Effect of waste tyre rubber size on physical, rheological and UV resistance of high-content rubber-modified bitumen

This study is focused on the effect of crumb rubber (CR) particle size on the physical, chemo-rheological, UV ageing resistance and storage stability of high-content rubberised bitumen. Two CR gradations, #30 (coarser) and #50 (finer), were selected for this study. The blends were prepared by mixing 22.5% CR (by weight of base bitumen) into 50/70 pen grade bitumen for 60 min at 177 ± 10 °C with a shear mixing speed of 700 rpm. Blends prepared with #30 CR were found to improve the elastic response and storage stability compared to #50 CR modified binder. The presence of a greater number of coarser CR particles – which mostly remained in the swollen-form at the end of the blending process – as compared to finer particles – which instead were depolymerized and released the absorbed oils back into the binder phase – proved to increase the flexibility of the binder and improve its rheological performance. This same factor also reduced the #50 modified binder's UV resistance as the C–C, C–H bonds in the saturates portion of the bitumen have a lower bond energy than the energy produced by the impact of UV rays. Overall, crumb-rubber modified bitumen resists better to ageing produced by UV radiation compared to standard unmodified bitumen. Further reprocessing of waste vehicle tyres into smaller #50 mesh size is deemed not to produce substantial improvements in the final bituminous blend other than the reduced viscosity for the same CR content.