## 3 Results and Discussion  
### Effect of Different CR Content on the Performance of AR  
#### 3.1.1 90# Matrix Asphalt  
In order to clarify the influence of different CR proportions on the asphalt rubber performance, five contents of CR (15%, 17.5%, 20%, 22.5% and 25%) were selected for comparison tests with SK90# matrix asphalt. All the AR samples were then produced with a heat fusion method. The basic physical performances of all samples were obtained through laboratory tests (Table 6). The results from Table 6 are graphically reproduced in Figure 3.  
As shown in Figure 3a, when the CR content increased from 15% to 20%, the penetration degree gradually decreased as the rubber powder absorbed the oil in the asphalt and indirectly changed the colloidal structure. The rubber powder was evenly distributed in the asphalt after swelling, and so were the micelles gradually changing from the free phase to the double continuous phase, and then from the double continuous phase to the rubber powder continuous phase and the free asphalt discontinuous phase [32]. This process resulted in a decrease of the asphalt rubber viscosity and penetration. As the amount of rubber powder increased from 20% to 25%, the rubber powder began to desulfurize and degrade, the long-chain structure formed was destroyed, and the asphalt became soft, resulting in gradual increase of the penetration degree. Therefore, the penetration of the AR, its flexibility and low-temperature performance can be significantly improved by increasing its CR content in the test blending range. As shown on Figure 3b, the softening point curve of AR increased with the increment of CR blending ratio, but overall, the softening point did not change significantly. In the range of 15-20%, the change of softening point was not obvious. In the range of 20-25%, the penetration curve showed a rapid growth trend with the increment of CR content. This was mainly because the rubber powder absorbed the light components in the asphalt and the cross-linking effect after the rubber powder particles swell [33]. Therefore, in the test blending range, the increment in CR content did not significantly change the softening point of the AR, however, it slightly increased the viscosity of the AR, which, in its turn improved the high temperature resistance to permanent deformation (to some extent only). As shown on Figure 3c, the ductility curve of AR increased with the increment of CR content. When the CR content was in the range of 15-25%, the ductility curve of AR continued to rise and the ductility index increased by nearly 80%. This was because the rubber molecular segment in the asphalt rubber was gradually increasing, which enhanced the low temperature flexibility of the asphalt, and the cross-linking, affected between the rubber powder in the AR, was gradually enhancing. Therefore, it can be concluded that the low temperature ductility will increase when the rubber powder content increases. \begin{table}
\begin{tabular}{c c c c c c} \hline
**ProportionProperties** & **Penetration (0.1 mm)** & **Softening Point** & **Ductility (\({}^{\circ}\)C)** & **Elastic Recovery** & **180 \({}^{\circ}\)C Rotational** \\  & & **(\({}^{\circ}\)C)** & **(cm)** & **(\%)** & **Viscosity (Pa*s)** \\ \hline
15\% & 55.2 & 58.67 & 5.8 & 55.00 & 0.486 \\
17.5\% & 52.67 & 59.55 & 7.27 & 71.00 & 0.877 \\
20\% & 51.23 & 59.15 & 7.9 & 73 & 1.497 \\
22.5\% & 63.77 & 60.55 & 8.53 & 71.67 & 1.494 \\
25\% & 70.27 & 61.65 & 10.37 & 81.00 & 4.464 \\ Technical standard & 60–80 & \textgreater{}58 & \textgreater{}10 & \textgreater{}55 & 1–4 \\ \hline \end{tabular}
\end{table} 
Table 6: Basic physical properties of different asphalt rubber (AR). From Figure 3d, the viscosity curve of AR continued to rise with the increment of CR content. This would cause difficulties in handling the AR and aggregate and increase the operation time during construction.