Analysis of flexible pavement response and performance using isotropic and anisotropic material properties

Several research studies have shown that unbound pavement layers exhibit anisotropic properties. Anisotropy is caused by the preferred orientation of the aggregate, to which both the shape characteristics of the aggregate and the compaction force itself contribute. The result is that unbound pavement layers have higher stiffness in the vertical direction than in the horizontal direction. The efficacy of using anisotropic properties to represent unbound layers is demonstrated by comparing pavement surface deflection measurements under wheel loads to finite element predictions based on models that incorporate isotropic and anisotropic properties for the unbound base and subbase layers. The surface deflections in the flexible pavements of the AASHO road test were selected for this comparison because the AASHO road test is such a widely used database and because of the tight control of traffic, pavement cross sections, and material quality at the road test. The paper also analyzes the influence of characterizing pavement layers as isotropic and anisotropic on the predictions of fatigue and permanent deformation performance of flexible pavements using the recently developed National Cooperative Highway Research Program 1-37A models. The results show that the anisotropic behavior of pavement layers explains part of the shift and calibration factors used to relate laboratory measurements to field performance.