Permanent Deformation Characterization of Asphalt Mixtures by Using Incremental Repeated Load Testing

Currently, flow number (FN) is being used for measuring the permanent deformation resistance of asphalt mixtures. The provisional AASHTO TP 79 test method specifies the requirements of the FN test; however, there are undefined levels of test variables, such as temperature, axial stress, and confinement. Agreeable FN criteria that can reliably discriminate between various mixtures have not been established. As the asphalt industry continues to develop more sophisticated mixtures (warm mix, reclaimed asphalt pavement, and reclaimed asphalt shingles), the FN value has failed to capture the true complexity of the asphalt mixtures. These shortcomings and the unpredictable testing time of the FN test have affected its usefulness for evaluating high-temperature performance of asphalt mixtures. A new test procedure for evaluation of the rutting susceptibility of asphalt mixtures is being proposed. The new procedure is conducted at one temperature and three stresses on the same replicate in three increments of 500 cycles and takes only 25 min to complete. The property of the test is the permanent strain attributable to the last cycle of each test increment (minimum strain rate, MSR). A master curve is developed by plotting the MSR values versus parameter TP, which is a product of temperature and pressure. The MSR master curve represents the unit rutting damage (rut per axle) of asphalt mixtures at any stress and temperature and can be used in the laboratory for material characterization, mix design verification, mixture ranking, or pavement design applications to predict rut depth for project climate and design traffic.