TECHBRIEF

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research

Determination of In-Place 

Elastic Layer Modulus: LTPP 

Backcalculation Methodology 

and Procedures

Publication No.: FHWA-HRT-15-037

FHWA Contact: Y. Jane Jiang, HRDI-30, 202-493-3149, 

jane.jiang@dot.gov

This document is a technical summary of the Federal Highway 

Administration report Long-Term Pavement Performance 

Program Determination of In-Place Elastic Layer Modulus: 

Backcalculation Methodology and Procedures (FHWAHRT-15-036).

Objective

This TechBrief provides an overview of the methodology and 

procedures used to backcalculate the elastic layer modulus from 

deflection basins for all test sections included in the Long-Term 

Pavement Performance (LTPP) database.

Background

Deflection data have been measured at periodic intervals with 

the falling weight deflectometer (FWD) on all rigid, flexible, 

and composite pavement test sections included in the LTPP 

program. These data are used to determine the load-response 

properties of the pavement structure and subgrade. Currently, 

there are 16,364 FWD testing days and more than 2,400 test sections in the LTPP database.

A common use of deflection data is to backcalculate in-place 

layered elastic modulus values. These modulus values, and 

how the values change over time, are inputs for estimating 

remaining life and determining an appropriate rehabilitation 

and design strategy. In addition, many agencies have used 

the LTPP deflection data for calibrating mechanistic-empirical 

distress transfer functions, including those in the MechanisticEmpirical Pavement Design Guide (MEPDG).(1)

Several years ago, the Federal Highway Administration (FHWA) 

sponsored studies to backcalculate elastic layer modulus 

The Long-Term Pavement Performance 

(LTPP) program is a large research 

project for the study of in-service 

pavements across North America. Its 

goal is to extend the life of highway 

pavements through various designs 

of new and rehabilitated pavement 

structures, using different materials and 

under different loads, environments, 

subgrade soil, and maintenance 

practices. LTPP was established under 

the Strategic Highway Research 

Program and is now managed by 

the Federal Highway Administration.
2

values from deflection basins and included 

the computed values in the LTPP database. 

(See references 2 through 5.) Since then, the 

amount of deflection data in the LTPP database 

has increased substantially, especially for the 

Specific Pavement Studies (SPS) sites. Therefore, 

FHWA sponsored a follow-up project to revisit 

the methods used in the first round of backcalculation and to calculate the elastic layer modulus 

values for the deflection data that did not exist 

during the first round of backcalculation.

Project Goals

The major goal of the backcalculation project 

was to determine the in-place elastic layer moduli from deflection basin measurements for the 

LTPP test sections and execute those methods 

for all flexible, rigid, and composite pavement 

sections included in the LTPP program. Another 

key goal was to automate the process and make 

it less dependent on the user so that others not 

directly involved in the development of the tools 

and procedures can recreate the results.

Challenges with Backcalculation

One of the difficulties in backcalculation is that 

it does not determine a unique solution or set 

of elastic layer moduli for a specific set of measured deflection basins. More importantly, the 

quality of the results depends greatly on the 

expertise of the user in setting up the problem. In 

other words, different users can obtain different 

results for the same set of deflection basins. 

The non-uniqueness of these solutions has been 

a major deterrent for some agencies to take 

full advantage of backcalculation methods for 

routine rehabilitation design. In addition, many 

agencies limit use of the deflection data to only 

determine the subgrade elastic modulus. 

This TechBrief provides a general overview of 

the backcalculation process and summarizes the 

steps that were automated in calculating the 

elastic layer moduli from deflection basin data. 

It also describes the tools written to simplify 

the backcalculation process while taking full 

advantage of the entire deflection basin dataset 

for rehabilitation design in accordance with the 

MEPDG.(1) The automated process is specific to 

LTPP and the data tables included in the LTPP 

database. The activities presented here, however, can be used to improve the backcalculation of elastic layer moduli for any rehabilitation 

project. 

Products from Backcalculation 

Project

The backcalculation project resulted in the 

development of two major products: (1) a set of 

computed parameter tables (CPT) stored in the 

LTPP database and (2) analytical tools for the 

pre-and post-processing of the deflection basin 

data. A user’s guide is available for executing the 

automated backcalculation process, as well as 

for the utility and software tools for organizing 

the results included in the CPTs.

