UV-induced gradient aging of bitumen films: A comprehensive study

The aging of bitumen materials under ultraviolet (UV) radiation is a highly intricate process involving volatilization of components and multiple photochemical reactions. To gain a comprehensive understanding of the mechanism and progression of UV aging in bitumen films within bitumen mixtures, this study investigates the formation and evolution of UV aging gradients in bitumen films from various perspectives, including morphology, composition, macro-, and micro-mechanical aspects. Initially, specific bitumen film specimens were subjected to aging, and a protocol for sample handing and testing was developed. Optical microscopy images, functional group indices, and component contents of the bitumen films were then measured and calculated to track the UV aging process from the surface to the interior. Additionally, profiling micro-mechanical tests and layer-by-layer dynamic mechanical tests were conducted to characterize the variations in mechanical properties induced by UV aging gradients within the bitumen films. The results indicate that UV radiation causes significant volatilization of light components on the film surface, resulting in wrinkling. Concurrently, photochromic reactions take place in both bitumen macromolecules (asphaltenes and resins) and small molecules (saturates and aromatic), while molecular photodegradation may also occur. Fluorescence imaging, infrared spectroscopy analysis, and component determination reveal that the degree of UV aging in the bitumen films increases with radiation time and gradually propagates towards the interior. C-S bonds are more sensitive to UV radiation than C = C bonds, leading to oxidation and the formation of sulfoxide groups. Due to the adsorption of light components by aggregates, the modulus values at the surface of aged bitumen films and the aggregate interface are higher, while the intermediate region exhibits lower values. Rheological indicators demonstrate that UV aging, to a certain extent, results in the decomposition of bitumen molecules, leading to a decrease in complex shear modulus and an increase in phase angle. This work significantly enhances our understanding of the mechanism of UV aging in bitumen films and holds important implications for the design and development of UV-blocking materials.