Effect of Simulated Oxidative Aging on Short- and Long-Term Cracking in Oklahoma Asphalt Mixtures

Abstract: The implementation of Balanced Mix Design (BMD) is already underway by many state-agencies with strong considerations for the adoption of performance tests such as the indirect tensile cracking test (IDEAL-CT) to assess cracking tolerance of asphalt mixes. Understanding the effect of aging on mix cracking tolerance through extended heat conditioning is a critical aspect of BMD implementation. An ongoing research project at Oklahoma State University focuses on evaluating the effects of different heat conditioning approaches on the cracking tolerance of Hot Mix asphalt (HMA) and Warm Mix Asphalt (WMA) mixtures in Oklahoma. The objective is to help the Oklahoma Department of Transportation (ODOT) select the most suitable heat conditioning approach for their BMD implementation efforts. An extensive review of existing protocols, including AASHTO R30 and NCHRP standards, provided insight into current practices for simulating oxidative aging of asphalt mixtures in the laboratory. Short-term oxidative aging during production and placement is being simulated in the laboratory using procedures specified in AASHTO R30 and NCHRP 9-43. Subsequently, long–term oxidative aging is being simulated using protocols prescribed in AASHTO R30 and NCHRP 9-54. The IDEAL-CT is being used as an indicator of mix cracking tolerance for both laboratory–mixed laboratory–compacted (LMLC) and plant–mixed, laboratory–compacted (PMLC) HMA and WMA specimens. A dense-graded asphalt mixture with a nominal maximum aggregate size of 19 mm, comprising a PG 64-22 binder is being used for this purpose. WMA samples are being produced at reduced temperatures, using chemical warm-mix additives. Two short–term heat conditioning approaches being considered are: (1) two hours at compaction temperature (149 °C for HMA and 135 °C for WMA mixes); and (2) four hours at 135 °C for HMA and four hours at 116 °C for WMA. Next, the two long–term heat conditioning protocols being considered are: (1) twenty hours at 110 °C; and (2) eight hours at 135 °C for both HMA and WMA mixtures. The mixtures are being tested for IDEAL-CT after different simulated aging conditions, and the CT_Index values are being compared against trends reported in the literature. This paper will present findings from this ongoing research effort, with primary focus on what approaches should be adopted by ODOT during their BMD implementation efforts.