LTPP Computed Parameter Tables

The backcalculated elastic layer modulus values, 

simulated pavement structures for each LTPP 

test section, and other related parameters were 

integrated into the CPTs of the LTPP database 

and are now available for use. Six CPTs were 

designed to store the results from the backcalculation process and assist in interpreting the 

results. One set of tables stores the EVERCALC

or MODCOMP results, and a second set of tables 

stores the BEST FIT results. A third set of tables 

stores the non-backcalculated data and values 

for each test section. The backcalculated modulus values from EVERCALC or MODCOMP and 

the BEST FIT method are organized by agency 
3

in separate files. Table 1 includes a listing of the 

following CPTs: 

• Non-backcalculated values table (the third 

column in table 1): This table includes 

general information on each test section 

analyzed through the backcalculation 

programs and compares the pavement 

structure included in the LTPP database 

to the structure simulated in the 

backcalculation process. It also includes 

the number of basins analyzed and the 

number of basins and stations with 

acceptable moduli, as well as whether a 

rigid layer is present.

• Backcalculated values from individual 

deflection basins (the first column in 

table 1): This table includes all results for 

individual deflection basins measured 

along a test section for all test dates. 

The calculated layer moduli, even 

those identified as errors, are included 

in this table. Test time and pavement 

temperatures are included in this table, 

as well as other factors that can be used to 

determine how the results are influenced 

from the different factors included in the 

backcalculation process. This information 

can be used to evaluate the stress 

sensitivity, temperature sensitivity, and 

seasonal sensitivity of the results.

• Summary of results or processed data 

(the second column in table 1): This 

table includes the average and standard 

deviation of elastic layer moduli along the 

test section on a test day basis. These values 

are the representative values for each test 

section that are layer dependent, test lane 

dependent, drop height dependent, season 

dependent, temperature dependent, etc. 

The averages and standard deviations were 

determined using the individual deflection 

basins not identified as an error.

Backcalculation Software Packages

The following three software packages were 

used to calculate the elastic layer moduli from 

Tables for Backcalculated Modulus 

Values from Individual Basins

Tables for Summary of Results and 

Processed Data from a Test Day Tables for Non-Backcalculated Values

Elastic layer moduli that are basinspecific from the EVERCALC or 

MODCOMP software packages for 

all test sections (rigid, flexible, and 

composite) included in the LTPP 

program.

Average and standard deviation of 

the elastic layer moduli for a test day 

(all rigid, flexible, and composite test 

sections), lane number, and drop 

height. This set of tables excludes 

the backcalculation results that were 

rejected from the acceptance criteria.

Backcalculation section information 

that identifies the number of basins 

used to calculate the elastic layer 

moduli and whether a rigid layer was 

included in the simulation.

Elastic layer moduli from the BEST 

FIT method for test sections with PCC 

surfaces for bonded and unbonded 

simulations that are basin-specific.

Average and standard deviation of the 

elastic layer moduli from the BEST FIT

method for the bonded and unbonded 

simulations for a test day, lane 

number, and drop height for LTPP test 

sections with PCC surfaces.

Structure used in the backcalculation 

process using EVERCALC or 

MODCOMP for a specific test day 

(layer thickness and material type).

LTE for tests performed at transfer 

joints and crack locations that are 

basin-specific.

Average and standard deviation of the 

LTE for a test day, lane number, and 

drop height for LTPP test sections with 

PCC surfaces.

Structure used in the calculation 

process using the BEST FIT method 

for a specific test day (layer thickness 

and material type).

Table 1. Computed parameter tables stored in the LTPP database.

CPT = Computed parameter table.

LTE = Load transfer efficiency.

LTPP = Long-Term Pavement Performance.

PCC = Portland cement concrete.
4

deflection basins measured on flexible, rigid, 

and composite pavements: 

• EVERCALC 5.0 was the primary program 

used for the analyses and was used for 

all data and all pavement types. The 

pre- and post-processing utility tools 

for the EVERCALC analyses were fully 

automated. The automation process 

included generating the input files based 

on pavement simulation rules, executing 

EVERCALC, and post-processing the results. 

• MODCOMP 6.0 was used as the auxiliary 

program to backcalculate results for those 

LTPP sections that did not yield acceptable 

results with EVERCALC. The MODCOMP

analyses were semi-automated as an 

iterative approach, and the simulated 

backcalculation structure was selected 

on a case-by-case basis until the results 

converged within the selected criteria. A 

few test sections or portions of test sections 

did not converge to produce satisfactory 

results even after multiple efforts using 

EVERCALC and MODCOMP. 

