A model for oxidative aging of rubber-modified asphalts and implications to performance analysis
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abstract
Oxidative aging of asphalt binders is a primary cause of binder-related long-term road failures. Viscosity hardening is primarily due to the oxidative conversion of polar aromatics to asphaltenes; oxidation is indicated by carbonyl formation. The aging of unmodified asphalts yields a constant, aging-temperature independent "hardening susceptibility" (HS) relation between viscosity change and the growth of the infrared carbonyl peak. Crumb-rubber-modified asphalts (CRMA) exhibit superior aging characteristics, lower hardening rates, and, often, lower oxidation rates, throughout the aging simulation range. However, CRMA materials may exhibit a hardening susceptibility that varies with aging temperature, suggesting either enhanced diffusion resistance or a kinetic competition between the oxidation sites of the rubber polymer and the asphalt's polar aromatics. This evidence suggests that the commonly accepted high-temperature, high-pressure, long-term aging technique is of questionable value when applied to CRMA materials. The aging characteristics of CRMA were found to depend most heavily upon the curing method, the fractional content of the rubber, and the chemical composition of the binder. The curing method has a major effect on the material and chemical properties. Curing at lower temperatures and shear rates leads to a mere introduction of the rubber material into the binder, producing a swollen, gelatinous particle phase, the aging consequences of which are difficult to assess. Progressively higher levels of mixing shear and temperature partially degrade the long polymer chains and cross-link structures. Polymer chains freed by thermo-mechanical shear are integrated in the binder, shielding or altering by competitive means the oxidation of the binder.
