Integrated Asphalt (Overlay) Mixture Design, Balancing Rutting and Cracking Requirements

The focus of this project is to develop an integrated hot-mix asphalt (HMA) mixture design method which balances both rutting and cracking requirements. The Hamburg Wheel Tracking Test (HWTT) and Overlay Tester (OT) devices were used to evaluate the rutting and cracking resistance of HMA mixtures, respectively. Eleven mixtures commonly used in Texas were designed following the current Texas Department of Transportation (TxDOT) mixture design process and then evaluated under the HWTT and the OT. It was found that the Dense-Graded and Superpave mixtures designed following current TxDOT mixture design procedures were rut resistant, but generally not crack resistant. However, all three Stone-Matrix Asphalt (SMA) mixtures were both rut and crack resistant. These observations are consistent with the past experience and field performance The balanced design procedure proposed in this project recommends minor changes to TxDOT's current mixture design procedure. Seven mixtures including dense-graded and Superpave mixtures were used to verify and demonstrate this balanced mixture design approach. It was found that a balanced HMA mixture could always be designed providing the aggregates used were not highly absorptive. Statistic analyses on the OT results showed that Performance-Grade (PG) of asphalt binder, effective asphalt content in volume (VBE), film thickness (FT), and surface area (SA) had significant impact on crack resistance of mixtures. Note that the influence of asphalt absorption by aggregates was included in the VBE and FT. The influence of air void content was not significant on crack resistance. Similarly, statistic analyses indicated that the following factors had significant influence on rutting resistance: 1) PG, 2) voids in the mineral aggregate, 3) FT, 4) SA, and 5) air void content. Additionally, the minimum and maximum asphalt contents for different mixtures to pass the cracking and rutting criteria were recommended based on extensive laboratory testing results. The recommended values were preliminarily verified by field performance data from IH20, WesTrack, and National Center for Asphalt Technology test track. Furthermore, a simplified version of the balanced HMA mixture design procedure was also proposed. Instead of volumetric design, trial asphalt contents for different mixtures were recommended for performance evaluation in the simplified mixture design procedure. The procedure was verified in two case studies.