Mitigating RAP variability through detailed particle size classification: Applications in high RAP content asphalt mixtures

High recycled asphalt pavement (RAP) content in asphalt mixtures provides substantial economic and environmental benefits. However, its application is hindered by performance instability caused by RAP variability. This study investigates a refined RAP particle size classification method that increases the grading levels from two to four, aiming to mitigate variability and enhance the performance of high RAP content mixtures. Laboratory-produced RAP was designed to simulate in-situ RAP properties with controlled variability in asphalt content, asphalt properties, and gradation. Recycled asphalt mixtures with RAP contents of 45 %, 60 %, and 100 % were evaluated for moisture sensitivity using immersion Marshall (retained stability) and freeze-thaw splitting tests (tensile strength ratio), low-temperature cracking resistance using semi-circular bending tests (fracture energy), and high-temperature stability using rutting tests (dynamic stability). Results demonstrate that RAP gradation variability predominantly impacts water stability, while asphalt content variability critically influences low-temperature performance. The refined classification method significantly reduced variability, enhanced mechanical properties, and stabilized overall performance. This approach offers a practical solution for improving the quality of high RAP content mixtures and advancing sustainable pavement construction practices.