........................................................................................................................4 

CHAPTER 3 – NONDESTRUCTIVE TESTING EQUIPMENT...........................................................................5 

10. CATEGORIES OF EQUIPMENT....................................................................................................5 

11. GENERAL REQUIREMENTS FOR NDT EQUIPMENT.............................................................6 

12. STATIC DEVICES.............................................................................................................................6 

13. VIBRATORY DEVICES. ..................................................................................................................7 

14. IMPULSE DEVICES. ........................................................................................................................7 

CHAPTER 4 – REQUIREMENTS FOR NONDESTRUCTIVE TESTING EQUIPMENT................................9 

15. NEED FOR STANDARIZATION. ...................................................................................................9 

16. FAA SENSITIVITY STUDY.............................................................................................................9 

17. SUMMARY OF FAA POLICY.......................................................................................................10 

CHAPTER 5 – TEST PLANNING ..........................................................................................................................11 

18. JUSTIFICATION FOR NDT. .........................................................................................................11 

19. NDT TEST OBJECTIVES. .............................................................................................................11 

20. NDT TEST TYPES...........................................................................................................................12 

21. TEST LOCATIONS AND SPACING.............................................................................................12 

22. NDT TEST SKETCHES. .................................................................................................................12 

23. SPECIAL CONSIDERATIONS......................................................................................................13 

CHAPTER 6 – TEST PROCEDURES ....................................................................................................................14 

24. EQUIPMENT MOBILIZATION....................................................................................................14 

25. STARTUP OPERATIONS. .............................................................................................................14 

26. DATA COLLECTION.....................................................................................................................15 

27. SPECIAL TEST CONDITIONS. ....................................................................................................16 

28. ONSITE REVIEW OF DATA.........................................................................................................17 

CHAPTER 7 – DEFLECTION DATA ANALYSES..............................................................................................18 

29. OVERVIEW OF PROCESS............................................................................................................18 

30. PROCESS RAW DEFLECTION DATA. ......................................................................................18 

31. BACK-CALCULATION ANALYSIS. ...........................................................................................20 

32. PCC JOINT ANALYSIS..................................................................................................................28 

33. PCC VOID ANALYSIS. ..................................................................................................................29 

34. PCC DURABILITY ANALYSIS. ...................................................................................................30 

35. SUMMARY.......................................................................................................................................31 

CHAPTER 8 – NDT-BASED EVALUATION AND DESIGN INPUTS ..............................................................32 

36. STATISTICALLY DERIVED INPUTS.........................................................................................32 

37. USING NDT RESULTS IN FAA ANALYSIS PROGRAMS. ......................................................33 

38. PCC LOSS OF SUPPORT...............................................................................................................36 

39. SUMMARY.......................................................................................................................................36 

APPENDIX 1 - FIGURES....................................................................................................................................... 1-1 

APPENDIX 2 - TABLES ........................................................................................................................................ 2-1 

APPENDIX 3 - GLOSSARY...................................................................................................3-1 
AC 150/5370-11A 12/29/2004 

Page ii 

FIGURES 

Appendix 1 

Figure 1. Illustration of an Impulse Load Created by FWD.............................................................................. 1-1

Figure 2. Sensors Spaced Radially From the Load Plate ................................................................................... 1-1

Figure 3. Schematic of Deflection Basin............................................................................................................... 1-2

Figure 4. Comparison of Deflection Basin of Three Pavements ........................................................................ 1-2

Figure 5. Illustration of Static and Dynamic Force Components for Vibratory NDT..................................... 1-3

Figure 6. Illustration of Time to Peak Load for Impulse-Based NDT Equipment........................................... 1-3

Figure 7. Benkleman Beam ................................................................................................................................... 1-4

Figure 8. Dynaflect Deflection Trailer ................................................................................................................. 1-4

Figure 9. Road Rater ............................................................................................................................................. 1-5

Figure 10. KUAB FWD ........................................................................................................................................... 1-5

Figure 11. Dynatest FWD........................................................................................................................................ 1-6

Figure 12. PHOENIX FWD (Redesigned by Viatest) ........................................................................................... 1-6

Figure 13. JILS HWD.............................................................................................................................................. 1-7

Figure 14. Evaluation of HWD Force Linearity in Terms of ISM....................................................................... 1-7

Figure 15. Evaluation of HWD Force Linearity in Terms of Subgrade Elastic Modulus ................................. 1-8

Figure 16. NDT Test Locations within a PCC Slab .............................................................................................. 1-8

Figure 17. Illustration of Load Transfer Across a PCC Joint.............................................................................. 1-9

Figure 18. Example Runway or Taxiway Sketch when Centerline Lies on Slab Joint...................................... 1-9

Figure 19. Example Runway or Taxiway Sketch when Centerline Does Not Lie on Slab Joint ..................... 1-10

Figure 20. Example Runway or Taxiway Sketch for HMA Pavements ............................................................ 1-10

Figure 21. Thermal Curling in PCC Slab From Temperature Changes........................................................... 1-11

Figure 22. Location of Additional Sensors for Corner and Joint Testing......................................................... 1-11

Figure 23. NDT Data Analysis and Design Flowchart........................................................................................ 1-12

Figure 24. ISM Plot Used to Identify Pavement Section Breaks........................................................................ 1-12

Figure 25. Normalized Deflection Plot Used to Identify Pavement Section Breaks ......................................... 1-13

Figure 26. Normalized Subgrade Deflection Plot Used to Identify Pavement Sections ................................... 1-13

Figure 27. Process for Data Preparation and Back-Calculation Method Selection ......................................... 1-14

Figure 28. Flowchart for Closed-Form Back-Calculation Using AREA Method............................................. 1-14

Figure 29. Illustration of Basin AREA for SHRP Four-Sensor Configuration................................................ 1-15

Figure 30. Comparison of Measured and Calculated Deflection Basins........................................................... 1-15

Figure 31. Back-Calculation Procedures for an Elastic-Layer Based Analysis................................................ 1-16

Figure 32. Initial BAKFAA Run for Example 2.................................................................................................. 1-17

Figure 33. Second BAKFAA Run for Example 2................................................................................................ 1-17

Figure 34. Output from Second BAKFAA Run for Example 2 ......................................................................... 1-18

Figure 35. Deflection vs. Stress LTE Relationship for 12-inch (300 mm) Diameter Load Plate..................... 1-18

Figure 36. Example Plot of Transverse Joint LTE∆ for a 10,000-foot (3,050 m) Taxiway ................................ 1-19

Figure 37. Void Detection Beneath PCC Slabs.................................................................................................... 1-19

Figure 38. Example Plot of Transverse Joint Voids for a 10,000-foot (3,050 m) Taxiway .............................. 1-20

Figure 39. Example Plot of ratio ISM for Transverse Joint for HMA Overlaid PCC......................................... 1-20

Figure 40. Histogram of ISM Values for Section 3 in Figure 24........................................................................ 1-21
AC 150/5370-11A 12/29/2004 

Page iii 

TABLES 

Appendix 2 

Table 1. Summary of Nondestructive Testing Measuring Equipment ............................................................ 2-1 

Table 2. Detailed Specifications for Selected FWDs and HWDs ......................................................2-2 

Table 3. ASTM Standards for Deflection Measuring Equipment.................................................................... 2-2 

Table 4. Common Sensor Configurations .......................................................................................................... 2-3 

Table 5. Typical Runway and Taxiway Test Locations and Spacing, feet (m) ............................................... 2-3 

Table 6. Typical Apron Test Locations and Frequency.................................................................................... 2-3 

Table 7. FAA Software Tools for Pavement Analysis, Evaluation, and Design.............................................. 2-4 

Table 8. Theoretical Basis of FAA Software Tools............................................................................................ 2-4 

Table 9. Required Sensor Distance (inch) from Load Plate with 12-inch (300 mm) Diameter ..................... 2-4 

Table 10. Type of Back-Calculation Software Tool that is Required for Each Load Scenario ....................... 2-5 

Table 11. AREA-based Constants for Equation 8............................................................................................... 2-5 

Table 12. Constants for *

r d (Equation 11)............................................................................................................. 2-5 

Table 13. Typical Modulus Values and Ranges for Paving Materials............................................................... 2-6 

Table 14. Typical Poisson's Ratios for Paving Materials.................................................................................... 2-6 

Table 15. Linear Analysis Back-Calculation Programs...................................................................................... 2-7 

Table 16. Nonlinear Analysis Back-Calculation Programs ................................................................................ 2-8 

Table 17. Seed Modulus and Poisson's Ratios for Example Problem 2............................................................. 2-8 

Table 18. Pavement Joint Performance Ratings.................................................................................................. 2-9 

Table 19. Statistical Summary of ISM Values for Each Section in Figure 24................................................... 2-9

Table 20. Required FAA Advisory Circular Evaluation and Design Inputs................................................... 2-10 

Table 21. Allowable Modulus Values for LEDFAA (AC 150/5320-6), psi (MPa)........................................... 2-11 

Table 22. HMA Pavement Base and Subbase Modulus and Equivalency Factor Inputs............................... 2-11 

Table 23. Recommended Reduced k-Values for Loss of Support Conditions................................................. 2-11 
AC 150/5370-11A 12/29/2004 

Page 1 

CHAPTER 1–INTRODUCTION 

1. GENERAL. Nondestructive testing (NDT) can 

make use of many types of data-collection equipment 

and methods of data analysis. In most cases, the data 

can be used to evaluate the structural or functional 

condition of a pavement. This AC focuses on 

collecting and analyzing NDT data, which are used to 

accomplish the following:

a. Evaluate the load-carrying capacity of 

existing pavements. 

b. Provide material properties of in-situ 

pavement and subgrade layers for the design of 

pavement rehabilitation alternatives that include 

extensive maintenance and repair work (restoration), 

functional and structural overlays, partial 

reconstruction (e.g., runway keel), and complete 

reconstruction. 

c. Compare parts of a pavement system to each 

other to gain relative strength and/or condition within 

that section. The results of the NDT can show which 

segments are the strongest and which are the weakest. 

These results can then be used to focus follow-on 

testing. 

d. Provide structural performance data to 

supplement pavement condition index (PCI) survey 

data in an Airport Pavement Management System 

(APMS). 

To accomplish these objectives, this AC provides an 

overview of the various types of NDT equipment; 

identifies those scenarios where NDT provides the 

most benefit to the engineer and owner; describes 

how NDT test plans should be developed for data 

collection; presents several methods for using the 

NDT data to characterize a pavement; and describes 

how the results from NDT should be used as inputs to 

evaluation, design, and pavement management 

analyses that comply with FAA policy. 

There are many software programs that can be used 

to collect and analyze NDT data, and this AC will 

reference many of them. The FAA's back-calculation 

program, BAKFAA, can be downloaded from the 

FAA website and can be used to analyze NDT data, 

subject to the limitations discussed herein. 

2. BACKGROUND. Recent advances in hardware 

and software technology have significantly improved 

NDT equipment, data collection, and analysis 

software. Not only has NDT work been conducted 

on hundreds of airport pavements throughout the 

world, it has been extensively used to evaluate and 

design interstate highways, state highways, tollways, 

county roads, city streets, and seaports. NDT is also 

being used by researchers to improve pavement 

evaluation and design methodologies.

The Federal Highway Administration (FHWA) uses 

NDT equipment to collect data at hundreds of test 

section sites throughout the U.S. The FAA currently 

uses NDT equipment to collect data at the National 

Airport Pavement Test Facility (NAPTF) in Atlantic 

City, NJ to advance airport pavement evaluation and 

design methods. 

There are several advantages to using NDT, in lieu 

of, or supplement traditional destructive tests. Most 

important, is the capability to quickly gather data at 

several locations while keeping a runway, taxiway, or 

apron operational during these 2-minute to 3-minute 

tests, provided the testing is under close contact with 

Air Traffic Control. Without NDT, structural data 

must be obtained from numerous cores, borings, and 

excavation pits on an existing airport pavement. This 

can be very disruptive to airport operations. For 

example, to conduct a plate load test for measuring 

in-situ modulus of subgrade reaction, k, tests, 4-foot 

(1.2 m) by 6-foot (1.8 m) pits are prepared by 

removing each pavement layer until the subgrade is 

exposed. Once the plate-bearing test is completed, 

the repair of a test pit can be expensive and may keep 

the test area closed for several days. 

Nondestructive tests are economical to perform and 

data can be collected at up to 250 locations per day. 

The NDT equipment measures pavement surface 

response (i.e., deflections) from an applied dynamic 

load that simulates a moving wheel. The magnitude 

of the applied dynamic load can be varied so that it is 

similar to the load on a single wheel of the critical or 

design aircraft. Pavement deflections are recorded 

directly beneath the load plate and at typical radial 

offsets of 12 inches (300 mm), out to typical 

distances of 60 inches (1,500 mm) to 72 inches 

(1,800 mm). 

The deflection data that are collected with NDT 

equipment can provide both qualitative and 

quantitative data about the strength of a pavement at 

the time of testing. The raw deflection data directly 

beneath the load plate sensor provides an indication 

of the strength of the entire pavement structure. 

Likewise, the raw deflection data from the outermost 

sensor provides an indication of subgrade strength. 
AC 150/5370-11A 12/29/2004 

Page 2 

In addition, when deflection or stiffness profile plots 

are constructed with deflection data from all test 

locations on a pavement facility, relatively strong and 

weak areas become readily apparent. 

Quantitative data from NDT include material 

properties of each pavement and subgrade layer that 

engineers use with other physical properties, such as 

layer thicknesses and interface bonding conditions, to 

evaluate the structural performance of a pavement or 

investigate strengthening options. Most of the 

material property information is obtained using 

software programs that process and analyze raw NDT 

data. Once material properties, such as modulus of 

elasticity, E, and modulus of subgrade reaction, k, are 

computed, the engineer can conduct structural 

evaluations of existing pavements, design structural 

improvements, and develop reconstruction pavement 

cross-sections using subgrade strength data. 

3. LIMITATIONS TO NDT. Although NDT has 

many advantages, it also has some limitations. NDT 

is a very good methodology for assessing the 

structural condition of an airfield pavement; however, 

engineers must use other methods to evaluate the 

functional condition of the pavement, e.g., visual 

condition, smoothness, and friction characteristics. 

The visual condition is most frequently assessed 

using the PCI in accordance with American Society 

for Testing and Materials (ASTM) D 5340, Standard 

Test Method for Airport Pavement Condition Index 

Surveys, and AC 150/5380-6, Guidelines and 

Procedures for Maintenance of Airport Pavements. 

Once the NDT-based structural and functional 

conditions are known, the engineer can assign an 

overall pavement condition rating.

The differentiation between structural and functional 

performance is important in developing requirements 

for pavement rehabilitation. For example, a 

pavement can have a low PCI due to environmental 

distress, yet the pavement has sufficient thickness to 

accommodate structural loading. The converse may 

also be true, whereby a pavement may be in good 

condition, but has a low structural life due to 

proposed heavier aircraft loading. 

In addition, while NDT may provide excellent 

information about structural capacity, the engineer 

may still require other important engineering 

properties of the pavement layers, such as grain-size 

distribution of the subgrade to determine swelling 

and heaving potential. For subsurface drainage 

evaluation and design, grain-size distribution and 

permeability tests may help assess the hydraulic 

capacity of the base, subbase, and subgrade. 

It should also be noted that quantitative results 

obtained from raw NDT data are model dependent. 

The results depend on the structural models and 

software algorithms that are used by programs that 

process NDT data and perform a back-calculation of 

layer material properties. 

Because of the model dependencies of NDT software 

analysis tools, the engineer should exercise caution 

when evaluating selected pavement types, such as 

continuously reinforced concrete pavement, posttensioned concrete, and pre-tensioned concrete. The 

structural theory and performance models for these 

pavement types are significantly different than 

traditional pavements, which include Asphalt Cement 

(HMA), jointed plain Portland Cement Concrete 

(PCC), jointed reinforced PCC, HMA overlaid PCC, 

and PCC overlaid PCC. 

Finally, NDT conducted at different times during the 

year may give different results due to climatic 

changes. For example, tests conducted during spring 

thaw or after extended dry periods may provide nonrepresentative results or inaccurate conclusions on 

pavement at subgrade strength. 

4. RELATED ADVISORY CIRCULARS. The 

following ACs provides additional information 

regarding NDT and structural analysis of airport 

pavements:

a. 150/5320-6, Airport Pavement Design and 

Evaluation. 

b. 150/5320-12, Measurement, Construction, 

and Maintenance of Skid Resistant Airport Pavement 

Surfaces. 

c. 150/5335-5, Standardized Method of 

Reporting Airport Pavement Strength PCN. 

d. 150/5380-6, Guidelines and Procedures for 

Maintenance of Airport Pavements.

5. ORGANIZATION OF THIS AC. The 

following chapters in this AC present an overview of 

the NDT data collection process and equipment that 

are used to collect the field data. The AC then 

focuses on how to prepare a test plan and develop 

procedures that should be used for data acquisition. 

The final chapters focus on processing the raw data to 

obtain pavement material characteristics that can then 

be used to evaluate a pavement's load-carrying 

capacity, remaining structural life, or strengthening 

requirements.
AC 150/5370-11A 12/29/2004 

Page 3 

CHAPTER 2–DESCRIPTION OF NDT PROCESS 

6. GENERAL. NDT, using static or dynamic 

testing equipment, has proven useful in providing 

data on the structural properties of pavement and 

subgrade layers. The data are typically used to detect 

patterns of variability in pavement support conditions 

or to estimate the strength of pavement and subgrade 

layers. With this information, the engineer can 

design rehabilitation overlays and new/reconstructed 

cross-sections, or optimize a rehabilitation option that 

is developed from an APMS.

This AC focuses on nondestructive testing equipment 

that measures pavement surface deflections after 

applying a static or dynamic load to the pavement. 

NDT equipment that imparts dynamic loads creates 

surface deflections by applying a vibratory or 

impulse load to the pavement surface through a 

loading plate. For vibratory equipment, the dynamic 

load is typically generated hydraulically, as is the 

case for the Road Rater, or by counter rotating 

masses, as is the case for the Dynaflect. For impulse 

devices, such as the Falling Weight Deflectometer 

(FWD), the dynamic load is generated by a mass free 

falling onto a set of rubber springs, as shown in 

Figure 1 in Appendix 1. The magnitude of the 

impulse load can be varied by changing the mass 

and/or drop height so that it is similar to that of a 

wheel load on the main gear of an aircraft. 

For both impulse and vibratory equipment, pavement 

response is typically measured by a series of sensors 

radially displaced from the loading plate, as shown in 

Figure 2. For static devices, such as the Benkleman 

Beam, a rebound deflection from a truck or other 

vehicle load is measured. Typically, the rebound 

deflection is measured only at the location of the load 

and not at the other radially spaced sensors shown in 

Figure 2. 

7. PAVEMENT STIFFNESS AND SENSOR 

RESPONSE. The load-response data that NDT 

equipment measure in the field provides valuable 

information on the strength of the pavement 

structure. Initial review of the deflection under the 

load plate and at the outermost sensor, sensors D1 

and D7 in Figure 2, respectively, is an indicator of 

pavement and subgrade stiffness. Although this 

information will not provide information about the 

strength of each pavement layer, it does provide a 

quick assessment of the pavement's overall strength 

and relative variability of strength within a particular 

facility (runway, taxiway, or apron).

Pavement stiffness is defined as the dynamic force 

divided by the pavement deflection at the center of 

the load plate. For both impulse and vibratory 

devices, the stiffness is defined as the load divided by 

the maximum deflection under the load plate. The 

Impulse Stiffness Modulus (ISM) and the Dynamic 

Stiffness Modulus (DSM) are defined as follows for 

impulse and vibratory NDT devices, respectively: 

I(D)SM = L / do 

Where: I(D)SM = Impulse and Dynamic Stiffness 

 Modulus (kips/inch) 

 L = Applied Load (kips) 

 do = Maximum Deflection of Load 

 Plate (inches) 

8. DEFLECTION BASIN. After the load is 

applied to the pavement surface, as shown in Figure 

1, the sensors shown in Figure 2 are used to measure 

the deflections that produce what is commonly 

referred to as a deflection basin. Figure 3 shows the 

zone of load influence that is created by a FWD and 

the relative location of the sensors that measure the 

deflection basin area. The deflection basin area can 

then be used to obtain additional information about 

the individual layers in the pavement structure that 

cannot be obtained by using deflection data from a 

single sensor. 

The shape of the basin is determined by the response 

of the pavement to the applied load. The pavement 

deflection is the largest directly beneath the load and 

then decreases as the distance from the load 

increases. Generally, a weaker pavement will deflect 

more than a stronger pavement under the same load. 

However, the shape of the basin is related to the 

strengths of all the individual layers. 

To illustrate the importance of measuring the 

deflection basin, Figure 4 shows a comparison of 

three pavements. Pavement 1 is PCC and pavements 

2 and 3 are HMA. As expected, the PCC distributes 

the applied load over a larger area and has a smaller 

maximum deflection than the other two pavements. 

Although pavements 2 and 3 have the same crosssection and the same maximum deflection under the 

load plate, they would presumably perform 

differently under the same loading conditions 

because of the differences in base and subgrade 

strengths. 
AC 150/5370-11A 12/29/2004 

Page 4 

In addition to each layer's material properties, other 

factors can contribute to differences in the deflection 

basins. Underlying stiff or apparent stiff layers, the 

temperature of the HMA layer during testing, 

moisture contents in each of the layers, and PCC slab 

warping and curling can affect deflection basin 

shapes. An important component in the evaluation 

process, then, is analysis of the NDT data to estimate 

the expected structural performance of each 

pavement layer and subgrade. 

9. USE OF NDT DATA. There are many ways to 

use the NDT data to obtain those pavement 

characteristics that are needed to identify the causes 

of pavement distresses, conduct a pavement 

evaluation, or perform a strengthening design. 

Engineers can evaluate the NDT data using 

qualitative and quantitative procedures. Subsequent 

chapters present several methods that can be used to 

compute and evaluate such pavement characteristics 

as: 

a. ISM, DSM, and normalized deflections. 

b. Back-calculated elastic modulus of 

pavement layers and subgrade. 

c. Correlations to conventional 

characterizations (e.g., California Bearing Ratio 

[CBR], k). 

d. Crack and joint load transfer efficiency. 

e. Void detection at PCC corners and joints. 

These NDT-derived pavement characteristics can 

then be used in the FAA's evaluation and design 

procedures. 

 
AC 150/5370-11A 12/29/2004 

Page 5 

CHAPTER 3–NONDESTRUCTIVE TESTING EQUIPMENT 

This chapter introduces the various types of NDT 

equipment that are used to evaluate pavements. 

Although the AC focuses on NDT equipment, other 

types of nondeflection measuring equipment are 

introduced to illustrate how NDT data can be 

supplemented with other test data to improve the 

quality and reliability of the pavement evaluation. 

