Evaluation of Laboratory Conditioning Protocols for Warm-Mix Asphalt

Warm-Mix Asphalt (WMA) refers to the asphalt concrete paving material produced and placed at temperatures approximately 50?F lower than those used for Hot-Mix Asphalt (HMA). Economic, environmental and engineering benefits have boosted the use of WMA technology across the world during the past decade. While WMA technology has been successfully utilized as a paving material, several specifications and mix design protocols remain under development. For example, currently, there is no consistent laboratory conditioning procedure for preparing WMA specimens for performance tests, despite being essential for mix performance.

Based on previous studies, several candidate conditioning protocols for WMA Laboratory Mixed Laboratory Compacted (LMLC) and off-site Plant Mixed Laboratory Compacted (PMLC) specimens were selected, and their effects on mixture properties were evaluated. Mixture stiffness evaluated in a dry condition using the Resilient Modulus (MR) test (ASTM D-7369) was the main parameter used to select a conditioning protocol to simulate pavement stiffness in its early life. The number of Superpave Gyratory Compactor (SGC) gyrations to get 7?0.5% air voids (AV) was the alternative parameter. Extracted binder stiffness and aggregate orientation of field cores and on-site PMLC specimens were evaluated using the Dynamic Shear Rheometer (DSR) (AASHTO T315) and image analysis techniques, respectively. In addition, mixture stiffness in a wet condition was evaluated using the Hamburg Wheel-Track Test (HWTT) (AASHTO T324) stripping inflection point (SIP) and rutting depth at a certain number of passes.

Several conclusions are made based on test results. LMLC specimens conditioned for 2 hours at 240?F (116?C) for WMA and 275?F (135?C) for HMA had similar stiffnesses as cores collected during the early life of field pavements. For off-site PMLC specimens, different conditioning protocols are recommended to simulate stiffnesses of on-site PMLC specimens: reheat to 240?F (116?C) for WMA with additives and reheat to 275?F (135?C) for HMA and foamed WMA. Additionally, binder stiffness, aggregate orientation, and overall AV had significant effects on mixture stiffness. Mixture stiffness results for PMFC cores and on-site PMLC specimens in a wet condition as indicated by HWTT agree with those in a dry condition in MR testing.