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Publication Number: FHWA-RD-97-086
Date: September 1997


Backcalculation of Layer Moduli of LTPP General Pavement Study (GPS) Sites

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A key challenge faced by engineers using the 1993 AASHTO Guide for Design of Pavement Structures (AASHTO Guide) is the selection of appropriate design values for the subgrade soil and for the pavement materials. Until now, the information available to help engineers choose appropriate values has been incomplete. This report documents one aspect of the analysis of the Long-Term Pavement Performance (LTPP) data conducted to develop more complete information on the backca1culation of pavement layer moduli from the LTPP deflection data. The specific guidelines and procedures developed through this analysis are presented in a series of three design pamphlets addressing: (1) the selection of appropriate design values to characterize the subgrade soil, (2) interpretation of pavement deflection data, and (3) characterization of the pavement materials. These pamphlets are Design Pamphlet for the Determination of Design Subgrade Moduli in Support of the 1993 AASHTO Guide for the Design of Pavement Structures (FHWA-RD-97 -083), Design Pamphlet for the Backcalculation of Pavement Layer Moduli in Support of the 1993 AASHTO Guide for the Design of Pavement Structures (FHWA-RD-97-076), and Design Pamphlet for the Determination of Layered Elastic Moduli for Flexible Pavement Design in Support of the 1993 AASHTO Guide for the Design of Pavement Structures (FHWA-RD-97 -077).

Application of the procedures and guidelines developed through this analysis will facilitate and improve application of the AASHTO Guide flexible pavement design procedures. Their use will provide: (1) improved designs, (2) more realistic estimates of pavement performance, and (3) more consistent use of the AASHTO design parameters. Furthermore, although the procedures are specifically developed for use with the 1993 AASHTO Guide, their use will give agencies a "leg up" on implementation of the design procedures being developed for inclusion in the 2002 AASHTO Guide for Design of New and Rehabilitated Pavement Structures.

This report is a valuable resource for those who use - or are considering using - pavement deflection data to evaluate the structural properties of pavement materials. It will also be of interest to those who wish to fully understand the technical basis for the referenced design pamphlets.

Charles J. Nemmers, P.E.
Office of Engineering
Research and Development



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