Moisture susceptibility of rubberised Stone Mastic Asphalt mixtures

Moisture damage is a critical distress mechanism in crumb rubber–modified asphalt mixtures, leading to premature loss of stiffness and strength in flexible pavements. This study systematically investigates the combined effects of ageing and moisture susceptibility in Crumb Rubber-Stone Mastic Asphalt (CR-SMA) produced via the dry process using a volume-based aggregate gradation adjustment technique. The influence of key variables, including binder content, CR content, CR size and gradation, and short-term ageing on moisture damage resistance of CR-SMA is evaluated through indirect tensile strength, fracture toughness, resilient modulus, and ultrasonic pulse velocity tests. Experimental data are analysed using response surface methodology (RSM) and ANOVA to develop predictive models, quantify factor interactions and identify optimal factor ranges. The results demonstrate that binder content is the primary factor governing moisture resistance, while CR content and particle size exert significant secondary effects. Short-term ageing shows minimal impact at high binder contents (≥7.0%), where moisture resistance remains stable. Optimum moisture performance was achieved at 1%–1.5% CR content, with finer CR gradation, and a 1.5-hour curing duration. The volume-based aggregate gradation adjustment technique markedly improved the water resistance of CR-SMA mixtures, highlighting its value for optimising pavement performance and sustainability.