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.
The aging of bitumen is a major contributor to the failure of asphalt pavements. Realistic and accurate laboratory aging methods can predict bitumen durability and guarantee the use of high-quality components in asphalt pavement. However, current standardized aging methods do not incorporate atmospheric parameters, besides elevated temperatures and molecular oxygen. Crucial chemical components like reactive oxygen species (ROS), e.g. nitrogen oxides (NOx) or ozone (O3), are completely neglected. This study focusses particularly on the reactivity of individual ROS, such as nitrogen monoxide (NO), nitrogen dioxide (NO2) and O3, in regards to the long-term aging (LTA) of three unmodified bitumen. For LTA an adapted version of the Viennese Binder Aging method was used and the aged bitumen samples were analyzed with the dynamic shear rheometer and Fourier-Transform-Infrared spectroscopy, respectively. The results show that NO as a single component does not induce significant aging, whereas NO2 leads to severe bitumen deterioration, which is even more accelerated when a second oxygen source is present. In comparison, the reactivity of O3 is rather mild and it did not cause additional aging for two of the investigated binders. This study provides evidence, that ROS play a crucial role in bitumen aging and should thus not be neglected when addressing realistic aging conditions in the laboratory.
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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