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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number:  FHWA-HRT-15-036    Date:  December 2015
Publication Number: FHWA-HRT-15-036
Date: December 2015


Long-Term Pavement Performance Program Determination of In-Place Elastic Layer Modulus: Backcalculation Methodology and Procedures

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The Long-Term Pavement Performance (LTPP) database includes deflection basins measured on thousands of test sections across the United States. These deflection data were used to backcalculate the elastic layer modulus of both flexible and rigid pavements. This report documents the tools, data analyses, backcalculation and forward calculation packages, and procedures used to calculate the in-place elastic layer modulus of the LTPP test sections. It summarizes the backcalculated elastic layered modulus of both new and rehabilitated flexible and rigid pavements in the LTPP program and demonstrates their use in day-to-day practice for pavement design, rehabilitation, and management. Many agencies have used the LTPP deflection data for calibrating mechanistic-empirical distress transfer functions, including those in the Mechanistic-Empirical Pavement Design Guide.(1)

An important outcome presented in this report is the documentation of methods and procedures for calculating in-place elastic layer modulus, including the pre- and post-processing tools so the results can be recreated by others to make the process less user-dependent. In addition, the elastic modulus computed parameter tables included in the LTPP database (Standard Data Release 27.0 released in 2013) are defined and explained in this report so agencies can use these results in multiple areas.(2) This final report is intended for use by pavement researchers as well as by practicing engineers involved in rehabilitation design and management of agencies’ pavements.

Jorge E. Pagán-Ortiz
Director, Office of Infrastructure
Research and Development


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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Technical Report Documentation Page
1. Report No.


2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle

Long-Term Pavement Performance Program Determination of In-Place Elastic Layer Modulus: Backcalculation Methodology and Procedures

5. Report Date

December 2015

6. Performing Organization Code
7. Author(s)

Harold L. Von Quintus, P.E., Chetana Rao, and Lynne Irwin

8. Performing Organization Report No.


9. Performing Organization Name and Address

Applied Research Associates, Inc.
100 Trade Centre Drive, Suite #200
Champaign, IL 61820

10. Work Unit No. (TRAIS)

11. Contract or Grant No.


12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered

Draft Final Report
September 30, 2011–October 30, 2014

14. Sponsoring Agency Code


15. Supplementary Notes

The Contracting Officer’s Technical Representative (COTR) was Ms. Yan "Jane" Jiang.

16. Abstract

Deflection data have been measured at periodic intervals with a falling weight deflectometer on all rigid, flexible, semirigid, and rehabilitated pavement test sections included in the Long-Term Pavement Performance (LTPP) program. A common use of deflection data is to backcalculate in-place layered elastic modulus values. The Federal Highway Administration sponsored earlier studies to backcalculate elastic layer modulus values from deflection basins measured on all LTPP test sections and included those computed values in the LTPP database.(3–5) While the earlier studies focus on the use of nonlinear methods and were considered to have minimal success, Some of the methods used have advanced within the past decade. As such, there was a need to revisit the methods used to improve on the results.

This report summarizes all activities completed to backcalculate the elastic layered modulus from deflection basins measured on all test sections included in the LTPP program. Specifically, the report documents the tools, data analyses, backcalculation and forward calculation packages, and procedures used to calculate, on a production basis, the in-place elastic layer modulus of the LTPP test sections. Multiple packages (including BAKFAA, EVERCALC©, MICHBACK©, MODULUS, MODCOMP©, and the area and best fit methods) were considered and evaluated for estimating layered elastic modulus values on a production basis. (See references 6–10.) The methods used in production for backcalculating elastic layer moduli include a combination of EVERCALC© and MODCOMP© for all types of pavements and the best fit method for rigid pavements. The methods and procedures, including the pre- and post-processing tools, have been automated so the results can be recreated by others not directly involved in development of the tools and procedures. The report also summarizes the results from the production runs to calculate the elastic layered modulus of flexible and rigid pavements (new construction and rehabilitation) as well as demonstrate the application and use of the results.

17. Key Words

Backcalculation, Deflection data, Elastic modulus, EVERCALC©, Falling weight deflectometer, Long-Term Pavement Performance, LTPP, MODCOMP©, MODTAG©, Modulus, Mechanistic-Empirical Pavement Design Guide, Pavement evaluation

18. Distribution Statement

No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161.

19. Security Classification
(of this report)


20. Security Classification
(of this page)


21. No. of Pages


22. Price
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors

Table of Contents

Chapter 1. Introduction

Chapter 2. Backcalculation Methodology and Packages

Chapter 3. Case Studies

Chapter 4. Backcalculation Process

Chapter 5. Backcalculation Results

Chapter 6. Application and Use of Backcalculation Results

Chapter 7. Conclusions

Appendix A: Best Fit Method—Calculating PCC Elastic Layer Moduli
Appendix B: LTPP Test Sections with a Moderate and High Percentage of Errors
Appendix C: Development of the Simulated Layer Structure for the Backcalculation Process Using Evercalc© and Modcomp©

List of figures

List of tables

List of Acronyms and Symbols
AASHTO American Association of State Highway and Transportation Officials
AC asphalt concrete
ANN artificial neural network
ATB asphalt-treated base
CPT computed parameter table
CRCP continuously reinforced concrete pavement
FHWA Federal Highway Administration
FWD falling weight deflectometer
GB granular base
GPR ground-penetrating radar
GPS General Pavement Studies
GS granular subbase
HMA hot mix asphalt
JPCP jointed plain concrete pavement
JRCP jointed reinforced concrete pavement
LTE load transfer efficiency
LTPP Long-Term Pavement Performance
MEPDG Mechanistic-Empirical Pavement Design Guide
PCC portland cement concrete
RAP reclaimed asphalt pavement
RMSE root mean squared error
SDR Standard Data Release
SHRP Strategic Highway Research Program
SLIC Sensor Location Independent Check
SMP Seasonal Monitoring Program
SPS Specific Pavement Studies
SS subgrade soil
TB treated base
TS treated subgrade




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