10. CATEGORIES OF EQUIPMENT. 

Nondestructive testing equipment includes both 

deflection and nondeflection testing equipment. 

Deflection measuring equipment for nondestructive 

testing of airport pavements can be broadly classified 

as static or dynamic loading devices. Dynamic 

loading equipment can be further classified according 

to the type of forcing function used, i.e., vibratory or 

impulse devices. Nondeflection measuring 

equipment that can supplement deflection testing 

includes ground-penetrating radar, infrared 

thermography, dynamic cone penetrometer, and 

devices that measure surface friction, roughness, and 

surface waves. 

a. Deflection Measuring Equipment. There 

are several categories of deflection measuring 

equipment: static, steady state (e.g., vibratory), and 

impulse load devices. A static device measures 

deflection at one point under a nonmoving load. 

Static tests are slow and labor intensive compared to 

the other devices. Examples of a static device 

include the Benkleman Beam and other types of plate 

bearing tests. 

Vibratory devices induce a steady-state vibration to 

the pavement with a dynamic force generator, as 

illustrated in Figure 5. As this figure shows, there is 

a small static load that seats the load plate on the 

pavement. The dynamic force is then generated at a 

precomputed frequency that causes the pavement to 

respond (deflect). The pavement deflections are 

typically measured with velocity transducers. There 

are several types of steady-state vibratory devices, 

including Dynaflect and Road Rater. 

Impulse load devices, such as the FWD or HeavyFalling Weight Deflectometer (HWD), impart an 

impulse load to the pavement with free-falling weight 

that impacts a set of rubber springs, as illustrated in 

Figure 6. The time from A to B in this figure is the 

time required to lift the FWD weight package to the 

required drop height. The magnitude of the dynamic 

load depends on the mass of the weight and the 

height from which the weight is dropped. 

The resultant deflections are typically measured with 

velocity transducers, accelerometers, or linear 

variable differential transducers (LVDT). 

Table 1 in Appendix 2 provides a summary of the 

various types of static, vibratory, and impulse load 

NDT equipment that are in use or in production 

today. The most popular and widely used NDT 

equipment falls in the impulse-based category. This 

category of NDT equipment is used extensively for 

airport, road, and seaport pavement testing. 

b. Nondeflection Measuring Equipment. 

Several other types of nondestructive testing 

equipment are available that may assist the engineer 

in conducting a pavement evaluation, performing a 

pavement design, or implementing a pavement 

management system. The data that are collected 

from nondeflection measuring equipment often 

supplement NDT data or provide standalone 

information in pavement analysis work. While 

deflection data from NDT equipment are used 

primarily to evaluate the structural capacity and 

condition of a pavement, the following nondeflection 

measuring equipment can also be used:

(1) Ground-Penetrating Radar (GPR)⎯The 

most common uses of GPR data include measuring 

pavement layer thicknesses, identifying large voids, 

detecting the presence of excess water in structure, 

locating underground utilities, and investigating 

significant delamination between pavement layers. 

(2) Spectral Analysis of Surface Waves 

(SASW)⎯SASW equipment provides data that can 

supplement NDT data. Unlike NDT equipment, 

which imparts much higher loads to the pavement, 

SASW equipment consists of small portable units 

that evaluate pavements from Rayleigh wave 

measurements that involve low strain levels. 

Engineers can then evaluate these data to compute 

the approximate thickness of pavement layers, layer 

modulus of elasticity values for comparison to NDT 

computed elasticity values, and approximate depth to 

rigid layers. 

(3) Infrared Thermography (IR)⎯One of 

the most common uses of IR data is to determine if 

delamination has occurred between asphalt pavement 

layers. 
AC 150/5370-11A 12/29/2004 

Page 6 

(4) Friction Characteristics⎯There are 

types of equipment that are available to conduct 

surface friction tests on a pavement. The methods of 

testing and several common types of friction testers 

for airports are addressed in AC 150/5320-12. 

(5) Smoothness Characteristics⎯There are 

also several types of equipment that are available to 

collect surface profile data and to determine how 

aircraft may respond during taxi, takeoff, and 

landing. 

(6) Dynamic Cone Penetrometer (DCP)⎯A 

DCP is another piece of equipment that can be used 

to supplement NDT data. If cores are taken through 

the pavement to verify the thickness of an HMA or 

PCC layer, the DCP can help evaluate the stiffness of 

the base, subbase, and subgrade. Data are recorded 

in terms of the number of blows per inch that is 

required to drive the cone-shaped end of the rod 

through each of the layers. Plots of these data 

provide information about the changes in layer types 

and layer strengths. 

11. GENERAL REQUIREMENTS FOR NDT 

EQUIPMENT. If deflection measuring equipment 

is being considered for use in a pavement study, the 

engineer should first evaluate project requirements. 

To provide meaningful results, several general 

requirements should be considered regarding 

equipment capabilities. The quality of the NDT 

results will depend on several factors, such as the 

quality of the test plan, test procedures, and data 

analyses procedures, as described in subsequent 

chapters of this AC.

In general, the value of NDT will be greater for 

primary airports compared to general aviation (GA) 

airports. However, if a GA airport supports, or will 

support, aircraft with a maximum gross takeoff 

weight greater than 30,000 pounds (13,500 kg), or 

heavy aircraft are expected to use the airport on an 

infrequent basis, NDT may be useful in evaluating 

the pavement. Also, because of the increasing 

number of business jets that operate from reliever and 

GA airports, NDT may add significant value to a GA 

pavement study. 

If nondestructive testing is indicated, the airport 

sponsor should consider the operational impacts of 

operating the equipment on the airside. While NDT 

equipment can collect data at many locations over a 

relatively short period of time, the airport may not be 

able to close a particular facility during peak periods 

of aircraft operations. 

Depending on the frequency and types of NDT tests, 

the work on a typical runway that is 9,000 feet (2,750 

m) long and 150 feet (45 m) wide normally takes 1 to 

2 days. If peak traffic occurs during daylight hours, 

it may be more efficient to conduct the NDT at night 

when the facility can be closed for 6 to 8 hour 

periods. 

If the sponsor and engineer decide to conduct NDT, 

they should carefully consider the type of equipment 

that will be used for the study. In general, the 

equipment should be capable of imparting a dynamic 

load to the pavement that creates deflections and 

loads that are large enough to be accurately recorded 

with the sensors on the pavement surface. The 

required magnitude of the dynamic load will depend 

primarily on the thickness and strength of the 

pavement layers. If the deflections are adequate for 

the structure and type of aircraft that will use the 

pavement, the NDT equipment sensors should 

provide accurate and repeatable deflection 

measurements at each sensor location. 

Repeatability is important for two reasons. First, 

NDT may be conducted at multiple load levels to 

learn more about the pavement structure, such as 

whether voids exist or if the subgrade soil is stress 

sensitive and appears to get harder or softer with 

increasing load. To characterize the pavement 

properly, the sensors must accurately and consistently 

record deflection data. Second, because pavements 

deteriorate over time, subsequent pavement 

evaluation and NDT work may be important. To 

quantify the rate of deterioration, it is important to 

have reliable deflection data at different times during 

the pavement's design life. 

12. STATIC DEVICES. The most common static 

device is the Benkleman Beam, although several 

other devices have been built to automate its use. 

Examples of automated beams include the Swedish 

La Croix Deflectograph; the British Transport and 

Road Research Laboratory Pavement Deflection Data 

Logging (PDDL), which is a modified La Croix 

Deflectograph; and Caltran's California Traveling 

Deflectometer. Figure 7 shows a Benkleman Beam 

that has not been automated. 

The Benkleman Beam measures the deflection under 

a static load, such as a truck or aircraft. The truck 

weight is normally 18,000 pounds (8,165 kg) or a 

single axle with dual tires. The tip of the beam is 

placed between the dual tires and the rebound 

deflection is measured as the vehicle moves away 

from the beam. 
AC 150/5370-11A 12/29/2004 

Page 7 

The primary advantages that are associated with the 

Benkleman Beam are its simplicity and the numerous 

design procedures that have historically used beam 

data. Disadvantages to its use include longer testing 

time and the lack of repeatability of results as 

compared with more modern devices. The 

Benkleman Beam also does not typically provide 

deflection basin data for back-calculation of 

pavement layer moduli. 

13. VIBRATORY DEVICES. Vibratory devices 

include the Dynaflect and the Road Rater. 

a. Dynaflect. The Dynaflect, shown in Figure 

8, is an electromechanical device for measuring 

dynamic deflection. It is mounted on a two-wheel 

trailer and is stationary when the measurements are 

taken. A 1,000-pound (5 kN) peak-to-peak 

sinusoidal load is applied through two rubber coated 

steel wheels at a fixed 8Hz frequency. The counterrotating masses produce a sinusoidal pavement 

deflection, which is recorded by velocity transducers. 

Advantages of the Dynaflect include high reliability, 

low maintenance, and the ability to measure the 

deflection basin. A major disadvantage of the 

equipment is the low dynamic load amplitude, which 

is significantly less than normal aircraft loads. The 

relatively light load may not produce adequate 

deflections on heavy airport pavements and the backcalculated subgrade moduli may not be accurate. 

Therefore, the use of this device is only 

recommended for light load pavements serving 

aircraft less than 12,500 pounds (5,670 kg). 

b. Road Rater. The Road Rater, shown in 

Figure 9, also measures dynamic deflection using a 

sinusoidal force generated by a hydraulic acceleration 

of a steel mass. Several models are available that 

have peak-to-peak loading that ranges from a low of 

500 pounds (2 kN) to a high of 8,000 pounds (35 

kN). Pavement response is measured at the center of 

the loading plate and at radial offset distances using 

four to seven velocity transducers, depending on the 

model. The Road Rater can measure deflection 

basins, as well as dynamic response over a broad 

range of frequencies. It has a rapid data acquisition 

system and its wide use has resulted in the 

availability of large amounts of data on pavement 

response and performance. The major disadvantage 

of the Road Rater is low force amplitude on some 

models. 

14. IMPULSE DEVICES. These devices measure 

deflection using a free-falling mass onto rubber 

springs to produce an impulse load. The magnitude 

of the calculated dynamic load and the resultant 

pavement deflections are recorded. Generally, these 

devices fall into one of two categories: FWD and 

HWD. Most impulse devices are classified as a 

HWD when they are able to generate a maximum 

dynamic load that is greater than 34,000 pounds (150 

kN).

There are several manufacturers of FWDs and 

HWDs, including KUAB America, Dynatest Group, 

Phoenix Scientific, Inc., Foundation Mechanics, Inc., 

and Viatest. These impulse devices all share several 

common advantages for this type of deflection 

measuring equipment. The FWD and HWD are 

believed to better simulate moving wheel loads, can 

measure the extent of the deflection basin, have 

relatively fast data acquisition, and require only a 

small preload on the pavement surface. The 

disadvantages of the equipment are minimal and 

related more to the overall systems and different 

pulse durations used on different models. Table 2 

provides a detailed summary of the impulse 

equipment specifications. 

a. KUAB America. KUAB manufactures a 

FWD (Figure 10) and HWD, which include five 

models with load ranges up to 66,000 pounds (294 

kN). The load is applied through a two-mass system, 

and the resultant dynamic response is measured with 

seismometers and LVDTs through a mass-spring 

reference system. The load plate is segmented to 

provide a uniform pressure distribution to the 

pavement. 

b. Dynatest Group. Dynatest manufactures 

both a FWD (Figure 11) and a HWD with models 

that generate dynamic loads up to 54,000 pounds 

(240 kN). The weights are dropped onto a rubber 

buffer system. Seven to nine velocity transducers are 

then used to measure the load and dynamic response. 

c. Carl Bro Group. Carl Bro manufactures the 

FWD and the HWD generating dynamic loads up to 

56,000 pounds (250 kN). Furthermore, Carl Bro 

manufactures the portable FWD (LWD) – PRIMA 

100 – for on-site measurement and analysis of 

collected data. 
AC 150/5370-11A 12/29/2004 

Page 8 

All equipment types are module-designed. With 

PRI models, this means that it it possible to move the 

measuring equipment from a trailer chassis into a van 

and to upgrade from 7-150 kN to 7-250 kN. With 

PRIMA 100, this means that one or two additional 

geophones can be connected depending on the work 

to be performed and wireless and GPS operation. The 

LWD uses 1-3 velocity transducers and FWD and 

HWD equipment uses 9-12 velocity transducers to 

measure load and dynamic response. Weights are 

dropped on a rubber buffer system and the load plates 

are four-split allowing maximum contact to the 

surface measured upon. 

d. Foundation Mechanics, Incorporated. 

Foundation Mechanics also manufactures a JILS 

FWD and a JILS HWD (Figure 13) that generate 

loads from 1,500 pounds (7 kN) to 54,000 pounds 

(240 kN). The FWD and HWD use two mass 

elements and a four-spring set combination to impose 

a force impulse in the shape of a half-sine wave. 

Load magnitude, duration, and rise time are 

dependent on the mass, mass drop height, and 

arresting spring properties. Seven velocity 

transducers are typically used to measure the 

dynamic response. 

Although impulse deflection measuring equipment 

are widely used in the pavement industry, vibratory 

and static equipment are still in operation, and 

extensive amounts of data using these devices have 

been collected over many years. Since historical data 

are important in a pavement study, Chapter 7 

discusses how those data, or data from older devices, 

can be used in the pavement study. 

In addition to the deflection and nondeflection 

measuring equipment discussed above, ongoing 

research in the development of a rolling wheel 

deflectometer may produce deflection measuring 

equipment that collects continuous deflection profiles 

at speeds of 50 miles (80 km) per hour. However, 

these devices are still in the development stage. 
AC 150/5370-11A 12/29/2004 

Page 9 

CHAPTER 4–REQUIREMENTS FOR NONDESTRUCTIVE TESTING EQUIPMENT 

This chapter addresses key issues that an airport 

sponsor or engineer should consider when selecting 

or approving a specific NDT device for an airport 

pavement study. The FAA does not have an 

approved list of deflection measuring equipment but 

does want to ensure that standards are established for 

the collection of deflection data. 

15. NEED FOR STANDARDIZATION. The 

analysis of raw deflection data can lead to varying 

conclusions regarding the strength of a pavement. 

Therefore, it is important to ensure that deflection 

data are consistent and repeatable among the various 

types of equipment within the static, vibratory, and 

impulse NDT categories. Because of Federal 

participation in pavement studies, the FAA must have 

standards to ensure reliable data collection.

A valuable benefit of NDT data is the ability to 

record relative variations in pavement strength 

between test locations. Variations in pavement 

strength are typically the result of variations in layer 

thicknesses and strength, temperature susceptibility 

of paving materials, seasonal effects, water table 

heights, frost depths, and NDT equipment itself. 

This chapter provides guidance on standardization for 

the various components of deflection measuring 

equipment so equipment or test variance can be 

minimized. Table 3 provides ASTM references for 

the equipment categories addressed in this AC. As 

previously described, the most common type of NDT 

equipment in use today is the impulse load device, 

(i.e., FWD or HWD). ASTM D 4694, Standard Test 

Method for Deflections with a Falling-Weight-Type 

Impulse Load Device, addresses key components of 

this device, which include instruments exposed to the 

elements, the force-generating device (e.g., falling 

weight), the loading plate, the deflection sensor, the 

load cell, and the data processing and storage system. 

Calibration of the equipment is very important to 

ensure accurate recordation of deflection data. 

ASTM D 4694 recommends the following calibration 

schedule for the impulse load device: 

a. Force-Generating Device (prior to testing 

or other component calibration). This calibration 

involves preconditioning the device by dropping the 

weight at least five times and checking the relative 

difference in each loading. 

b. Deflection Sensors (at least once a month 

or as specified by the manufacturer). During this 

calibration, the deflection measurements for each 

sensor are adjusted so they will produce the same 

deflection measurement within the precision limits of 

the sensors, as specified by the manufacturer.

16. FAA SENSITIVITY STUDY. Assuming the 

NDT device is correctly calibrated and functioning 

properly, the engineer or equipment operator will 

make several decisions concerning testing options for 

the deflection measuring equipment. 

a. Load Plate Diameter. Many impulseloading equipment manufacturers offer the option of 

a 12-inch (300 mm) or an 18-inch (450 mm) diameter 

load plate. There are several important factors that 

should be considered when selecting the load plate 

size for a pavement study, including the following: 

(1) Most Common Plate Size⎯It is much 

easier to evaluate NDT data if all the data has been 

collected using one plate size. Most analysis 

software has been written for both 12-inch (300 mm) 

and 18-inch (450 mm) plate sizes. 

(2) Pavement Layer Compression⎯A 

larger load plate has the advantage of distributing the 

impulse load over larger areas and minimizing the 

amount of layer compression. The importance of the 

plate size depends on the magnitude of the load, 

surface temperature, and if the surface layer consists 

of unbound or bounded material. Since most NDT 

work is conducted on HMA and PCC surfaces when 

the pavement is not extremely hot, compression is 

generally not a significant concern. However, if 

NDT is conducted on an unbound granular base, 

subbase, or subgrade, the larger plate may be more 

advantageous. 

(3) Plate Seating on Pavement Surface⎯If 

the surface of the pavement is very rough, the larger 

plate may not seat properly on the surface and cause 

a nonuniform distribution of the impulse load. A 

segmented load plate helps mitigate the effects of a 

rough surface. 

(4) Summary⎯The 12-inch (300 mm) load 

plate is normally used when testing on bound surface 

materials. If NDT is to be performed on unbound 

base, subbase, or subgrade materials an 18-inch (450 

mm) load plate should be used. If the manufacturer 

does not provide the larger load plate, the engineer 

can use the smaller load plate, but should rely more 

on the deflection sensors away from the load plate. 
AC 150/5370-11A 12/29/2004 

Page 10 

b. Sensor Spacing and Number. The number 

of available sensors depends on the manufacturer and 

equipment model. As a result, the sensor spacing 

will depend on the number of available sensors and 

the length of the sensor bar. Although most NDT 

equipment allows for the sensors to be repositioned 

for each pavement study, it is desirable to conduct 

NDT work using the same configuration, regardless 

of the type of pavement structure. 

Table 4 shows common sensor configurations that are 

used by various agencies. In general, those NDT 

devices that have more sensors can more accurately 

measure the deflection basin that is produced by 

static or dynamic loads. Most agencies prefer to limit 

the distance between sensor spacing to no more than 

12 inches (300 mm). The exception is the seventh 

sensor in the Strategic Highway Research Program 

(SHRP) configuration, where there are 24 inches 

(600 mm) between the sixth and seventh sensors. 

Accurate measurement of the deflection basin is 

especially important when analyzing the deflection 

data to compute the elastic modulus of each 

pavement layer. However, while accurate 

measurement of the deflection basin is important, it is 

also very important to ensure that the magnitude of 

deflection in the outermost sensor is within the 

manufacturer's specifications for the sensors. The 

magnitude of the deflection in the outermost sensor 

depends primarily on the magnitude of the dynamic 

load, the thickness and stiffness of the pavement 

structure, and the depth to an underlying rock or stiff 

layer. 

c. Pulse Duration. For impulse-load NDT 

equipment, the force-pulse duration is the length of 

time between an initial rise in the dynamic load until 

it dissipates to near zero. Both the FAA and ASTM 

recognize a pulse duration in the range of 20 to 60 

milliseconds as being typical for most impulse-load 

devices. Likewise, rise time is the time between an 

initial rise in the dynamic load and its peak before it 

begins to dissipate. Typical rise times for impulseload devices are in the range of 10 to 30 milliseconds.

d. Load Linearity. During the analysis of 

deflection data, engineers often assume that all layers 

in the structure respond in a linear elastic mode. For 

example, this means that a 10-percent increase in the 

magnitude of the dynamic load from the NDT device 

will lead to a 10-percent increase in the response to 

the dynamic load increase. For most pavement 

structures and testing conditions, traditional paving 

materials will behave in a linear elastic manner 

within the load range that the tests are conducted. 

At the NAPTF, the FAA studied the response of the 

flexible pavement test items. The test sections 

included flexible pavement on aggregate and 

stabilized bases that were constructed on low-, 

medium-, and high-strength subgrade. The FAA 

tested each test section using HWD loads of 12,000 

pounds (50 kN), 24,000 pounds (107 kN), and 36,000 

pounds (160 kN). 

Figures 14 and 15 show the linear behavior of the 

HMA test sections in terms of the ISM and backcalculated subgrade elastic modulus. The procedures 

for back-calculation of the subgrade modulus are 

discussed in Chapter 7. For the ISM and computed 

subgrade modulus, results of the sensitivity study 

showed there is little difference in the pavement 

response when the HWD impulse load is changed, 

provided the measured deflections are within the 

specified limits of the sensors. A linear response was 

also observed when the FAA conducted similar tests 

on the instrumented PCC runway test section at 

Denver International Airport (DIA), CO. 

Based on the results from the sensitivity studies at the 

NAPTF and DIA, the amplitude of the impulse load 

is not critical provided the generated deflections are 

within the limits of all deflection sensors. The key 

factors that will determine the allowable range of 

impulse loads are pavement layer thicknesses and 

material types. Thus, unless the pavement is a very 

thick PCC or HMA overlaid PCC structure, most 

FWD devices will be acceptable since they will be 

able to generate sufficient deflections for reliable data 

acquisition. 

Generally, the impulse load should range between 

20,000 pounds (90 kN) and 55,000 pounds (245 kN) 

on pavements serving commercial air carrier aircraft, 

provided the maximum reliable displacement sensor 

is not exceeded. Lighter loads may be used on 

thinner GA pavements. 

17. SUMMARY OF FAA POLICY. This section 

provides guidance on the equipment options that are 

associated with most types of deflection measuring 

equipment. Proper configuration of the NDT device 

regarding load plate size, sensor number and spacing, 

and impulse load magnitudes will ensure that 

consistent, reliable, and reusable deflection data can 

be recorded with the equipment. Before mobilizing 

to the field, the engineer should develop an NDT test 

plan, as described in Chapter 5, that can be properly 

executed, as described in Chapter 6. 
AC 150/5370-11A 12/29/2004 

Page 11 

CHAPTER 5 – TEST PLANNING 

Chapter 4 presented several equipment options for 

various NDT devices. This chapter discusses how to 

prepare an NDT plan before mobilizing to the field. 

Chapter 6 focuses on executing that NDT plan in the 

field. Together, all three chapters stress the 

importance of standardization so the deflection data 

that is recorded in the field is consistent, repeatable, 

and reliable. Data collection methods that meet these 

requirements will help ensure that future deflection 

data for the same pavement section can be compared 

to previous results to determine how quickly the 

pavement may be deteriorating at various stages of its 

design life. 