• The BEST FIT procedure was used to 

analyze LTPP sections with a portland 

cement concrete (PCC) surface to obtain 

the subgrade k-value and the elastic moduli 

of the PCC and base layers. The PCC and 

base modulus values were determined for 

bonded and unbonded interface conditions.

All three programs were executed in a batch 

mode process on a State-by-State basis to handle the computational effort needed for the 

large volume of deflection basins in the LTPP 

database. The BEST FIT method provides unique 

solutions for PCC-surfaced pavements using 

a three-layered structure. The EVERCALC and 

MODCOMP analyses were performed in four 

main analysis phases. Each phase progressively 

identified and filtered out those cases with poor 

quality results and made appropriate adjustments to the analysis parameters to improve the 

convergence in the subsequent phase. The first 

phase included all LTPP data, while the second 

phase used a subset from the first phase. The 

third and fourth analysis phases, if required, 

used a subset from the second phase. Phases 1 

and 2 used EVERCALC, and phases 3 and 4 used 

MODCOMP. 

Computational Tools

The backcalculation methods and procedures, 

including the pre- and post-processing tools, 

were automated to minimize manual errors and 

enable independent users (those not directly 

involved in the development of the tools and 

procedures) to recreate the results. In addition, 

the automation process was designed to enable 

users to generate results for additional data 

collected by LTPP in the future. 

A user’s manual and source code were prepared 

for all software packages and tools that were 

modified, revised, or developed as a part of the 

backcalculation project. In addition, source codes 

were prepared for each of the pre- and postprocessing utility tools used in the backcalculation process. Source codes were not prepared 

for any unchanged program that was already 

available in the public domain.

The computational tools utilize raw data in 

Microsoft® Access format from the LTPP 

database as the starting point to generate the 

backcalculation results. The results are merged 

back into Microsoft® Access tables with common reference fields with the original deflection tables. Therefore, Microsoft® Access-based 

macros, Microsoft® Windows-based utilities, 

and necessary graphical user interfaces were 

developed to do the following:
5

• Perform general data validity checks.

• Use the original LTPP tables to merge 

deflection, sensor spacing, and temperature 

data.

• Establish data flags to filter out problem 

basins (i.e., those with non-decreasing 

deflections, missing deflection data, or 

missing sensor spacing data). Missing 

temperature data were flagged but not 

discarded for backcalculation.

• Process pavement layer information 

from raw data in LTPP tables to develop 

backcalculation structure suitable for use 

in the EVERCALC program. This process 

involved combining pavement layers 

to reduce the pavement structure to an 

equivalent backcalculation layer structure 

meeting EVERCALC requirements.

• Assign reasonable seed modulus values 

and a range of modulus values for use in 

the backcalculation process.

• Generate the input files in the format 

necessary for use in the EVERCALC

program.

• Generate input files in the format necessary 

for use in the MODCOMP analyses. 

Because the MODCOMP analyses followed 

EVERCALC, the input files for MODCOMP

were created by modifying the EVERCALC

input files.

• Generate the input files in the format 

necessary for the BEST FIT analyses 

program. These files were created directly 

from the Microsoft® Access data tables 

and were independent of the EVERCALC or 

MODCOMP analyses.

• Execute the EVERCALC analyses in a batch 

mode.

• Execute the MODCOMP analyses in a batch 

mode.

• Execute the BEST FIT analyses in a batch 

mode.

• Calculate the load transfer efficiency across 

the transverse and longitudinal joints in 

jointed plain concrete pavement sections 

and across transverse cracks in continuously 

reinforced concrete pavement sections.

• Read output data files, merge results, and 

compute summary tables for inclusion in 

the LTPP results database.

The macros were developed in Microsoft® 

Access primarily because the raw data resided in 

Microsoft® Access tables. The macros were used 

to perform the preliminary data checks and create supplementary data tables. Supplementary 

data include all data other than the raw FWD 

deflection data that are necessary to perform 

the backcalculation analyses, including sensor 

spacing; temperature data; backcalculation layer 

structure and layer thicknesses; layer material information, including specified ranges for 

EVERCALC analysis; and beta factors for the 

BEST FIT analysis methods. 

