Warm-Mix Asphalt Study: Evaluation of Hot and Warm Mix Asphalt with Respect to Binder Aging

Warm-mix asphalt (WMA) technologies are added to asphalt binders to lower production and construction temperatures, reduce emissions, allow for increase haul distances, and improve the workability. Reduced temperatures at the plant and during laydown and compaction are hypothesized to impact long-term oxidative aging behavior. This study attempted to quantify these impacts through characterization of field-aged unmodified and rubber-modified binders extracted and recovered from cores sampled from 13 test sections representing seven different WMA technologies and associated hot-mix controls. A dynamic shear rheometer (DSR) was used to evaluate the binder rheological properties at high temperatures with respect to expected rutting performance. The cup-and-bob DSR testing procedure was assessed as an alternative approach for testing rubberized binders. A bending beam rheometer (BBR) was used to characterize low-temperature properties. Test results did not appear to be influenced by the warm-mix technology chemistry. However, the organic wax additive consistently showed better rutting resistance across all tests, and this was attributed to the residual crystallization wax structure in the binder. All test results appeared to be influenced by production and placement temperatures, indicating that some mixes produced at very low temperatures could be more susceptible to early rutting on pavements experiencing high ambient temperatures and high traffic loading. Air-void content appeared to have very little effect on the rheological properties of the extracted binder over the aging period assessed, which was not expected. The results indicated that zero shear viscosity (ZSV) is a good indicator of the rheological behavior of asphalt binders with respect to rutting performance, as observed from accelerated load testing. ZSV was also found to be more suitable for describing the rutting performance of rubberized binders than the current Superpave G*/sinδ criterion. Viscosity-shear susceptibility is a suitable parameter for understanding the shear sensitivity of rubberized binders. It increases during long-term oxidative aging due to the increased association of polar compounds. The non-recoverable creep compliance and percent recovery parameters obtained from the multiple stress creep recovery test are useful parameters for understanding expected field rutting performance. Bending beam rheometer results indicated that the WMA technologies tested did not result in a grade change with respect to thermal cracking properties at low temperatures, with all binders meeting the Superpave criteria at all ages tested. Performance trends for individual binders were consistent with rutting test results. The warm-mix additives and associated lower production and placement temperatures generally had limited effect on aging kinetics with respect to long-term field aging, with the exception of the organic wax. Laboratory binder aging, specifically the rolling thin film oven test, did not always correspond to field performance.