Pre-swelling-induced surface property changes in waste crumb rubber for impact-absorbing pavement applications: A novel sustainable sidewalk solution

Limited compatibility between waste crumb rubber (WCR) and bitumen, along with swelling-induced instability, restricts dry-process WCR recycling in asphalt pavements. This study investigated whether epoxidized soybean oil (ESO) pre-swelling can improve the surface state and volumetric stability of WCR while retaining partial elasticity as flexible aggregates in cold-mixed impact-absorbing pavement with reclaimed asphalt pavement (RAP). First, ESO diffusion and WCR expansion were evaluated under different temperatures and durations, followed by physicochemical, thermal, and rheological characterization. Subsequently, cold mixtures containing pre-swollen WCR, RAP, emulsified bitumen, and filler were prepared, and their volumetric stability, cross-sectional structure, indirect tensile strength (ITS), and indirect tensile stiffness modulus (ITSM) were assessed. Results show that WCR swelling exhibits logarithmic growth over time, with temperature exerting a stronger influence than size. Fine and coarse WCR reach equilibrium expansions of 4–24% and 1–14%, respectively, from 30 to 150 °C, consistent with ESO diffusion coefficients that are 4–7 times higher in fine WCR and over three orders of magnitude higher at 150 °C than at 30 °C. Pre-swelling was associated with lower contact angle, enriched oxygen-containing groups, and a softer viscoelastic response of WCR, suggesting improved surface affinity and reduced thermal sensitivity. Treatment at 150 °C for 2 h gave the best overall performance, increasing volumetric stability, ITS, and ITSM by 36%, 23%, and 62%, respectively. These findings support the waste-to-value use of WCR and RAP in safer urban slow-mobility pavements. However, ESO’s rejuvenation on RAP reduces mixture strength and requires control. • Mixtures using 100% recycled aggregates through dry and cold processes were tested. • Enhanced mixture stability and strength by using compatible and stable pre-swollen WCR. • Explored effects of temperature and duration on WCR swelling and ESO diffusion. • Analyzed physicochemistry and rheological changes in WCR at macro and micro levels.