Backcalculation Tool

A standalone software program, Back Calculator, 

was developed for bulk processing and filtering 

the deflection data, executing EVERCALC and 

BEST FIT programs, and processing the backcalculation results. Figure 1 shows a simplified flowchart of the Back Calculator program using the 

EVERCALC software package. MODCOMP analyses were not included in this program because 

only a limited number of sections were analyzed 

using MODCOMP. MODCOMP analyses were 

performed in a batch mode by executing the 

application file from an MS-DOS® prompt.
6

Figure 1. Simplified flowchart for the Back Calculator program using EVERCALC as the backcalculation software package.
7

Decision Criteria for Defining 

Acceptable Results

Two criteria were used to establish a quantitative measurement for categorizing the validity 

of the backcalculated modulus values: the root 

mean squared error (RMSE) and a range of the 

elastic modulus values. The RMSE criterion was 

set at 3 percent (i.e., for the results to be valid, 

the RMSE needed to be at or below 3 percent). 

The range of backcalculated modulus values 

for each layer had to fall within an acceptable 

or atypical range for each layer type and layer 

category based on the default range of values 

included in the MEPDG.(1)

Another factor included in the automated 

process for evaluating the acceptability of 

the results was to determine whether the 

calculated elastic layer moduli are characteristic 

of a normal distribution. Nearly all volumetric 

and structural properties exhibit a normal distribution unless some type of bias was created 

during construction or a boundary condition. 

Thus, results from a single test day were evaluated to determine whether those results exhibited a normal distribution. The verification for 

normal distribution was performed using the 

Jarque-Bera test statistic, which asymptotically 

has a chi-squared distribution with 2 degrees of 

freedom.

Basic Facts and Data About the 

Backcalculation Process

The deflection data were extracted from LTPP 

Standard Data Release 27.0 and included specific 

data from all LTPP test sections for all days of 

deflection basin testing. The data needed for the 

backcalculation of layer modulus values were 

grouped into two categories: direct and indirect. 

The direct data were from those tables needed 

to establish the inputs for the backcalculation 

and forward calculation of layer moduli, and the 

indirect data included were from tables needed 

to evaluate the results from the backcalculation 

process. The direct data elements were extracted 

from the LTPP database for all test sections on an 

experimental basis. The indirect data elements 

were extracted for those test sections that had an 

appreciable number of results that were considered unacceptable to identify potential reasons 

for the unacceptable or poor results.

The number of sections by each LTPP experiment included 1,744 sections with an asphalt 

concrete (AC) surface and 1,008 sections with 

a PCC surface. There were a total of 7,771 test 

days, of which 4,534 sections had an AC surface and 2,237 sections had a PCC surface. The 

surface type changed in 381 of the test sections. 

For example, a jointed plain concrete pavement 

that was eventually overlaid with AC is counted 

in two categories or experiments. Therefore, 

there were a total of 3,133 unique sections in 

all States combined. The number of deflection 

basins considered acceptable and unacceptable 

for the backcalculation process is as follows: 

• Deflection basins considered acceptable for 

backcalculation: 5,847,770.

• Deflection basins considered unacceptable 

for backcalculation: 195,709.

• Total deflection basins with backcalculated 

elastic layer moduli : 5,662,494.

Nearly 97 percent of all deflection basins were 

considered acceptable for backcalculation. This 

high percentage is a testament to the LTPP quality control procedures. Slightly fewer of the 

data were reported with backcalculated moduli 

because of a lack of convergence in the backcalculation analysis and missing critical data 

elements. In addition some of the acceptable 

basins were borderline.
8

Of the deflection basins included in the backcalculation process, 4,311,814 resulted in elastic 

layer moduli that met the acceptance criteria, 

for a slightly more than 76 percent success rate. 

More importantly, some 75 to 90 percent of the 

deflection basins measured along most of the 

LTPP test sections exceeded the acceptance 

criteria.

Application of Results

The following sections provide some examples of how the results can be used related to 

pavement design and rehabilitation, as well as 

research topics that can be tackled with this data. 

