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Advanced Characterization of Crumb Rubber-Modified Asphalts, Using Protocols Developed for Complex Binders

The use of crumb rubber modifiers (CRMs) to change asphalt binders was evaluated. Blending two base asphalt binders of performance grade 70-22 and 64-22 with three sizes of a CRM at two rubber contents resulted in varying grades of asphalt binders. In addition to these binders, two binders modified with reacted crumb rubber using a patented process were included in the study. Testing was conducted at different temperatures, testing frequencies, and strain conditions, using methods recently developed as part of the NCHRP 9-10 project (Superpave Protocols for Modified Asphalt Binders) for complex binders. The new testing includes the particulate additives test (PAT), which determines the volume of particulate material greater than 75 μm in asphalt, and the laboratory asphalt stability test (LAST), which measures the potential for separation and degradation of additives in asphalt. Strain sweeps, fatigue testing, and low-temperature failure and creep properties were also measured. The results indicate that the concentration and size of crumb rubber affect the viscosity of the resulting modified binders significantly and influence the volume of residue collected in PAT. It was observed that concentration and size of rubber are, however, not important compared with other factors in LAST and that keeping the binders agitated during storage can reduce separation significantly. On the basis of the rheological testing, it was found that both fatigue and strain dependencies are highly sensitive to asphalt binder type, rubber size, and rubber content. The failure properties of CRMs measured using the direct tension test indicate that failure stress and strain are reduced by increasing rubber size and concentration.