Fatigue performance prediction of North Carolina mixtures using the simplified viscoelastic continuum damage model

Fatigue performance modeling is a major topic in the field of asphalt concrete modeling work. Currently, the only standard fatigue test available for asphalt concrete mixtures is the flexural bending fatigue test, AASHTO T-321. Several issues are associated with flexural fatigue testing, the most important being that the stress state is not uniform but varies with the depth of the specimen, and that the beam specimen fabrication equipment is not widely available. Viscoelastic continuum damage (VECD) fatigue testing is a promising alternative to flexural fatigue testing. Different researchers have successfully applied the VECD model to asphalt concrete mixtures using constant crosshead rate direct tension tests. However, due to the load level limitation of the newly released Asphalt Mixture Performance Tester (AMPT) testing equipment, there is an immediate need to develop a model that can characterize fatigue performance quickly using cyclic test data. In this study, a simplified VECD model, developed at North Carolina State University, is applied to various North Carolina mixtures that are part of the NCDOT project, Local Calibration of the MEPDG for Flexible Pavement Design. A failure criterion that is based on pseudo stiffness is developed from the test data. The application of the VECD model using this failure criterion results in very good agreement between the measured and predicted fatigue life for the eleven mixtures. In addition, a completely independent verification study is conducted for the FHWA ALF mixtures, including both unmodified and modified mixtures. Again, it is found that the simplified VECD model predicts the fatigue life of the ALF mixtures well. It is shown that the simplified VECD model based on the data from a single temperature and a single strain level can predict fatigue test results fairly accurately under various temperature conditions and at various strain levels. It is also shown that the model can be utilized further to simulate both controlled strain and controlled stress direct tension fatigue testing and gain insight into the impact of various mixture design variables, such as asphalt content, binder grade, NMAS, and the inclusion of RAP materials, on the fatigue performance.