Effect of RAP Gradation on Fracture Tolerance of Asphalt Mixtures

This study investigated the effect of RAP gradation on the fracture tolerance of asphalt mixtures in terms of fracture energy density (FED). Experimental factors included two RAP sources, one virgin binder and four RAP contents of 0, 20, 30 and 40%, resulting in a total of seven mixtures. Binder fracture energy tests were conducted to determine the FED of blends of virgin and extracted RAP binders. Moreover, interstitial component direct tension (ICDT) tests were employed to measure the FED of the interstitial component, which is the fine portion of a mixture that governs cracking performance. Finally, Superpave IDT tests were performed to obtain the FED of RAP mixtures. Increasing RAP content led to reductions in FED at binder, IC and mixture levels. More importantly, the reduction in FED was almost identical for mixtures and IC, whereas a less pronounced drop was observed for binders. This finding substantiates that full blending between RAP and virgin asphalt as artificially created in binder specimens did not occur in the actual RAP mixtures. Coarse RAP yielded greater IC and mixture FED values than fine RAP despite the lower RAP binder stiffness of the latter, substantiating that RAP gradation controls the distribution of RAP binder in mixtures and consequently, high RAP binder content in IC portion reduces the fracture tolerance of a mixture. A new design guideline that considers RAP binder content and properties along with RAP gradation is proposed to determine the maximum allowable RAP content, so that cost savings and environmental benefits can be maximized by reducing the need for virgin pavement materials and landfill space.