Laboratory versus Nondestructive Testing for Pavement Design

A detailed comparison of moduli measured in the laboratory by resilient modulus testing with moduli measured in the field by falling weight deflec‐tometer (FWD) testing is made. This comparison includes an analysis of whether good agreement between lab and field values should be expected as well as an assessment of relative quality of the results and relative testing costs. Good agreement between lab and field measured moduli should not be expected when following the usual practice of sampling and testing only the top 1 or 2 ft (0.3 or 0.6 m) of the subgrade. Agreement is not to be expected because different volumes of the material are being tested in the laboratory compared to field testing by FWD. Because different volumes of material are tested, heterogeneity in any layer contributes to different measured moduli by lab and field testing. It is also concluded that the cost of determining moduli by laboratory resilent modulus testing is typically 60–80 times greater than the cost of an equivalent field testing program with the FWD. When the added costs of traffic control during sampling and project delays are included, the cost ratio may exceed 100. Furthermore, the moduli from field FWD testing are typically of higher quality and more appropriate for mechanistic pavement design than lab‐measured moduli. This conclusion arises from the fact that it is very difficult, if not impossible, to erase completely the effects of sample disturbance on lab‐measured moduli. In addition, when moduli are back‐calculated from FWD testing, a proper weighting factor is automatically applied to all zones contributing to deformations, it is also possible to take advantage of compensating errors in back‐calculated moduli.