## 3 Results and Discussion
### Test Result of Superpave Method for SBS Modified Asphal
#### 3.2.2 Brookfield Rotary Viscosity Test  
Another index to measure the performance of asphalt binder is the viscosity. The smaller the viscosity of asphalt at high temperature, the better the asphalt mixture can be mixed and compacted [18]. Therefore, the SHRP method requires the rotary viscosity at 135 \({}^{\circ}\)C shall not exceed 3 Pa.s. The Brookfield rotary viscometer was adopted in this paper to determine the rotational viscosity of SBS modified asphalt. In the test, the rotor speed was 20 rpm/min, the rotor of SBS modified asphalt was 27%, and the mass was 10.5 g, the mass of 90# matrix asphalt sample was 8.5 g using 21# rotor. The test temperature were 135 \({}^{\circ}\)C, 140 \({}^{\circ}\)C, 150 \({}^{\circ}\)C, 160 \({}^{\circ}\)C, 170 \({}^{\circ}\)C, 177 \({}^{\circ}\)C and 190 \({}^{\circ}\)C respectively, and the dosages of SBS modifier were 0%, 4.5%, 5%, 5.5% and 6%. The test results are shown in Figure 5.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **0\%** & **4.5\%** & **5\%** & **5.5\%** & **6\%** \\ \hline
0\% & – & S & S & S & S \\
4.5\% & & – & S & S & S \\
5\% & & & – & S & S \\
5.5\% & & & – & S & S \\
6\% & & & – & N & \\ \hline \hline \end{tabular} Note: S = Significant; N = Non-significant.  
\end{table}
Table 6: Difference analysis of G*/sin \(\delta\) at different SBS modifier dosages.  
Figure 5: \(Cont\).  
From Figure 5, the addition of SBS modifier could significantly improve the rotary viscosity of asphalt binder. Compared to the rotary viscosity at 135\({}^{\circ}\)C of matrix asphalt, when the SBS modifier dosages were 4.5%, 5%, 5.5% and 6% respectively, the rotary viscosity of asphalt binder increased by 2.42 times, 9.14 times, 16.96 times and 22.31 times respectively. And with the increase of SBS modifier dosage, the rotary viscosity of asphalt binder increased significantly. In general, the viscosity-temperature curve was used to characterize the relations of viscosity and temperature of SBS modified asphalt, as shown in Equation (1) [19].  
\[\log\log(\eta)=n-VTS\log(T) \tag{1}\]  
Which, \(\eta\) is the asphalt viscosity (cPa-s); \(T\) is the test temperature; \(n\) is the regression coefficient; VTS is represents the temperature sensitivity of asphalt binder.  
The viscosity of SBS modified asphalt with different SBS modifier dosage was fitted by Equation (1), and the relations of viscosity and temperature was determined. The results are shown in Table 7.  
From Table 7, the fitting coefficient R\({}^{2}\) of viscosity-temperature curve of SBS modified asphalt were all greater than 0.99, which meant that the viscosity-temperature relationship of SBS modified asphalt could be characterized better by Refutas curve. The VTS represent the temperature sensitivity of SBS modified asphalt, and the bigger value of VTS, the more distinct temperature sensitivity [20]. From Table 7, the VTS value decreased with the increase of SBS modifier dosage, which meant that the SBS modifier could significant lower the temperature sensitivity of asphalt binder.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**SBS Modifier Dosage** & **N** & **VTS** & **R\({}^{2}\)** \\ \hline
4.5\% & 3.426 & 0.869 & 0.997 \\
5\% & 3.351 & 0.743 & 0.997 \\
5.5\% & 3.254 & 0.715 & 0.995 \\
6\% & 3.139 & 0.637 & 0.993 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Fitting result of viscosity-temperature curve for SBS modified asphalt.  
Figure 5: Rotary viscosity of asphalt binder.