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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations

This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-05-150
Date: February 2006

Review of The Long-Term Pavement Performance (LTPP) Backcalculation Results

Chapter 1. Introduction


The Long-Term Pavement Performance Program (LTPP) has incorporated within its database a set of computed parameter tables. These tables include backcalculated moduli and other related pavement layer data from falling weight deflectometer (FWD) deflections taken before the summer of 1998. Backcalculated data, however, do not exist for all of the pre-1998 data. Some of these data were gathered at joints and other noncontinuous pavement features (where backcalculation is not used), while other data produced results that were already identified as unreliable or out of range.

Deflection data, corresponding backcalculated moduli, or other deflection-based parameters strongly relate to pavement performance, and the premise of mechanistic-empirical design methods is to control stresses and strains as a response to traffic loads. In addition, although many of the available backcalculated moduli certainly appear reasonable, some do not, which has raised some concerns. Based on earlier work where these data have been used, it became desirable to review and screen the backcalculated moduli database to ascertain whether, and to what extent, unreasonable or unlikely values currently exist.

This report documents the procedures and results of a comprehensive review of the LTPP backcalculation results. For this study, only the existing backcalculation data derived for the 1998 or earlier FWD were evaluated. All work was completed under the LTPP Data Analysis Contract (Federal Highway Administration (FHWA) Contract No. DTFH61-02-D-00138).


The main objective of this project is to assess and improve the quality of the present and future backcalculated modulus data in the LTPP database. The following are the main activities carried out under this project to meet the primary objective:

  • Provide an assessment of the validity of the LTPP backcalculated modulus parameters that have been derived through the backcalculation process.
  • Develop additional procedures and criteria that can be applied to evaluate the results of backcalculation (whether for LTPP or for other purposes) to ensure that the backcalculated layer parameters are valid.

An additional product from this study is a screening method to assist the analyst in assessing whether the backcalculated moduli obtained, using any backcalculation program, are reasonable. A separate report documents generic draft guidelines for review and evaluation of backcalculated pavement layer parameters.


The complete LTPP backcalculation and supporting data were requested and received from the FHWA-LTPP customer support center at the beginning of this project. The research team used LTPP Data Release 16.0—July 2003 Upload throughout this project. These data included all available asphalt concrete (AC) and portland cement concrete (PCC) LTPP sections where backcalculation was carried out, including both level E data and nonlevel E data.


This report is organized into seven chapters, including an introduction, background methods, development of the forwardcalculation methodology, initial evaluation with backcalculation screening results, backcalculation screening methods, summary of the LTPP backcalculation data screening results, and an overall summary with conclusions and recommendations.

This chapter presents a background of the problem, study objectives, LTPP data source, and report organization. Chapter 2 gives a summary of the literature reviewed with an introduction of both the backcalculation and the forwardcalculation techniques. Chapter 3 documents the development of the forwardcalculation methodology that was used in this study, followed by the results of the initial phase I pilot screening results in chapter 4. Chapter 5 outlines the subsequent method for screening the entire LTPP database of the backcalculated computed parameter tables, followed by a summary of the screening results and recommendations for identifying backcalculated data anomalies in chapter 6. Finally, chapter 7 contains a summary, conclusions, and recommendations to complete the report.

Microsoft® Excel spreadsheets have been prepared containing all formulae used in phase I of this study. All forwardcalculation input quantities are totally transparent to those who wish to use the methodology, whether for screening or in rehabilitation design. To this end, four spreadsheets are available—two for asphalt-bound surfaces (using SI and U.S. Customary units) and two for cement-bound surfaces (SI and U.S. Customary). These spreadsheets can be obtained by contacting LTPP Customer Support Services: by phone at 202–493–3035 or by e-mail at ltppinfo@fhwa.dot.gov.


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