Characterization of microstructural evolution of asphalt due to water damage using atomic force microscopy

Water erosion significantly impacts the microstructure of asphalt, which in turn affects the macroscopic performance of asphalt mixtures, such as reducing the adhesion between asphalt and aggregates as well as diminishing water stability. This study utilizes atomic force microscopy (AFM) to investigate the micromorphology, surface height distribution, nanosurface roughness, and microsurface energy of asphalt under different water immersion durations. Furthermore, it considers the influence of asphalt's penetration grade, oil source, and modifiers on the evolution of its microstructure. AFM analysis reveals that, with the exception of crumb rubber modified asphalt (PJ90/CR), the quantity and area of beelike structures in the asphalt decrease with increasing water immersion time. The surface height distribution of asphalt exhibits characteristics of an approximate normal distribution, with an increase in nanoscale heights over time due to the dissolution of surface components by water and physical erosion effects. Notably, the surface height of PJ90/CR and SBS modified asphalt (PJ90/SBS) is significantly lower than that of PJ90 before and after water immersion. Water damage causes asphalt surface particles to dissolve and re-aggregate, forming distinct nano-protrusions and pits, thereby increasing the asphalt's roughness. Among them, the roughness of SK90 are generally higher than those of SK70, with the average roughness (Ra) and peak-to-valley roughness (Rt) increasing by 28.93 % and 60.56 %, respectively, before immersion. Moreover, water damage leads to a decrease in the surface energy of asphalt to varying degrees, indicating that water more easily penetrates the asphalt film through adsorption and diffusion actions, altering its microstructure and, consequently, reducing its surface energy. The variations in the microstructure of SK90, Shell90, and PJ90 under different water immersion durations highlight the significance of oil source on asphalt performance.