Laboratory and field performance investigation of pre-swollen crumb rubber modified asphalt mixtures

The pre-swollen crumb rubber (PSCR) is a pelletized rubber produced by the reaction of scrap tire rubber particles with bitumen-compatible oil at elevated temperatures. It does not require a prior blending with bitumen, and it can be added directly at an asphalt plant. A laboratory investigation was carried out to evaluate the performance of plant-produced PSCR modified mixtures for surface and intermediate courses and analyze their suitability for low-traffic volume road in cold environment applications. Linear viscoelastic properties, rutting and thermal cracking performance were investigated in the laboratory. The presence of pre-swollen rubber particles and, in particular, the bitumen-compatible oil softened the mixtures as indicated by the dynamic modulus master curves. The different mechanical responses led to an enhancement of thermal cracking resistance but increased the rutting susceptibility of the PSCR mixtures. Despite the laboratory findings, no rutting was observed after five years of service life based on visual inspection of the field test sections constructed in Michigan with PSCR-modified Hot-Mix Asphalt (HMA). A few transverse cracks were generated in the lanes where the control mix with neat bitumen was used. Those cracks did not propagate into the PSCR modified material, in agreement with the laboratory findings.