18. JUSTIFICATION FOR NDT. Before 

developing an NDT test plan, the airport sponsor and 

engineer should decide if the current situation 

warrants the collection of deflection data. Visual 

condition surveys, such as the PCI procedure, provide 

excellent information regarding the functional 

condition of the pavement. However, visual distress 

data can only provide an indirect measure of the 

structural condition of the pavement structure. 

Nondestructive testing combined with the analytical 

procedures described herein can provide a direct 

indication of a pavement’s structural performance.

Most commercial hub airports have fleet mixes that 

contain heavy narrow- and wide-body aircraft at a 

significant number of annual departures. The 

potential for structural damage typically depends on 

the number of annual departures and the maximum 

gross takeoff weights (MGTOW) of aircraft 

exceeding 100,000 pounds (45,360 kg).

On the other end of the scale, most GA airports do 

not support routine operation of aircraft with 

MGTOWs exceeding 60,000 pounds (27,200 kg). 

However, there are scenarios where one or two 

departures of a heavy aircraft could cause significant 

damage to the pavement structure. Therefore, the 

ability to evaluate whether the pavement can 

accommodate occasional overload situations 

significantly benefit airport operation. Also, many 

GA airports service high tire pressure corporate jet 

operations of 20,000 pounds (9,100 kg) to 60,000 

pounds (27,200 kg) that could justify an NDT 

program. 

Once the airport sponsor and engineer have decided 

to include NDT in their pavement study, they should 

focus on the number and types of tests that will be 

conducted. The total number of tests will depend 

primarily on three factors: 

a. The area of the pavements to be included in 

the study. 

b. The types of pavement. 

c. The type of study, which is typically 

referred to as a project or network-level investigation. 

Project-level investigations refer to studies that are 

conducted in support of pavement rehabilitation, 

reconstruction, and new construction designs. 

Network-level studies generally support the 

implementation and updates of pavement 

management systems. The frequency of the NDTs is 

greater in a project-level study that may typically 

include only one or two pavement facilities. This is 

in contrast to a network-level study, which may 

include all airside pavements, all landside pavements, 

or both. 

19. NDT TEST OBJECTIVES. The objective of 

the NDT program is to collect deflection data that 

will support the objectives of a project or networklevel pavement study. The data should be collected 

efficiently with minimal disruption to aircraft or 

vehicle traffic operations on the airside and landside 

of an airport. The NDT test plan should support the 

project and network-level objectives, which can be 

categorized as follows:

a. Project-Level Objectives: 

(1) Evaluate the load-carrying capacity of 

existing pavements. 

(2) Provide material properties of in-situ 

pavement layers for the design of pavement 

rehabilitation alternatives, which include restoration, 

functional and structural overlays, partial 

reconstruction (e.g., runway keel), and complete 

reconstruction. 

b. Network-Level Objectives: 

(1) Supplement PCI survey data that may 

be stored in an APMS database for those scenarios 

where the NDT data will lead to the development of a 

multiyear Capital Improvement Program (CIP). 

(2) Generate Pavement Classification 

Numbers (PCN) for each airside facility in 

accordance with AC 150/5335-5. 
AC 150/5370-11A 12/29/2004 

Page 12 

20. NDT TEST TYPES. There are several types of 

tests that may be conducted during a pavement study. 

For all types of pavements, the most common test is a 

center test. For jointed PCC and HMA overlaid PCC 

pavements, this is a test in the center of the PCC slab. 

For HMA pavements, this is a test in the center of the 

wheel path away from any cracks that may exist. The 

center test serves primarily to collect deflection data 

that form a deflection basin that can be used to 

estimate the strength of the pavement and subgrade 

layers.

For PCC and HMA overlaid PCC pavements, there 

are several other types of tests that will help 

characterize the structure. All of these tests focus on 

the fact that most PCC pavements have joints and 

most HMA overlaid PCC pavements have surface 

cracks that have reflected up from PCC joints. NDT 

at various locations on the joints, as shown in Figure 

16, provides data regarding pavement response to 

aircraft loads and changes in climatic conditions. 

Testing at longitudinal and transverse joints shows 

how much of an aircraft's main gear is transferred 

from the loaded slab to the unloaded slab, as shown 

in Figure 17. As the amount of load transfer is 

increased to the unloaded slab, the flexural stress in 

the loaded slab decreases and the pavement life is 

extended. The amount of load transfer depends on 

many factors, including pavement temperature, the 

use of dowel bars, and the use of a stabilized base 

beneath the PCC surface layer. 

Corner testing is another common location to test, as 

shown in Figure 16. This is an area where a loss of 

support beneath the PCC slab occurs more often than 

other areas in the slab. Voids or a loss of support 

generally first occur in the slab corner because this is 

where deflections are the greatest in a PCC slab. 

Therefore, if concrete slabs have corner breaks there 

is a possibility that voids exist. Corner slab testing 

on uncracked slabs in the area would be important in 

this case. Often, concrete midslab, joint, and corner 

tests are performed on the same slab to evaluate the 

relative stiffness at different locations. 

21. TEST LOCATIONS AND SPACING. Once 

the types of NDT have been selected, the next step is 

to select the location and testing interval for each 

pavement facility. Depending on the operating 

conditions and types of tests, the NDT operator can 

typically collect deflection data at 150 to 250 

locations per 8-hour shifts. 

While NDT will provide much better coverage of the 

pavement than destructive testing (e.g., bores and 

cores), a balance should be obtained between 

coverage, cost, and time.

Table 5 provides general guidance on the spacing and 

location of testing for taxiways and runways. The 

offset recommendations are based on an assumed 

longitudinal joint spacing of approximately 18 feet (6 

m) for PCC pavements. The offset distance refers to 

the distance from the taxiway and runway centerline. 

The third offset distances of 60 feet (18 m) and 65 

feet (20 m) are applicable for runways that are wider 

than 125 feet (38 m). Table 6 provides general 

guidance on the frequency and location of testing for 

aprons. 

The total number of tests for each facility should be 

evenly distributed in a grid. Each adjacent NDT pass 

in the grid should be staggered to obtain 

comprehensive coverage. For testing of airside 

access roads, perimeter roads, and other landside 

pavement, the recommendations provided in ASTM 

D 4695, Standard Guide for General Pavement 

Deflection Measurements, should be followed. This 

ASTM standard refers to network level testing as 

"Level I" and project level testing as "Level II" and 

"Level III." 

22. NDT TEST SKETCHES. Once the test types, 

locations, and spacing have been established for the 

pavement study, the next step is to prepare a sketch, 

such as those shown in Figures 18 through 20, that 

clearly shows this information. In addition, the test 

plan should show the beginning station for each test 

facility and the direction of travel. Absent an airport 

wide stationing plan, the low-number end of a 

runway (e.g., end 16 of RW 16-34) can be 

established as NDT Station 0+00. 

Figures 18 and 19 show ways to standardize the 

deflection recording process in the field. For 

example, the centerline joint in Figure 18 is annotated 

as joint "9.2" and the centerline in Figure 19 is noted 

as "Lane 9." 

Figures 18 through 20 provide one example of how to 

develop an NDT sketch so that the engineer and NDT 

equipment operator can efficiently obtain deflection 

data in the field and minimize potential errors or 

misunderstandings. 

In addition to the test lane nomenclature, the engineer 

should also develop standard designations for each 

type of test that will be conducted. 
AC 150/5370-11A 12/29/2004 

Page 13 

This is very important since each type of data should 

be grouped for analysis, as discussed in Chapter 7. 

An example of numerical designations or coding that 

could be used for HMA, PCC, and HMA overlaid 

PCC pavements are: 

a. Center of PCC slab and HMA 

b. Transverse joint 

c. Longitudinal joint 

d. Corner 

e. Transverse crack 

f. Longitudinal crack 

23. SPECIAL CONSIDERATIONS. It is 

important to consider how the climate and weather 

will affect NDT results. In northern climates, NDT is 

generally not conducted during the winter if frost has 

penetrated into the base, subbase, or subgrade. In 

addition, spring thaw represents a seasonal period 

when the pavement may be very weak for a short 

period of time. While it may be beneficial to know 

the strength of the pavement during spring thaw, it 

does not represent the typical strength of that 

structure throughout the year. Therefore, if 

deflection data are not going to be collected more 

than once, the engineer should select a test period 

that best represents the strength of the pavement for a 

majority of the year.

For both HMA and PCC pavements, NDT should not 

be conducted near cracks unless one of the test 

objectives is to measure load transfer efficiency 

across the crack. For HMA pavements, NDT passes 

should be made so that deflection data are at least 1.5 

feet (0.5 m) to 3 feet (1 m) away from longitudinal 

construction joints. 

Another concern for NDT work on PCC pavements is 

slab curling. Slab curling occurs when the corners or 

center of the slab lifts off of the base due to 

differences in temperature between the top and 

bottom of the slab. As shown in Figure 21, the slab 

corners may lift off the base during nighttime curling, 

while the slab center and midjoints may lift off 

during daytime curling. The amount of curling 

depends primarily on joint spacing, PCC layer 

thickness, temperature differential between the 

bottom and top of the slab, and the stiffness of the 

base. 

It is important for engineers to be aware of possible 

curling so they are not confused by the results when 

they are attempting to conduct a void analysis. 

Voids, or loss of support, may occur from 

temperature curling, moisture warping, or erosion of 

the base. In most instances, the engineer is 

attempting to determine if voids exist because of 

erosion, consolidation, or expansive soils. As 

discussed in Chapter 7, for this purpose, engineers 

should conduct NDTs at a time when the change in 

temperature is relatively constant between the day 

and night. 

Finally, NDT test plans should consider that several 

analysis procedures require more than one test per 

location. Void analysis techniques generally require 

at least three load levels at each location. Likewise, 

if there is concern about stress sensitivity of the 

subgrade, multiple tests at different load levels will 

also be needed. 
AC 150/5370-11A 12/29/2004 

Page 14 

CHAPTER 6–TEST PROCEDURES 

Chapter 5 presented guidelines for the development 

of a NDT plan that will meet the objectives of 

project-level or network-level studies. If the NDT 

equipment is properly configured, as discussed in 

Chapter 4, and a comprehensive NDT plan has been 

developed, the last step in the collection of the raw 

deflection data is to mobilize to the airport and safely 

conduct the NDT work. To ensure that quality data 

are collected in accordance with the NDT plan, the 

equipment operator should follow several procedures, 

as described below. 

24. EQUIPMENT MOBILIZATION. Prior to 

mobilizing to the field site, the equipment operator 

should run through a pre-departure checklist, one 

designed for use with all NDT projects. The 

following list highlights several key items that should 

appear on the checklist: 

a. Airport management notified and facility 

closures coordinated with Airport Operations staff. 

b. Appropriate aircraft security and access 

security clearances obtained. 

c. A copy of the NDT test plan and sketch. 

d. An airport map with access roads and gates 

shown. 

e. A check of airport identifiers and radio 

frequencies. 

f. An airport layout plan with all pavements 

and facilities labeled. 

g. A list of key airport personnel and their 

telephone numbers. 

h. Pavement construction history reports. 

i. Verification that all badging requirements 

have been met. 

j. Properly configured deflection sensors. 

k. Equipment and supplies: 

(1) Beacon and flag. 

(2) Spray paint for marking key locations. 

(3) NDT equipment spare parts. 

(4) Radios. 

(5) Small drill for temperature holes. 

(6) Safety vests. 

(7) Equipment lights for nighttime testing. 

l. 24-Hour "go-no-go" checks: 

(1) Weather acceptable. 

(2) NDT equipment checks. 

m. Load cell and deflection sensor calibration 

in check. 

Within 24 hours of mobilization, the operator should 

check to see that weather enroute and at the project 

site is acceptable. In addition, the operator should 

conduct tests using the anticipated loads in 

accordance with the test plan. A nondestructive 

testing device is a high-technology piece of 

equipment that often requires maintenance and repair. 

It is much better to discover mechanical problems 

prior to setting off for the job site. 

25. STARTUP OPERATIONS. Equipment 

preparation for the start of data collection should be 

accomplished prior to accessing the Airport 

Operations Area (AOA). The equipment operator 

should develop the checklist and reuse it for each 

NDT project. The following checklist includes some 

items that should be addressed prior to entering the 

AOA:

a. Has air traffic control been contacted to 

verify testing schedule? 

b. If required, have escorts been contacted? 

c. Are badges properly displayed? 

d. Are all supplies readily available? 

e. Are radios working? 

f. Are copies of the NDT plan, maps, and 

contact telephone numbers on hand? 

g. Has the NDT equipment been run to ensure 

it is working correctly? 
AC 150/5370-11A 12/29/2004 

Page 15 

Conducting these operations prior to entering the 

AOA has several advantages. Most importantly, the 

NDT equipment will be ready to collect deflection 

data as soon as it is allowed on the AOA. It also 

demonstrates to air traffic control that preparations 

have been made to operate on the airside and collect 

data as quickly, safely, and efficiently as aircraft 

traffic operations will permit. Finally, if minor 

maintenance or repair work is required, better 

lighting conditions will exist outside the AOA if the 

work is being done at night. 

26. DATA COLLECTION. Deflection data may 

be collected under several operational scenarios. The 

NDT operator may be working on a small, 

uncontrolled GA airport or on a large commercial 

hub airport. Most large commercial airports require 

the NDT operator to be escorted and may issue a 

Notice to Airmen to close very busy facilities, such 

as a primary runway. Small airports, such as 

relievers, may allow the NDT operator to collect 

deflection data on a "give way" basis during slow 

traffic periods. The operator must be prepared to 

work under all conditions in a safe and efficient 

manner.

a. Industry Standards. The references in 

Table 3 provide guidance on field testing procedures. 

Key field testing issues that are addressed in these 

documents include sensor accuracy and repeatability, 

recording of time and test locations, and 

measurement of pavement temperature 

b. Orientation Photos. Prior to the start of 

testing on a pavement facility, the operator should 

spray paint test lane (PCC) and pass (HMA) locations 

at Station 0+00. Each lane and pass should be at the 

centerline offsets or grid locations specified in the 

NDT test plan. If the offsets need to be adjusted or 

other elements of the plan need to be modified, the 

NDT operator should carefully note the changes and 

reasons for the changes. After Station 0+00 and test 

lanes have been marked, the operator should 

photograph or video tape the markings and an 

overview of the pavement facility, including typical 

surface distresses. 

c. Record and Monitor Pavement 

Temperature. A pavement's response under load is 

often temperature dependent. This is especially true 

for HMA pavements where the modulus of elasticity 

may change dramatically as the temperature rises. 

Also, load transfer across a non-doweled joint on a 

PCC pavement may also change significantly with a 

change in temperature. 

A good location to monitor the mid-depth 

temperature of an HMA or PCC surface layer is at 

Station 0+00. A hole large enough to accommodate a 

thermometer can be chilled by the operator and filled 

with oil to record temperatures every 2 hours or as 

necessary. This location should be marked with 

spray paint so the operator can easily find the 

temperature hole prior to starting NDT on the next 

lane or pass. Ambient and pavement surface 

temperatures should also be recorded, hourly. 

d. Selection of Input Force. The NDT test 

plan should estimate the magnitude of the dynamic 

load for the vibratory or impulse equipment. 

However, one of the first checks that the operator 

should make is to ensure the pre-selected loads are 

generating deflections that are within the 

manufacturer-specified limits of the sensors, as 

discussed in Chapter 4. If multiple tests are being 

conducted at each location at different force levels, 

the operator should ensure that all load levels 

produce deflections within the sensor limits. 

The NDT operator may have to adjust the load levels 

throughout the data collection work if the pavement 

types and thicknesses vary significantly. Given the 

anticipated variation in pavement structures, the 

operator should select intermediate load levels that 

will generate deflections that are large enough for 

thick pavements, but not too large for thin structures. 

If the range of deflections is within the limits of the 

sensor accuracy, the testing operations will be more 

efficient and potential errors that may occur through 

reconfiguration of the NDT device and operating 

software will be minimized. In addition, NDT work 

at large commercial airports may require movement 

from one pavement facility to another prior to 

completion of testing on a particular facility to 

accommodate air traffic control directives. An 

intermediate load-level configuration will be 

especially beneficial in this situation. 

e. Recording Deflection Measurements. 

Once proper input force levels have been established, 

the operator can begin collecting deflection data in 

accordance with NDT test plan. The NDT operation 

software that is used to record test data should allow 

the operator to record the following information, in 

addition to the input force and sensor deflections: 

(1) Configuration setup (e.g., sensor 

spacing, load plate size, and project description). 

(2) Air, pavement surface, and surface layer 

mid-depth temperatures. 
AC 150/5370-11A 12/29/2004 

Page 16 

(3) Optional comments about pavement 

condition at a test location. 

(4) Test location, as discussed in Chapter 5 

and depicted on Figures 18 and 19. 

(a) Station (automatically recorded 

with most software). 

(b) Lane (PCC) or pass (HMA) 

number (e.g., 1.X). 

(c) Location within each PCC lane 

(e.g., X.1 or X.2 for center or joint). 

(d) Location within each PCC slab 

(e.g., 2 for transverse joint) 

If the equipment operating software allows this 

information to be recorded and a comprehensive test 

plan has been developed, the data can be efficiently 

processed to allow comprehensive evaluation of 

results. If data analysis software is available, the 

NDT operator should record all deflection data for a 

pavement facility in a single file. The NDT operator 

should use a file naming nomenclature that allows the 

engineer to quickly understand what data are in the 

file, such as "RW12_30" for Runway 12-30. Before 

the NDT equipment is shut down for the day, the 

operator should copy all NDT data files from the 

laptop computer to disks or a CD-ROM. 

f. Monitoring Deflection Data. It is very 

important for the NDT operator to continuously 

monitor the data. In addition to the mirrors on a 

towing vehicle, most NDT devices are equipped with 

a monitor in the tow vehicle and cameras surrounding 

the load plate that allow the operator to get a good 

view of the pavement surface in the vicinity of the 

load plate. As the equipment is moved into a testing 

position, the surface area should be checked to be 

sure it is clean and free of debris. A clean test 

location ensures the load plate will seat properly and 

that reliable deflection data will be recorded. 

If the deflection data look suspicious, the operator 

should rerun the test sequence at the same location. 

Typical deflection anomalies include nondecreasing 

deflections from the load plate and high D1 sensor 

(load plate) deflections compared to previous tests in 

that test lane. These anomalies may occur because of 

anomalies in the pavement structure or may indicate 

an equipment problem. In either case, the NDT 

operator should repeat the test at least once. 

As testing proceeds along the test lane and pavement 

facility, differences in the pavement construction may 

require alteration of the NDT test plan. For example, 

the width of an HMA paving lane may change and 

require the planned offset distance from the 

centerline to be changed to avoid being within 3 feet 

(1 m) of the longitudinal construction joint. These 

changes should be annotated on the NDT plan sketch 

and should include information as to why the changes 

are needed and at what lane and station location the 

change was made. 

27. SPECIAL TEST CONDITIONS. The 

engineer and NDT operator should use good 

judgment in the execution of a test plan. For the 

following scenarios, testing time and locations may 

have to be changed to safely collect accurate 

deflection data.

a. High or Low Temperatures. Many NDT 

devices will not operate properly when the ambient 

air temperature is too high or too low. If the 

temperature is too low, many manufacturers do not 

recommend conducting NDT because of the stress 

put on the equipment, especially those devices that 

are hydraulically operated. Likewise, if the 

temperature is too high, an HMA surface layer may 

compress excessively, leading to the recording of 

low-quality data. For these scenarios, the testing 

should be postponed until more moderate 

temperatures are encountered. 

b. High-Volume Landside Roads. The 

airport manager and engineer should exercise good 

safety practices for NDT work on landside roads that 

have high traffic volumes during the day. If testing 

must be conducted during the day, traffic control 

procedures should be used similar to those typically 

used by state Department of Transportation agencies. 

Alternatively, testing should be conducted late at 

night when traffic levels have decreased to an 

acceptable level. 

c. Sensor Configurations for PCC Testing. 

As discussed in Chapter 5, the NDT plan may require 

tests to be performed at the center, corner, transverse 

joint, and longitudinal joint of PCC slabs. These tests 

may also have to be performed at these locations for 

HMA overlaid PCC when cracks have reflected to 

the surface from underlying joints. The 

manufacturer's NDT devices and operation software 

may permit additional sensors to be installed on both 

sides and behind the load plate, as shown in Figure 

22. 
AC 150/5370-11A 12/29/2004 

Page 17 

As Figure 22 indicates, no additional sensors are 

required to collect slab center and transverse joint 

deflections along the PCC lane. However, an 

additional sensor behind the load plate will allow for 

transverse joint load transfer with the load plate on 

either side of the joint. This may be important if the 

aircraft or vehicle traffic is largely unidirectional. 

On PCC pavement, the NDT operator should conduct 

corner tests so the load plate is within 6 inches (150 

mm) of the transverse and longitudinal joints. An 

additional side sensor allows the operator to 

efficiently move the NDT equipment forward and test 

for load transfer across the longitudinal joint. 

Properly positioned cameras on the NDT device 

around the load plate make this maneuver much 

easier. 

28. ONSITE REVIEW OF DATA. At the end of 

each day's testing, the operator should review the 

NDT data files to ensure the data has been properly 

recorded. This is also a good time to add any 

additional comments to the file while the testing 

events are still fresh in the operator's mind. The 

operator should review all files prior to leaving the 

job site. If there are significant errors or anomalies in 

the test data, additional tests should be conducted 

before leaving the site. This eliminates the need to 

remobilize should these errors not be discovered until 

after the NDT equipment has left the site.

Although Chapter 7 discusses NDT data analysis, it is 

important to mention that an onsite preliminary 

analysis of the deflection data may be extremely 

valuable. A plot of raw deflection data and ISM 

values versus station for each pavement facility may 

show suspect areas where additional test points 

would improve the reliability of the analysis results. 
AC 150/5370-11A 12/29/2004 

Page 18 

CHAPTER 7–DEFLECTION DATA ANALYSES 

This chapter presents the next step in the pavement 

study -- analyzing the deflection data to obtain 

pavement structural characteristics that are needed to 

complete the project-level or network-level study. 

This chapter introduces several analysis procedures 

that can be used to obtain the desired pavement 

characteristics for an HMA, PCC, or HMA overlaid 

PCC pavement. 

29. OVERVIEW OF PROCESS. Figure 23 

provides an overview of the NDT data analysis 

process. Chapter 7 focuses on the analysis of the 

deflection data to evaluate the characteristics of the 

existing pavement structure. Chapter 8 then focuses 

on how engineers use these pavement characteristics 

in structural capacity analyses, rehabilitation designs, 

and the development of a multiyear CIP for a 

pavement management system.

