## 3 Experimental Design of Composite-Modified Asphal
### Determination of Compound Modifier Dosing
#### 3.3.2 Ductility and Brookfield Viscosity  
Ductility characterizes the plasticity of asphalt, and the ductility test is commonly used in China to evaluate its low-temperature performance. Asphalt viscosity reflects its shear deformation resistance under high-temperature. However, it should be noted that excessively high viscosity may increase mixing and construction difficulties despite indicating stronger shear deformation resistance. Therefore, the asphalt viscosity should be controlled within a reasonable range. In accordance with the technical requirements of \"Highway Engineering Asphalt and Asphalt Mixture Test Procedure\", prepared asphalt was tested for 5 \({}^{\circ}\)C ductility and 180 \({}^{\circ}\)C Brookfield viscosity [23]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rubber Powder (\%)** & **0** & **3** & **6** & **9** & **12** \\ \hline
20 & 54.3 & 44.2 & 38.4 & 33.4 & 29.6 \\ \hline
25 & 45.3 & 37.7 & 33.2 & 29.3 & 26.4 \\ \hline
30 & 40.5 & 33.8 & 30.6 & 27.5 & 25.2 \\ \hline
0 & — & & 60.2 & — & 55.1 \\ \hline
70\# & & & 68.3 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Test results of penetration (0.1 mm). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rubber Powder (\%)** & **0** & **3** & **6** & **9** & **12** \\ \hline
20 & 65.5 & 72.5 & 77.5 & 82.0 & 84.0 \\ \hline
25 & 75.5 & 81.5 & 86.0 & 87.5 & 88.5 \\ \hline
30 & 80.5 & 85.5 & 88.5 & 89.5 & 90.0 \\ \hline
0 & — & & 50.5 & — & 54.5 \\ \hline
70\# & & & 44.0 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Test results of softening points (\({}^{\circ}\)C). The results of the ductility test are presented in Table 8. It is evident that the addition of rock asphalt to the matrix asphalt significantly reduces its ductility, with a greater reduction observed as the mass of rock asphalt added increases. When compared to 70# asphalt, the asphalt modified with 12% rock asphalt becomes brittle and fractures at an early stage during tensile testing. This suggests that rock asphalt has a pronounced weakening effect on low-temperature properties. Additionally, compared to 70# asphalt, the addition of 20%, 25%, and 30% rubber powder-modified asphalt increases ductility by 150%, 202%, and 275%, respectively. This indicates that rubber powder modification significantly improves the low-temperature performance of matrix asphalt. As the mass of rubber powder increases, the ductility of rock asphalt gradually increases while maintaining a content of 6%. This suggests that rubber powder can compensate for weakened low-temperature performance in rock asphalt. To produce high-modulus asphalt with adequate ductility in low-temperature environments, it is recommended that the ductility should exceed 5 cm at 5 \({}^{\circ}\)C and a minimum rubber powder content of 20% be used. In addition to the same factors that contribute to decreased penetration and increased softening point, the analysis of rock asphalt must also consider its physical premixing with rubber powder and matrix asphalt. In rubber powder-modified asphalt, the distribution of rock asphalt particles is relatively independent; however, stress concentration and early fracture may occur on the surface of rock asphalt particles at low temperatures. The results of the viscosity test for modified asphalt are presented in Table 9. It is evident from the table that adding rubber powder and rock asphalt can significantly enhance the viscosity of asphalt. Maintaining a 6% admixture of rock asphalt and increasing the content of rubber powder from 20% to 30% resulted in a viscosity increase of 0.7 Pa\(\cdot\)s. Similarly, maintaining the rubber powder content at 25% and increasing the rock asphalt content from 0% to 12% led to a viscosity increase of 1 Pa\(\cdot\)s. These indicate a positive contribution of rock asphalt and mastic powder to improving asphalt viscosity.