Stress Path Testing for Proper Characterization of Unbound Aggregate Base Behavior

Abstract

Realistic pavement stresses induced by moving wheel loads were examined in granular base layers, and the significant effects of rotation of principal stress axes were addressed for a proper characterization of unbound aggregate behavior. Granular material resilient moduli are commonly determined at the centerline of wheel loading without taking into account the effects of moving wheel loads and the constantly rotating field principal stress states. Differences that exist between field- and laboratory-applied stress states were identified by comparing the field stresses simulated using a nonlinear axisymmetric finite element program, GT-PAVE, with the stress states commonly used in the standard resilient modulus test procedure, AASHTO T294-94. Three sets of complete triaxial test data obtained from testing aggregates under various realistic in situ stress paths caused by moving wheel loading were analyzed. Eight different granular material modulus models were developed from the experimental test data to include the applied mean stress, the applied shear stress, and the slope of stress path loading. Because of the complex loading regimes followed in the laboratory tests, characterization models that simultaneously analyzed the static and dynamic components of the applied mean and deviator stresses produced a high degree of accuracy. Models that consider the stress path slope variations predict the best stress path dependency of aggregate behavior caused by moving wheel loads. Such advanced models that allow for the effects of principal stress rotation better describe the granular material behavior under the actual field loading conditions.