Mechanical and meso-structural behavior of multi-recycled asphalt mixtures with high reclaimed asphalt pavement (RAP) content under moisture damage

Moisture damage is a major factor threatening the durability of asphalt pavements. However, while many studies have addressed this issue in conventional or low-reclaimed asphalt pavement (RAP) mixtures, the long-term performance of high-RAP mixtures under repeated recycling remains insufficiently understood. This study examined a dense-graded mixture containing 60 % RAP subjected to five successive recycling cycles with a commercial bio-oil rejuvenator, using surface free energy (SFE) analysis, indirect tensile strength ratio (ITSR) testing, dynamic modulus testing under the moisture-induced stress tester (MIST), computed tomography (CT) scanning, and principal component analysis (PCA). The results showed that recycling progressively weakened interfacial adhesion, with the work of adhesion ( W add ) and compatibility ratio (CR) decreasing by 28.31 % and 40.38 %, respectively, while the work of adhesion with water ( W adw ) increased by 20.1 %. These changes were reflected at the mixture scale, where ITSR remained above 80 % through the third cycle but declined to 75 % in the fifth cycle, accompanied by an apparent loss of stiffness under dynamic conditioning. CT analysis further indicated that the first cycle slightly improved compactness, whereas subsequent recycling promoted coarsening and more connected air-void networks. PCA confirmed a mechanism shift, with adhesion-related factors sustaining resistance in early cycles, while moisture intrusion became the main driver of deterioration thereafter. Overall, high-RAP mixtures maintained adequate moisture resistance for up to two recycling cycles, whereas durability degraded sharply beyond this limit due to cumulative binder aging, interfacial weakening, and pore-network coarsening.