The Influence of Modifiers on the Performance of Recycled Asphalt Mixtures

To enhance the performance of reclaimed asphalt pavement and evaluate its suitability under severe environmental conditions, this study systematically assessed the reinforcing effects of two modifiers: styrene–butadiene rubber (SBR) and polyurethane (PU). The stability and mechanical properties of mixtures with varying recycled asphalt contents were tested, with a focus on the effects of water immersion and high-temperature (150 °C) aging. The underlying mechanisms were elucidated using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). Results indicate that recycled asphalt incorporation significantly degraded the mixture’s performance; stability and mechanical strengths decreased with recycled asphalt content following a cubic function, with maximum deterioration rates of 37.3%, 67.2%, 73.6%, and 72.1%. Water and thermal aging further accelerated performance decay, with deterioration rates reaching up to 61.2% and 63.9%, respectively. Modifiers effectively counteracted this trend: SBR and PU enhanced the aforementioned properties by up to 102.6%, 50%, 60.3%, and 56.9% and 139.5%, 81.4%, 113.2%, and 120.3%, respectively. Microstructural analysis revealed that RAP led to decreased C and O contents, increased Si and Fe presence, and a more porous structure. In contrast, the modifiers increased binder content and interfacial density. This research provides a theoretical foundation and technical pathway for the engineering application of high-performance recycled asphalt mixtures in complex environments.