## 3 Results and Discussion
### Effect of Different Production Methods on AR  
Conventional performance tests were conducted on asphalt rubber using different stirring methods. The test results of 25 \({}^{\circ}\)C penetration, softening point, 5 \({}^{\circ}\)C ductility, 25 \({}^{\circ}\)C elastic recovery and 180 \({}^{\circ}\)C viscosity are all shown in Figure 6. It can be seen from Figure 6a,b that a dosage of 20% was a turning point. When the CR content was less than 20%, the AR penetration of the two production methods decreased gradually from 59 and 55 to 53 and 52 cm, respectively. The AR produced by the high-speed shearing method had higher penetration than that of the heat fusion method. When the CR content was more than 20%, the trend changed, and the penetration increased gradually with the CR content increasing. Meanwhile, the AR produced by the heat fusion method had higher penetration and a faster recovery speed. For the softening point, AR had a much higher softening point than that of the asphalt and increased remarkably with increasing the CR content. For the two different methods, the heat fusion method showed a higher softening point, which brought a good high temperature performance to the AR. It can be seen from Figure 6c that the change of ductility was not apparent with the rubber powder content increasing. Almost all the values of ductility were less than 10 cm. When the rubber powder content was less than 25%, the asphalt rubber produced by the shearing method had larger ductility values, and the trend changed only when the content reached 25%. If the rubber powder content was less than 25%, the shearing method was a better choice for obtaining good low temperature properties. According to the test data, the hot melt method was better than the shearing method when the content was higher than 25%. However, good ductility came at the expense of high temperature performance, which led to asphalt aging and influenced the asphalt properties. Overall, the shearing method would be the first choice for AR in the road construction. Based on Figure 6d, the heat fusion method received better elastic recovery with the content less than 15%, which reflected better swelling of rubber powder. Nevertheless, with the content higher than 15%, the shearing method showed its superiority and elastic recovery increased obviously. A similar problem with ductility occurred at 25% content, the elastic recovery with the hot melt method was better than that of the shearing method. It required much higher temperature to swell the rubber powders, which lead to asphalt aging. The test of viscosity highlighted the difference between the two methods. From Figure 6e, the viscosity values with the heat fusion method were always larger than that with the shearing method. Higher viscosity increases the difficulty of road construction and also affects its quality and progress. When the rubber powder content reached 25%, the AR with the heat fusion method had both better elasticity and low temperature property, but the fluidity of AR became very poor. To conclude, it is possible to improve the elasticity and high temperature property of AR with the heat fusion method. However, the shearing method can further make the rubber particles finer and give better melt reaction with the asphalt, which is more favorable for the large-scale engineering application of AR. SEM Test of AR  
In order to characterize the micromorphology and rheological properties of asphalt rubber in the previous test results, SK90# asphalt with crumb rubber content of 20% was selected. Scanning electron microscopy and rheological properties testing were conducted. The SEM test was conducted for virgin asphalt and AR, and the results are shown in Figure 6(a),b, respectively. It could be clearly seen that the surface of virgin asphalt was very smooth. The asphalt appearance became uneven after mixing it with rubber powder. The rubber powder was completely covered by asphalt.