Determination of Laboratory Aging Conditions for Hot Mix Asphalt Cracking Test

With the increased use of recycled materials in asphalt mixtures, many highway agencies are investigating the use of and implementing cracking tests for mix design evaluation, quality assurance, and/or quality control to identify cracking-susceptible mixtures. One of the key factors affecting cracking test results is the aging conditions. Optimal laboratory aging conditions should mimic the aging level in the field at the time of cracking initiation. Other studies and research, including the current aging protocol in AASHTO R121 (formerly R30), were based on matching the change in binder or mixture properties (e.g., modulus) that do not correspond to field aging data and do not account for binder absorption or selective absorption by aggregate during aging. The objective of this study is to develop an aging protocol for the Indirect Tensile Asphalt Cracking Test specific to Washington’s climatic conditions. It was found that the average crack initiation age was approximately 5 years for both eastern and western Washington, which is an intermediate-term pavement performance issue rather than a long-term issue. As part of this study, roadway cores from projects that were 5 years old were acquired for solvent extraction/recovery and residue testing to determine how much these binders aged 5 years after placement. Additionally, plant-produced mixtures were acquired to determine the amount of laboratory aging (temperature and time) it would take to match the binder aging measured in the roadway cores. A total of 4 h aging at 135°C, including short-term and intermediate-term aging, is recommended as an aging protocol for cracking sample preparation.