## 3 Results and Discussion
### Low-Temperature Properties  
In this study, BBR test temperatures of \(-12\)\({}^{\circ}\)C, \(-18\)\({}^{\circ}\)C, and \(-24\)\({}^{\circ}\)C were set and S-values and m-values of all GF-WTB/SBS composite-modified binders are in Tables 2 and 3. Tables 2 and 3 present the average of the measurement results. Three BBR specimens of each asphalt were measured to ensure reliable test results. The maximum coefficient of variation of each asphalt sample group in the BBR test was 5.0%. Table 2 shows under \(-12\)\({}^{\circ}\)C and 60 s loading conditions, with S-values and m-values of four SBS2 + WTB binders and four SBS3 + WTB binders all satisfying S \(\leq\) 300 MPa and m \(\geq\) 0.3 SHRP specification demands, respectively, and meeting performance-grade PG-22. In addition, for SBS3 + WTB1 and SBS3 + WTB4, the performance grades also reached PG-28 demand. Table 3 shows four SBS4 + WTB binders and four SBS5 + WTB binders that can meet performance-grade PG-34. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**Temp**} & \multirow{2}{*}{**Index of BBR**} & \multicolumn{8}{c}{**Code of the Asphalt Binder**} \\ \cline{3-10}  & & **SBS2 +** & **SBS2 +** & **SBS2 +** & **SBS2 +** & **SBS3 +** & **SBS3 +** & **SBS3 +** & **SBS3 +** \\ \multirow{2}{*}{(\({}^{\circ}\)C)} & & **WTB0** & **WTB1** & **WTB2** & **WTB4** & **WTB0** & **WTB1** & **WTB2** & **WTB4** \\ \hline \multirow{3}{*}{\(-12\)} & S (MPa) & 175 & 142 & 160 & 121 & 121 & 112 & 106 & 115 \\  & m & 0.352 & 0.315 & 0.385 & 0.309 & 0.312 & 0.306 & 0.294 & 0.321 \\ \multirow{3}{*}{\(-18\)} & k (\(\times 10^{-4}\) MPa\({}^{-1}\)) & 20.11 & 22.18 & 24.06 & 25.54 & 25.79 & 27.32 & 27.74 & 27.91 \\  & S (MPa) & 368 & 303 & 208 & 209 & 187 & 198 & 171 & 167 \\ \multirow{3}{*}{\(-18\)} & m & 0.312 & 0.307 & 0.252 & 0.261 & 0.279 & 0.301 & 0.292 & 0.305 \\  & k (\(\times 10^{-4}\) MPa\({}^{-1}\)) & 8.48 & 10.13 & 12.12 & 12.49 & 14.92 & 15.20 & 17.07 & 18.26 \\ \multirow{3}{*}{\(-24\)} & S (MPa) & 661 & 615 & 594 & 375 & 391 & 340 & 302 & 281 \\  & m & 0.240 & 0.257 & 0.250 & 0.259 & 0.273 & 0.265 & 0.290 & 0.295 \\ \multirow{3}{*}{\(-24\)} & k (\(\times 10^{-4}\) MPa\({}^{-1}\)) & 3.63 & 4.18 & 4.21 & 6.91 & 6.98 & 7.79 & 9.60 & 10.50 \\ \multirow{3}{*}{\(\text{ Performance grade}\)} & PG-22 & PG-22 & PG-22 & PG-22 & PG-22 & PG-28 & PG-22 & PG-28 \\ \end{tabular}
\end{table} 
Table 2: BBR test results of SBS2 + WTBx and SBS3 + WTBx. Figure 6: The rutting factor and high-temperature PG of GF-WTB/SBS composite-modified asphalt: (a) SBS2 + WTBx; (b) SBS3 + WTBx; (c) SBS4 + WTBx; (d) SBS5 + WTBx. Relative research shows that when the low-temperature grade of PG classification is set every 6 \({}^{\circ}\)C, although different asphalts under the same PG low-temperature grade have the same classification standard, the difference in low-temperature performance between them cannot be reflected [30; 31]. Based on the S and m indexes, a new low-temperature evaluation index k = m/S is proposed by combining the two together; that is, the creep rate of asphalt under the unit creep stiffness. It can be seen from the representation that the larger the k index, meaning small creep stiffness and large stiffness change rate, the better the low-temperature crack resistance of asphalt. The index k-values under \(-\)12 \({}^{\circ}\)C, \(-\)18 \({}^{\circ}\)C, or \(-\)24 \({}^{\circ}\)C vs. GF-WTB contents of certain SBS dosage-modified asphalt are shown in Figure 7 and the k-values data are in Tables 2 and 3. Figure 7 shows that k-values decrease with the test temperature decreasing for all the asphalt samples and k-value increases evidently with the SBS dosage increasing. Under the same SBS dosage, GF-WTB content elevates the k-value and the low-temperature crack resistance of SBS-modified asphalt.