1. Introduction
Asphalt pavement stands as the preferred choice for road construction due to its exceptional leveling, driving comfort, low noise, resistance to salt decay, and ease of maintenance. Nevertheless, asphalt pavements inevitably confront adverse factors like sunlight, oxygen, varying temperatures, and moisture over prolonged periods, leading to aging-related challenges. This results in decreased durability and a shortened service life for asphalt pavements [
1]. The primary cause of asphalt pavement deterioration lies in the aging of the asphalt binder, which binds aggregates, mineral powders, and additives together [
2]. Effectively enhancing asphalt pavement durability and implementing precise pavement maintenance measures constitute urgent challenges to address. Solving these problems necessitates a more precise and profound comprehension of the principles governing asphalt binder aging.
Aging typically manifests in macroscopic properties, marked by hardening, reduced penetration, decreased ductility, irreversible creep flexibility, and phase angle, as well as an increase in the softening point and zero shear viscosity. Zhang et al. [
3] observed a decrease in the ductility with increasing aging, albeit with an elevated critical low-temperature point. SBS-modified asphalt binder displayed superior plastic deformation capability and robust resistance to aging. Liu et al. [
4] noted that UV aging led to higher softening point and viscosity values, accompanied by decreased ductility and penetration of asphalt binder. Layered double hydroxides enhanced asphalt binder’s aging resistance. Valtorta et al. [
5] revealed an increase in complex modulus and temperature sensitivity of aged asphalt binder. Asphalt pavement exhibited signs of hardening and loosening due to aging. On the microscopic front, aging is typified by the thermal evaporation of light components, the transformation of light fractions into heavier ones, an uptick in polar functional groups, and a rise in average molecular weight. Traxler [
6] concluded that asphalt binder aging primarily stems from light component volatilization, oxidation, structural changes, photopolymerization, and thermal condensation. Zhao et al. [
7] observed an augmentation in the macromolecular structure and average molecular weight following aging. Zhang et al. [
8] detected an increase in straight chain length, a higher number of large molecules, and heightened internal friction in asphalt binder molecules during motion after aging.
Presently, much of the research on asphalt binder aging relies on laboratory-accelerated simulated aging [
9]. The Thin Film Oven Test (TFOT) or Rolling Thin Film Oven Test (RTFOT) simulates asphalt binder aging during production, transportation, and paving. Pressure Aging Vessels (PAV) simulate aging during service life [
10]. Researchers also have developed UV aging equipment. Asphalt binder UV aging tests are now increasingly becoming standardized [
11,
12,
13,
14,
15]. Ozone, water, and multifactorial coupled aging are also garnering increased attention [
10,
16,
17,
18]. Qian [
19] delved into the effects of UV radiation and various water environments on asphalt binder, uncovering that water environments and UV aging exacerbated asphalt binder aging and diminished its low-temperature crack resistance. Interactions and synergistic effects between diverse aging factors are evident. Multifactorial effects on asphalt binder are substantial. Part of the multifactorial effects can induce notable deviations between the actual environment and the simulated environment.
However, asphalt binder undergoes forced aging through accelerated laboratory simulations. This process aims to reduce the aging time of the binder by subjecting it to higher temperatures, greater pressures, or more intense radiation than it experiences in its natural environment. Theoretically, the parameters for this acceleration simulation are scientifically grounded in energy conversion principles. Nevertheless, the environment of pavements is shaped by climate, geography, and other factors. Consequently, replicating the aging of various binders in such complex conditions through laboratory simulations remains challenging [
20]. Moreover, the study of asphalt binder exposed to real-world conditions presents difficulties due to extended test durations and associated costs. In many areas of Northwest China, including the focus of this study, there are approximately 3360 h of sunshine annually, with extreme temperatures reaching as high as 45.1 °C. Additionally, these regions contend with various deteriorating factors, such as strong winds, saline soils, and intense light-oxygen radiation. These conditions accelerate the aging of binders exposed to the elements, leading to highly intricate aging behaviors.
The primary aim of this study is to elucidate the impact of diverse environmental factors on asphalt binder aging and uncover the aging processes experienced by asphalt binders exposed to real-world conditions. Initially, various aging modes were established to age asphalt binder. Subsequently, high-magnification cameras were employed to capture the apparent morphology of samples exposed to outdoor conditions, facilitating an analysis of aged asphalt binders. Finally, it characterized the effects of various environmental factors on the chemical composition and rheological parameters of these two types of asphalt binders at multiple scales, revealing the mechanisms behind asphalt binder aging. The results of this investigation provide a deeper understanding of the behavior of asphalt binder in actual service conditions, aiding in the prediction of performance changes during service.