The modulus of pavement layer materials is a necessary input to mechanistic-type design and analysis procedures for flexible pavements. Resilient moduli (MR) were initially introduced in the first U.S. mechanistic design procedure developed in 1972 by the Asphalt Institute. The MR response of both hot-mix asphalt (HMA) and unbound materials was universally introduced in the 1986 edition of the AASHTO Guide for Design of Pavement Structures.
NCHRP Project 1-28 was completed in 1997. Its primary objective was to develop and recommend laboratory test procedures for determining resilient moduli of component materials in a flexible pavement structure that could account for varying field conditions, particularly temperature and moisture content. Project 1-28 produced recommended MR methods for HMA and unbound materials that represent enhancements of previously developed methods presented by AASHTO, ASTM, and the SHRP LTPP program. These recommended enhancements yield a substantial increase in precision, accuracy, and practicality over the existing standard methods.
The objective of Project 1-28A was to develop two implementable products from the results of the Project 1-28 research: (1) a single test method for measurement of the resilient modulus of HMA that harmonizes the protocol proposed by Project 1-28 with the existing AASHTO TP31 method and the FHWA LTPP Laboratory Start-Up and Quality Control Procedure; and (2) a single test method for measurement of the resilient modulus of unbound granular base/subbase materials and subgrade soils that harmonizes the protocol proposed by Project 1-28 with the existing AASHTO TP46, T 292, and T 294 methods and the FHWA LTPP Laboratory Start-Up and Quality Control Procedure.
The recommended draft protocols are presented in NCHRP Research Results Digest 285, Laboratory Determination of Resilient Modulus for Flexible Pavement Design.
PDFs of each volume of the researcher's two-volume final report are available here: