The Hamburg rutting test - Effects of HMA sample sitting time and test temperature variation

The Hamburg Wheel Tracking Test (HWTT) is a widely used routine laboratory test with a proven history of successfully identifying and screening hot-mix asphalt (HMA) mixes that are prone to rutting and/or susceptible to moisture damage (stripping). Based on Texas specification Tex-242-F, the HWTT is typically conducted at a single test temperature of 50 °C (122 °F) with 12.5 mm rut depth as the standard HMA pass-fail screening criteria. However, with the record high summer temperatures of the recent years in Texas, several premature field rutting failures have occurred with some HMA mixes that had passed the HWTT screening criteria in the laboratory. This laboratory study was thus initiated to review and evaluate if the current Texas HWTT protocol and its associated Texas test specification (Tex-242-F) are simulative of the current summer field conditions for routine HMA mix-design and screening to optimize rutting resistance performance. Specifically, two key aspects were evaluated, namely the HMA sample sitting time and test temperature. The corresponding laboratory test results indicated that a maximum sample sitting time of five days should be consistently adapted, i.e., lab-molded samples should be tested within five days of fabrication, particularly for HMA mix-design screening purposes. For HMA mixes to be used in high temperature/stress environments, the study findings suggested that higher (>50 °C), and/or multiple HWTT test temperatures should be considered.