Asphalt-rubber interactions

Abstract

Three experiments were designed to evaluate the influence of increasing percentages of rubber (0, 10, 15, and 20% by weight of binder) and rubber type (passenger and industrial tires), pretreatment of rubber (none, tall oil pitch), and asphalt chemistry (Strategic Highway Research Program materials reference library asphalts AAD, AAG, AAF, and AAM) on asphalt-rubber interactions. Viscosity was measured by a Brookfield rotational viscometer; testing variability was also estimated. Untreated rubber was included to represent interaction characteristics representative of the wet process. Treated rubber was included as a way of stabilizing the rubber before use in the dry process. Results confirmed that rubber (either passenger or industrial tires) reacts more readily with a softer grade of asphalt from a given refinery. Pretreatment of passenger tire rubber with various percentages of tall oil pitch significantly reduces both the viscosity and the influence of aging. Pretreatment of industrial tire rubber appears to have a significant effect only when lighter grades of binder are used. The same rubber will react differently with various sources of the same grade of asphalt cement. This is a function of the molecular weight of the asphalt cement, which is related to solubility, and the viscosity of the binder at the storage temperature, which is related to the rate of penetration and swell.