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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-05-152
Date: February 2006

Guidelines for Review and Evaluation of Backcalculation Results

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This report documents a new approach, called forward calculation, used for determining layered elastic moduli from in situ load-deflection data. Guidelines are provided for carrying out forward calculation procedures as well as screening Backcalculation results using forward calculated moduli.

This report will be of interest to highway agency engineers involved in pavement analysis, design, construction, and deflection data collection, as well as researchers who will use the Long-Term Pavement Performance (LTPP) load-deflection data to improve design procedures and standards for constructing and rehabilitating pavements.

Gary L. Henderson
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. This report does not constitute a standard, specification, or regulation.

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.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

Technical Report Documentation Page


1. Report No.


2. Government Accession No.

3. Recipient's Catalog No.

4. Title and Subtitle

Guidelines for Review and Evaluation of Backcalculation Results

5. Report Date

February 2006

6. Performing Organization Code

7. Author(s)

R.N. Stubstad, Y.J. Jiang, and E.O. Lukanen

8. Performing Organization Report No.

9. Performing Organization Name and Address

Applied Research Associates, Inc.
7184 Troy Hill Drive, Suite N
Elkridge, Maryland 21075-7056

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, Virginia 22101-2296

13. Type of Report and Period Covered

Guidelines-Final Report

14. Sponsoring Agency Code

15. Supplementary Notes

Contracting Officer's Technical Representative (COTR): Larry Wiser, HRDI LTPP Data Analysis Contract.

16. Abstract

This document presets a new approach to determining layered elastic moduli from in situ load-deflection data, which was developed under the Federal Highway Administration's project for reviewing Long-Term Pavement Performance (LTPP) Backcalculation data. This approach is called forward calculation, and it differs from Backcalculation in that modulus estimates are calculated directly from the load and deflection data using closed-form formulae rather than through iteration. The closed-form forward calculation equations are used for the subgrade and the bound surface course, respectively, for both flexible and rigid pavement falling weight deflectometer (FWD) data. Intermediate layer moduli are estimated through commonly used modular ratios between adjacent layers.

The audience for this document includes highway agency engineers, researchers, and consultants who are involved in pavement analysis, design, construction, and deflection data analysis.

17. Key Words

Pavements, LTPP, FWD, deflection data, elastic modulus, Backcalculation, forward calculation, screening of pavement moduli.

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


List of Figures

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