RAP agglomeration and partial blending of recycled hot mix asphalt: A literature review

Current mix design practices typically assume total blending and use the white curve of reclaimed asphalt pavement (RAP) to determine the gradation and optimum asphalt content (OAC) of recycled hot mix asphalt (HMA), often overlooking the effects of RAP agglomeration and partial blending. This oversight can result in unsatisfactory performance, particularly when higher RAP content is used. Therefore, this paper reviews and discusses strategies for adjusting the mix design of recycled HMA to enhance its in-service performance. The discussion begins with RAP particle agglomeration, a significant phenomenon that significantly impacts the aggregate gradation of recycled HMA. Subsequently, detection methods to clarify the blending between virgin and RAP binders are described. Partial blending between RAP and virgin binders is common, and various indexes have been proposed to quantify the blending degree. Finally, the adjusted mix design method of recycled HMA is presented, emphasizing gradation optimization and corrected OAC. Gradation optimization should account for RAP agglomeration, while the corrected OAC should consider particle blending. Recycled HMA using the adjusted mix design exhibits improved crack resistance and fatigue life without substantially impairing rutting performance. This review aims to help both academics and highway agencies maximize the utilization of RAP materials within sustainable pavement frameworks.