A comprehensive review on the blending condition between virgin and RAP asphalt binders in hot recycled asphalt mixtures: Mechanisms, evaluation methods, and influencing factors

The application of hot recycled asphalt mixtures is receiving growing attention in pavement engineering because of the environmental and economic benefits. In the hot recycled asphalt mixture, the degree of blending (DOB) between virgin and reclaimed asphalt pavement (RAP) asphalt binders directly affects the mixture performance. Therefore, an in-depth interpretation of the DOB is of great practical significance for the improvement of design and production of hot recycled asphalt mixtures. The present paper reviews and discusses the state-of-the-art studies in the field of the blending condition between virgin and RAP asphalt binders. Firstly, a few theories proposed to interpret the blending process between virgin and RAP asphalt binders are introduced and discussed. Then, the methods commonly used to evaluate the DOB between virgin and RAP asphalt binders are summarized and classified into five categories based on the test principle, including indirect measurement methods, identi-fiable difference methods, staged extraction methods, indicator marking methods, and in-situ microscopic observation methods. Afterwards, critical factors affecting the DOB between virgin and RAP asphalt binders are described in detail. Finally, some challenges for the investigation of the blending condition between virgin and RAP asphalt binders are further discussed, and the corresponding suggestions are recommended for future investigation.