## 3 Results and Discussion
### Tensile Strength  
Table 4 and Figures 5 and 6 present the tensile strength results versus test temperature for AC-13 and SMA-13 under different degrees of aging. As shown in Figures 5 and 6, the tensile strength of both SMA-13 and AC-13 gradually increases with the increase in aging degree when test temperature remains the same. This phenomenon can be explained as follows: under a thermo-oxidative aging environment, light components can be transformed into asphaltene by the chemical reaction between the hydrocarbon (derivatives) in asphalt and oxygen, which undoubtedly increases the number of polar molecules in bitumen. A high temperature can also enhance the polarity of aggregate molecules [33]. It is worth mentioning that the adhesion between asphalt and aggregate is generated by the linkage of polar molecules. Therefore, it follows that the adhesion between bitumen and aggregate can be strengthened and the load-bearing capacity of the asphalt mixture can be enhanced after thermo oxidative aging. Figure 4: Stress–strain plots of AC-13 and SMA-13 under different aging degrees, plotted at 40 \({}^{\circ}\)C.  
Figure 3: Stress–strain plots of AC-13 and SMA-13 under different aging degrees, plotted at 30 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Condition**} & **Data** & **Parameter** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** \\ \cline{2-16}  & **Clean** & **Parameter** & **Clean** & **Parameter** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** \\ \cline{2-16}  & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** & **Clean** \\ \hline \hline \multirow{4}{*}{**ACC-10**} & **10  
tensile strength of SMA-13 fluctuates, but it is worth noting that the LTOA8d condition exhibits the most remarkable elevation effect by a growth gradient of 20% (10 \({}^{\circ}\)C) and 13% (20 \({}^{\circ}\)C). At 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C, while the STOA condition presents the most obvious increasing effect on the tensile strength of SMA-13 by a growth gradient of 20% (30 \({}^{\circ}\)C) and 13% (40 \({}^{\circ}\)C). The growth gradient declines with the increase in aging degree at 30 \({}^{\circ}\)C; however, it fluctuates at 40 \({}^{\circ}\)C.  
With the elevation in test temperature, the tensile strength at each degree of aging shows a decreasing trend for both AC-13 and SMA-13. The cause of the phenomenon may be illustrated as follows: the increase in test temperature can intensify the periodic thermal movement of macromolecules inside the asphalt mixture, thus enlarging the distance between the macromolecules, weakening the mutual attraction between the macromolecules, and eventually reducing the load required to destroy the structure of the asphalt mixture [34]. For AC-13 under LTOA2d, LTOA5d and LTOA8d conditions, tensile strength decreases approximately linearly, with an average decline rate of 24% with every increase of 10 \({}^{\circ}\)C. Comparatively, for AC-13 under STOA conditions, tensile strength decreases unsteadily by 12% (from 10 \({}^{\circ}\)C to 20 \({}^{\circ}\)C), 30% (from 20 \({}^{\circ}\)C to 30 \({}^{\circ}\)C) and 21% (from 30 \({}^{\circ}\)C to 40 \({}^{\circ}\)C).