Quantification and Performance Implications of Recycled Binder Availability in Asphalt Mixtures Containing Reclaimed Asphalt Materials

Abstract

Asphalt mixtures containing recycled asphalt materials (RAM), including reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), are commonly designed and produced under the assumption that all recycled binder fully mobilizes and blends with the virgin binder. In practice, only part of the recycled binder mobilizes during mixing and blends with the virgin binder to form the active binder phase. The portion that becomes active is referred to as the Recycled Binder Availability (RBA). When the actual RBA is lower than the assumed value, the mixture contains less active binder than intended, which can reduce cracking resistance and shorten pavement service life. A reliable determination of RBA would allow binder content to be adjusted to compensate for the inactive fraction and restore the intended active binder content. However, whether this adjustment improves long‑term pavement performance and is economically justified has not been established for asphalt mixtures containing RAM. This dissertation quantified RBA in asphalt mixtures and evaluated how it affected mixture performance and life‑cycle cost. Three RBA determination methods were examined. The first was a volumetric method based on the differences in optimum binder content between the actual RBA and 100% RBA conditions. The second was a high-temperature indirect tensile (HT-IDT) interpolation method based on the mixture mechanical response of the actual RBA condition and reference specimens produced at 0%, partial, and 100% RBA conditions. The third was the Glass Beads method, based on comparing the performance grade of the active binder phase, recovered via glass beads, against the 0% and 100% RBA reference conditions. Glass beads were incorporated as tracers for virgin aggregate to capture the binder mobilized during mixing for that comparison. These methods were first compared using laboratory mixtures prepared with a single RAP source aged to three controlled levels. The two methods that successfully detected differences across the RAP aging levels were then applied to a second mix design containing a different RAP source to preliminarily evaluate their ability to differentiate RBA across mix designs and RAP sources. The Glass Beads method demonstrated the highest repeatability and sensitivity and was selected for the remaining investigations. The Glass Beads method was then applied to mixtures containing a wide range of field RAM sources, mix design variables, and production conditions. The measured RBA varied substantially and was influenced by RAM type, content, and source, virgin binder type and source, aggregate type, recycling agent use, mixing temperature, and simulated silo storage. Within a given RAM type, RBA decreased as RAP content or binder stiffness increased, whereas the use of a recycling agent increased RBA. Although RAS binders were stiffer, RAS mixtures exhibited RBA values within the range observed for RAP mixtures, indicating that factors such as particle agglomeration, particle size, and virgin binder proportion also influenced RBA. These findings confirmed that RBA was governed by the combined effects of RAM, virgin materials, and production conditions rather than by RAM alone. Because only part of the recycled binder becomes active, binder content can be increased to compensate for the inactive fraction and restore the intended active binder content, an approach referred to as the RBA-adjusted optimum binder content (A-OBC). The performance implications of this adjustment were evaluated for four high-RAM mixtures representing two mix designs and two virgin binder types. At A-OBC, the mixtures exhibited lower dynamic modulus (|E*|) and improved cyclic fatigue resistance compared with the volumetric optimum binder content (V-OBC), which has no correction for inactive recycled binder. FlexPAVE simulations of a perpetual pavement structure translated this improved performance into lower predicted fatigue damage accumulation and fewer rehabilitation events over the analysis period for the RBA-adjusted designs. These predicted performance differences were then carried into a life-cycle cost analysis (LCCA), which accounted for both the higher initial construction cost from the additional binder at A-OBC and the savings from fewer rehabilitation events. The LCCA projected net present value (NPV) savings ranging from 0.8% to 7.7% across the four simulation scenarios, indicating that the value of fewer rehabilitations outweighed the higher upfront cost from a life-cycle cost perspective. Overall, the results demonstrated that measuring and accounting for RBA improved the performance reliability and economic efficiency of asphalt mixtures containing RAM, though the magnitude of these benefits depended on mix design and virgin binder type. These findings supported the integration of RBA determination as an effective strategy to design durable and cost-effective asphalt mixtures containing RAM.