Evaluation of the effects of crumb rubber on mechanical properties of hot mixtures

In this study the use of crumb rubber (CR) recycled from waste tires using an ambient grinding process was evaluated at two stages as in bitumen and hot mixture. First, bitumen modified with crumb rubber was evaluated by rotational viscometery (RV), dynamic shear rheometry (DSR) and conventional binder tests. Hot asphalt mixtures including crumb-rubber-modified bitumen were then evaluated by determining the permanent and fatigue characteristics and stiffness moduli of control and modified mixtures. Because styrene?butadiene?styrene (SBS) is a polymer modifier that is widely used in bitumen to improve the performance properties of hot-mix asphalt, the properties of the crumb-rubber-modified bitumen and asphalt mixtures were compared to different contents of SBS-modified-bitumen and asphalt mixtures. The rheological binder tests showed that to achieve the same performance, as with SBS-modified binders, the CR-modified binders must be used at much higher contents than SBS, particularly at low frequency. However, the CR-modified binder displayed a lower phase angle than the SBS-modified binder at the same complex modulus at low frequency, indicating an increased elastic response. For acceptable workability and pumpability according to Superpave specifications, it was determined that the SBS and CR contents must be lower than 6.7% and 10.5% by weight, respectively. In conventional binder tests, it was determined that both SBS- and CR-modified binders exhibit less temperature susceptibility with increased additive content. However, this trend was more pronounced for the SBS-modified binder. The dynamic creep tests showed that a crumb-rubber content twice that of the SBS content in the hot asphalt mixtures provided the same performance. An 8%-CR modification exhibited better results than 2%, 3%, or 4% SBS modifications in the fatigue test. Furthermore, 8% CR modified mixtures displayed 50% higher stiffness modulus values than the control mixture.Key Words: Crumb Rubber, SBS, Modification, Rheology, Mechanical Properties