## 1 Introduction  
With the development of China's social economy in recent years, the number of motor vehicles has increased significantly, resulting in a sharp increase in the number of used tires. China became the world's largest tire producer in 2006, and the tire production increased from 190 million units in 2003 to 816 million units in 2018 with more than 10 million tons of used tires are accumulated every year [1; 2; 3; 4; 5]. The large number of used tires has caused severe environmental and ecological safety problems, known as \"black pollution\", and resulted in the accumulation of large quantities of used tires in the suburbs of large cities [6; 7; 8; 9]. What is worse, not proper storage of the tires may lead to fires [10; 11; 12]. Recycling these tires can create economic benefits while addressing the environmental issues. The processing of used tires into rubber powder by mechanical means for asphalt modification or asphalt mixture modification is recognized as a harmless treatment method for used tires [13]. Since rubber and asphalt are both polymeric organic materials, and have a natural affinity, which means they are easy to react, thus, the use of waste rubber powder to modify asphalt (asphalt rubber) have a bright application prospect [14; 15; 16; 17; 18]. Asphalt rubber (AR) can significantly improve the performance of the road surface against rutting, high temperature performance, reflection cracks, water stability and fatigue cracking. In addition, AR also has the characteristics of reducing road maintenance, improving anti-sliding and reducing road noise [19; 20; 21; 22; 23]. [PERSON] [24] found that the mesh number of rubber powders could significantly affect the dispersion of rubber powder in asphalt during the initial miscibility process. In the later reaction process, the reaction temperature was the key influencing factor responsible for rubber powder swelling and desulfurization degradation. [PERSON] [25; 26; 27] established that temperature was the most important factor affecting the dissolution rate of rubber powder, and that the performance of asphalt changed with the influence of rubber powder in asphalt. [PERSON] [28] showed that the fatigue life of the asphalt rubber mixture increased with the crumb rubber content increasing, but only to a certain point before it started to decrease again, meaning that there was an optimal crumb rubber content. [PERSON] [29] proposed that the most ideal state of rubber powder distribution in asphalt rubber was that where the macroscopic dispersion was uniform, and the microadhesive powder and asphalt components absorbed and cross-linked each other. In addition, the poor compatibility caused the rubber powder particles to flocculate, granular and even caused delamination, which significantly affects the performance of asphalt rubber. [PERSON] [30] found that the asphalt rubber produced under different mixing modes had little difference except for the viscosity. When examining these mixing methods, the asphalt rubber produced by the high-speed shear method had the worst viscosity, while the asphalt rubber produced by the high-speed stirring method had better viscosity. When examining the different stirring temperatures, the indexes changed significantly, showing that higher or lower production temperature was not conducive to the performance of asphalt rubber. The AR performance produced at 180-200 \({}^{\circ}\)C provided the best results. Therefore, the production temperature, different crumb rubber content, the mesh number and the different production method all have a certain effect on the performance of the asphalt rubber. However, it is not clear how exactly these different factors affect the performance properties of AR, which in their turn have an effect on its wide range of use. Compared to the previous studies, most of the researchers focused on the performance improvement of AR with different additives. All the produced samples were in accordance with the related specifications, which made the preparation process and parameters not concerned.