Investigation of the Black Rock Issue for Recycled Asphalt Mixtures

As part of the NCHRP 9-12 research, Incorporation of Reclaimed Asphalt Pavement in the Superpave System, a study was conducted top investigate whether Reclaimed Asphalt Pavement (RAP) acts like a black rock when added to hot mix asphalt mixtures. If RAP performs like a black rock, the assumption behind using a blending chart in the design of recycled asphalt mixtures is not valid. In addition, with the implementation of Superpave for selection of materials and design of the asphalt mixtures, it is necessary to characterize asphalt mixtures containing RAP with the Superpave parameters. In this study, three RAPs with different stiffnesses, two new binders, and two RAP ratios were used. Three possible interactions between new and old binders in the RAP were considered and three mixture cases were made. These three mixture cases were total blending, standard practice (real world) and black rock. Using the Superpave Shear Tester (SST) different shear tests, including repeated shear at constant height, simple shear and frequency sweep tests, were conducted on all mixtures to compare the three mixture cases. The results showed that at a low RAP ratio, 10 percent, there was not a significant difference between the three mixture cases but for 40 percent RAP ratio, the standard practice (real world) mixtures were closer to the total blending that the black rock. Statistical analysis verified this observation for most studied cases.