As shown in Figure 23, there are several 

characteristics that are used to evaluate the structural 

condition of an existing pavement structure. The 

most common use of deflection data is to measure the 

strength of the structure as a whole and each 

individual layer within the structure. Because most 

PCC pavements are built using expansion, 

contraction, and construction joints, several 

additional characteristics are used to evaluate the 

condition of the concrete pavements. These 

discontinuities in the PCC create opportunities for the 

joint to deteriorate and transfer less load to the 

adjacent slab, lead to higher deflections at slab 

corners that may create voids beneath the slab, and 

provide opportunities for excessive moisture 

accumulation at the joints that may accelerate PCC 

material durability problems. 

a. FAA Software Tools. Table 7 shows the 

software tools that are available to analyze the 

deflection data, conduct a structural evaluation, 

perform a rehabilitation design, or develop a new 

pavement cross-section. The tools shown in Table 7 

will require varying levels of pavement engineering 

expertise to correctly use them. Before using these 

tools, it is important to understand their theoretical 

basis. Most software is accessible from the FAA web 

site www.faa.gov.

b. Background on FAA Software Tools. 

Most of the design tools shown in Table 7 are based 

on structural models that use static material property 

characteristics. Vibratory and impulse NDT devices 

generate dynamic loads that measure the pavement's 

response to those dynamic loads.

Therefore, for selected material properties, 

adjustments may be required before using the 

programs in Table 7. These adjustments are 

discussed throughout this chapter.

Although engineers have several choices regarding 

FAA software tools, they should select programs that 

have the same theoretical basis for a study. Stated 

differently, the back-calculation methods used should 

be consistent with the forward computational 

procedure that will be used for structural evaluation 

and design. As shown in Table 8, the FAA software 

are based on CBR, elastic layer, or the Winkler 

foundations. Since there are no NDT data analysis 

tools that are based on CBR, the engineer must use 

elastic layer theory or the Winkler foundation to 

perform a NDT data analysis. If the engineer wants 

to use elastic layer theory for the study, elastic layerbased programs should be used for NDT data 

analysis, pavement evaluation, and pavement design. 

The same would be true for studies based on the 

Winkler foundation. For CBR-based studies, the 

engineer should use an elastic layer program, such as 

BAKFAA, for NDT data analysis and then convert 

the elastic modulus values to CBR values, as 

discussed in Chapter 8. 

The design theory and, hence, the software tool that 

should be used to analyze pavement performance is 

based primarily on pavement type. HMA or flexible 

pavements are analyzed and designed using CBR or 

elastic layer theory. PCC or rigid pavements are 

routinely evaluated using the Winkler foundation and 

Westergaard's theory, layered elastic theory, or a 

finite element analysis. Composite pavements or 

HMA overlaid PCC are normally analyzed using the 

Winkler foundation if the HMA overlay is thin. 

However, if the HMA overlay is very thick relative to 

the thickness of the PCC, elastic layer analysis may 

be more appropriate. 

30. PROCESS RAW DEFLECTION DATA. The 

boundary limits of pavement sections within a facility 

may have already been defined in a pavement 

management system or through a review of the 

construction history. In a pavement management 

system, a section is defined as an area of pavement 

that is expected to perform uniquely because of 

aircraft traffic levels, pavement age, or pavement 

cross-section. Deflection data can be used to define 

or refine the limits of all sections within a pavement 

facility.
AC 150/5370-11A 12/29/2004 

Page 19 

If the deflection data have been collected as 

described in Chapter 6, the data for a pavement 

facility may be contained in one electronic file. This 

file may contain several types of deflection data, such 

as PCC center, slab joint, and slab corner tests. The 

center deflection data should be extracted from the 

file and reviewed to identify pavement section limits 

within the facility. 

A preliminary analysis of the center deflection data is 

routinely conducted by plotting normalized 

deflections or the ISM along the length of an apron, 

taxiway, or runway. The ISM or DSM is computed 

as shown in Chapter 2. Raw data deflections are 

normalized by adjusting deflections to a standard 

load. For example, one may want to normalize the 

deflections to a critical aircraft wheel load of 40,000 

pounds (18,000 kg), although deflections were 

recorded at impulse load levels of 31,500 pounds 

(140 kN), 36,000 pounds (160 kN), and 42,500 

pounds (190 kN). Each deflection recorded at these 

load levels would have to be adjusted as follows to 

obtain three normalized deflections at a load level of 

40,000 pounds (178 kN): 

0 0 d

L

L d

applied

norm

n ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

= [1]

Where: d0n = Normalized deflection 

Lnorm = Normalized load 

Lapplied = Applied load 

d0 = Measured deflection at selected 

sensor location 

When reviewing the profile plots of normalized 

deflections or ISM values, the engineer should look 

for patterns of variability. The normalized 

deflections under the load plate and ISM values 

provide an indication of the overall strength of the 

entire pavement structure (i.e., pavement layers and 

subgrade) at each NDT test location. For a given 

impulse load (e.g., 40,000 pounds (178 kN)), 

increasing ISM values or decreasing normalized 

deflections indicate increasing pavement strength. 

Example profile plots of ISM and normalized deflects 

are as illustrated in Figures 24 and 25, respectively. 

Figure 24 illustrates how the ISM profile plots were 

used to identify four unique pavement sections within 

this pavement facility. It is very clear from this 

figure that section 1 is the strongest of all four 

sections since its average ISM value is significantly 

higher than all other sections. Although the mean 

ISM values for sections 2, 3, and 4 are similar, ISM 

variability is much higher in section 3. 

Likewise, section 2 may be the weakest of the 

sections because the HMA layer is less than 5 inches 

(125 mm) thick or the stabilized base may be very 

weak. Profile plots can identify locations for coring 

and boring work that will provide additional 

information on layer thickness and strength. 

Figure 25 shows that normalized deflection profile 

plots can also be used to identify the limits of 

pavement sections within a particular facility. As 

these profile plots show, stronger pavement sections 

have lower normalized deflections. The engineer can 

use either normalized deflections or ISM values to 

identify section limits. ISM values are used more 

frequently and provide information independent of 

force. 

Deflection data can also be used to identify variations 

in the subgrade strength along a pavement facility. A 

sensor that is located a precomputed distance from 

the center of the load plate may provide a good 

estimate of the subgrade strength. The American 

Association of State Highway and Transportation 

Officials’ (AASHTO’s) 1993 design procedures 

provide guidance for the distance the sensor should 

be from the load plate to reflect the subgrade strength 

(e.g., outside of the stress bulb at the subgradepavement interface). 

For typical modulus values of HMA and PCC and 

three subgrade strengths, Table 9 shows the 

approximate distance the sensor should be from a 12-

inch (300 mm) diameter load plate to reflect the 

subgrade strength. Using Table 9 as a guide, the 

engineer should select the sensor that is closest to the 

value shown in the table, which is not necessarily the 

outermost sensor on the NDT device. Since most 

NDT devices do not record deflections beyond 72 

inches (1,800 mm), this table shows that the 

outermost sensor may not provide a good indication 

of the subgrade strength for thick PCC pavements. 

Figure 26 shows that deflections measured 24 inches 

(600 mm) from the center of the load plate can be 

used to compare subgrade strengths along the 

pavement facility. Table 9 shows that for all four 

sections, the NDT sensor at a distance of 24 inches 

(600 mm) from the load plate, is acceptable and 

should provide a good indication of subgrade 

strength. Figure 26 shows that subgrade strengths for 

all four sections are significantly different. 

Once the boundary limits of the pavement sections 

have been refined or defined within a pavement 

facility, the raw deflection data for each section must 

be separated from the facility data file for analysis. 
AC 150/5370-11A 12/29/2004 

Page 20 

After the deflection test data has been separated by 

pavement section, the data needs to be further 

subdivided by NDT test type. As shown in Figure 

23, the following types of deflection data collection 

will be required to conduct the analysis work 

described in this chapter: 

a. Center Deflection (Deflection Basin) Data. 

Pavement layer strengths and material durability. 

b. Joint and Crack Deflection Data. Joint 

condition and material durability. 

c. PCC Slab Corner Deflection Data. 

Support conditions and material durability. 

31. BACK-CALCULATION ANALYSIS. The 

engineer can use deflection basin data from flexible 

pavements and rigid center NDT tests to compute the 

strength of pavement layers. The process that is used 

to conduct this analysis is referred to as backcalculation because the engineer normally does the 

opposite of traditional pavement design. Rather than 

determining the thickness of each pavement layer 

based on assumed layer strengths, back-calculation 

typically involves solving for pavement layer 

strengths based on assumed uniform layer 

thicknesses. Throughout the remainder of this 

chapter, layer strength is referred to in terms of 

Young's modulus of elasticity or simply the elastic 

modulus.

As discussed in this AC, the types of loads that are 

applied through the use of NDT equipment fall into 

two general categories: static loads and dynamic 

loads. Dynamic loads include vibratory and impulse 

load NDT devices discussed in Chapter 3. For both 

static and dynamic loads, the pavement can respond 

linearly or nonlinearly to the applied loads. 

Table 10 shows the possible scenarios that may exist 

during back-calculation work. In addition, this table 

shows that finite element-based tools are required for 

theoretical modeling of pavements that are 

dynamically loaded by vibratory- and impulse-based 

NDT devices. However, for routine fieldwork, the 

FAA supports those software tools that are 

commonly used by all agencies and consultants who 

work in the pavement industry. This includes those 

tools in Table 10 that fall in the static-linear category. 

Back-calculation analysis work that falls in the staticlinear category is typically conducted using two 

procedures. The first category allows the engineer to 

use closed-form procedures that directly compute the 

elastic modulus of each layer by using layer 

thicknesses and deflections from one or more 

sensors. The second category uses an iterative 

mechanistic process to solve for the elastic modulus 

by using layer thicknesses and deflections from at 

least four sensors. 

Before conducting an analysis, the engineer should 

review the deflection tests that have been separated 

by pavement facility and section for back-calculation. 

Regardless of the software tool that will be used in 

the analysis, linear-elastic theory requires that 

pavement deflections decrease as the distance from 

the NDT load plate increases. In addition, for typical 

NDT sensor configurations, the deflections should 

gradually decrease from the load plate to the 

outermost sensor. 

Deflection basin anomalies could occur for several 

reasons, including the presence of a crack near the 

load plate, a nonfunctioning sensor, sensor and NDT 

equipment configuration error, sensors not properly 

calibrated, voids, loss of support, temperature curling 

or moisture warping of PCC slab, or several other 

reasons. The engineer should review the deflection 

data and remove those data that have the following 

anomalies. 

• Type I Deflection Basin. In this scenario, the 

deflections at one or more of the outer sensors 

are greater than the deflection under the load 

plate. This type of anomaly will produce the 

largest error during back-calculation analysis. 

• Type II Deflection Basin. Another less obvious 

anomaly is an unusually large decrease in 

deflection between two adjacent sensors. While 

elastic layer theory requires deflections to 

decrease as the distance from the load plate 

increases, the amount of decrease should be 

gradual and relatively consistent between all 

sensors. 

• Type III Deflection Basin. Similar to Type I, the 

deflection at the outermost sensor of two 

adjacent sensors is greater than the deflection at 

the sensor that is closest to the load plate. 

Figure 27 summarizes the deflection data preparation 

and analysis tool selection processes. The AASHTO 

closed-form procedure for flexible pavements is 

appropriate for landside pavements and airside access 

roads where truck and other vehicle traffic are the 

predominant types of loading, and the analysis will 

be based on the AASHTO Design Guide. For PCC 

pavement analysis, HMA overlays are considered to 

be thin if they are less than 4 inches (100 mm) thick 
AC 150/5370-11A 12/29/2004 

Page 21 

and the PCC layer thickness is less than 10 inches 

(250 mm). The HMA overlay is also considered to 

be thin if it is less than 6 inches (150 mm) thick and 

the PCC layer is greater than 10 inches (250 mm) 

thick. 

a. Closed-Form Back-Calculation. Closedform back-calculation algorithms are commonly used 

when the results will be used for two specific design 

methodologies. The first methodology is based on 

the 1993 AASHTO Design Guide for HMA 

pavements that relies on the resilient modulus (Mr) of 

the subgrade as computed by laboratory testing. The 

second algorithm, commonly referred to as the 

AREA-based methodology, is used primarily for 

PCC or HMA overlaid PCC pavements when the 

design procedure relies on the modulus of subgrade 

reaction, k, as discussed in Chapter 8.

Equation 2 shows that the Mr value for the subgrade 

can be calculated by using the deflections from the 

appropriate sensor of the NDT equipment. Referring 

to Figure 26, this would be the 24-inch (600 mm) 

from the center of the load plate based on guidelines 

presented in Table 9. 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = d r

P M

r

r

0.24 [2] 

Where: Mr = Resilient modulus, psi 

P = Applied load, pounds 

dr = Measured deflection at distance r 

from applied load, inch 

r = Radial distance at which the 

deflection is measured, inch 

For the four pavement sections shown in Figure 26, 

the dynamic resilient modulus values for the 

subgrade would be as follows if the mean 12-inch 

(600 mm) sensor deflections were used in the above 

equation: section 1, 72,150 psi (497 MPa); section 2, 

26,780 psi (185 MPa); section 3, 43,650 psi (301 

MPa); and section 4, 33,170 psi (229 MPa). The four 

mean deflections for this sensor position are 2.76, 

7.48, 4.57, and 6.02 mils. 

As expected, the subgrade strength for section 1 is 

the highest because it has been stabilized. These 

subgrade modulus values would have to be adjusted 

to laboratory resilient modulus values using a 

correction factor (typically, 0.33), as discussed in the 

1993 AASHTO Design Guide, before conducting an 

HMA pavement design or evaluation in accordance 

with those design procedures. 

Another commonly used closed-form backcalculation procedure is the AREA-based 

methodology. This methodology, also used in the 

1993 AASHTO Design Guide, is predominantly used 

for PCC and HMA overlaid PCC pavements when 

the design tool uses the subgrade modulus, k. AREA 

is computed by using NDT deflections that form the 

deflection basins described in Chapter 2. 

Figure 28 shows the steps that are required to 

compute the elastic modulus and subgrade modulus 

of reaction, k, when an AREA-based procedure is 

used in the back-calculation analysis. AREA 

equations 5 and 6 are used to account for the 

compression that occurs in the HMA overlay or in 

very thick PCC pavements. These equations do not 

include the deflection under the load plate where 

compression occurs. 

The following AREA equations are based on the four 

and seven-sensor SHRP and U.S. Air Force 

configurations. Equations 3 and 4 use the deflections 

from all sensors on the NDT device to calculate the 

basin AREA. Equation 5 uses the outer five sensors 

from the SHRP seven-sensor configuration. 

Likewise, equation 6 uses the outer six sensors from 

the Air Force seven-sensor configuration. 

 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

0

36

0

24

0

12

4 6 12 12 6

d

d

d

d

d

d AREA Sensor

 [3] 

 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

0

60

0

36

0

24

0

18

0

12

0

8

7 4 6 5 6 9 18 12

d

d

d

d

d

d

d

d

d

d

d

d AREA Sensor

 [4] 

 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

12

60

12

36

12

24

12

18

5 3 6 9 18 12

d

d

d

d

d

d

d

d AREA Outer

 [5] 

 ⎥

⎦

⎤ ⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

12

72

12

60

12

48

12

36

12

24

6 6 12 12 12 12 6

d

d

d

d

d

d

d

d

d

d AREA Outer

 [6] 

Where: 

AREA = AREA in inches for SHRP four 

sensor, SHRP seven sensor, SHRP 

outer five sensors, and Air Force 

outer six sensors 

d0 = Maximum deflection at the center 

of the load plate, mils 

di = Deflections at 8, 12, 18, 24, 36, 48, 

60, and 72 inches (200, 300, 450, 

600, 900, 1200, 1500, and 1800 

mm) from the load plate center, 

mils 
AC 150/5370-11A 12/29/2004 

Page 22 

Figure 29 illustrates how AREA is calculated for the 

SHRP four-sensor configuration. The same 

principles can be used to compute the AREA for any 

arbitrary sensor configuration. After the correct 

AREA calculations are performed, the radius of 

relative stiffness, as defined in equation 7, can be 

used to compute the k-value and effective PCC 

modulus. 

λk 4 2 12(1 )k

E pcc h pcc

− μ = [7] 

Where: 

λk = Winkler foundation radius of 

relative stiffness, inch 

Epcc = Effective elastic modulus of bound 

layers above the subgrade, psi 

hpcc = Total thickness of all rigid layers 

above the subgrade, inch 

μ = PCC Poisson's ratio 

k = modulus of subgrade reaction, 

psi/inch 

There is a unique relationship between the areas that 

are calculated by equations 3 through 6 and the radius 

of relative stiffness,λk , which takes the general form 

that is shown in equation 8. 

D

k C

B

A AREA

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎣

⎡ ⎟

⎠

⎞ ⎜

⎝

⎛ −

=

ln

λ

 [8] 

Where: 

λk = Winkler foundation radius of 

relative stiffness, inch 

AREA = AREA as calculated in 

equations 3 through 6 

A, B, C, D = AREA-based constants as 

shown in Table 11 

After the radius of relative stiffness has been 

computed, the subgrade k-value and effective 

modulus can be computed by simultaneously solving 

for either the k-value or the modulus using two 

independent equations. The first is equation 7 for the 

radius of relative stiffness, and the second is 

Westergaard's equation for the deflection at the center 

of the slab directly beneath an applied load. 

However, since Westergaard's equation does not 

apply for outer AREA scenarios where deflections 

are not normalized to the deflection directly under the 

load plate, the following equations are used to 

compute the k-value and effective modulus: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = 2

*

r k

r

d

Pd k

λ

 [9] 

Where: 

k = Modulus of subgrade reaction, psi/inch 

P = Applied NDT load, pounds. 

λk = Winkler foundation radius of relative 

stiffness, inch. *

r d = Nondimensional deflection coefficient 

for radial distance r. 

r d = Measured deflection at radial distance 

r, inch 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − = 3

2 2 * 12(1 )

d h

P d E

r

μ λk r [10] 

Where: 

E = Effective elastic modulus, psi 

μ = PCC Poisson's ratio 

 P = Applied NDT load, pounds 

λk = Winkler foundation radius of relative 

stiffness, inch *

r d = Nondimensional deflection coefficient 

for radial distance 

r d = Measured deflection at radial distance 

r, inch 

h = Thickness of all bound layers above 

the subgrade, inch 

The nondimensional deflection coefficient, * dr , can 

be calculated as follows: 

( ) ( ) * k z ye

r d xe − λ − = [11] 

Where: * dr = Nondimensional coefficient for 

radial distance r 

λk = Winkler foundation radius of 

relative stiffness. 

x, y, z = Constants, as shown in Table 12 

As shown in Figure 28, the next step in the AREAbased method of conducting back-calculation is to 

adjust the k-value and effective modulus to account 

for the effect of a finite slab size. Slab size 

corrections may be necessary to account for the 

infinite slab assumption that is inherent in 

Westergaard's analyses. 
AC 150/5370-11A 12/29/2004 

Page 23 

If the engineer believes this to be necessary, 

procedures for slab size adjustments can be found in 

Report FHWA-RD-00-086, Back-calculation of 

Layer Parameters for LTPP Test Section, or the 

AASHTO Design Guide for Rigid Pavements. 

If the PCC structure does not contain a stabilized 

base, HMA overlay, or PCC overlay, the backcalculated dynamic effective modulus is the PCC 

modulus of elasticity. However, the back-calculated 

dynamic k-value must be adjusted to obtain a static kvalue that is the basis for conventional FAA 

evaluation and design programs that use a k-value. 

National Cooperative Highway Research Program 

(NCHRP) Report 372, Support Under Portland 

Cement Concrete Pavements, reported that the statick value is equal to one-half of the dynamic k-value. 

The static-k value is the value that would be obtained 

by conducting plate bearing tests as described in 

AASHTO T222. 

If the PCC structure contains a stabilized base, thin 

HMA overlay, or PCC overlay, the back-calculated 

dynamic effective modulus may be used to compute 

two modulus values. Possible modulus scenarios are 

as follows: bonded or unbonded PCC overlay and 

PCC layer, thin HMA overlay and PCC layer, PCC 

layer and lean concrete or cement-treated base, or 

PCC layer and HMA stabilized base. 

During back-calculation, most engineers should 

assume that for more than 95 percent of the cases, the 

two layers will appear to be bonded because of the 

high-friction forces that result from the falling weight 

of impulse-load NDT devices. This includes 

scenarios where coring results show delamination 

between the layers, bond breakers are placed between 

the layers (including two layers of polyethylene 

sheathing), and HMA separation layers are 

constructed (e.g., traditional unbonded PCC overlay 

on PCC). For the bonded layer scenario, the neutral 

axis must first be calculated as shown below: 

1 2

2

2 1

2

1

2 2

h h

h h h h

x β

β

+

⎥

⎦

⎤ ⎢

⎣

⎡ ⎟

⎠

⎞ ⎜

⎝

⎛ + +

= [12] 

Where: x = Depth to the neutral axis, inch 

hi = Thickness of upper layer (e.g., plate), 

inch 

β = E2 divided by E1 (estimated by 

engineer) 

 Where: E2 = Modulus of lower layer, psi. 

 E1 = Modulus of upper layer, psi. 

Before estimating the depth to the neutral axis, the 

engineer must estimate the ratio of the elastic 

modulus of the lower layer to the upper layer, as 

shown in equation 12. Typical ratio values, or Beta 

(β) values, would be as follows, depending on HMA, 

PCC, and stabilized base mix designs: HMA 

overlaid PCC = 10, PCC overlaid PCC = 1.4, PCC 

with lean concrete base = 0.40, PCC with cement 

treated base = 0.25, and PCC with asphalt treated 

base = 0.10. 

After estimating and computing the depth to the 

neutral axis, the elastic modulus for the upper layer, 

and, subsequently, the lower layer using the assumed 

value for β can be computed for a bonded condition 

using equation 13. 

[ ]

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡ ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟ + − + ⎠⎞ ⎜

⎝

⎛ + + −

+ = 2

2

2 1

2

1

1

3

2

3

1

3

1 2

1

2

12

2

12

( )

h h h x h h h h x

E h h E e

β β

 [13] 

Where: 

Ee = Back calculated effective modulus 

(equation 10), psi 

If the condition between the two layers is unknown, 

the engineer may want to run the analysis for both an 

unbonded and a bonded condition. Using β, Ee, h1, 

and h2 from above, the elastic modulus values for the 

upper and lower layers may be computed for an 

unbonded condition using equation 14: 

[ ]

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ +

+ =

3

2

3

1

3

1 2

2

( )

h h

E h h E e

β

 [14] 

The following example, which illustrates the use of 

the AREA-based methodology, shows the 

computation of the material properties of a PCC 

apron that has been constructed as follows and tested 

at the center of a slab using an impulse-load NDT 

device: 

Example Problem 1: Given Inputs

15-inch (375 mm) jointed PCC (3 years old) 

9-inch (225 mm) CTB (bonded interface) 

6-inch (150 mm) unbound AGBS 

20-foot (6 m) transverse joint spacing 

18.75-foot (5.75 m) longitudinal joint spacing 

SHRP seven-sensor NDT configuration 

NDT impulse load of 22,582 pounds (100 kN) 

Measured deflections at one test location 
AC 150/5370-11A 12/29/2004 

Page 24 

Sensor D1 D2 D3 D4 D5 D6 D7 

Spacing, inches 0 8 12 18 24 36 60 

Deflections, 

mils 22.90 21.40 20.30 19.10 17.90 16.30 10.70

The first step is to ensure the deflection data do not 

contain Type I, II, or III errors. A review of the 

deflections shows that all deflections gradually 

decrease from the center of the load plate. Since the 

deflection data appears to be consistent, the next step 

is to compute what AREA equation should be used in 

the back-calculation. 

