Effects of RAP Asphalt Mortar Migration on the Cracking Performance of Recycled Asphalt Mixtures: Evaluation, Mechanism, and Regulation Strategy

The dispersion behavior of reclaimed asphalt pavement (RAP) particles during hot mixing directly affects the blending of aged and virgin materials and consequently influences the cracking performance of plant-mixed hot recycled asphalt mixtures (PHRAMs). This study investigated the effects of material composition and mixing process parameters on RAP particle dispersion and cracking performance of PHRAMs. PHRAMs containing 40% RAP were prepared using an orthogonal experimental design considering gradation type, asphalt binder content, mixing time, and RAP preheating temperature. Using a developed magnetite tracer method, the RAP asphalt mortar migration rate index was calculated from the mass of adhering mineral particles smaller than 2.36 mm, normalized by tracer aggregate mass and expressed relative to the corresponding loading in a disintegrated-RAP reference mixture. Semi-circular bending tests were conducted to evaluate intermediate-temperature crack propagation and low-temperature cracking resistance. Finally, the relationship between the RAP asphalt mortar migration rate and cracking performance was comprehensively analyzed. The results showed that gradation type and asphalt binder content were the dominant factors affecting RAP asphalt mortar migration, whereas asphalt binder content was the key factor governing cracking performance. Although RAP asphalt mortar migration was correlated with cracking resistance, it was not the primary controlling factor. Comprehensive analysis indicated that asphalt film thickness and gradation characteristics were more closely associated with cracking performance, together with the effects of RAP asphalt mortar migration and aged asphalt binder proportion. Accordingly, a multi-objective optimization framework was proposed to regulate RAP particle dispersion under cracking performance constraints and determine reasonable ranges of material composition and mixing process parameters.