Resilient modulus is an important material property required for characterization of pavement layers for use in modern mechanistic pavement design. The resilient modulus can be determined in the laboratory using various types of tests, which measure a material's response under simulated field loading conditions. It can also be estimated from nondestructively measured pavement surface deflections and the so-called backcalculation procedures. Unfortunately, studies have shown that the resilient modulus determined from laboratory testing can differ significantly from that determined from nondestructive test (NDT) based backcalculation procedures.

This paper further addresses the issue by presenting the results of a site-specific comparison of the resilient modulus determined through laboratory testing and NDT-based backcalculation analysis. Unlike some of the earlier studies that focused more on developing a correlation between the two, this study explores some of the flaws and weaknesses of the two processes that are likely to result in the observed differences. It is also important to point out that the results of this study are based on an intensive amount of both laboratory and NDT data gathered from a pavement constructed to exceptional standards of construction quality, i.e., the WesTrack full-scale accelerated load test facility in northern Nevada.

The emphasis of the paper is on the unbound layers (i.e., base and recompacted subgrade soil layers) below the pavement surface. Although laboratory testing of the hot-mix asphalt (HMA) surface layer was completed, the data were not yet to a level of quality acceptance to produce a meaningful comparison with the NDT-based backcalculation results of the surface layer.