Figure 28 shows that AREA equations 5 or 6 can be 

used when the PCC pavement is more than 16 inches 

(400 mm) thick. Our PCC layer thickness is only 15 

inches (375 mm) thick, but a 9-inch (225 mm) 

cement treated base (CTB) underlies the slab. 

Therefore, to better account for compression of the 

PCC and CTB under the NDT falling weight, 

equation 5 (SHRP configuration) can be used with a 

total layer thickness of 24 inches (600 mm). 

Equation 5, AREA based on outer five sensors, leads 

to an AREA of 47.15 inches (1,175 mm). With the 

AREA now known, the radius of relative stiffness 

can be computed using equation 8, yielding a 

λk value of 47.15 inches (1,175 mm). Before the k 

value and effective modulus can be computed using 

equations 9 and 10, the nondimensional deflection 

coefficient, * dr , must be calculated using equation 11. 

For the outer AREA method, the distance from the 

load plate for the SHRP seven-sensor configuration is 

12 inches (300 mm). Using Table 12 and equation 

11, d*

r is equal to 0.1185. Using this value, a dr

deflection value of 0.00203 inches (0.051562 mm), 

an impulse load of 22,582 pounds (100 kN), a 

combined PCC and CTB thickness of 24 inches (600 

mm), and a λk value of 47.15 inches (1,175 mm), the 

initial dynamic k and effective modulus values are 

593 psi/in and 2,486,767 psi (17146 MPa), 

respectively. 

After correcting for the finite slab size effects, the 

adjusted dynamic k and effective modulus values are 

702 psi/in and 2,541,877 psi (17526 MPa), 

respectively. As shown in Figure 28, the static k 

value is then computed as one half of the adjusted 

dynamic k value, or 351 psi/in. This is the k value 

that should be used in FAA design and evaluation 

programs. 

The effective elastic modulus of the PCC and CTB 

layers can be used to compute the individual layer 

modulus values. If one assumes the ratio of the CTB 

to the PCC elastic modulus, β, is 0.25 and that the 

two layers are bonded, the depth to the neutral axis 

from the PCC surface is 9.07 inches (230 mm), as 

calculated from equation 12. Using equation 13 for a 

bonded interface condition results in elastic modulus 

values of 5,064,615 psi (34919 MPa) and 1,266,154 

psi (8730 MPa) for the PCC and CTB layers, 

respectively. 

For an unbonded PCC and CTB interface condition, 

equation 13 is used to calculate elastic modulus 

values of 9,878,110 psi (68107 MPa) and 2,469,528 

psi (17027 MPa) for the PCC and CTB layers, 

respectively. Based on knowledge of the condition 

and age of the PCC and CTB layers, the bonded 

interface condition yields more reasonable results for 

the NDT deflections that were recorded at this test 

location. 

The procedures that were used in this example and 

illustrated in Figure 28 should be used to backcalculate elastic modulus and k values for all center 

slab test locations in a pavement section. The 

methods and equations for the AREA method 

computations can be incorporated into a spreadsheet 

program to make the computational process a 

straightforward effort. 

b. Iterative Elastic Layer Back-Calculation. 

Unlike the closed-form procedures presented earlier, 

this methodology relies on elastic-layer theory and a 

computationally intensive process to compute the 

modulus values of all layers in the pavement 

structure, including the subgrade. As shown in 

Figure 27, this methodology is most appropriate for 

HMA pavements, HMA overlaid PCC pavements, 

and PCC pavements when LEDFAA is used as the 

evaluation and design tool. 

An alternative method for back-calculation of 

concrete elastic modulus and subgrade k-value based 

on plate theory can also be used for rigid pavement. 

This method, termed “Best Fit” is described in 

FHWA Report RD-00-086, Back-calculation of 

Layer Parameters for LTPP Test Sections, Volume I: 

Slab on Elastic Solid and Slab on Dense-Liquid 

Foundation Analysis of Rigid Pavements. The Best 

Fit method solves for a combination of radius of 

relative stiffness, λ, and modulus of subgrade 

reaction, k, that, similar to layered elastic backcalculation methods, produces the best possible 

agreement between computed and measured 

deflections at each sensor. 
AC 150/5370-11A 12/29/2004 

Page 25 

However, detailed Long-Term Pavement 

Performance (LTPP) studies show that elastic-layer 

back-calculation procedures may not always work 

well for PCC pavement sections. Therefore, if tools 

based on this methodology are used in backcalculation work, the results should be carefully 

scrutinized to ensure they are reasonable and 

consistent with typical modulus values. 

As with the closed-form back-calculation, the elastic 

layer methodology is used to compute the layer 

moduli based on the deflection basin data, layer 

thicknesses, and the composition of the pavements. 

This is achieved by estimating an initial set of layer 

elastic moduli (seed values) and a reasonable range 

of moduli for each material type. Modulus and 

Poisson's ratio values for typical pavement materials 

are shown in Tables 13 and 14. 

After a range of a modulus values has been assigned 

for each layer, an iterative process is used to obtain 

the best match between the measured and calculated 

deflections. The calculation process is started by first 

estimating the initial modulus, often referred to as the 

"seed” value, of each layer within the specified 

allowable range. The selection of this initial value is 

based on the material type and environmental 

conditions at the time of testing. The initial 

deflection basin is then calculated using the NDT 

device load. 

The initial deflection basin is then compared to the 

measured basin, as shown in Figure 30. If the basins 

are significantly different, the "seed" modulus values, 

range in modulus values, and/or pavement 

thicknesses are adjusted. The process is repeated 

until the back-calculated deflections from the 

estimated moduli converge with the measured NDT 

deflections at an acceptable error level. 

Successfully obtaining convergence to an acceptable 

error level depends on several factors. When 

reviewing error levels and back-calculation results, it 

is important to understand that a unique solution is 

not obtained during a back-calculation process. 

Rather, one of many feasible solutions is obtained 

based on the set of constraints that exist or have been 

defined in the back-calculation setup. The magnitude 

of the error and the layer modulus values that are 

obtained for a feasible solution depend on several 

factors. 

The magnitude of the error and the results that are 

obtained through iterative back-calculation using 

elastic layer theory are influenced by many factors, 

including the following: 

• Number of Layers. As the number of layers is 

increased in the back-calculation analysis, the 

error level may increase and result in an 

unfeasible solution. 

• Layer Thicknesses. As the thickness of a layer is 

decreased in the analysis, the error level may 

increase. In addition, if the estimated thickness 

of a layer is substantially different than the actual 

thickness, error levels may also increase. 

• Layer Interface Condition. The strength of the 

bond between any two layers in a multilayer 

analysis will also affect the results and error 

levels. 

• HMA Layer Temperature. An asphalt concrete 

layer is very sensitive to changes in temperature. 

When the air temperature changes significantly 

on a hot sunny summer day, the HMA modulus 

will also change significantly. This can be 

reflected in the error levels and analysis results. 

• Layer "Seed” Values. The initial modulus value 

that is selected for each layer can have an impact 

on the results. The magnitude of the error will 

depend on the iteration algorithm that is used by 

the back-calculation software program. 

• Adjacent Layer Modulus Ratios. Larger errors 

can occur when the estimated modulus between 

two adjacent layers in a pavement structure is 

significantly different. For example, the error 

and results obtained from analysis of a 4-inch 

(100 mm) thick HMA overlay on a 15-inch (375 

mm) PCC may be quite high. 

• Underlying Stiff Layer. Likewise, if a relatively 

stiff layer is within 10 feet (3.0 m) of the 

pavement surface, the error level may be quite 

large if the back-calculation tool does not 

consider this layer, often referred to as the "depth 

to bedrock." However, this layer does not have 

to be bedrock, it can be a layer that is much 

stiffer than the unbound layers above it. 

• Pavement Cracks. Elastic layer theory assumes 

that there is no discontinuity in any layer in the 

pavement structure. Therefore, if the NDT load 

plate is close to a crack of any type, or an 

underlying joint in an HMA overlaid PCC 

pavement, large errors may occur. 

• Sensor Errors. If the NDT sensors are not 

calibrated or the measured deflections are 

outside the sensor specification limits, the error 

level may also increase. 
AC 150/5370-11A 12/29/2004 

Page 26 

• NDT Load Plate. If the load plate is not in 

uniform contact with the pavement surface, the 

error level may increase. 

• Pulse Duration. For impulse-load NDT devices, 

the pulse duration of the applied load may also 

affect the results. 

• Frequency Duration. For vibratory-load NDT 

devices, the load frequency may also affect the 

results. 

• Seasonal Effects. The water table level may 

change throughout the year. In addition, for 

northern climates, frost penetration and spring 

thaws may affect the error levels and analysis 

results. 

• Material Property Variability. Pavements are 

constructed on a subgrade or fill material whose 

thickness and characteristics may change along 

the transverse or longitudinal profile of the 

pavement facility. Subgrades may be nonlinear, 

inhomogeneous, or anisotropic. Subgrade 

properties may change considerably in a 

relatively short distance. 

The long list of key factors that may affect the backcalculation error and results illustrate why backcalculation is a laborious process that requires a high 

degree of skill and experience. Because so many 

factors impact the error level and results and, because 

there is no one unique solution, iterative elastic-layer 

back-calculation requires good engineering judgment. 

Figure 31 shows how elastic-layer back-calculation 

can be conducted to obtain reasonable modulus 

values that will provide reliable inputs for airport 

pavement evaluation, design, and management. 

ASTM D 5858, Standard Guide for Calculating InSitu Equivalent Elastic Moduli of Pavement 

Materials using Layered Elastic Theory, and LTPP's 

Design Pamphlet for the Back-Calculation of 

Pavement Layer Moduli in Support of the 1993 

AASHTO Guide for the Design of Pavement 

Structures provides guidance on how to set up the 

initial back-calculation analysis and then make 

adjustments as required to obtain a feasible and 

reasonable solution. Although back-calculation of 

reasonable layer moduli depends on many factors, the 

following suggestions may improve the probability 

that success is obtained in the evaluation of the 

deflection data for each pavement section. 

(1) Conduct Dynamic Cone Penetrometer 

(DCP) Tests. If cores are being obtained for the 

pavement study, conduct DCP tests through the core 

holes and unbound bases and 18 inches (450 mm) 

into the subgrade. Evaluate DCP data and estimate 

CBR and elastic modulus values for each layer. 

(2) Verify Layer Thicknesses. Check 

construction history, cores, borings, and DCP tests to 

ensure thicknesses are reasonable. Eliminate or 

combine thin layers that are less than 3 inches (75 

mm) thick with other layers. ASTM D 5858 and the 

LTPP back-calculation guide define thin as those 

thicknesses that are less than one quarter the diameter 

of the loaded area. For the most common load plate 

size of 12 inches (300 mm), this would be a thickness 

of 3 inches (75 mm). 

(3) Optimize Number of Layers. Excluding 

the apparent stiff layer, keep the number of layers in 

the back-calculation analysis to five or less, with 

three being the ideal number. As stated in ASTM D 

5858 and LTPP documents, the minimum number of 

deflections (e.g., sensors) should not be less than the 

number of layers in the analysis. Therefore, for the 

SHRP four sensor configuration, no more than four 

layers should be included in the analysis. 

(4) Combine Problem Layers. For two 

adjacent unbound granular layers, consider 

combining these layers into one equivalent layer 

using the combined thicknesses. For PCC, HMA 

overlaid PCC pavements with a granular base or 

subbase, consider eliminating the granular layers 

from the analysis (i.e., use “composite” subgrade 

layer). For thin HMA overlays on PCC pavements, 

consider removing the HMA layer from the backcalculation. 

(5) Verify Depth to Stiff Layer. This depth 

can have a significant impact on the analysis results. 

The magnitude of this impact will vary depending on 

the software program that is used in the analysis. If 

initial results are not reasonable, review additional 

geotechnical data from local agencies to verify type 

and variability of depth to rock or underlying stiff 

layer. 

(6) Analyze Each Deflection Basin. Rather 

than computing a representative deflection basin by 

averaging the deflection values for each sensor, backcalculation should be conducted for each basin. Use 

of a representative deflection basin may cause the 

engineer to overlook localized weak layers within a 

pavement section. 
AC 150/5370-11A 12/29/2004 

Page 27 

(7) Subdivide Subgrade Layers. If the 

water table is within 10 feet (3.0 m), the error levels 

may be reduced by dividing the subgrade into two 

layers: one above the water table and the other below 

it. The primary reason for this division is that the 

modulus of the subgrade in a saturated condition may 

be significantly lower than it is in an unsaturated 

condition. 

(8) Use Outer Sensors. There are scenarios 

when better results may be obtained by not using the 

deflection underneath the load plate. If NDT was 

conducted on a hot day, there may be significant 

compression of the HMA layer. The amount of 

compression may be even greater when testing an 

HMA overlaid PCC on a hot day. For these 

scenarios, use of the outer sensors may reduce the 

error level during back-calculation. 

The preceding discussion provides guidelines on 

procedures that should lead to consistent and 

reasonable back-calculation results. High-speed 

computers have allowed engineers to use software 

programs to more efficiently solve for modulus 

values. Tables 15 and 16 show the features of several 

linear and nonlinear elastic-layer based programs, 

respectively. Notes for each of these tables provide 

additional information regarding the features of each 

software program. 

Several studies have been conducted to determine 

which programs may produce better results. 

However, because elastic-layer back-calculation does 

not produce a unique solution and because experience 

has a significant impact on the results that are 

obtained, evaluation criteria that have been used in 

past studies are influenced by the back-calculation 

experience of the research team. However, some 

programs are more widely distributed and used than 

others in Tables 15 and 16. For example, the 

WESDEF and MODULUS programs have been used 

extensively in the analysis of test data in the FHWA's 

LTPP research program, and WESDEF has been 

extensively used for the evaluation of military airfield 

pavements. 

Several factors should be considered when selecting a 

back-calculation program. In addition to the features 

listed in Tables 15 and 16, the engineer should 

evaluate the quality of the software documentation or 

help features, technical support, purchase or lease 

costs, the proprietary nature of the product, user 

friendliness, and other factors that may be important 

to an agency or a firm. 

To assist airport owners and engineers, the FAA has 

developed a back-calculation program, titled 

"BAKFAA," that is free and available to all agencies 

by downloading from the FAA website.

To illustrate the elastic-layer back-calculation 

program, BAKFAA will be used to solve a problem 

for one deflection basin in section 4 that was shown 

in Figure 24. The pavement cross-section consists of 

the following layers. Back-calculation will be 

conducted for deflection data at one test location. 

Example Problem 2: Given Inputs

5-inch (125 mm) HMA layer 

16-inch (400 mm) aggregate base 

Boring data indicates depth to bedrock is 13 feet 

(3.9 m) 

SHRP seven-sensor NDT configuration 

NDT impulse load of 20,000 pounds (90 kN) 

Measured deflections at one test location 

Sensor D1 D2 D3 D4 D5 D6 D7 

Spacing, in 0 8 12 18 24 36 60 

Deflections, 

mils 20.79 14.96 12.02 9.66 7.30 5.28 3.37 

The first step is to ensure the deflection data do not 

contain Type I, II, or III errors. As with the previous 

example, a review of the deflections shows that all 

deflections gradually decrease from the center of the 

load plate. Since the deflection data appears to be 

consistent, the next step shown in Figure 31 is to 

compute the depth to bedrock or an apparent stiff 

layer and compute the initial pavement cross-section. 

In the example, the given depth to a stiff layer is 13 

feet (3.9 m). With the cross-section that is provided, 

an initial attempt is made to back-calculate layer 

moduli for three layers using the “seed” moduli and 

Poisson's ratios shown in Table 17. These initial 

values are obtained from Tables 13 and 14. Results 

from the initial run of BAKFAA are shown in Figure 

32. 

The results from the initial back-calculation run show 

that the root mean square (rms) error is 0.3872 mils, 

which is in the acceptable coefficient of variation 

(COV) range of 2 to 5 percent for the BAKFAA 

algorithm. The HMA modulus is reasonable, albeit, 

lower than is expected for an aged HMA layer. As 

was previously discussed, the HMA layer thickness 

may be quite variable. To illustrate the impact on 

thickness, the back-calculation was rerun using an 

HMA layer thickness of 4 inches (100 mm). The 

output from the second run is shown in Figure 33. 
AC 150/5370-11A 12/29/2004 

Page 28 

The results from the second BAKFAA run show that 

the HMA modulus has increased from 160,000 to 

215,000 psi (1103 to 1482 MPa) because of the 1-

inch (25 mm) decrease in HMA layer thickness. The 

back-calculated modulus values for the granular base 

and subgrade remain reasonable, although their 

moduli have increased slightly from the first run. 

The output from the second BAKFAA run is shown 

in Figure 34. 

A review of the output from the second run shows 

that the RMS error has decreased slightly to 0.3845 

from the first run. The HMA modulus is reasonable, 

and the granular base modulus is 44,486 psi (307 

MPa). The subgrade modulus of 8,395 psi (58 MPa) 

is also acceptable. 

Another concern during back-calculation is the 

presence of a soft layer directly above or below a 

very stiff layer. If cores had been taken as part of the 

pavement study, it would be very beneficial to have 

taken NDT tests directly over the core locations to 

develop a higher level of confidence in the backcalculation results at the core locations. Using the 

core thicknesses and the NDT results, backcalculation runs could then be conducted for Example 

Problem 2 to compare the results with the NDT 

results for the remainder of section 4 shown in the 

pavement facility in Figure 24. 

The discussion thus far has focused on backcalculation of elastic modulus values for each layer in 

a pavement. While it is important to know the 

strength of each layer in a pavement evaluation or 

design study, PCC pavements often require additional 

testing and evaluation of characteristics that are 

important for rigid pavements. As shown in Figure 

23, these characteristics include joint and crack 

conditions, support conditions, and material 

durability. 

32. PCC JOINT ANALYSIS. The analysis of PCC 

joints or cracks is very important because the amount 

of load that is transferred from one PCC slab to the 

adjacent slab can significantly impact the structural 

capacity of the pavement. 

As discussed in Chapter 5, NDT tests are conducted 

at joints and cracks to estimate what percentage of 

the total main gear weight is transferred from the 

loaded slab to the unloaded slab. As the amount of 

load that is transferred to the unloaded slab increases, 

the flexural stress in the loaded slab decreases and the 

pavement life is extended. 

The amount of load transfer depends on many 

factors, including gear configuration, tire contact 

area, pavement temperature, use of dowel bars, and 

use of a stabilized base beneath the PCC surface 

layer. 

Deflection load transfer efficiency (LTE∆) is most 

frequently defined as shown in equation 15. If the 

LTE∆ is being calculated for an HMA overlaid PCC 

at the joint reflective crack, compression of the HMA 

overlay may result in an inaccurate assessment of the 

load transfer. 

For this scenario, the engineer may want to have 

NDT tests conducted using the second and third 

sensors of the NDT device and then use these sensors 

in the LTE∆ computation. 

100%

_

_

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

∆

∆

∆ =

loaded slab

unloaded slab LTE

 [15] 

Where: 

LTE∆ = Deflection load transfer 

efficiency, percent 

∆unloaded_slab = Deflection on loaded slab, 

normally under load plate, 

mils 

∆loaded_slab = Deflection on adjacent 

unloaded slab, mils 

Once LTE∆ values are computed, they must be related 

to the stress load transfer efficiency (LTEσ) to 

understand how load transfer will impact the 

structural capacity of a pavement section. This is 

necessary because the FAA design and evaluation 

procedures in AC 150/5320-6 assume that the amount 

of load transfer is sufficient to reduce the free edge 

flexural stress in a PCC slab by 25 percent. Since the 

relationship between LTE∆ and LTEσ is not linear, 

additional analysis work is required to compute if the 

stress load transfer efficiency is 25 percent. Equation 

16 shows how LTEσ is defined. 

100%

_

_

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

loaded slab

unloaded slab LTE σ

σ

σ

 [16] 

Where: 

LTEσ = Stress load transfer 

efficiency, percent 

σunloaded_slab = Stress on loaded slab, psi 

σloaded_slab = Stress on adjacent unloaded 

slab, psi 
AC 150/5370-11A 12/29/2004 

Page 29 

Figure 35 illustrates one relationship between LTE∆

and LTEσ for a 12-inch (300 mm) load plate. Other 

relationships for 12-inch and 18-inch (300 mm and 

450 mm) load plates can be found in FAA Report 

DOT/FAA/PM-83/22, Investigation of the FAA 

Overlay Design Procedures for Rigid Pavements 

(copy can be obtained from William J. Hughes 

Technical Center). The relationship between these 

efficiencies depends on the ratio of the load contact 

radius to the radius of relative stiffness of the slab, a 

/λk For a load plate radius of 6 inches (150 mm), 

λk is the only variable that changes during analysis 

of the NDT deflection data. 

The two curves shown in Figure 35 provide a 

boundary for the range of λk values that are 

expected for airfield pavements. From this figure, a 

LTE∆ of 70 percent leads to a LTEσ of 25 percent 

when λk is 20 inches (500 mm). A LTEσ of 25 

percent is the value used in FAA design and 

evaluation procedures. Likewise, a LTE∆ of 90 

percent leads to a LTEσ of 25 percent when λk is 130 

inches (3,300 mm). 

Using Figure 35 and a target LTEσ of 25 percent, 

Table 18 provides general recommendations for three 

ranges of LTE∆. Through interpretation between 

λk values of 20 inches and 130 inches (500 mm and 

3,300 mm) in Figure 35, the engineer can compute 

the performance rating of a pavement's load transfer 

efficiency. 

Earlier in this chapter, the use of ISM plots along the 

length of a pavement facility were used to 

demonstrate how they can be used to identify the 

boundaries of pavement sections. If the LTEσ values 

are also plotted for a jointed PCC or HMA overlaid 

PCC, differences in joint performance may also 

become evident. The variations in performance may 

be associated with the type of joint (e.g., doweled 

versus nondoweled) or a deterioration in aggregate 

interlock for nondoweled pavements. Furthermore, 

changes in performance may be an indication of the 

amount of load transfer that is provided by stabilized 

bases under the PCC. 