Confirm In-Place Damage Concept 

for Rehabilitation Design 

Figure 2 and figure 3 illustrate the change in the 

AC modulus over time for two LTPP test sections in Minnesota. These data can be used to 

determine the in-place damage for rehabilitation 

designs and forensic investigations, as defined 

by the MEPDG.(1) Figure 2 shows no damage for 

test section 27-1018, while figure 3 shows damage in the later years of testing for test section 

27-6251. The backcalculated elastic moduli can be 

used to confirm or reject the concept included in 

the MEPDG for rehabilitation designs in determining the in-place damage of bound layers. In 

addition, the backcalculation results can be used 

to determine the temperature-modulus relationship of various hot mix asphalt (HMA) mixtures.

Evaluate the In-Place Modulus of 

Fractured PCC Slabs

The backcalculated elastic modulus data can be 

used to determine the in-place layer modulus 

for different PCC repair techniques. Table 2 lists 

the average PCC elastic layer modulus values 

calculated from the deflection basins measured 

on two test sections from the Oklahoma SPS-6 

project before and after using the two fractured 

slab concepts (crack and seat and rubblization). 

Significantly different modulus values were 

obtained above and below the reinforcing steel 

for the rubblization test sections. These data can 

be used to confirm or revise the default values 

included in the MEPDG.(1)

Figure 4 shows the elastic modulus of the fractured PCC slabs from the crack and seat and 

rubblization methods used in rigid pavement 

rehabilitation in comparison with the intact PCC 

slabs for the Michigan SPS-6 test sections. The 

backcalculation data can be used to determine 

the default modulus values at the time of rehabilitation and define how those values change 

over time. 

More importantly, the current version of the 

MEPDG assumes that these modulus values 

remain constant over time. The backcalculation 

data can be used to confirm or recommend 

changes to that hypothesis.

Seasonal Variation of Pavement Response and 

Its Effect on Pavement Performance

Figure 5 shows the change in elastic modulus 

over time from the Idaho Seasonal Monitoring 

Program (SMP) test section. The backcalculation data can be used to estimate the effect of 

seasonal factors on the response of different 

unbound aggregate base layers, as well as for 

the subgrade soils and embankments. 

Another important application of the backcalculation data is to define characteristics of the 

deflection basin that can be used to identify 

subsurface features that could have a significant 

impact on the rehabilitation or reconstruction 

project.
9

Figure 2. Minnesota section 27-1018.

Figure 3. Minnesota section 27-6251.
10

Section Fracture Method

Prior to Fracturing; 

Intact Slabs After Fracturing Slabs

40-0606 Crack and seat method 4,799 ksi 3,615 ksi

40-0608

Rubblization method

Above reinforcing 

steel

5,082 ksi 284 ksi

Below reinforcing 

steel

2,013 ksi

Table 2. Average PCC elastic layer moduli before and after fracturing, Oklahoma SPS-6 project.

Figure 4. Michigan SPS-6 project.

Figure 5. Idaho SMP project.
11

Evaluate Differences of the In-Place Mixture 

Responses 

Figure 6 compares the elastic modulus of different HMA mixtures (with and without reclaimed 

asphalt pavement (RAP)) in the Minnesota SPS-5 

project for flexible pavement rehabilitation. 

The backcalculation data can be used to compare the load-response characteristics between 

different mixtures, which can be compared with 

laboratory-measured dynamic values in determining whether there will be any significant 

difference in performance. Most of the SPS-5 

projects exhibit no significant difference between 

the in-place elastic modulus values for mixtures 

with and without RAP.

Determination of Coefficients for the MEPDG 

Fatigue Cracking Transfer Function

Figure 7 shows a relationship between the 

in-place damage and the amount of fatigue 

cracking for the Georgia LTPP test sections and 

other roadway segments that were used in their 

local calibration project.(6) The in-place damage 

is defined as the ratio of the backcalculated elastic modulus values and laboratory-measured 

dynamic modulus values in accordance with the 

MEPDG.(1)

The backcalculated elastic layer moduli and 

fatigue cracking data were used to determine 

the coefficients of the fatigue cracking transfer 

function independent of executing the MEPDG 

software. The coefficients from this relationship were found to result in no bias between 

the predicted and measured fatigue cracking. 

Confirmation of this concept and determination 

of the coefficients independent of the software 

reduces the amount of field work and materials testing required for local calibration. More 

importantly, confirmation of this damage concept permits agencies to make more use of their 

deflection basin data for rehabilitation design in 

day-to-day practice, as well as in selecting and 

planning preventive maintenance type projects.

