## 4 Conclusions and Recommendations  
This study investigated the effects of the three aging and rejuvenation cycles with three types of rejuvenators designated as Type I, II, and III with different dosages (6%, 10%, and 14%, respectively). The rheological (thermal, rutting, and fatigue resistance), chemical, and morphological characteristics of SBSMA were evaluated using the DSR, LAS, FTIR, and AFM tests. From this study, the following conclusions were drawn:  
* The superiority ranking for the rutting factor of the repetitively aged asphalt-binders and fatigue factor of the repetitively rejuvenated asphalt-binders was Type II > Type I > Type III, respectively. Multiple rejuvenation cycles reduced the percentage recovery of the SBSMA with varying rejuvenators and made the asphalt-binder depend more sensitively on the strain levels. The fatigue life of the rejuvenated SBSMA with the three rejuvenators increased at low stress level and decreased at high strain level as the rejuvenation cycles were increased. Considering the high- and intermediate-temperature performance, the Type I rejuvenator exhibited a satisfactory rejuvenation effect. * From FTIR analysis, the carbonyl and sulfoxide groups of SBSMA increased whilst the chain segments of butadiene decreased after repeated aging and rejuvenation cycles. The addition of the rejuvenator could not restore the chemical aging indices to the same level as the RTFO residue. During the multiple aging and rejuvenation cycles, the aging resistance of SBSMA gradually decreased. This inherently limited the expected rejuvenation effects under the repeated rejuvenation cycles. * From AFM morphological analysis, PAV aging decreased the quantity of the beelike structures and the surface roughness of the asphalt-binders. The rejuvenators significantly changed the microscopic morphology and phase dispersion of the aged asphalt-binders. The three rejuvenators showed different rejuvenation effects, but generally increased the morphological undulations and surface roughness. Overall, plausible results were obtained in this laboratory study with promising potential for using rejuvenators to improve the rheological properties of asphalt-binders. The study has also shown that rejuvenators have the potential of achieving satisfactory repeated aging and rejuvenation in terms of performance characteristics (rutting, thermal, and fatigue cracking resistances) and morphological features of SBSMA. Nonetheless, the conclusions drawn are not exhaustive nor exclusive as the test results presented herein pertain to the specific materials evaluated in this study. Future research should include a wide array of different materials including high-activity rejuvenators to further supplement this study's findings including field application trials and validation. Conceptualization, X.C. and Y.N. ; methodology, Y.N. and J.W. ; validation, X.Z. and Y.G. ; formal analysis, Y.N. and W.W.; investigation, X.Z. ; resources, X.C. and R.Z. ; writing--original draft preparation, Y.N. and J.W. ; writing--review and editing, X.C. and X.Z. ; visualization, Y.N. ; supervision, X.C. and J.T. ; funding acquisition, Y.G. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Science Foundation of China (Grant No. 51778140 and No. 52078130), Suzhou Transport Investment, Planning Design and Management Co., Ltd. (Grant No. 8521008637), and Kunshan Transportation Development Holding Group Co., Ltd. (Grant No. 8521002599 and No. 8521008420). Not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article. The authors would like to acknowledge sincerely the financial support provided by the National Science Foundation of China (Grant No. 51778140 and No. 52078130), Suzhou Transport Investment, Planning Design and Management Co., Ltd. (Grant No. 8521008637), and Kunshan Transportation Development Holding Group Co., Ltd. (Grant No. 8521002599 and No. 8521008420)for this study.