Center for Transportation Research and Education

About the research

High-traffic (HT) asphalt mixtures in Wisconsin are designed using 100 gyrations, however concerns have been raised regarding the ability to achieve balanced performance and adequate field compaction using such a high gyration level. This research aims to propose modification to WisDOT’s HT mix design specifications to improve constructability while maintaining or enhancing performance.

The study began with a synthesis of HT mix design requirements in several states. A benchmarking study was then conducted using eight HT mixes, sampled from different projects in Wisconsin, using the Indirect Tensile Asphalt Cracking Test (IDEAL-CT) and Hamburg Wheel Tracking Test (HWTT) to assess cracking and rutting resistance, respectively. The mixes underwent extraction and recovery, and the recovered binders were evaluated for Performance Grade (PG) and Multiple Stress Creep Recovery (MSCR). The virgin binders and recycled asphalt materials (RAM) used in the mixes were also collected and evaluated to determine their PG and MSCR. Mix results were benchmarked against WisDOT HT mixes, leading to the selection of three mixes for redesign and further testing. The mixes were reproduced as lab-mixed lab-compacted (LMLC) specimens, redesigned using the Bailey Method and Asphalt Film Thickness (AFT), and compacted at 75 gyrations to 4.0% air voids without air-void regression.

Overall, it was noted that the cracking resistance was significantly influenced by the stiffness of the recovered binder’s PG, and the recycled binder ratio. Performance testing showed that the redesigned mixes achieved similar or improved cracking resistance, rutting resistance, and compactability compared to baseline HT mixes designed at 100 gyrations with air-void regression. To evaluate the impact of air-void regressed design accompanied with reduced gyration level, a fourth mix was selected for redesign using 3.0% regressed air voids and 75 gyrations, resulting in improved cracking and rutting resistance.

The findings of this study support using 75 gyrations at 4.0% air voids for HT mixes in Wisconsin with air-void regression. The study also highlighted the impact of the recovered binder’s PG on the mixture cracking resistance. Additionally, the Bailey Method and AFT can be used to guide the selection of aggregate gradation, and asphalt binder content. A special provision is proposed for initial implementation.