Figure 36 shows how a profile plot of the LTE∆ can 

help determine which sections of a pavement facility 

have good joint performance. 

This figure of a taxiway shows that the LTE∆ is 

acceptable or fair for 2,000 feet (610 m) and then 

starts to deteriorate in the next 1,600 feet (488 m) 

until it becomes poor for 3,300 feet (1,005 m). 

Only after Station 69+00, does the LTE∆ start to 

improve again. A review of the aircraft traffic flow 

for this taxiway showed that the connector taxiway at 

Station 20+00 is where most of the departing aircraft 

entered the parallel taxiway. 

33. PCC VOID ANALYSIS. In addition to joint 

load transfer, another important characteristic of a 

PCC pavement is the slab support conditions. One of 

the assumptions that is made during PCC backcalculation is that the entire slab is in full contact 

with the foundation. The presence of surface 

distresses such as corner breaks, joint faulting, and 

slab cracking, indicates that a loss of support may 

exist in the pavement section. As with a joint 

condition analysis, the focus of the void analysis is 

near joints or slab corners. 

A loss of support may exist for one of three reasons. 

Erosion may have occurred in the base, subbase, or 

subgrade. It is important to recognize that a 

stabilized base or subbase may erode unless key 

criteria are followed in the design of the stabilized 

layer. 

In addition to erosion, settlement may have occurred 

beneath the PCC layer. The most frequent reason for 

settlement is inadequate compaction during 

construction. Finally, a loss of support may occur 

due to temperature curling or moisture warping. The 

amount of warping in a PCC slab is relatively 

constant throughout the year, but the amount of 

curling can vary significantly throughout the day, 

depending on key factors such as PCC layer 

thickness, base type, and the change in temperature. 

A void analysis should be conducted at the slab 

corners and midjoint locations. 

Figure 37 is a plot made for three NDT test drops at 

three load levels at different test locations. If the plot 

passes through the X-axis near the origin, good 

support exists beneath the slab. The further the line 

passes to the right of the origin, the greater the loss of 

support. In general, a deflection intercept greater 

than 3 mils indicates the presence of a void. It is 

important to note that this procedure provides an 

estimate of the void depth, but not the area of the 

void beneath the slab. 
AC 150/5370-11A 12/29/2004 

Page 30 

Research results at the NAPTF show that the 

presence of voids at midjoints may affect the LTE∆

analysis results that were presented in section 32. 

Typically, the sum of the deflections on both sides of 

a joint increase proportionally as slab curling or 

warping increases. 

Therefore, if deflection measurements are taken at 

two different times during a hot day or two different 

seasons, the sum of the deflections should remain 

constant if no curling or warping is present. If NDT 

is conducted only once to measure LTE∆, the void 

detection plots shown in Figure 37 cannot be used to 

ensure that voids are not affecting the results of the 

LTE∆ calculation. 

Figure 38 shows how a profile plot of the void 

analysis results can help identify which sections of a 

pavement facility may have loss of support problems. 

For the same taxiway shown in the LTE∆ profile plot 

in Figure 36, Figure 38 shows that very few voids are 

present at the transverse joint in this taxiway. 

In addition, this void plot shows that the LTE∆

analysis results in Figure 36 have not been affected 

by slab curling. 

Although NDT can provide a good indication that 

voids exist under concrete slabs, other methods, such 

as coring, ground penetrating radar, or infra-red 

thermography should be used to confirm the presence 

of voids. 

34. PCC DURABILITY ANALYSIS. The backcalculation analysis procedures presented in this 

chapter assume that the PCC layer is homogenous. 

Furthermore, the back-calculation results are based 

on center slab deflections and the condition of the 

slab in the interior. However, PCC pavements can 

experience durability problems as a result of poor 

mix designs, poor construction, reactive and 

nondurable aggregates, wet climates, and high 

numbers of freeze-thaw cycles. In general, durability 

problems are most severe along PCC joints and at 

slab corners because moisture levels are the highest 

at these locations. This paragraph focuses on how to 

evaluate PCC slab durability problems. 

NDT deflection data may be very useful in assessing 

the severity of durability-related problems because 

surface conditions may not be a good indicator of the 

severity several inches below the PCC surface. This 

is especially true if a PCC pavement with durability 

problems has been overlaid with HMA. 

Often, the severity of the durability distresses 

increases after an HMA overlay has been constructed 

because more moisture is present at the HMA and 

PCC interface. 

The extent of the durability problem can be assessed 

by evaluating the ISM (or DSM) obtained from the 

center of the slab and comparing it to the ISM (or 

DSM) at a transverse or longitudinal joint or at the 

slab corner. 

The ISMratio will not be equal to one for a perfect slab 

because slab deflections are highest at the slab corner 

and lowest at the slab center. If a joint load transfer 

or loss of support analysis has been conducted, the 

same raw deflection data can be used to compute the 

ISMratio. 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

_ int

_

_

_

slab jo

slab center

slab corner

slab center

ratio ISM

ISM or ISMISM

ISM Κ Κ [17] 

Where: 

ISMratio = Impulse stiffness modulus 

ratio 

ISMslab_center = Impulse stiffness modulus 

at slab center, lbs/inch 

ISMslab_corner = Impulse stiffness modulus 

at slab corner, lbs/inch 

ISMslab_joint = Impulse stiffness modulus 

at slab joint, lbs/inch 

An ISMratio greater than 3 may indicate that the PCC 

durability at the slab corner or joint is poor. If it is 

between 3 and 1.5, the durability is questionable. 

Finally, if the ratio is less than 1.5, the PCC is 

probably in good condition. These ranges are based 

on the assumption that the durability at the PCC 

interior is excellent. This assumption can be verified 

by reviewing the modulus values obtained from backcalculation analysis of the PCC layer. 

Figure 39 is an example ISMratio plot for an HMA 

overlaid PCC runway. As this figure indicates, there 

are very few locations where the durability of the 

PCC is poor. For this scenario, cores should be taken 

in suspect areas and compared to interior, joint, and 

corner cores taken from locations where the PCC 

appears to be in excellent condition. The cores in the 

suspect locations should be inspected and additional 

laboratory tests (e.g., petrographic analysis, split 

tensile, etc.) conducted as necessary to evaluate the 

severity of the durability distress. 
AC 150/5370-11A 12/29/2004 

Page 31 

Use of the ISMratio for HMA overlaid PCC pavements 

has the advantage of eliminating the "HMA 

compression" effect that occurs during NDT. 

Assuming that the HMA layer is the same thickness 

throughout the PCC slab and that its condition (e.g., 

stiffness and extent of shrinkage cracks) is relatively 

constant throughout the slab, there should be 

approximately the same amount of HMA 

compression at the slab center, corner, and joint. The 

net effect is that the ISMratio will primarily reflect the 

durability of the PCC layer. 

35. SUMMARY. This chapter has focused on the 

development of several procedures to compute 

pavement layer modulus values and subgrade 

strengths, evaluate PCC joint load transfer efficiency, 

conduct PCC void analyses, and assess PCC material 

durability. The engineer should develop a statistical 

summary for each layer characteristic so that the 

results from this chapter can be used for pavement 

evaluation and design, as discussed in Chapter 8. 
AC 150/5370-11A 12/29/2004 

Page 32 

CHAPTER 8 – NDT-BASED EVALUATION AND DESIGN INPUTS 

Chapter 7 described several analysis procedures that 

an engineer can use to characterize a pavement. 

Characteristics that are required as inputs for 

pavement evaluation and design include layer elastic 

moduli, CBR values, subgrade elastic moduli, and kvalues. This chapter provides guidance on how to 

use these NDT-based inputs for structural evaluation 

and design in accordance with ACs 150/5320-6 and 

150/5335-5. 

Development of evaluation and design inputs 

requires a two-step approach. First, the engineer 

must use a statistical approach to decide what input 

values should be used for each pavement 

characteristic. In Chapter 7, the raw deflection data 

were used in conjunction with construction history, 

cores, and borings to identify boundaries of each 

pavement section within a facility. Since each 

section typically has many NDT test locations, a 

representative value must be selected for evaluation 

and design. 

The second step requires the engineer to use 

pavement characteristics that are appropriate for FAA 

evaluation and design programs. AC 150/5320-6 

requires traditional FAA inputs such as CBR for 

flexible (e.g., HMA) pavements and modulus of 

subgrade reaction (k-value) for rigid (e.g., PCC) 

pavements. Chapter 7 of AC 150/5320-6 requires 

elastic modulus values as inputs for all layers in the 

structure. As discussed in Chapter 7, engineers 

should know what evaluation or design program they 

will use when conducting back-calculation analyses. 

To obtain more reliable evaluation and design results, 

the structural theory should be the same for both the 

"backward" and "forward" analyses. 

36. STATISTICALLY DERIVED INPUTS. 

There are two sources of error, or variation, that the 

engineer should recognize when selecting NDTbased input values. The first source is "systematic 

error", which is caused by errors in the NDT devices. 

Sources of systematic error in a NDT device include 

sensors, microprocessors, and operational software. 

The second source of error is "random error." For 

pavement deflection measurements and 

characterization, random error sources can be 

categorized as follows:

a. Environmental Noise. The deflection 

responses that are recorded by NDT devices will vary 

because of changes in air temperature, solar radiation, 

types and amounts of precipitation on the pavement 

surface, subsurface frost, and water table height 

fluctuation. The properties of many paving materials 

vary with changes in environmental conditions. 

These condition changes affect the overall deflection 

response of the pavement when it is tested with a 

NDT device.

b. Time-Dependent Noise. The recorded 

NDT deflections and material characteristics of a 

pavement are valid for one point in the design life of 

a pavement. The pavement layer characteristics will 

change as the materials age and as the number and 

magnitude of aircraft and vehicle load applications 

increase. Changes may occur from stripping and 

oxidation in HMA layers, alkali-silica reactivity and 

durability cracking in PCC layers, flexural strength 

and elastic modulus changes in PCC layers, and 

corrosion of dowel bars or steel reinforcement.

c. As-Built Noise. Even if the engineer is able 

to accurately define the boundaries of each pavement 

section within a facility, there may be significant 

variation within a section. There is inherent 

variability associated with construction of a 

pavement cross-section. Sources of "as-built" 

variability include deviations from specified layer 

thicknesses, HMA job mix formula, HMA 

compaction densities, PCC flexural strengths, PCC 

air voids, aggregate sources, and application rates of 

curing compounds.

Systematic error can be minimized by ensuring the 

NDT equipment is regularly maintained and 

calibrated as described in Chapter 4. Random 

variability associated with environmental noise can 

be minimized by following the guidelines that were 

established in Chapter 6 and not risking data 

collection during conditions that may compromise 

the integrity of the deflection data. Random 

variability associated with time-dependent and asbuilt noise is one of the primary reasons that 

deflection data are collected and analyzed. Accurate 

characterization of pavement and subgrade properties 

will provide reliable inputs for pavement evaluation 

and design. 

Statistics are used in pavement engineering to 

develop evaluation and design inputs. In general, as 

the number of data points (e.g., deflection test 

locations) increase, confidence that the computed 

mean and standard deviation values are close to true 

values increases. For most pavement characteristics, 

it is assumed that all values are normally distributed, 

as shown in Figure 40. 
AC 150/5370-11A 12/29/2004 

Page 33 

Figure 40 is a histogram of all ISM values for section 

3 in Figure 24, i.e., 5-inch (125 mm) HMA layer with 

a 28-inch (700 mm) aggregate base. Figure 40 shows 

that the ISM values for section 3 are approximately 

normally distributed with a median ISM value of 

1,884 kips/inch. The median value is the middle 

value in a data set for which 50 percent of all ISM 

values are greater and lesser. The mean value, or 

average, is equal to the sum of the ISM values 

divided by the number of values contained in the data 

set. For section 3, the mean ISM value is 2,010 

kips/inch. Since the median is less than the mean, the 

ISM data for section 3 are said to be slightly skewed 

to the right, as illustrated in Figure 40. 

Another statistical parameter that is useful in 

selecting input values for evaluation and design is the 

standard deviation. Once the ISM mean is computed, 

the standard deviation can be computed as shown by 

equation 18. For section 3, the standard deviation is 

560 kips/inch. For a normal distribution of values, 

approximately 68 percent of the ISM values will fall 

within plus or minus one standard deviation of the 

mean value. Likewise, approximately 96 percent of 

the values will fall within plus or minus two standard 

deviations of the mean. 

( )

1

2

−

− = ∑

n

x x s [18]

Where: 

s = Standard deviation of pavement 

characteristic (e.g., ISM)

x = Computed value (e.g., ISM) from one 

NDT test location

x = Mean of all values 

n = Number of sample values (i.e., test 

locations) in a pavement section 

Another statistical parameter that is useful in 

selecting input values is the coefficient of variation, 

Cv. This value is simply the standard deviation 

divided by the mean as shown by equation 19. Cv 

values for pavement characteristics measured in the 

field vary significantly. A Cv value of less than 20 

percent is normally acceptable for NDT-based 

pavement characteristics. However, it is not 

uncommon for Cv values to range between 20 and 50 

percent, or higher if there are several significant 

sources of environmental, time-dependent, and asbuilt variability. 

⎟100% ⎠

⎞ ⎜

⎝

⎛ = x

s Cv [19] 

Where: Cv = Coefficient of variation, percent. 

In the selection of an evaluation or design input 

value, the engineer should simultaneously consider 

the mean, standard deviation, and coefficient of 

variation. Many evaluation and design procedures 

recommend that the mean value be used in the 

analysis. Table 19 shows the results of a statistical 

analysis for all ISM data in each of the four pavement 

sections that were presented in Figure 24. 

For a more conservative evaluation or design 

approach, AC 150/5320-6 recommends that in 

general, the mean minus one standard deviation may 

be used for establishing evaluation and design inputs. 

If the coefficient of variation is large, outliers should 

be removed to compute the mean minus one standard 

deviation. If outliers are not removed, this approach 

leads to the use of a pavement characteristic value 

(e.g., ISM or elastic modulus) that is less than 85 

percent of all section values for a normally 

distributed population. If the outliers are removed 

and the use of a mean minus one standard deviation 

continues to lead to unreasonable low input values, 

the engineer should consider division of the existing 

pavement section into two or more subsections. 

37. USING NDT RESULTS IN FAA ANALYSIS 

PROGRAMS. Table 20 shows the inputs that are 

required for airport pavement and design as 

referenced in AC 150/5320-6. Once NDT-based 

values are statistically selected for pavement 

evaluation and design, they may be directly used as 

described in the ACs, except for those scenarios that 

are described in the remaining paragraphs of this 

section. As shown in Table 20, NDT analysis results 

can assist the engineer in selecting values for 10 of 

the 15 inputs that are required to use the elastic-layer 

based LEDFAA program, and the CBR and k-value 

based designs as required in AC 150/5320-6.

a. Use of Back-Calculated HMA and PCC 

Surface Moduli. For rigid pavement analysis in 

Chapter 7 of AC 150/5320-6, the modulus of rupture, 

Mr, is required for the existing PCC layer in 

pavement evaluation and overlay designs. For the 

design of bonded PCC overlays on an existing PCC 

and the evaluation of an existing PCC pavement, the 

existing PCC modulus of rupture should be used in 

the analyses. The allowable ranges of Mr in 

LEDFAA (AC 150/5320-6, Chapter 7) are 650 to 800 

psi (4 to 5.5 MPa) and 500 to 900 psi (3 to 6 MPa), 

respectively. The back-calculated PCC elastic 

modulus can be used to estimate the modulus of 

rupture, as shown in equation 20.
AC 150/5370-11A 12/29/2004 

Page 34 

The results from equation 20 should be compared 

with the results obtained using the splitting tensile 

strength correlation shown in equation 21 and in AC 

150/5320-6.

488.5

10

43.5 6 +⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = pcc

r

E

M [20] 

M ( ) f psi r =1.02 t

′+ 200 [21] 

Where: 

Mr = PCC modulus of rupture, psi 

Epcc = Mean back-calculated PCC elastic 

modulus values, psi 

t f ′ = Splitting tensile strength, psi 

(1) AC 150/5320-6, Chapter 7. As shown 

in Table 21, P-401 and P-501 surface layer moduli 

are fixed in LEDFAA. The following general 

assumptions can be made for the evaluation of 

allowable aircraft loads, computation of the 

remaining structural life, and overlay designs. If the 

back-calculated HMA and PCC moduli are greater 

than the fixed modulus values shown in Table 21, the 

design will be more conservative. Likewise, if the 

back-calculated moduli are less than the fixed 

moduli, the overlay design will be less conservative. 

(2) AC 150/5320-6. Use of the design 

nomographs, F806FAA flexible design program, and 

the R805FAA rigid design program does not require 

an HMA modulus value, and the PCC modulus value 

is fixed at 4,000,000 psi (27579 MPa). As mentioned 

above, rigid pavement evaluations and overlay 

designs will be more or less conservative, depending 

on the back-calculated PCC modulus value. 

b. Use of Back-Calculated Stabilized Base 

and Subbase Moduli. 

(1) AC 150/5320-6, Chapter 7. Table 21 

shows the allowable range of modulus values for 

HMA and cement-stabilized bases in LEDFAA that 

are contained in AC 150/5320-6, Chapter 7. For both 

flexible and rigid pavement evaluation and overlay 

designs, statistically selected stabilized layer modulus 

values obtained through back-calculation should be 

used to select the appropriate LEDFAA input 

modulus values, as shown in Table 22. 

(2) AC 150/5320-6. For flexible pavement 

evaluation and overlay designs, the back-calculated 

modulus values can be used to select the appropriate 

equivalency factors, as shown in Table 22. 

For rigid evaluation and overlay designs of a 

pavement with a stabilized base or subbase, the 

subgrade k-value is adjusted using the stabilized layer 

thickness, as shown in Figure 3-16 in AC 150/5320-

6. 

c. Use of Back-Calculated Granular Base 

and Subbase Moduli. 

(1) AC 150/5320-6, Chapter 7. Table 21 

shows that P-209 and P-154 layer modulus values 

have fixed initial “seed” values but are automatically 

adjusted during LEDFAA analysis runs, based on the 

lower-layer moduli and thicknesses. For both 

flexible and rigid pavement evaluation and overlay 

designs, the engineer should select a P-209 layer 

when the back-calculated base or subbase modulus is 

greater than 40,000 psi (276 MPa) and a P-154 layer 

when the back-calculated modulus is less than or 

equal to 40,000 psi (276 MPa), as shown in Table 22. 

(2) AC 150/5320-6. For flexible pavement 

evaluation and overlay designs, an equivalency factor 

of one is always used, regardless of the backcalculated modulus values for a granular base and 

subbase, as shown in Table 22. For rigid evaluation 

and overlay designs of a pavement with no stabilized 

base or subbase, the subgrade k-value should not be 

adjusted to account for the presence of the granular 

base or subbase during the analysis if the closed-form 

solution, or layered elastic solution with a single 

composite subbase/subgrade, was used. 

d. Use of Back-Calculated Subgrade Elastic 

Moduli. 

(1) AC 150/5320-6, Chapter 7. The 

allowable range of subgrade modulus values that can 

be used in LEDFAA is shown in Table 21. The 

allowable range of 1,000 to 50,000 psi (7 to 345 

MPa) is characteristic of most types of subgrade 

soils. For both flexible and rigid pavements, the 

statistically selected modulus value can be directly 

input into LEDFAA. However, since many subgrade 

soils are stress sensitive and because apparent stiff 

layers can significantly affect back-calculations 

results, the engineer should carefully review the 

statistically selected modulus to ensure it is consistent 

with field DCP and CBR tests, laboratory CBR tests, 

and soil classification data. CBR and back-calculated 

subgrade modulus values can be compared by using 

equation 22, which is one of the more commonly 

used correlations between E and CBR. 

Esubgrade = 1500 CBR [22] 
AC 150/5370-11A 12/29/2004 

Page 35 

Where: 

CBR = California Bearing Ratio, percent 

 Esubgrade = Back-calculated subgrade elastic 

modulus, psi 

(2) AC 150/5320-6. For flexible pavement 

evaluation and overlay designs, the analysis is 

conducted using the subgrade CBR. Equation 22 can 

be used to estimate the CBR by using the backcalculated subgrade modulus. For rigid evaluation 

and overlay designs, the analysis is conducted by 

using the subgrade k-value. The k-value can be 

estimated from the back-calculated elastic modulus 

by using equation 23. 

Esubgrade = 26k1.284 [23] 

Where: 

k = Modulus of subgrade reaction, 

psi/in. 

 Esubgrade = Back-calculated static subgrade 

elastic modulus, psi. 

Alternatively, the subgrade k (or composite 

subgrade/subbase k) can be computed directly using a 

closed-form solution, as discussed in Chapter 7. 

e. Use of Back-Calculated Subgrade kValue. 

(1) AC 150/5320-6, Chapter 7. For flexible 

pavement evaluation and overlay designs, the 

analysis is conducted using the subgrade CBR or 

elastic modulus. Before k-values can be used in 

LEDFAA for a flexible pavement analysis, the 

selected static k-value must be converted to an elastic 

modulus, which is estimated using equation 23. For 

rigid pavement evaluation and overlay designs, the 

statistically selected k-value can be directly input into 

LEDFAA. The allowable range of static subgrade 

modulus values that can be input into LEDFAA is 17 

to 362 psi/in. However, a k-value that is less than 50 

psi/in should not be used. Back-calculated k-values 

lower than 50 psi/in may indicate that other problems 

exist, such as a loss of support, which is addressed 

later in this chapter. 

(2) AC 150/5320-6. For flexible pavement 

evaluation and overlay designs, the analysis is 

conducted using the subgrade CBR. Equations 22 

and 23 can be used to estimate the subgrade CBR 

from closed-form back-calculated subgrade k-value. 

For rigid evaluation and overlay designs, the analysis 

is conducted by directly using the static subgrade kvalue that is obtained from the closed-form NDT 

back-calculation work. 

f. Selecting PCC Overlay Condition 

Factors. Overlay condition factors are used when 

HMA or PCC overlays are to be constructed on an 

existing PCC pavement. In AC 150/5320-6, Cb and 

Cr are used for HMA and PCC overlays, respectively. 

These empirical factors account for past structural 

damage by effectively reducing the existing PCC 

layer thickness in the overlay designs. The Cb and Cr

condition factors can be selected from AC 150/5320-

6 or from the Structural Condition Index (SCI) using 

PCI distress data and equations 25 and 26 

SCI = 93.2Cr + 7.1 [25] 

Where: SCI = Structural condition index computed 

from PCI data 

Cr = Condition factor for PCC overlays of 

a PCC 

=100 − 25 0.75 ≤ ≤1.0 Cb Cb SCI [26] 

Where: Cb = Condition factor for HMA overlay of a 

PCC 

When SCI data are not available, the back-calculated 

PCC modulus can be used subjectively with the 

photos in AC 150/5320-6 to estimate Cb and Cr for 

use in the design of overlays for rigid pavements. 

