Study of Measures to Design Asphalt Mixes Including High Percentages of Recycled Asphalt Pavement and Recycled Asphalt Shingles

With the increased use of recycled materials and various additives, the performance of asphalt mixes, especially their cracking resistance, has become a primary concern. Researchers have developed performance test methods and parameters to simulate field performance in the laboratory. When these performance tests are included in specifications, contractors must adjust their mix design parameters accordingly. To aid contractors, this study evaluated rut depths obtained from Hamburg wheel-tracking tests of 261 asphalt mixes and cracking test (CT) index values from IDEAL-CT tests of 69 mixes designed by contractors and verified by the Washington State Department of Transportation. These mixes had different gradations; binder contents; air void contents; recycled asphalt pavement (RAP); recycled asphalt shingles (RAS); recycling agents; binder performance grades; and other volumetric properties. The results show that (1) rut depth is highly sensitive to binder content, performance grade, aggregate angularity, and RAP/RAS content and (2) the CT index is sensitive to binder content, air void content of the mixture, aggregate gradation, and RAP/RAS content. The low-RAP mixes also showed worse cracking performance than the mixes without RAP. Therefore, the performance grade of blended binder may need to match the target binder grade even for low-RAP mixes. It was found that the design air void of 3.5% is optimal in respect of cracking resistance. The use of softer virgin binder or rejuvenator may not be effective in completely mitigating the cracking potential of high-RAP/RAS mixes.