## 3 UV Aging Law of Asphalt
### Analysis of Test Results  
#### 3.2.1 Penetration, Softening Point and Ductility  
Penetration is an important index to reflect the viscosity of asphalt. It is generally considered that with the deepening of aging degree, the penetration of both base asphalt and modified asphalt decreases in varying degrees, and the reduction of penetration is usually considered as an effective index to reflect the aging degree of asphalt. Figure 4 shows the asphalt penetration (25 \({}^{\circ}\)C) at the UV irradiation intensity of 157.5 W/m\({}^{2}\) under different UV irradiation times. Figure 5 shows the penetration loss rate of asphalt under different UV irradiation times. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Technical Parameters**} & \multicolumn{6}{c}{**Asphalt**} \\ \cline{2-8}  & **50\({}^{\circ}\)** & **70\({}^{\circ}\)** & **90\({}^{\circ}\)** & **110\({}^{\circ}\)** & **SBS I-C** & **SBS I-D** \\ \hline Penetration at 25 \({}^{\circ}\)C, 0.1 mm & 53.8 & 68.5 & 91.7 & 109.2 & 72.7 & 55.1 \\ \hline Softening point, \({}^{\circ}\)C & 50.3 & 47.4 & 46.3 & 43.8 & 66.3 & 71.5 \\ \hline Ductility at 10 \({}^{\circ}\)C, cm & 18.7 & 27.5 & 34.6 & 46.9 & — & — \\ \hline Ductility at 5 \({}^{\circ}\)C, cm & — & — & — & — & 35.3 & 27.8 \\ \hline \multirow{4}{*}{RTFOT 1} & Mass Change, \% & 0.5 & 0.45 & 0.67 & 0.71 & 0.87 & 0.73 \\  & Residual penetration ratio at 25 \({}^{\circ}\)C, \% & 71.1 & 68.9 & 66.5 & 62.3 & 75.8 & 71.3 \\ \cline{1-1}  & Residual ductility at 10 \({}^{\circ}\)C, cm & 7.6 & 9.1 & 10.2 & 13.4 & — & — \\ \cline{1-1}  & Residual ductility at 5 \({}^{\circ}\)C, cm & — & — & — & 25.3 & 19.7 \\ \hline \hline \end{tabular}  
* RTFOT: Rolling Thin Film Oven Test.  
\end{table} 
Table 1: Technical parameters of asphalt used in this paper. Figure 4: The penetration of asphalt with different UV irradiation time. With the extension of UV irradiation time, the penetration of asphalt obviously decreased. From the point of decline, the lower the asphalt grade, the greater the decrease in penetration, but the absolute value of decrease was not very high. The decrease in penetration of SBS modified asphalt was obviously smaller than that of base asphalt, which shows that the anti-UV performance of SBS modified asphalt was better than that of base asphalt. In addition, the decrease in penetration of SBS I-D was lower than that of SBS I-C.  
Generally, 87.21% of the temperature interval between the dropping point and the hardening point was taken as the softening point. Soft point is generally considered as an indicator of the thermal stability of asphalt materials, and also a measure of the conditional viscosity of asphalt. Relevant data show that UV radiation can increase the softening point of asphalt [7; 8; 9]. In this paper, the ring and ball method was used to determine the softening point. Figure 6 shows the asphalt softening point at the UV irradiation intensity of 157.5 W/m\({}^{2}\) under different UV irradiation times. Figure 7 shows the softening point loss rate of asphalt under different UV irradiation times. Figure 5: The penetration loss rate of asphalt with different UV irradiation time. Figure 6: The softening point of asphalt with different UV irradiation time. With the extension of UV irradiation time, the softening point of asphalt increased gradually. With the gradual increase in asphalt grade, the increase in softening point was larger, but the absolute value of increase was relatively small. The absolute decrease in the softening point of SBS modified asphalt was obviously smaller than that of base asphalt, which indicated that the anti-UV performance of SBS modified asphalt was better than that of base asphalt, and the increase in softening point of SBS I-D was lower than that of SBS I-C.