Figure 6. Minnesota SPS-5 project.
12

Conclusions

Backcalculation of elastic properties is not an 

exact science and requires user interaction in 

some cases. To reduce the impact of user interaction, bias, and/or inexperience, the process 

was automated through a series of utility functions and tools. Results from this automated 

procedure provide elastic layer load response 

properties that are consistent with previous experience and laboratory material studies related 

to the effect of temperature, stress state, and 

seasonal effects on the material load-response 

behavior. The following lists some of the important findings from this study:

• Over 76 percent of the deflection basins 

analyzed with EVERCALC and MODCOMP

resulted in layer moduli with an RMSE 

less than or equal to 3 percent and layer 

moduli within an expanded typical range of 

modulus values.

• There is no consensus on the “best” 

backcalculation package that provides 

the most reliable and accurate results. 

Nonetheless, the findings from the 

backcalculation project were similar to the 

findings documented in a previous study.(7)

Specifically, EVERCALC consistently resulted 

in lower error terms and a higher number of 

successful modulus determinations when 

considering all deflection basins. When only 

considering those deflection basins that 

ran successfully, however, the MODCOMP

program resulted in lower RMSE values. It 

should also be noted that the MODCOMP

program was used only for sections that 

were resulting in poor RMSE values from 

the EVERCALC analysis.

• The use of four layers generally resulted in 

lower RMSEs than the use of three layers. 

In many cases, separating the subgrade 

Figure 7. Georgia LTPP test sections used in local calibration.
13

into at least two layers improved the match 

between the measured and calculated 

deflection basins.

• In an earlier study, Von Quintus and 

Killingsworth recommended one software 

package to be used because of large 

differences between multiple programs.(8)

The current backcalculation project used 

three programs: EVERCALC, MODCOMP, 

and BEST FIT, all of which are available 

in the public domain. EVERCALC and 

MODCOMP were used to calculate the 

elastic layer moduli of all pavement sections 

in the LTPP database, while the BEST FIT

method was used for all PCC-surfaced 

test sections. EVERCALC and MODCOMP

provide non-unique solutions (i.e., the 

results are dependent on the user inputs of 

layer structure and the range of modulus 

values for iterations). In contrast, the BEST 

FIT method provides unique solutions for 

subgrade modulus and effective modulus; 

however, user decision on the layers 

representing the base and the condition of 

the PCC-base interface bond are needed for 

selecting slab and base modulus values. 

The following list summarizes some of the 

findings as related to the results and their 

interpretation between different software 

programs: 

• EVERCALC and MODCOMP resulted 

in statistically indifferent results for 

deflection basins classified as typical 

or in conformance with elastic layer 

theory as defined by Von Quintus 

and Simpson and Von Quintus and 

Killingsworth.(5,8) For type 2 deflection 

basins, the EVERCALC and MODCOMP

programs resulted in statistically 

different elastic moduli for the 

intermediate layers (the weathered 

soil layer and thinner aggregate base 

layers, which accounted for about 

25 percent of the deflection basins 

analyzed). The pavements exhibiting 

deflection-softening behavior with 

type 2 deflection basins were the most 

difficult to analyze and were generally 

found to have higher RMSEs. Some 

of these deflection basin analyses 

resulted in no reasonable solution, or 

the solution provided unrealistic layer 

moduli for the type of material defined 

in the LTPP database. Unrealistic is 

defined as the resulting moduli values 

being outside the expanded typical 

range for at least one of the layers 

within a test section.

• The PCC elastic moduli calculated 

with EVERCALC resulted in 

significantly greater values than the 

moduli calculated with the BEST 

FIT unbonded method. Thus, results 

from the two methods should not 

be combined, or a factor needs to 

be used to adjust the EVERCALCgenerated PCC moduli to the BEST 

FIT unbonded condition method or 

laboratory-measured elastic moduli. 

• Historically, a constant modulus for 

the PCC rubblized slabs has been used 

for rehabilitation design projects. 

However, the elastic modulus of the 

rubblized PCC slabs was found to 

steadily increase over time for some 

of the LTPP SPS-6 test sections.

• No consistent difference was found in 

the load-response properties (stiffness) 

between the RAP and virgin mixtures of the 

SPS-5 experiment.
14

• Similar c-factor values were found in 

this project to the values included in the 

MEPDG.(1) More importantly, no significant 

difference or bias was found between the 

laboratory-derived dynamic modulus and 

the field-derived values for the AC layers.