Back-calculated moduli below 4,000,000 psi (27,579 

MPa) would indicate the use of lower Cb and Cr

values. However, the use of the SCI from a properly 

conducted pavement condition survey is the preferred 

and recommended method to establish the condition 

of the PCC layer for layered elastic or conventional 

overlay designs in accordance with AC 150/5320-6. 

g. Other Inputs. As shown in Table 20, 

remaining input items 11 through 15 are not obtained 

from NDT analysis results. Layer thicknesses and 

interface bonding conditions can be obtained from 

cores, borings, GPR, and DCP analysis results. 

Aircraft traffic data are very important. For the two 

or three most critical aircraft, the most important 

characteristic is the operational gross takeoff weight 

and the number of annual departures. Unless 

approved by the FAA, the design life is typically 20 

years. For a remaining structural life analysis, in 

years, the allowable gross takeoff weight may have to 

be restricted to extend the life of the pavement to 

meet the owner's operational needs.
AC 150/5370-11A 12/29/2004 

Page 36 

38. PCC LOSS OF SUPPORT. After 

confirmation, if the NDT analysis results from 

Chapter 7 indicate that extensive voids exist 

throughout the PCC section, the engineer has three 

options. The recommended option is to conduct the 

evaluation and design assuming that support will be 

restored through undersealing operations. In this 

instance, statistically selected back-calculated kvalues can be used in the analysis.

The second option is to conduct a finite element 

analysis to compute the increase in PCC flexural 

stress that is caused by a loss of support at the slab 

corner or joints. 

The third option, and least desirable, is to conduct an 

analysis using a reduced k-value, similar to the 

approach used in AC 150/5320-6 for frost protection. 

For both frost and loss of support scenarios, the 

subgrade support is reduced to a k-value that is less 

than the value obtained from an NDT analysis. 

For frost-based reduced subgrade strengths, the kvalue ranges from 25 to 50, depending on the frost 

group, as discussed in AC 150/5320-6. Table 23 

presents recommendations for a reduced k-value 

based on the results of the void analysis in Chapter 7. 

39. SUMMARY. The engineer can use the NDT 

results that are obtained using the procedures 

discussed in Chapter 7 to select inputs for the FAA 

analysis procedures that are described in AC 

150/5320-6. If the engineer has analyzed the NDT 

deflection data, as discussed in Chapter 7, the NDT 

results can be directly used in many of the FAA's 

evaluation and design programs. Otherwise, several 

correlations must be used to estimate evaluation and 

design inputs, as discussed in this chapter. In either 

case, the use of NDT-based evaluation and design 

inputs may increase the reliability of the analysis 

because in-situ properties of the pavement structure 

and subgrade are used for evaluation and design.
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-1 

APPENDIX 1-FIGURES 

Sp ring constant

Mass

Drop height

Load p late

FIGURE 1. ILLUSTRATION OF AN IMPULSE LOAD CREATED BY FWD 

D2 D3 D4 D5 D6 D7

FWD Load Plate (Sensor D1 Located In Load Plate)

Sensor Designation

 Pavement Surface

Sensor

FIGURE 2. SENSORS SPACED RADIALLY FROM THE LOAD PLATE 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-2 

NDT Load

Unloaded Surface

Surface Layer

Base Layer

Subgrade

Deflection Basin

Zone of Load Influence

Apparent Stiff Layer

FIGURE 3. SCHEMATIC OF DEFLECTION BASIN 

-25

-20

-15

-10

-5

0

-72 -60 -48 -36 -24 -12 0 12 24 36 48 60 72

PAVEMENT 1

PAVEMENT 2

PAVEMENT 3

 8" PCC E-4,000,000 psi

 6" AGG E-80,000 psi

SG E-12,000 psi

 4" HMA E-500,000 psi

 8" AGG E-20,000 psi

SG E-24,000 psi

PAVEMENT 1 PAVEMENT 2

15,000

PAVEMENT 3

 4" HMA E-500,000 psi

 8” AGG E-80,000 psi

SG E-12,000 psi

FIGURE 4. COMPARISON OF DEFLECTION BASIN OF THREE PAVEMENTS 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-3 

FIGURE 5. ILLUSTRATION OF STATIC AND DYNAMIC FORCE COMPONENTS FOR VIBRATORY NDT 

FIGURE 6. ILLUSTRATION OF TIME TO PEAK LOAD FOR IMPULSE-BASED NDT EQUIPMENT 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-4 

FIGURE 7. BENKLEMAN BEAM 

FIGURE 8. DYNAFLECT DEFLECTION TRAILER 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-5 

FIGURE 9. ROAD RATER 

FIGURE 10. KUAB FWD 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-6 

FIGURE 11. DYNATEST FWD 

FIGURE 12: CARL BRO FWD/HWD AND LWD 

TRAILER, VAN-INTEGRATED OR PORTABLE) 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-7 

FIGURE 13. JILS HWD 

FIGURE 14. EVALUATION OF HWD FORCE LINEARITY IN TERMS OF ISM

0

500

1,000

1,500

2,000

2,500

1

Test Item

ISM (k/in)

LFS MFS

HFS

LFS

MFS

HFS

Applied Force

12, 24, 36 kip
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-8 

FIGURE 15. EVALUATION OF HWD FORCE LINEARITY 

 IN TERMS OF SUBGRADE ELASTIC MODULUS 

D1

Transverse Joint

D1

Center of Slab

D1

D1

Slab Corner Longitudinal Joint

Direction of Testing

FIGURE 16. NDT TEST LOCATIONS WITHIN A PCC SLAB 

12, 24, 36 kip 

Applied Force

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

1

Test Item

Subgrade Modulus (psi)

LFS

MFS

HFS

LFS

MFS

HFS

Applied Force

12, 24, 36 kip
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-9 

Aircraft Wheel 

Loaded PCC Slab Unloaded PCC Slab 

FIGURE 17. ILLUSTRATION OF LOAD TRANSFER ACROSS A PCC JOINT 

 Taxiway or Runway Centerline 

Direction of 

Testing

 Station 

 0+00 

PCC

Lane 2

PCC

Lane 1

PCC

Lane 4

PCC

Lane 3

9.2

3.1

1.2 3.2

2.1 1.1

2.2

4.1

4.2

NOTE: First number indicates PCC lane number; second number indicates location within lane (e.g., along slab center 

or slab joint). 

FIGURE 18. EXAMPLE RUNWAY OR TAXIWAY SKETCH WHEN CENTERLINE 

 LIES ON SLAB JOINT 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-10 

 Taxiway or Runway Centerline 

Direction of 

Testing

Station 0+00 

PCC 

Lane 2 

PCC 

Lane 9 

PCC 

Lane 1 

2.2 

1.1

1.2 3.2

2.1 9.1

4.2 

NOTE: First number indicates PCC lane number; second number indicates location within lane (e.g., along slab center 

or slab joint). 

FIGURE 19. EXAMPLE RUNWAY OR TAXIWAY SKETCH WHEN CENTERLINE 

 DOES NOT LIE ON SLAB JOINT 

Taxiway or Runway Centerline 

Direction of

Testing

Station 0+00

4.1 3.1 2.1 9.1 1.1

NOTE: First number indicates HMA lane number; second number indicates a "center" test for HMA pavements. 

FIGURE 20. EXAMPLE RUNWAY OR TAXIWAY SKETCH FOR HMA PAVEMENTS 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-11 

Positive (daytime) curling

Negative (nighttime) curling 

Thermal Curling 

FIGURE 21. THERMAL CURLING IN PCC SLAB FROM TEMPERATURE CHANGES 

Slab Center

Test

Direction of Testing

Transverse

Joint Test

Slab Corner

Test

Longitudinal

Joint Test

Plate

Plate

Plate

Plate

FIGURE 22. LOCATION OF ADDITIONAL SENSORS FOR CORNER AND JOINT TESTING 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-12 

Process and Group Raw

Deflection Data by

Pavement Facility

Conduct Preliminary Analysis

by Normalizing Deflections

and Computing ISM

Separate Facility Raw

Deflection Data into

Pavement Section Subsets

Conduct Analysis to Obtain

Desired Pavement

Characteristics

Pavement Layer Strength

Back-calculate Elastic

Modulus Values

Joint/Crack Condition

Compute Load Transfer

Efficiency

Material Durability

Determine Extent of

Material Deterioration

Support Conditions

Determine if Voids

Exist Under PCC Slab

Statistically Determine

Design and Evaluation Inputs

Structural Capacity

Determine Allowable

Loads and Remaining Life

Rehabilitation Design

Develop Overlay and

Reconstructed Cross-sections

Airport Pavement Management

Develop Criteria to Supplement PCI

in Capital Program Development

FIGURE 23. NDT DATA ANALYSIS AND DESIGN FLOWCHART 

0

 1,000

 2,000

 3,000

 4,000

 5,000

 6,000

 7,000

0 500

 (150)

1,000

 (300)

1,500

 (455)

2,000

 (600)

2,500

 (760)

3,000

 (900)

3,500

(1060)

4,000

(1200)

4,500

(1370)

5,000

(1525)

5,500

(1675)

6,000

(1825)

6,500

(1980)

7,000

(2130)

7,500

(2285)

8,000

(2435)

8,500

(2590)

STATION, STA ft. (m)

ISM, (k/in)

SECTION 1 SECTION 2 SECTION 3 SECTION 4

8in(200mm)HMA

12in(300mm)stab. base

6in(150 mm)stab. subgr

 5in(125mm) HMA

15in(375mm) stab. base

 5in(125mm) HMA

28in(700mm) aggregate base

 5in(125 mm) HMA

16in(400 mm) aggregate base

Note: Bold bars indicate the

average ISM value for each

section.

FIGURE 24. ISM PLOT USED TO IDENTIFY PAVEMENT SECTION BREAKS 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-13 

0

5

10

15

20

25

30

35

0 500

 (150)

1,000

 (300)

1,500

 (455)

2,000

 (600)

2,500

 (760)

3,000

 (900)

3,500

(1060)

4,000

(1200)

4,500

(1370)

5,000

(1520)

5,500

(1675)

6,000

(1825)

6,500

(1980)

7,000

(2130)

7,500

(2285)

8,000

(2435)

8,500

(2590)

STATION, STA ft. (m)

20-kip Normalized D1, (mils)

SECTION 1 SECTION 2 SECTION 3 SECTION 4

 8in(200mm) HMA

12in(300mm) stab. base

 6in(150mm) stab. subgr.

 5in(125mm) HMA

15in(375mm) stab. base

 5in(125mm) HMA

28in(700mm) aggregate base

 5in(125mm) HMA

 16in(400mm) aggregate base

Note: Bold bars indicate the

average normalized deflection

value under the load plate for each

section.

FIGURE 25. NORMALIZED DEFLECTION PLOT USED TO IDENTIFY PAVEMENT SECTION BREAKS 

0

2

4

6

8

10

12

14

0 500

 (150)

1,000

 (300)

1,500

 (455)

2,000

 (600)

2,500

 (760)

3,000

 (900)

3,500

(1060)

4,000

(1200)

4,500

(1370)

5,000

(1520)

5,500

(1675)

6,000

(1825)

6,500

(1980)

7,000

(2130)

7,500

(2285)

8,000

(2435)

8,500

(2590

STATION, STA ft. (m)

20-kip Normalized D4, (mils)

SECTION 1 SECTION 2 SECTION 3 SECTION 4

 8in(200mm) HMA

12in(300mm) stab. base

 6in(150mm) stab. subgr.

5in(125mm) HMA

15in(375mm stab. base

 5in(125mm) HMA

28in(700mm) aggregate base

 5in(125mm) HMA

16in(400mm) aggregate base

Note: Bold bars indicate the

average normalized deflection

 value 24 inches (600 mm) from the load

plate for each section.

FIGURE 26. NORMALIZED SUBGRADE DEFLECTION PLOT 

 USED TO IDENTIFY PAVEMENT SECTIONS 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-14 

Remove Basin Deflection

Data with Type I, II, or III

Errors

Identify Pavement Type from

Section Construction History

Compare Layer Thicknesses

From Cores, Borings, and

Construction History

Select Back-calculation Method

Based on Pavement Type

HMA Pavements

Iterative Elastic Layer

AASHTO Closed-Form

PCC Pavements

Iterative Elastic Layer

AREA Closed-Form

Thick HMA Overlays on PCC

Iterative Elastic Layer

Thin HMA Overlays on PCC

Iterative Elastic Layer

AREA Closed-Form

FIGURE 27. PROCESS FOR DATA PREPARATION AND BACK-CALCULATION METHOD SELECTION 

Calculate AREA Based on NDT 

Sensor Configuration and 

Pavement Type

 PCC Pavements (<16in(400mm) thick

 AREA Equation 3 AREA Equation 4

HMA Overlaid PCC

AREA Equation 5

AREA Equation 6

PCC Pavements (> 16in(400mm) thick

AREA Equation 5 AREA Equation 6

Calculate Radius of Relative 

Stiffness from Equation 8 

Calculate Modulus of Subgrade 

Reaction, k, from Equation 9

Calculate Effective Elastic Modulus 

Above Subgrade from Equation 10 

Apply PCC Slab Size Correction Factor 

to Modulus and k-value

Bonded PCC or HMA OL, or St. Base

Equation 12 Equation 13 Static k-value = 1⁄2 back-calculated k

No Overlay or Stabilized Base

Effective Modulus Equals PCC 

Modulus of Elasticity

Static k-value = 1⁄2 back-calc. k

Unbonded PCC or HMA OL, or St. Base

Equation 14

Static k-value = 1⁄2 back-calculated k

Split Effective Modulus as Required 

Based on Overlay (OL) and Base Type

FIGURE 28. FLOWCHART FOR CLOSED-FORM BACK-CALCULATION USING AREA METHOD 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-15 

12 inches (300 mm)

LOAD PLATE

24 inches (600 mm)

36 inches (90 mm)

D0

D0

Basin AREA

D12

D0

D24

D0

D36

D0

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = +

0

36

0

24

0

12 ( ) 6 12 12 6

D

D

D

D

D

D AREA in

FIGURE 29. ILLUSTRATION OF BASIN AREA FOR SHRP FOUR-SENSOR CONFIGURATION 

MEASURED

CALCULATED

E1

E3

E2

FIGURE 30. COMPARISON OF MEASURED AND CALCULATED DEFLECTION BASINS 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-16 

Compare Layer Thicknesses 

from Cores, Borings, and 

Construction History

Identify Potential Problem Layers

1. Thin layers (<3 inches (75mm) thick) 

2. More than 3 layers 

3. Soft layer above strong layer 

4. Soft layer between 2 strong layers 

5. Depth to stiff layer (<10 ft. (3.0 m)) 

Determine Elastic Modulus 

“Seed Values” for each Layer

Determine Depth to Bedrock or 

Apparent Stiff Layer

Determine the Initial Pavement 

Cross-section and Layer 

Thicknesses 

Adjust Cross-section, Layer 

Thicknesses, and Modulus 

Limits as Necessary

Review Back - Calculation Results

1. Are the error levels within limits? 

2. Are the modulus values reasonable? 

3. For any layer, is the upper modulus 

limit being hit often for each basin? 

4. For any layer, is the lower modulus 

limit being hit often for each basin? 

5. Are moduli ratios between adjacent 

unbound layers unrealistic?

Compute Statistical 

Summary of Results 

for Each Layer 

Run ElasticLayer Back -

calculation Software Program

Results 

OK?

No Yes

FIGURE 31. BACK-CALCULATION PROCEDURES FOR AN ELASTIC LAYER BASED ANALYSIS 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-17 

FIGURE 32. INITIAL BAKFAA RUN FOR EXAMPLE 2 

FIGURE 33. SECOND BAKFAA RUN FOR EXAMPLE 2 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-18 

FIGURE 34. OUTPUT FROM SECOND BAKFAA RUN FOR EXAMPLE 2 

FIGURE 35. DEFLECTION VS. STRESS LTE RELATIONSHIP FOR 12-INCH (300 mm) 

DIAMETER LOAD PLATE 

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 70 80 90 100

Stress LTE (%)

Deflection LTE (%)

Radius of Relative Stiff=20in(500mm)

Radius of Relative Stiff=130in(3300mm)

25 % Stress LTE is FAA Target

V l
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-19 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0+00 20+00 40+00 60+00 80+00 100+00 120+00

Station

Transverse Joint LTE

NDT Lane 1-12ft(3.6m) right of CL

NDT Lane 2-12ft(3.6m) left of CL

LTE Acceptable

LTE Poor

LTE Fair

Section 1 Section 2 Section 3 Section 4

FIGURE 36. EXAMPLE PLOT OF TRANSVERSE JOINT LTE∆ FOR A 10,000-FOOT (3,050 m) TAXIWAY 

LINE A - NO VOIDS

LINE B - SMALL VOID

LINE C - LARGE VOID

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

0

5

10

15

20

25

HWD CORNER LOAD (kips) MAXIMUM PAVEMENT DEFLECTION (mils)

FIGURE 37. VOID DETECTION BENEATH PCC SLABS 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-20 

-5

-3

-1

1

3

5

7

9

11

13

15

0+00 10+00 20+00 30+00 40+00 50+00 60+00 70+00 80+00 90+00 100+00 110+00 120+00

Station

Voids, mils

NDT Lane 1-12ft(3.6m) right of CL

NDT Lane 2-12ft(3.6m) left of CL

Voids Present

Voids Present

No Voids

FIGURE 38. EXAMPLE PLOT OF TRANSVERSE JOINT VOIDS FOR A 10,000-FOOT (3,050 m) TAXIWAY 

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0+00 10+00 20+00 30+00 40+00 50+00 60+00 70+00 80+00

Station

ISM Center to Edge Ratio

NDT Lane 1-10ft(3.0m) right of CL

NDT Lane 2-10ft(3.0m) left of CL

NDT Lane 3-20ft(6.09m) right of CL

NDT Lane 4-20ft(6.09m) left of CL

Underlying PCC is in

fair to poor condition

Underlying PCC is in

good condition

Underlying PCC is in

excellent condition

FIGURE 39. EXAMPLE PLOT OF ISMratio FOR TRANSVERSE JOINT FOR HMA OVERLAID PCC 
AC 150/5370-11A 12/29/2004 

Appendix 1 

Page 1-21 

1

9

18

14 14

4

2 2

3

0

2

4

6

8

10

12

14

16

18

20

1182 1484 1787 2090 2392 2695 2998 3300 >3300

ISM (k/in)

Frequency (NDT Test Locations)

Mean

Median

FIGURE 40. HISTOGRAM OF ISM VALUES FOR SECTION 3 IN FIGURE 24 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-1 

APPENDIX 2-TABLES 

TABLE 1. SUMMARY OF NONDESTRUCTIVE TESTING MEASURING EQUIPMENT 

Category Equipment Manufacturer Load range, 

 lbs. (kN) 

Load transmitted 

by, in. (mm) 

Number of 

sensors 

Sensor spacing, 

in. (mm) 

Benkleman Beam Soiltest, Inc. Vehicle 

dependent 

Loaded truck or 

aircraft 1 NA 

La Croix 

Deflectograph Switzerland Vehicle dependent Loaded truck 1 NA Static 

Plate-bearing test 

Several, 

ASTM D 

1196 

Vehicle 

dependent Loaded truck 1 NA 

Dynaflect Geolog, Inc. 1,000 

(5) 

15 (2,400) 

diameter steel 

wheels 

4 

Variable, 

0 to 48 

(0 to 1,200) 

Road Rater 

Foundation 

Mechanics, 

Inc. 

500 to 8,000 

(2 to 35) 

18 (450) 

diameter plate 4 to 7 

Variable, 

0 to 48 

(0 to 1,200) 

Vibratory 

WES Heavy 

Vibrator 

U.S. Corps of 

Engineers 

500 to 30,000 

(2 to 130) 18 (450) 

 diameter plate 5 

Variable, 

0 to 60 

(0 to 1,500) 

Dynatest FWD Dynatest 

Engineering 

1,500 to 27,000

(7 to 120) 

12 or 18 

(300 or 450) 

diameter plate 

7 to 9 

Variable, 

0 to 90 

(0 to 2,250) 

Dynatest HWD Dynatest 

Engineering 

6,000 to 54,000 

(27 to 240) 

12 or 18 

(300 or 450) 

diameter plate 

7 to 9 

Variable, 

0 to 90 

(0 to 2,250) 

JILS FWD 

Foundation 

Mechanics, 

Inc. 

1,500 to 24,000 

(7 to 107) 

12 or 18 

(300 or 450) 

diameter plate 

7 

Variable, 

0 to 96 

(0 to 2,400) 

JILS HWD 

Foundation 

Mechanics, 

Inc. 

