Analysis of Critical Structure Responses for Flexible Pavements in NCHRP 1-37A Mechanistic-Empirical Pavement Design Guide

Accurate determination of critical structure responses plays a vital role in the ability of the Mechanistic-Empirical Pavement Design Guide (MEPDG) to achieve a reliable pavement design. In the MEPDG, the responses generated by a single axle are evaluated at a series of predefined locations, and responses under multiple axle gears are determined using the responses from a single axle following the principle of superposition. This research indicates that the approach currently employed in the MEPDG is able to identify the critical location and obtain the critical response for a tandem axle, while it is inadequate for a tridem or quad axle. The primary reason for the inadequacy is that the response along the outside axle of a tridem or quad gear is not computed and compared to responses at other locations. The erroneous critical location identified by the MEPDG for a tridem or quad axle will lead to an underestimated critical response and consequently an overpredicted pavement life. The results show that the fatigue lives of asphalt concrete (AC) and cement stabilized mixture (CSM) layers could be overpredicted by up to 50 and 200%, respectively. A new set of analysis locations are recommend in this study, and equations for computing responses under multiple axle gears using the principle of superposition are proposed for accurate determination of critical responses and prediction of pavement performance.