Influence of crumb rubber and digestion time on the asphalt rubber binders

The behavior of asphalt binders modified with recycled crumb rubber depends on several factors, such as: rubber content and type, temperature and time employed during the digestion process. Some of these aspects are investigated in this paper by means of a series of tests performed on asphalt-rubber produced via the wet process. Crumb rubber manufactured by ambient grinding and cryogenic processes were used in this work. Reduction of penetration and sharp increase of viscosity, softening point and resilience were observed for increasing rubber contents. The results show that the Brookfield viscosity limits the crumb rubber content incorporated into straight binder, once it tends to become too high above a certain critical amount of incorporated rubber. The effect of digestion time on the viscosity of the modified binders depended in this research on the rubber content. For high rubber contents, there seems to exist a critical time after which viscosity tends to decrease for continuing digestion process. A rubber-binder interaction model, which could explain such results, is proposed. The influence of rubber type occurred especially in terms of Brookfield viscosity and softening point. It was observed a decrease in these properties for the asphalt-rubber binders produced with crumb rubber obtained by cryogenic process in relation to those produced by the grinding process.