Multi-scale characterization of scraping and diffusion effects: mechanistic insights and design strategies for sustainable high-RAP asphalt mixtures

Clarifying the roles of the scraping effect and diffusion effect is essential for optimizing high-reclaimed asphalt pavement (RAP) mixtures. This study employed field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), dynamic shear rheometer (DSR), gel permeation chromatography (GPC), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) for multi-scale characterization to quantify the mechanistic dominance of the two effects, determine the critical mixing time, and develop an improved mix design integrating close packing and multi-support skeleton theories. Results show that the scraping effect dominates the degree of blending (DOB), while the diffusion effect becomes predominant when mixing reaches 360 seconds. RAP conditioning temperature most strongly influences the equivalent diffusion time, reaching up to 616 seconds. Nanomechanical properties correlate strongly with oxidation indices (R2 > 0.80). As RAP content increases, the improved design achieves enhanced low-temperature cracking resistance and water stability.