Warm-Mix Asphalt Moisture Susceptibility Evaluation for Mix Design and Quality Assurance

Guidelines for the evaluation of warm-mix asphalt (WMA) moisture susceptibility, including a flowchart and several thresholds for laboratory tests, were developed in a previous study on the basis of a limited number of WMA mixtures. In this study, 64 WMA mixtures were identified from a literature review and web survey with moisture susceptibility data available from mix design or quality assurance as well as field pavement performance. A desirable correspondence was achieved when the moisture susceptibility laboratory test parameters for those WMA mixtures were compared against the proposed thresholds and their field pavement performance, which showed the effectiveness of the guidelines in identifying moisture-susceptible WMA mixtures. In addition, a laboratory experiment was completed to evaluate the effect of various moisture conditioning protocols and specimen drying methods on moisture susceptibility laboratory test parameters. The 1,000-cycle moisture induced stress tester at 140 degrees F (60 degrees C) and 40 psi and the 3-day hot water bath at 140 degrees F (60 degrees C) produced an equivalent level of moisture damage as the modified Lottman protocol per AASHTO T 283 and, thus, are suggested as alternative moisture conditioning protocols in the guidelines. The testing of saturated surface dry specimens was problematic, possibly because of the incompressibility and flow resistance of water remaining inside the pore structure of the specimens, which yielded higher mixture stiffness and strength. Therefore, drying of the moisture-conditioned specimens with the CoreDry 48-h air dry at 77 degrees F (25 degrees C) or the 24-h oven dry at 140 degrees F (60 degrees C) method before testing is recommended. Finally, a revised flowchart including the findings from this study is proposed.