Influence of reclaimed asphalt pavement (RAP) and recycled asphalt shingle (RAS) binder availability on the composition of asphalt mixtures

Reclaimed asphalt pavements (RAP) and recycled asphalt shingles (RAS) are incorporated in the production of new asphalt mixtures to reduce cost, waste, and the consumption of virgin materials. The use of recycled asphalt materials (RAM) is limited by controlling agencies due to the uncertainties regarding their impacts on the volumetric composition and long-term performance of the asphalt mixtures. Recycled binder availability (RBA), which reflects the proportion of the recycled binder that is available to blend with virgin asphalt in an asphalt mixture, is currently neglected by the majority of mix design methods adopted by highway agencies, despite complete RBA being considered an unrealistic scenario. An incorrect assumption of the RBA in mixture design has important consequences on the inferred asphalt mixture volumetric properties, and consequently on the performance. A recently created method, so-called Availability Adjusted Mix Design (AAMD), proposes a revised interpretation of the volumetric properties of asphalt mixtures and design of the aggregate structure on the basis of partial RBA caused by agglomerations of RAM particles. The AAMD method is used in this study to evaluate the impacts of RBA on the volumetric properties of a diverse set of surface asphalt mixtures. The impacts of the RBA on the inferences of asphalt mixture composition are assessed for each mixture and the viability of using fixed RBA values for RAP and RAS sources in the state of North Carolina is assessed by comparing the inferred asphalt mixture composition yielded from the source-specific RBA versus the average RBA of the pool of recycled materials studied. The results show that the volumetric properties change considerably as a function of the assumed RBA, and the use of the AAMD becomes crucial for mixtures containing as little as 15 percent RAP and/or 4 percent or RAS. The use of a fixed RAS RBA value, calibrated as the average of a representative set of RAS stockpiles, versus the source-specific values has negligible impacts on the inferred volumetric properties of mixtures containing permissible RAS contents in North Carolina. Conversely, the use of source-specific RAP RBA values is recommended, which can be quantified through comparative sieve analysis of RAP and recovered RAP aggregate.