Influence of unbound materials on flexible pavement performance: A comparison of the AASHTO and MEPDG methods

Anecdotal reports have suggested that the new Mechanistic-Empirical Pavement Design Guide (MEPDG) assigns less structural credit to unbound materials than does the empirical AASHTO procedure. A limited sensitivity study was performed to quantify the effect of base layer thickness and stiffness and subgrade quality on service life as predicted by the AASHTO and MEPDG procedures. The study found that the AASHTO and MEPDG methods agreed well at low to moderate traffic levels but diverged at higher traffic volumes. The sensitivity of predicted service life to unbound granular base and subbase properties was found for the scenarios in the study to be consistently higher for the AASHTO method than for the MEPDG. Limited field data suggest that the actual sensitivity may lie somewhere between the limits of the two design methods.