A rapid performance test for Superpave HMA mixtures
Permanent deformation and shear instability of Hot-Mix Asphalt (HMA) have been major concerns in the asphalt paving industry for a long time because permanent deformation failure, e.g., rutting and shoving, significantly reduces the ride quality of asphalt pavements and may even cause hazardous hydroplaning of vehicles. Furthermore, the fact that truck tire pressures are increasing and most of rutting occurs in the top 3~4 inches of the HMA layer requires the production of more rut-resistant and stable mixtures.;HMA mix design has escaped from its empirical stage and become more rational with the advent of the Superpave system. Superpave Level-I mix design, however, entirely depends on the volumetric properties of the mixture, without evaluating the potential performance of mixtures. In order to cope with the increasing demand for a simple performance test, extensive research has been conducted recently across the U.S. and new test methods and/or testing equipment have been introduced. Unfortunately the equipment is expensive, and the test procedures and sample preparations are rather complicated.;This study sought to develop a performance test that is rapid and easy to perform so that it can be routinely used during mix design and during construction to differentiate stable from unstable or rut-susceptible mixtures. In order to be cost-effective, the Rapid Performance Test (RPT) presented in this study utilizes the existing Superpave Gyratory Compactor (SGC), without the need for new, elaborate or sophisticated equipment.;However, it has been recognized that the behavior of HMA mixtures observed during the conventional compaction procedure cannot properly represent the performance of the mixture due to the unrealistically elevated compaction temperatures used in testing, and the confined movements of the mixture inside the rigid mold. Therefore the indenter of 4&inches;-diameter is inserted between the SGC loading platen and the mixture in order to allow plastic flow or lateral/upward movements of the mixture. Also, the RPT is performed at in-service temperatures to better capture realistic shear strength of HMA mixtures.;Extensive experiments were conducted on Iowa mixes and the RPT was evaluated by the dynamic creep test using the Nottingham Asphalt Tester (NAT).