The Effects of Recycling Agents on Asphalt Mixtures with High Recycled Materials Content

Economic and environmental benefits motivate transportation agencies to increase the amount of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) used in asphalt mixtures. In the U.S., materials cost savings in 2017 totaled approximately $2.2 billion with RAP and RAS materials replacing virgin asphalt binder and aggregate in asphalt mixtures. However, recycled asphalt mixtures with high RAP and RAS content are usually less workable, difficult to compact in the field, and more prone to cracking, raveling, and other durability-related pavement distresses, primarily due to the presence of the severely aged, substantially stiffer and more brittle RAP/RAS binders. To meet these challenges, mixture adjustments are recommended including the use of recycling agents, or rejuvenators, to reduce mixture stiffness and improve cracking performance. The main objectives of this study were to provide tools for selecting the appropriate dose of recycling agent; optimizing recycled materials type and content; evaluating the effect of various factors on the long-term performance of rejuvenated asphalt mixtures; and providing practice-ready guidelines for evaluation, materials selection and optimization, and design of recycled asphalt mixtures with recycling agent. In this study, a summary of the current knowledge related to the use of recycling agents in the asphalt pavement industry was first provided, including recycling agent definition, advantages, and challenges. Rejuvenation mechanism of recycling agents, the effectiveness of recycling agents in improving the rheology and performance of recycled binder blends and recycled asphalt mixtures, and cost-effectiveness associated with the use of recycling agents were also reviewed. Then, different recycling agent dose selection methods were evaluated based on rheological parameters of the recycled binder blend. Furthermore, blending charts to balance recycled binder blend proportions and blending charts to select the appropriate dose of recycling agent to be added to an asphalt mixture during mix design was also established and verified. Then, the performance of rejuvenated asphalt mixtures produced in five field projects in the U.S. that include a wide spectrum of materials, mix designs, and climate was evaluated. Finally, a proposed method to quantify the available or effective RAP binder was introduced, and can be used to adjust the virgin binder content in RAP mixtures to ensure that the mix design optimum binder content is achieved