• The use of drop heights one through four 

in the LTPP deflection testing program did 

not result in significantly different elastic 

moduli for the unbound layers. As such, it 

was difficult to determine the coefficients 

of a constitutive equation to estimate the 

stress sensitivity of an unbound layer 

because of the variation in moduli over time 

and along a specific test section. The stress 

levels resulting from the four drop heights 

used in the LTPP deflection testing program 

are simply too narrow.

• Damage was determined in accordance 

with the MEPDG procedure for multiple 

LTPP test sections to demonstrate its use 

in rehabilitation design.(1) The results were 

positive and were used to support the MEDPG 

procedure. The damage concept as applied 

to interpreting the backcalculated moduli 

can be useful in explaining low moduli. 

The Florida Department of Transportation 

was one of the first agencies to monitor 

changes in the deflection basin (similar to 

this damage concept) over time in planning 

rehabilitation projects from a pavement 

management standpoint. From the case 

study sites and examples of data use, the 

damage concept can be used to evaluate 

the condition of the existing pavement and 

plan future rehabilitation projects.

References

1. AASHTO, Mechanistic-Empirical Pavement 

Design Guide—A Manual of Practice, 

American Association of State Highway and 

Transportation Officials, Washington, DC, 

2008.

2. Khazanovich, L., Tayabji, S.D., and Darter, 

M.I., Backcalculation of Layer Parameters for 

Long-Term Pavement Performance (LTPP) 

Test Sections, Volume I: Slab on Elastic 

Solid and Slab on Dense-Liquid Foundation 

Analysis of Rigid Pavements, FHWA 

Publication No. FHWA-RD-00-086, Federal 

Highway Administration, Washington, DC, 

December 2001.

3. Stubstad, R.N., Jiang, Y.J., and Lukanen, 

E.O., Guidelines for Review and Evaluation 

of Backcalculation Results, FHWA Publication 

No. FHWA-HRT-05-152, Federal Highway 

Administration, Washington, DC, February 

2006.

4. Stubstad, R.N., Y.J. Jiang, Y.J., Clevenson, 

M.L., and Lukanen, E.O., Review of the LongTerm Pavement Performance Backcalculation 

Results, FHWA Publication No. FHWAHRT-05-150, Federal Highway Administration, 

Washington, DC, February 2006.

5. Von Quintus, H.L. and Simpson, A.L., 

Backcalculation of Layer Parameters for 

LTPP Test Sections, Volume II: Layered Elastic 

Analysis for Flexible and Rigid Pavements, 

FHWA Publication No. FHWA-RD-01-113, 

Federal Highway Administration, 

Washington, DC, October 2002.

6. Von Quintus, H., et al., Implementation of 

the Mechanistic-Empirical Pavement Design 

Guide in Georgia, Draft Final Report Number 

FHWA/GA-014-11-17, Georgia Department of 

Transportation, Atlanta, GA, November 2014.

7. Smith, K.D., et al., Using Falling Weight 

Deflectometer Data with MechanisticEmpirical Design and Analysis—

Volume 1: Final Report, Federal Highway 
15

Administration, Washington, DC, December 

2010.

8. Von Quintus, H., and Killingsworth, B. 

Analyses Relating to Pavement Material 

Characterizations and Their Effects on 

Pavement Performance, FHWA Publication 

No. FHWA-RD-97-085, Federal Highway 

Administration, McLean, VA, January 1998.
16

Researchers—This study was performed by Applied Research Associates Inc., Contract No. DTFH61-

11-C-00051.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct 

distribution is being made to the FHWA Divisions and Resource Center.

Availability—The TechBrief may be obtained from the FHWA Product Distribution Center by e-mail to 

report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.

gov/research.

Key Words—Backcalculation, deflection data, elastic modulus, falling weight deflectometer, LongTerm Pavement Performance, MODCOMP, MODTAG, Mechanistic-Empirical Pavement Design Guide, 

EVERCALC. 

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no liability 

for the use of the information contained in this document. The U.S. Government does not endorse 

products or manufacturers. Trademarks or manufacturers’ names appear in this report only because 

they are considered essential to the objective of the document.

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AUGUST 2015 FHWA-HRT-15-037

HRDI-30/08-15(500)E