## 3 Results and Discussion
### Rheological Properties of PPA/SBS/SBR Composite-Modified Asphalt Cold Replacement Solution  
#### 3.3.1 High-Temperature Rheological Properties  
The complex shear modulus (G*) and phase angle (\(\delta\)) were obtained through DSR to reflect the viscoelastic properties of the asphalt. A higher G* value indicates better deformation resistance of the asphalt at high temperatures [22]. \(\delta\) indicates changes in the viscous and elastic parts of the asphalt materials. Because asphalt is an elastoplastic, a higher \(\delta\) value indicates it is more viscous and less elastic. The rutting factor (G*/sin\(\delta\)) is used to evaluate the high-temperature rutting resistance of asphalt, where a larger value indicates greater high-temperature rutting resistance [23]. Temperature scanning tests were conducted on H-ACRL, L-ACRL, and base asphalt using DSR. The modifiers, surfactants, diluents, and reinforcing agent content were different in H-ACRL and L-ACRL. The results of different asphalt temperature scanning tests are shown in Figure 5.  
Figure 5a and c show the G* values and G*/sin\(\delta\) of the three types of asphalt (\(\delta\)), all of which sharply decreased upon increasing the temperature, and then plateaued. This was because the asphalt gradually transitioned from a highly-elastic state to a viscous flow state upon increasing the temperature, resulting in a decrease in its deformation resistance. The G* and G*/sin\(\delta\) values of the two ACRLs at the same temperature were significantly higher than those of the base asphalt, indicating that the shear deformation resistance of H-ACRL and L-ACRL was significantly higher than that of the base asphalt. The addition of modifiers, surfactants, diluents, and reinforcing agent increased the elastic part and enhanced the deformation resistance of the base asphalt system. The G* and G*/sin\(\delta\) values of H-ACRL were greater than those of L-ACRL at the same temperature, indicating that the deformation resistance of H-ACRL was better than that of L-ACRL.  
Figure 5b shows that \(\delta\) increased with temperature, which was related to the transition of asphalt from a highly-elastic state to a viscous-flow state, as mentioned above. At the same temperature, both types of ACRL showed \(\delta\) values significantly lower than that of the base asphalt, indicating that the elastic recovery ability of the two ACRLs was significantly better than that of the base asphalt. The addition of modifiers, surfactants, diluents, and reinforcing agent endowed the asphalt with more elastic components, which improved its deformation recovery ability at high temperatures. Additionally, the \(\delta\) value of L-ACRL was less than H-ACRL at the same temperature, indicating that the contribution of elastic components in H-ACRL was higher, and its deformation recovery was better.  
Figure 4: Three major indicators after TFOT.  
Figure 5: Temperature scanning test results before TFOT.  
Figure 5(a)-c shows that after short-term aging, the complex shear modulus G* and rutting factor G*/sin\(\delta\) increased, while the phase angle \(\delta\) significantly decreased. These changes indicate that the high-temperature performance of the asphalt was significantly improved, mainly because the gel of polyphosphoric acid increases the content of green and medium-hard components. The gel effect of polyphosphoric acid on asphalt is not affected by the degree of asphalt aging. Under short-term aging, polyphosphoric acid-modified asphalt has a higher content of hard components. The increase in the content of hard components in asphalt enhances the elasticity of asphalt and improves high-temperature performance [24; 25; 26].