Improved Mix Design, Evaluation, and Materials Management Practices for Hot Mix Asphalt with High Reclaimed Asphalt Pavement Content

Abstract

The objectives of this project were to (1) develop a mix design and evaluation procedure that provides satisfactory long-term performance for asphalt mixtures containing high reclaimed asphalt pavement (RAP) contents—in the range of 25 to 50% or greater—and (2) propose changes to existing American Association of State Highway and Transportation Officials (AASHTO) standards to adapt them to the design of high RAP content mixtures. The project team conducted a comprehensive laboratory experiment to answer basic questions about preparing and characterizing RAP materials for mix designs. A series of mix designs was then prepared with materials from four different parts of the United States with different RAP contents and different virgin binders. Those mix designs were evaluated against standard Superpave criteria and a set of performance-related tests to further assess the mix designs for their susceptibility to common forms of distress, particularly fatigue cracking, low-temperature cracking, and moisture damage. A concurrent effort developed a set of best practices for RAP management in field production and construction from information obtained through a literature review, surveys of current practices in the industry, discussions with numerous contractor quality control (QC) personnel, and analysis of contractor stockpile QC data from across the United States The research found that only minor, though important, revisions to the current AASHTO standards for asphalt mix design, AASHTO R 35 (Superpave Volumetric Design for Hot Mix Asphalt) and M 323 (Superpave Volumetric Mix Design), were needed to adapt them for the successful design of high RAP content asphalt mixtures. As expected, high RAP contents substantially increased the dynamic modulus of the asphalt mixtures as well as their rutting resistance as measured by the confined flow number test. Tensile strength ratios of high RAP content mixtures as measured by AASHTO T 283 were comparable to those of control mixtures without RAP, indicating similar moisture damage susceptibilities. As might be expected, compared to control mixtures without RAP, the high RAP content mixtures generally had lower fracture energies at test temperatures used to evaluate susceptibility to fatigue and low-temperature cracking. This finding suggests that careful attention should be given to the selection of the performance grade of the virgin binder used in high RAP content mixtures to minimize any long-term risk of cracking distress.