Project 1-28 produced recommended resilient modulus methods for hot-mix asphalt and unbound materials that represent enhancements of previously developed methods presented by AASHTO, ASTM, and the SHRP LTPP program. These enhancements demonstrate a substantial increase in precision, accuracy, and practicality over the existing standard methods.

The resilient modulus of pavement materials and subgrades is an important input item for design of flexible pavement structures. Existing laboratory test procedures for determining resilient modulus have varied in approach, and they appeared complex and ambiguous, requiring a major investment in time and equipment, while providing questionable results. Moreover, they did not adequately simulate field conditions, and considerable differences existed between field-determined moduli and laboratory test results.

The primary objective of this study was to develop enhanced laboratory test procedures for determining resilient moduli of component materials in a flexible pavement structure and which are suitable for the design of both new pavements and rehabilitation of existing pavements and account for varying field conditions, such as temperature of the asphalt surface layer and moisture content of a subbase or subgrade layer.

The agency final report, "Laboratory Determination of Resilient Modulus for Flexible Pavement Design," is available on the Internet as NCHRP Web Document 14 where it can be browsed or copies can be ordered. The contractor's final report has been distributed to the state departments of transportation. Further work is underway in NCHRP Project 1-28A to reconcile the existing, standard methods for measurement of resilient modulus with those developed in Project 1-28 and produce a single method each for hot mix asphalt and unbound materials to replace the competing methods now available.