Multi-Scale Assessment of Photo-Oxidative Versus Thermal Aging in Extracted Layered Cores and Un-Extracted Bitumen Films

Asphalt pavements experience complex, depth-dependent aging driven by combined thermal-oxidative and UV photo-oxidative mechanisms. Current laboratory aging protocols often evaluate either only binder films or bulk mixtures exposed to thermal or photo oxidation; that is why this study aimed to contrast the differences in the chemical and rheological behaviors of both bitumen extractions and thin bitumen films exposed to heat only and heat combined with UV irradiation. Three dense-graded asphalt mixture samples were aged in an oven at 70 °C. For comparison, three samples with the same properties were aged in an ultraviolet (UV) weathering chamber at the same temperature but with UV irradiation at an intensity of 50 W/m2 for 98 days. Afterwards, samples were cut into three disks, and binder extractions from the top and middle disks were tested using a dynamic shear rheometer (DSR), saturates–aromatics–resins–asphaltenes (SARA) analysis, and Fourier-transform infrared spectrometry (FTIR) analysis. Samples of the same bituminous binder were also applied to steel plates at a thickness of 1 mm before being aged and tested under the same conditions. The results showed that UV irradiation plays a significant role in the aging process of bituminous binders in both asphalt mixtures and bitumen samples. In terms of DSR and SARA analysis results, extractions from the top disks of the UV- and oven-aged mixtures showed far more severe aging compared to the bituminous binder films, while the effect of pavement depth was found to have a more significant effect in samples that had been aged in the UV chamber. FTIR results followed the same trend, but the bituminous binder results were comparable to the results of the extractions from the aged mixture samples. It was concluded that binder aging research is better performed using binder specimens extracted from aged asphalt mixture samples, rather than thin films of binder, and that UV irradiation is essential for more realistic accelerated aging protocols.