6,000 to 54,000 

(27 to 240) 

12 or 18 

(300 or 450) 

diameter plate 

7 

Variable, 

0 to 96 

(0 to 2,400) 

KUAB FWD KUAB 1,500 to 34,000 

(7 to 150) 

12 or 18 

(300 or 450) 

diameter plate 

7 

Variable, 

0 to 72 

(0 to 1,800) 

KUAB HWD KUAB 3,000 to 66,000 

(13 to 294) 

12 or 18 

(300 or 450) 

diameter plate 

7 

Variable, 

0 to 72 

(0 to 1,800) 

Carl Bro FWD Carl Bro Group 1,500 to 34,000 

(7 to 150) 

12 or 18 

(300 or 450) 

diameter plate 

9 to 12 Variable, 

0 to 100 

(0 to 2,500) 

Carl Bro HWD Carl Bro Group 1,500 to 56,000 

(7 to 250) 

12 or 18 

(300 or 450) 

diameter plate 

9 to 12 Variable, 

0 to 100 

(0 to 2,500) 

Impulse 

Carl Bro LWD Carl Bro Group 225 to 3,400 

(1 to 15) 

4 or 8 or 12 

(100 or 200 or 300) 

diameter plate 

1 to 3 Variable, 

0 to 40 

(0 to 

1,000) 

NOTE: EQUIPMENT MENTIONED ABOVE IS FOR INFORMATION PURPOSES, ONLY. 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-2 

TABLE 2. DETAILED SPECIFICATIONS FOR SELECTED FWDS AND HWDS 

Equipment manufacturer Equipment 

Specifications Dynatest Foundation 

Mechanics, Inc. KUAB Carl Bro Group 

Load range, lbs (kN) 1,500 to 54,000 

(7 to 240) 

1,500 to 54,000 

(7 to 240) 

1,500 to 66,000 

(7 to 294) 

1,500 to 56,000 

(7 to 250) 

Load duration 25 to 30 millisecond Selectable 56 millisecond 25 to 30 millisecond 

Load rise time Variable Selectable 28 millisecond 12 to 15 millisecond 

Load generator One-mass One-mass Two-mass One-mass 

Type of load plate 

Rigid with 

rubberized pad or 

split plate 

Rigid with 

rubberized pad 

Segmented or 

nonsegmented with 

rubberized pads 

Four segmented plate 

with rubberized pad 

Diameter of load plate, 

in. (mm) 

12 and 18 

(300 and 450) 

12 and 18 

(300 and 450) 

12 and 18 

(300 and 450) 

12 and 18 

(300 and 450) 

Type of deflection 

sensors 

Geophones with or 

without dynamic 

calibration device 

Geophones 

Seismometers with 

static field calibration 

device 

Geophones with our 

without dynamic 

calibration device 

Deflection sensor 

positions, in. (mm) 

0 to 90 

(0 to 2250) 

0 to 96 

(0 to 2400) 

0 to 72 

(0 to 1800) 

0 to 96 

(0 to 2500) 

Number of sensors 7 to 9 7 7 9 to 12 

Deflection sensor range 

Mils (mm) 80 or 100 

(2 or 2.5 mm) 

80 (2) 200 (5) 90 (2.2) 

Deflection resolution 1 μm (0.04 mils) 1 μm (0.04 mils) 1 μm (0.04 mils) 1 ìm (0.04 mils) 

Relative accuracy of 

deflection sensors 2 μm ± 2% 2 μm ± 2% 2 μm ± 2% 2 ìm ± 2% 

Test time required 

(four loads) 25 seconds 30 seconds 35 seconds 20 seconds 

Type of computer Personal computer Personal computer Personal computer Personal computer 

TABLE 3. ASTM STANDARDS FOR DEFLECTION MEASURING EQUIPMENT 

NDT equipment type 

ASTM Static Vibratory Impulse 

D 1195, Standard Test Method for Repetitive Static Plate Load Tests 

of Soils and Flexible Pavement Components, for Use in Evaluation 

and Design of Airport and Highway Pavements

● 

D 1196, Standard Test Method for Nonrepetitive Static Plate Load 

Tests of Soils and Flexible Pavement Components, for Use in 

Evaluation and Design of Airport and Highway Pavements

● 

D 4602, Standard Guide for Nondestructive Testing of Pavements 

Using Cyclic-Loading Dynamic Deflection Equipment ● 

D 4694, Standard Test Method for Deflections with A Falling-WeightType Impulse Load Device ●

D 4695, Standard Guide for General Pavement Deflection 

 Measurements ● ● ●
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-3 

TABLE 4. COMMON SENSOR CONFIGURATIONS 

Sensor distance from center of load plate, inches (mm) Agency Configuration 

name Sensor1 Sensor2 Sensor3 Sensor4 Sensor5 Sensor6 Sensor7 

U.S. Air Force AF 7-Sensor 0 12 

(300) 

24 

(600) 

36 

(900) 

48 

(1,200) 

60 

(1,500) 

72 

(1,800) 

SHRP

4-Sensor 0 12 (300) 

24 

(600) 

36 

FHWA & State (90) 

DOTs SHRP 

7-Sensor 0 6 (150) 

12 

(300) 

18 

(450) 

24 

(600) 

36 

(900) 

60 

(1,500) 

TABLE 5. TYPICAL RUNWAY AND TAXIWAY TEST LOCATIONS AND SPACING, FEET (m) 

Jointed PCC and HMA overlaid PCC HMA 

Project level Network level Project level Network level Test type Offset, 

 ft. (m) Spacing, ft. (m) Offset, 

ft. (m)

Spacing, 

ft. (m)

Offset, 

ft. (m) Spacing, ft. (m) Offset, ft. (m)

Spacing, 

ft. (m)

Center 

10 (3) 

30 (9) 

65 (20) 

100 (30) 

100-200 (30-60) 

400 (120) 

10 (3) 200-400 

(60-120) 

10 (3) 

20 (6) 

65 (20) 

100 (30) 

100-200 (30-60) 

200-400 (60-120) 

10 (3) 200-400 

(60-120) 

Transverse 

Joint 

10 (3) 

30 (9) 

65 (20) 

100-200 (30-60) 

200-400 (60-120) 

400 (120) 

10 (3) 400 

(120) 

Longitudinal 

Joint 

20 (6) 

40 (12) 

60 (18) 

200 (60) 

400 (120) 

400 (120) 

Corner 

20 (6) 

40 (12) 

60 (18) 

200 (60) 

400 (120) 

400 (120) 

NOTE: For each centerline offset, there are two NDT passes, one to the left and one to the right; spacing is 

staggered between adjacent NDT passes; and a minimum of two NDT tests should be conducted per 

pavement section. 

TABLE 6. TYPICAL APRON TEST LOCATIONS AND FREQUENCY 

Jointed PCC and HMA overlaid PCC HMA, ft 2

 (m2) Test type Project level Network level Project level Network level

Center 1 test for every 

10 to 20 slabs 

1 test for every 

30 to 60 slabs 

1 test for every 

1,970 to 4,000 

(600 to 1200)

1 test for every 

5,750 to 11,490 

(1750 to 3,500)

Transverse 

Joint 

1 test for every 

10 to 40 slabs 

1 test for every 

60 slabs 

Longitudinal 

Joint 

1 test for every 

20 to 40 slabs 

1 test for every 

60 slabs 

Corner 1 test for every 

20 to 40 slabs 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-4 

TABLE 7. FAA SOFTWARE TOOLS FOR PAVEMENT ANALYSIS, EVALUATION, AND DESIGN 

NDT data analysis Evaluation1

 Design Tool function 

BAKFAA FEAFAA COMFAA LEDFAA F806FAA R805FAA 

Back-calculate HMA 

moduli ● 

Back-calculate PCC 

moduli ●

Back-calculate APC2

moduli ● 

Compute load 

transfer ● 

Conduct void 

analysis ●

Compute allowable 

loads ● ● ● ● ●

Compute remaining 

life ● ● ● 

Compute PCN ● 

Perform HMA 

overlay design ● ● 

Perform PCC overlay 

design ● ●

Design new HMA 

cross-section ● ● 

Design PCC 

Cross-section ● ●

NOTES: 1 These evaluation tools can be used for design checks but are not approved FAA design 

programs. 2 APC is asphalt overlaid PCC pavements. 

TABLE 8. THEORETICAL BASIS OF FAA SOFTWARE TOOLS 

Structural Theory 

FAA Tool Winkler Foundation CBR Elastic Layer Westergaard Finite Element 

BAKFAA ● 

FEAFAA ●

COMFAA ● ● 

LEDFAA ● 

F806FAA ● 

R805FAA ● 

NOTE: These programs can be downloaded from the FAA’s website at www.faa.gov. 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-5 

TABLE 9. REQUIRED SENSOR DISTANCE (INCH) FROM LOAD PLATE WITH 

12-INCH (300 MM) DIAMETER 

Subgrade CBR Surface Type Surface Layer 

Thickness, in (mm) 4 (Weak) 12 (Average) 20 (Strong) 

 4 (100 mm) 15 12 9 HMA 

 8 (200 mm) 30 24 18 

12 (300 mm) 84 60 48 PCC 20(500 mm) 144 108 84 

NOTE: The shaded areas show the required sensor distance beyond the typical NDT equipment maximum 

of 70 inches (1,775 mm). 

TABLE 10. TYPE OF BACK-CALCULATION SOFTWARE TOOL THAT IS REQUIRED 

FOR EACH LOAD SCENARIO 

Type of load application Material response Static Dynamic 

Linear Closed-form and iteration-based tools Finite-element tools 

Nonlinear Iteration-based tools Finite-element tools 

NOTE: Dynamic loads include those generated by vibratory and impulse load NDT equipment. 

TABLE 11. AREA-BASED CONSTANTS FOR EQUATION 8 

Constant Area method 

A B C D 

1. SHRP 4-sensor 

 (0 to 36 inches (0 to 900mm) 

36 1812.279 -2.559 4.387 

2. SHRP 7-sensor 

 (0 to 60 inches (0 to 1,500mm) 

60 289.078 -0.698 2.566 

3. SHRP 5 outer sensors 

 (12 to 60 inches (300 to 1,500mm) 

48 158.408 -0.476 2.220 

4. Air Force 6 outer sensors 

 (12 to 72 inches (300 to 1,800mm) 

60 301.800 -0.622 2.501 

TABLE 12. CONSTANTS FOR *

r d (EQUATION 11) 

Radial distance from Constant 

load plate, in. (mm) x y z 

0 0.12450 0.14707 0.07565 

12 (300) 0.12188 0.79432 0.07074 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-6 

TABLE 13. TYPICAL MODULUS VALUES AND RANGES FOR PAVING MATERIALS 

Material Low value, psi (MPa) Typical value, psi/MPa High value, psi/MPa 

Asphalt concrete 70,000 (483) 500,000 (3447) 2,000,000 (13790) 

Portland cement concrete 1,000,000 (6895) 5,000,000 (34474) 9,000,000 (62053) 

Lean-concrete base 1,000,000 (6895) 2,000,000 (13790) 3,000,000 (20684) 

Asphalt-treated base 100,000 (689) 500,000 (3447) 1,500,000 (10342) 

Cement-treated base 200,000 (1379) 750,000 (5171) 2,000,000 (13790) 

Granular base 10,000 (69) 30,000 (207) 50,000 (345) 

Granular subbase or soil 5,000 (34) 15,000 (103) 30,000 (207) 

Stabilized soil 10,000 (69) 50,000 (345) 200,000 (1379) 

Cohesive soil 3,000 (21) 7,000 (48) 25,000 (172) 

TABLE 14. TYPICAL POISSON'S RATIOS FOR PAVING MATERIALS 

Material Low value High value 

Asphalt concrete or asphalt-treated base 0.25 0.40 

Portland cement concrete 0.10 0.20 

Lean concrete or cement-treated base 0.15 0.25 

Granular base, subbase, or soil 0.20 0.40 

Stabilized soil 0.15 0.30 

Cohesive soil 0.30 0.45 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-7 

TABLE 15. LINEAR ANALYSIS BACK-CALCULATION PROGRAMS 

Program 

name 

Developed 

by 

Calculation 

subroutine 

Rigid layer 

analysis 

Layer 

interface 

analysis 

Maximum number of 

layers 

Convergence 

routine 

BAKFAA FAA LEAF Yes Variable 10 rms 

BISDEF 

U.S. Army 

Corps of 

Engineers - 

WES 

BISAR 

Proprietary Yes Variable 

Cannot exceed no. of 

deflections. Works 

best for three 

unknowns 

Sum of sq. of 

absolute 

error 

CHEVDEF 

U.S. Army 

Corps of 

Engineers - 

WES 

CHEVRON Yes Fixed 

(rough) 

Cannot exceed no. of 

deflections. Works 

best for three 

unknowns. 

Sum of sq. of 

absolute 

error 

ELSDEF 

Texas A&M 

Univ.; U.S. 

Army Corps 

of Engineers 

- WES 

ELSYM5 Yes Fixed 

(rough) 

Cannot exceed no. of 

deflections. Works 

best for three 

unknowns 

Sum of sq. of 

absolute 

error 

MODULUS Texas Trans 

Institute WESLEA Yes Variable Fixed Up to four unknowns, plus stiff layer 

Sum of 

relative sq. 

error 

WESDEF 

U.S. Army 

Corps of 

Engineers - 

WES 

WESLEA Yes Variable Up to five layers 

Sum of sq. of 

absolute 

error 

MICHBAK Michigan 

State CHEVRON Yes Fixed Up to four unknowns, plus stiff layer 

Sum of 

relative sq. 

error 

NOTES: All programs use multilayer elastic theory during the back-calculation. All programs, except 

MODULUS, use an iterative back-calculation method; MODULUS uses a database format. “Seed” moduli 

are required for all programs. A range of acceptable modulus values is required for all programs, except 

MICHBAK. All programs allow the user to fix the modulus value for a layer. All programs contain an 

error convergence function. 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-8 

TABLE 16. NONLINEAR ANALYSIS BACK-CALCULATION PROGRAMS 

Program name Developed 

by 

Calculation 

subroutine 

Rigid layer 

analysis 

Layer 

interface 

analysis 

Maximum number 

of layers Convergence routine 

BOUSDEF 

Zhou, et.al. 

Oregon State 

Univ. 

OdemarkBoussinesq Yes Fixed 

(rough) 

Five, works best for 

three unknowns. Sum of percent errors 

ELMOD/ 

ELCON 

P. Ullidtz, 

Dynatest 

OdemarkBoussinesq Yes 

Variable 

Fixed 

(rough) 

Up to four, 

excluding rigid 

layer 

Relative error on five 

sensors 

EMOD PCS/LAW CHEVRON No Fixed 

(rough) Three Sum of relative sq. error 

EVERCALC J. Mahoney 

et al. CHEVRON Yes Fixed (rough) 

Three, excluding 

layer Sum of absolute error 

FREDDI W. Uddin BASINPT Yes 

Variable 

Fixed 

(rough) Unknown Unknown 

ISSEM4 R. Stubstad ELSYM5 No Fixed 

(rough) Four Relative deflections 

error 

MOD-COMP3 L. Irwin, 

Szebenyi CHEVRON Yes Fixed (rough) 

Two to 15 layers, 

max. five unknown 

layers 

Relative deflections 

error at sensors 

PADAL S.F. Brown 

et.al Unknown Unknown Fixed Unknown Sum of relative sq. error 

RoSy DESIGN Carl Bro 

Group 

OdemarkBoussinesq No Fixed 

(rough) 

Up to four layers Relative error on six 

sensors 

NOTES: All programs, except BOUSDEF and ELMOD/ELCON, use multilayer elastic theory during the 

back-calculation; BOUDEF and ELMOD/ELCON use Odemark-Boussinesq. All programs use 

an iterative back-calculation method. Nonlinear analysis for ELMOD/ELCON, EMOD, and 

PADA is limited to the subgrade. “Seed” moduli are required for all programs, except 

ELMOD/ELCON and FREDDI. With the exception of ELMOD/ELCON, a range of acceptable 

modulus values is required. Unknown for FREDDI and PADA. All programs allow users to fix 

the modulus value for a layer; unknown for FREDDI and PADA. Only BOUSDEF contains an 

error convergence function; unknown for PADA. 

TABLE 17. SEED MODULUS AND POISSON'S RATIOS FOR EXAMPLE PROBLEM 2 

Layer Low value, 

psi (MPa) 

Typical value, 

psi (MPa) 

High value, 

psi (MPa) 

Poisson's 

Ratio 

5-inch (125 mm) HMA (E1) 70,000 

(483) 

500,000 

(3447) 

2,000,000 

(13790) 0.35 

16-inch (400 mm) 

aggregate base (E2) 

10,000 

(69) 

30,000 

(207) 

50,000 

(345) 0.40 

Granular soil (E3) 5,000 

(34) 

15,000 

(103) 

30,000 

(207) 0.40 

Bedrock Layer (E4) 1,000,000 

(6895) 

1,000,000 

(6895) 

1,000,000 

(6895) 0.50 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-9 

TABLE 18. PAVEMENT JOINT PERFORMANCE RATINGS 

Radius of relative stiffness, λk LTE∆ , percent 

20 inches (500 mm) 130 inches (3,300 mm) 

90 to 100 Acceptable Acceptable 

70 to 90 Acceptable Fair 

50 to 70 Fair Poor 

Less than 50 Poor Poor 

TABLE 19. STATISTICAL SUMMARY OF ISM VALUES FOR EACH SECTION IN FIGURE 24 

Section Mean, k./in. St.Dev. k./in. Cv, percent Mean minus 

1 St.Dev., k./in. 

1 4,505 1,016 22.6 3,489 

2 896 126 14.2 770 

3 2,010 560 27.7 1,450 

4 1,290 280 21.7 1,010 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-10 

TABLE 20. REQUIRED FAA ADVISORY CIRCULAR EVALUATION AND DESIGN INPUTS 

Required analysis AC 150/5320-6 

inputs 

Analysis 

type

Common 

input sources LEDFAA F806FAA R805FAA 

FE, RE, 

OF, OR 1. HMA or PCC NDT modulus, lab tests ● 

surface layer 

elastic moduli FD, RD Lab mix design ● 

FE, RE, 

OF, OR 2. Stabilized NDT modulus, lab tests ● 

base/subbase layer 

elastic moduli FD, RD Lab mix design ● 

FE, RE, 

OF, OR 3. Granular NDT modulus, DCP, lab tests ● 

base/subbase layer 

elastic moduli FD, RD Lab tests ● 

4. Subgrade elastic 

moduli 

All 

types 

NDT modulus, DCP, lab tests, 

field tests ● 

5. Subgrade 

modulus, k 

RE, RD, 

OR NDT modulus, field tests ●

6. Unbound layer 

and subgrade CBR 

All 

types 

NDT modulus correlation, 

DCP, lab tests, field tests ● 

7. Base/subbase 

equivalency factors 

FE, FD, 

OF 

NDT modulus correlation, 

specified in AC 150/5320-6 ● 

RE, OR NDT modulus correlation, lab 

mix design, lab split tensile 8. PCC modulus of ● ●

rupture RD Lab mix design ● ●

9. PCC Cb, Cr

overlay factors OR NDT results, illustrations in AC 150/5320-6 ●

10. PCC SCI 

overlay factor OR NDT results, PCI results per ASTM D 5340 ● 

11. Existing layer 

thicknesses 

FE, RE, 

OF, OR Cores, borings, DCP, GPR ● ● ●

FE, FD, 

RD 

Always assumed to be bonded 

in AC 150-5320-6 ● 

RE, OR As specified in AC 

150/5320-6 ● ● 12. Layer interface 

bonding condition 

OF Rigid unbonded & flexible 

bonded per AC 150/5320-6 ● 

13. Aircraft 

models, weights, 

annual operations 

All 

types 

Airport records, master plans, 

field observations ● ● ●

14. Design life FD, RD, 

OF, OR 

20 years per AC 150/5320-6, 1 

to 50 years per AC 150/5320-6 

Chapter 7, as approved 

● ● ●

15. Targeted 

remaining life FE, RE As specified by airport owner with aircraft weight restriction ● ● ●

NOTES: FE and RE are flexible and rigid evaluation, respectively. FD and RD are flexible and rigid 

design, respectively. OF is overlays on flexible pavement. OR is overlays on rigid pavement. A past 

records review should always be the first source of information. Frost considerations and swelling soils are 

addressed in AC 150/5320-6. 
AC 150/5370-11A 12/29/2004 

Appendix 2 

Page 2-11 

TABLE 21. ALLOWABLE MODULUS VALUES FOR LEDFAA (AC 150/5320-6), psi (MPa) 

Layer type FAA specified layer Rigid pavement Flexible pavement 

P-501 PCC 4,000,000 (27579) Surface 

P-401 HMA 200,000 (1379) 

P-401 HMA 400,000 (2758) 

Variable stabilized (flexible) 150,000 to 400,000 (1034 to 2758) 

Variable stabilized (rigid) 250,000 to 700,000 (1724 to 4826) 

P-306 econocrete 700,000 (4826) 

P-304 cement treated base 500,000 (3447) 

Stabilized base 

and subbase 

P-301 soil cement 250,000 (1724) 

Granular base P-209 crushed aggregate 75,000 (517)

and subbase P-154 uncrushed aggregate 40,000 (276)

Subgrade Subgrade 1,000 to 50,000 (7 to 345) 

Undefined Undefined layer 1,000 to 4,000,000 (7 to 27579)

NOTE: Initial values are automatically adjusted during analysis based on the moduli of lower layers and 

layer thicknesses; nonstandard FAA layer. 

TABLE 22. HMA PAVEMENT BASE AND SUBBASE MODULUS AND EQUIVALENCY FACTOR INPUTS 

Layer type 

Back-calculated 

modulus value, 

psi (MPa) 

AC 150/5320-6, Chapter 7 

input moduli (LEDFAA), 

psi (MPa) 

AC 150/5320-6 

equivalency factors 

> 400,000 (2758) 400,000 (2758) 1.6 

150,000 to 400,000 

(1034 to 2758) Back-calculated value 

Interpret between 1.2 and 

1.6 using back-calculated 

value 

Stabilized base1

and subbase for 

flexible pavement 

< 150,000 (1034) 150,000 (1034) 1.2 

> 700,000 (4826) 700,000 (4826) 1.6 

250,000 to 700,000 

(1724 to 4826) Back-calculated value N.A. 

Cement stabilized 

base1

 and subbase 

for rigid pavement < 250,000 (1724) 250,000 (1724) N.A. 

Granular base and > 40,000 (276) Use P-209 layer 1.0 

subbase ≤ 40,000 (276) Use P-154 layer 1.0 

NOTE: 1Equivalency factors are based on a P-209 granular base in 150/5320-6. 

TABLE 23. RECOMMENDED REDUCED VALUES FOR LOSS OF SUPPORT CONDITIONS 

Percent of slabs Statistically selected back-calculated k-value (static) 

with void depth > 

3 mils k < 100 100 ≤ k < 300 k ≥ 300 

0 ≤ % < 10 50 75 100 

10 ≤ % < 25 40 60 80 

25 ≤ % < 100 25 40 55 
AC 150/5370-11A 12/29/2004 

Appendix 3 

Page 3-1 

APPENDIX 3-GLOSSARY 

AASHTO American Association of State Highway and Transportation 

AC Advisory Circular 

ACN Airport Classification Number 

AGBS Aggregate Base 

AOA Airport Operations Area 

APC Asphalt Overlaid PCC Pavements 

APMS Airport Pavement Management System 

ASTM American Society for Testing and Materials 

CBR California Bearing Ratio 

CIP Capital Improvement Program 

CL Centerline 

COV Coefficient of Variations 

CTB Cement Treated Base 

DCP Dynamic Cone Penetrometer 

DIA Denver International Airport, CO 

DSM Dynamic Stiffness Modulus 

FAA Federal Aviation Administration 

FHWA Federal Highway Administration 

FWD Falling Weight Deflectometer 

GA General Aviation 

GPR Ground-Penetrating Radar 

HMA Asphalt 

HWD Heavy-Falling Weight Deflectometer 

IR Infrared Thermography 

ISM Impulse Stiffness Modulus 

LTPP Long-Term Pavement Performance 

LVDT Linear Variable Differential Transducers 

MGTOW Maximum Gross Takeoff Weights 

NAPTF National Airport Pavement Test Facility 

NCHRP National Highway Cooperative Research Program 

NDT Nondestructive Testing 

PCI Pavement Condition Index 

PCC Portland Cement Concrete 

PCN Pavement Classification Number 

PDDL Pavement Deflection Data Logging 

RMS Root Mean Square 

SASW Spectral Analysis of Surface Waves 

SCI Structural Condition Index 

SHRP Strategic Highway Research Program 

U.S. United States