# Study on the Aging Behavior of Asphalt Binder Exposed to Different Environmental Factors
## 4 Conclusions  
This study procured various types of aged asphalt binder samples through outdoor exposure tests. The impacts of diverse environmental factors on the two asphalt binders were assessed across multiple scales, encompassing apparent morphology, rheological properties, and chemical composition. The primary conclusions can be summarized as follows:  
1. Following exposure aging, micro-cracks manifested on the surfaces of asphalt binder. The prevalence of these microcracks and the escalation in surface roughness were chiefly associated with UV radiation, while the influence of O-aging on apparent morphology remained minimal.
2. Thermo-oxidative aging's effects on the asphalt binder primarily manifested during the mixing, transportation, and paving stages of asphalt mixture production. It becomes evident that light exposure significantly impacted the rheological properties.
3. The incorporation of SBS modifiers markedly heightened the risk of fatigue cracking in the virgin asphalt binder. However, it concurrently bolstered the aging resistance of asphalt binder. Moreover, the SBS-modified asphalt binder displayed a higher tolerance to external factors like wind, rain, and dust.
4. Outdoor exposure exerted a substantial influence on the fatigue resistance of asphalt binder. Among the various aging scenarios, the most pronounced impact was observed with All-aging for both types of asphalt binder.
5. Throughout the exposure process, the molecular chains within the asphalt binder underwent fracture, followed by an additional polymerization reaction involving the fractured and original molecular chains. This transformation resulted in elongated carbon chain lengths, cycloalkane aromatization, and an alteration in structural configuration toward macromolecules.
6. In comparison to virgin asphalt binder, natural aging had a lesser effect on the characteristic functional groups of SBS-modified asphalt binder. O+UV-aging and All-aging had the most significant impact on the sulfoxide index of virgin asphalt binder, surpassing even the effects of PAV aging.
7. This study focused only on samples aged for 3 months by outdoor exposure. The characterization of aging samples at different times will be a focus of future research. A dynamic relationship between the time of outdoor aging and the service condition of asphalt pavements will be established.  
Methodology, S.S., L.W., C.F., M.L. and L.Y.; Software, S.S., C.F. and X.J.; Validation, L.W.; Formal analysis, C.F., M.G. and L.Y.; Investigation, L.Y.; Resources, S.S. and X.J.; Data curation, C.F. and M.G.; Writing--original draft, S.S., X.J. and M.L.; Writing--review & editing, S.S., L.W., M.G. and M.L.; Project administration, L.W.; Funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Gansu Provincial Science and Technology Plan Project (Basic Research Program--Natural Science Foundation) (23 JRRA1375), Research project of Gansu Provincial Department of Transportation (No. 2022-33) and National Key Research and Development Program of China (2022 YFE0137300, No. 2019 YFE0117600), National Natural Science Foundation of China (52078018).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available in the article.  
**Conflicts of Interest:** Authors [PERSON] and [PERSON] were employed by the company Gansu Henglu Transportation Survey and Design Institute Co., Ltd. Author [PERSON] was employed by the company Gansu Provincial Highway Development Group Co., Ltd. Author [PERSON] was employed by the company Gansu Hengtong Road and Bridge Engineering Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.