Viscosity Characteristics of Rubber-Modified Asphalts

This study presents a state-of-the-art review of crumb-rubber modified (CRM) asphalt concrete technology and describes the results of viscosity measurements of CRM asphalt cements using a Brookfield viscometer. Literature was reviewed in relation to both the wet process (i.e., mixing of the crumb rubber with the binder prior to mixing with the aggregate) and the dry process (i.e., simultaneous mixing of the crumb rubber, the binder, and the aggregate). In wet processes, the reaction that takes place between rubber particles and asphalt binder is not chemical in nature, but rather a diffusion of the light aromatic components of the binder into the crumb rubber. This results in a reduction in the viscosity of the binder, which depends on the type of the binder, the type of the rubber, the mixing temperature, and the mixing energy (i.e., time). In dry processes, the crumb-rubber particles replace aggregate particles in the asphalt concrete mix, while due to the limited mixing time allowed, no significant reaction takes place between the crumb rubber and the binder. Brookfield viscometer testing of the CRM binder was conducted following a variation of the ASTM standards D2196-86 and D4402-87. Three binders were tested, namely AC-5, AC-10, and AC-20, with CRM percentages of 0, 3, 5, 7, 12, and 18. The CRM particle size was finer than sieve size No. 80. The viscosity of the CRM asphalt cement initially decreased in time, until the temperature of the crumb-rubber chips reached that of the binder. This took place in approximately 30-40 min, when a minimum viscosity value was reached, called the "stabilized" viscosity. The high-percentage CRM, however, experienced a linear increase in viscosity with time at a rate of approximately 180 cP hr as mixing continued past the stabilized viscosity.