Effect of Warm-Mix Additive USP on the Performance of Rubberized Asphalt and Fiber-Reinforced Rubberized Asphalt RAP Interlayer

To address the dual challenges of cryogenic performance degradation and excessive VOC emissions in rubberized asphalt, this study proposes a synergistic optimization strategy using a polymer-based warm-mix additive (USP). The effects of USP on the rheological behavior, VOC emission characteristics, and mechanical performance of polymer-modified asphalt and fiber-reinforced RAP interlayers were systematically investigated. The results indicate that 5% USP optimally improves low-temperature flexibility (141.1% increase in ductility, 28.48% reduction in creep stiffness) while maintaining adequate high-temperature stability, and simultaneously achieves an 82.01% reduction in total VOC emissions at 150 °C. Microscopic analysis and DIC tests confirm that USP enhances polymer–asphalt–aggregate interactions, leading to improved adhesion, reduced water permeability, and extended fatigue life. This work provides a fundamental understanding of polymer–binder–aggregate synergy and offers a practical pathway toward greener, high-performance recycled asphalt pavement technologies.