Viscoelastic Behavior and Characterization of Bitumen under Natural Exposure Aging in the Tibetan Areas

This study investigates the aging mechanisms and effects of outdoor natural exposure aging (NEA) on bitumen in the Tibetan Autonomous Region of China. Unmodified bitumen (No. 90) and styrene–butadiene–styrene (SBS)-modified bitumen were exposed to natural weathering for 0, 4, and 8 months, and their aged samples were collected and analyzed. Dynamic shear rheology (DSR) and Fourier transform infrared spectroscopy (FTIR) were employed to examine the changes in rheological properties and chemical composition of the samples during NEA. The study also assessed the aging extent of bitumen using the complex modulus aging index (CMI), phase angle aging index (PAI), and rutting factor growth rate. FTIR was utilized to quantitatively evaluate the variations in functional group indices with NEA time. The correlation analysis between the conventional binder properties and functional group indices facilitated a deeper understanding of the aging behavior. The results revealed the differences in temperature sensitivity and aging resistance between unmodified bitumen (No. 90) and SBS-modified bitumen under NEA. This research offers insights into bitumen aging in high-altitude regions and emphasizes the intrinsic role of chemical functional group changes in inducing macroscopic indicator changes during NEA.