## 3 Test Results, Analyses, and Discussion
### FTIR Analysis  
To quantitatively investigate the influence of multiple aging and rejuvenation cycles on the SBSMA chemical structure and the vibration of the functional groups under repeated aging and rejuvenation, the 600-2000 cm\({}^{-1}\) functional group peak area was selected as a reference to calculate the butadiene (CH=CH), carbonyl (C=O), and sulfoxide (S=O) relative contents. This helped to eliminate the influence of the thickness and concentration of the asphalt-binders. The chemical aging indices, namely butadiene index (BI), carbonyl index (CI), and sulfoxide index (SI), were determined using Equations (1)-(3) [38]:  
\[\text{CI}\ =\ \frac{\text{Area of the carbonyl centered around 1700 cm${}^{-1}$}}{\text{Area of spectral bands between 2000 and 600 cm${}^{-1}$}} \tag{1}\]  
\[\text{SI}\ =\ \frac{\text{Area of the sulfoxide centered around 1030 cm${}^{-1}$}}{\text{Area of spectral bands between 2000 and 600 cm${}^{-1}$}} \tag{2}\]  
\[\text{BI}\ =\ \frac{\text{Area of the butadiene centered around 966 cm${}^{-1}$}}{\text{Area of spectral bands between 2000 and 600 cm${}^{-1}$}} \tag{3}\]  
Figure 10 gives more intuitive information about the changes in the functional groups. As the aging cycles increased, repetitively aged asphalt-binders showed an obviously increasing trend in the CI value. The indistinctive increasing and decreasing trends in the SI and BI values, respectively, are obvious. In the aging process of SBSMA, due to oxidation, dehydrogenation, and cross-linking reactions of the asphalt-binder and SBS polymer, degradation occurred simultaneously, and the spatial network structure was destroyed [39,40]. By comparing the aged and rejuvenated asphalt-binders, it can be seen that the rejuvenating effect was obvious in each aging and rejuvenation cycle, and that the decrease in the BI, CI, and SI values were significant. Figure 9: LAS fatigue life results: (**a**) rejuvenated asphalt-binders; (**b**) aged asphalt-binders. Based on the theory of compositional harmonic on rejuvenation mechanisms [41], the rejuvenators rich in aromatics are added to the aged asphalt-binder to dilute the aged asphalt-binders, enhance the proportion of maltenes, and harmonize the different fractions in the asphalt-binder. This was not simply the reversal of the transformation from aromatics to the aged asphalt-binders. Figure 10: Chemical aging indices of repetitively aged and rejuvenated SBSMA: (**a**) AG\(-\)CI; (**b**) RE\(-\)CI; (**c**) AG\(-\)SI; (**d**) RE\(-\)SI; (**e**) AG\(-\)BI; (**f**) RE\(-\)BI. into asphaltenes during the aging process, expressing that the aromatics were consumed and asphaltenes increased. However, as the dosage of rejuvenators increased in each cycle, the CI and SI values of the rejuvenated asphalt-binders also increased, whilst the value of BI decreased during the repeated aging and rejuvenation cycles. In other words, the aging of the rejuvenated asphalt-binder was faster than the asphalt-binder after RTFO aging. This might be explained by the decrease of the aging resistance due to the oxidative degradation of SBS polymer in the multiple aging process [42]. Meanwhile, the oxidation and breaking of polybutadiene (PB) segments of SBS would produce more oxygen-containing groups such as -OH, C=O, -COOH, etc. [43, 44]. Hence, the increased aromatic hydrocarbon oil might have also led to the acceleration of the asphalt-binder aging. The addition of the rejuvenator indicated the potential to improve the rheological properties of the aged asphalt-binders but could not restore the chemical aging indices to the same level as the RTFO residue. As a result, the rejuvenating effect was not significant enough to achieve an appreciable repeated rejuvenation. With an increase in the rejuvenation cycles, the CI value decreased, especially for the Type III rejuvenator. This indicated that all the three types of rejuvenators had certain rejuvenating effects on the aged asphalt-binders, with the Type III rejuvenator being more impactful.