Pilot Test of Proposed Standard Practice for Use of Recycling Agents in Asphalt Mixtures with Recycled Asphalt Materials

Abstract

Recycled asphalt materials (RAM) such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) are routinely used in the construction of new asphalt concrete (AC) pavements. This yields economic and environmental benefits by reducing the amount of virgin materials used in the pavement. RAM is typically stiffer in nature and more prone to cracking than virgin asphalt. For this reason, the amount of RAM in new pavement is generally limited. Mitigation strategies can be employed to increase the amount of RAM while maintaining adequate performance. This includes the use of recycling agents and the substitution of the virgin binder for a softer binder. However, there is currently no standard method for incorporating these strategies in new AC pavement. This research focuses on verifying a draft AASHTO standard practice proposed in NCHRP report 927 that aims to standardize the use of recycling agents in asphalt mixtures with high RAM content. Major findings from this research include the following: revisions to a recycling agent dose selection method, binder availability factor (BAF) recommendations, revisions to previously suggested material selection guidelines, and test aging sensitivity results. It was found that an adequate and more economical recycling agent dose could be estimated using both the low and high-performance grades of binder blends. The BAF was confirmed to be of importance when designing a high RAM mixture. To improve reliability, it is recommended that only one variable be adjusted in a mixture to mitigate the increased stiffness from recycled binders. Some test parameters were found to be very sensitive to aging including flexibility index (FI) from the semicircular bend test and the cracking test index (CTindex) from the ideal cracking test while others were not seen to be sensitive to aging including the fracture energy (Gf) measured by the low temperature disc shaped compact tension test. These finding were used to revise the draft AASHTO standard practice proposed in NCHRP report 927.