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Aging of asphalt binders is one of the main causes of its hardening, which negatively affects the cracking and fatigue resistance of asphalt binders. Understanding asphalt aging is crucial to improve the durability of asphalt pavements. In this regard, this study aims at understanding and differentiating the effect of temperature and oxygen uptake on the aging mechanisms of unmodified asphalt binders. For that, four laboratory aging procedures were employed. The two standardized procedures, rolling thin-film oven test (RTFOT) and pressure aging vessel (PAV), were considered to simulate the short-term and long-term aging of the asphalt binders, respectively. In addition, two thin-film aging test procedures, the nitrogen atmosphere oven aging test (NAAT) and ambient atmosphere oven aging test (OAAT) were employed to assess the effect of thermal and oxidative aging on unmodified asphalt binder properties. The NAAT procedure is based on the principle that the inert gas minimizes the oxidative aging. The rheological and chemical characterization showed that the high temperatures considered during the NAAT procedure did not change the properties of the unmodified asphalt binders. Therefore, it can be hypothesized that no significant thermal and oxidative aging was observed during NAAT aging procedure for the considered binders and that oxidative aging is the main cause for the hardening.

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The objective of this study is to investigate the microevolution of the bitumen microstructure, polymer phase, and polymer–bitumen interaction of high-viscosity modified bitumen (HVMB) in reactive oxygen species (ROS) aging using the Viennese binder aging (VBA) method. First, the VBA system was utilized to age HVMB with different ROS concentrations. Then, an optical inverse bright-field, dark-field, and fluorescence microscope (OIM) was used to observe the bitumen microstructure, its evolution, and polymer structure degradation of HVMB during ROS aging. Afterward, ImageJ software was applied to quantitatively assess the bee structure size characteristics of bitumen and the distribution characteristics of the polymer. Finally, the differences in the distribution features of bee structures in the polymer zone (PZ) and non-polymer zone (NPZ) were counted to understand the polymer–bitumen interaction during ROS aging. The results show that the existence of polymers affects the distribution state of bee structures in bitumen significantly. The area ratio of bee structures and the average area of bee structures in HVMB both progressively rise with an increase in ROS concentration, although both are much less than those in base bitumen (BB). The destruction of the polymer phase starts from the inside of polymers during ROS aging. As the ROS concentration increases, the polymer degrades from the intact network structure to isolated small molecules. Polymer–bitumen interactions are widespread in HVMB. A large number of bitumen bee structures are also present in the PZ of HVMB. Moreover, the bee structures considerably grow in the PZ as opposed to the NPZ during ROS aging. The clarification of these microstructural characteristics lays the foundation for a deeper comprehension of the aging mechanism of HVMB induced by ROS.

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