Investigation of ultraviolet radiation aging gradient in asphalt binder

The phenomenon of ultraviolet (UV) radiation aging gradient in asphalt binder is existent but usually neglected in most of UV radiation aging simulations and real asphalt pavement. Therefore, it is necessary to investigate the developing velocity and causing mechanism of UV aging gradient in asphalt binder. In this paper, the 3 mm-thickness asphalt films that suffered different UV radiation aging periods were divided into average three layers by peeling procedures. Then, rheological properties, molecular sizes distributions, chemical functional groups contents and fluorescence tests were conducted on different asphalt layers. The results indicate that by rheological indexes, the effect of 6-day UV aging is almost negligible, but the UV aging depth can exceed 1 mm after 9-day UV aging period and further exceed 2 mm after 15-day UV aging period. In addition, the vertical gradients of molecular sizes and oxidation products contents tend to form in asphalt film as UV aging process proceeds. Moreover, the observation of inorganic fluorescent powders being able to permeate into fresh asphalt film in lower layers by fluorescence microscope objectively demonstrates the “diffusion aging model” ─ UV aged asphalt molecules diffuse downwards into fresh asphalt film in lower layer.