## 4 Results and Analysis
### Results of Low-Temperature Bending Test  
According to the low-temperature bending test method, three-point bending tests were carried out on NHMA, 40%HMA, 40%RHMA, 40%RHMA-1C, 40%RHMA-1.5C and 40%RHMA-2.5C. The results are shown in Table 6. It can be seen that 40%HMA had greater flexural tensile strength than NHMA. This is because the old asphalt binder was harder and had a smaller penetration than the new asphalt binder, resulting in the higher strength of 40%HMA. After adding waste engine oil regenerant to 40%HMA, the flexural tensile strength became smaller, even smaller than NHMA. After adding carbon nanotubes into 40%RHMA, the flexural tensile strength increased, which was close to that of NHMA. Moreover, the flexural tensile strength of 40%RHMA increased with the increasing content of carbon nanotubes.  
Because of the poor elongation of the old asphalt binder, brittle fractures occurred easily. The deformation capacity of 40%HMA was poor, and the maximum flexural strain was much smaller than that of NHMA. After the addition of the waste engine oil regenerant, the maximum flexure strain of 40%RHMA rose, but there was still a big difference between it and NHMA. The maximum flexural tensile strain required in the specifications should be no less than 2000 \(\mu\varepsilon\) for common areas and no less than 2600 \(\mu\varepsilon\) for some cold areas. Obviously, the performance of 40%RHMA would not be up to standard in cold regions. The \(\varepsilon_{B}\) increased significantly after the addition of carbon nanotubes to 40%RHMA, reaching 3018 \(\mu\varepsilon\).  
The bending stiffness modulus reflects the ability of an asphalt mixture to resist bending deformation within the elastic limit. The greater the modulus of bending stiffness, the smaller the ability of an asphalt mixture to resist bending deformation within the elastic limit. Therefore, a larger bending strain of an asphalt mixture corresponds to smaller the flexural tensile strength and bending stiffness modulus, as well as better low-temperature cracking resistance. The bending stiffness modulus of 40%HMA was the largest, indicating that RAP can greatly reduce the low-temperature cracking resistance of recycled asphalt mixtures. The loss of low-temperature cracking resistance caused by the addition of RAP was recovered by adding waste engine oil regenerant. This is because the waste engine oil regenerant restored part of the performance of the old asphalt binder in RAP and enhanced the flow performance of the old asphalt binder. In this way, brittle damage did not easily occur in 40%RHMA, but at the same time, the waste engine oil reduced the strength of the recycled asphalt mixture, resulting in the low-temperature crack resistance being improved, but it still could not reach the level of NHMA. With the addition of carbon nanotubes to 40%RHMA, the bending stiffness modulus decreased first and then increased with the increase in carbon nanotubes. This means the low-temperature cracking resistance increased first and then decreased. Due to the excellent viscoelastic properties of 40%RHMA modified by carbon nanotubes, the asphalt mixture was less prone to cracking. However, with the increase in the carbon nanotube content, the increasing hardness hindered the elastic deformation of 40%RHMA. 40%RHMA-1.5C had excellent viscoelastic properties and low hardness. Therefore, 40%RHMA-1.5C had the smallest bending stiffness modulus and the best crack resistance at low temperature, even better than NHMA.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\mathbf{R_{B}}\) **(MPa)** & \(\varepsilon_{B}\) (\(\mu\varepsilon\)) & \(\mathbf{S_{B}}\) **(MPa)** \\ \hline NHMA & 9.13 & 2850 & 3203.51 \\
40\%HMA & 12.16 & 2234 & 5443.15 \\
40\%RHMA & 8.78 & 2346 & 3742.54 \\
40\%RHMA-1C & 9.01 & 2554 & 3527.8 \\
40\%RHMA-1.5C & 9.23 & 2882 & 3202.64 \\
40\%RHMA-2.5C & 10.34 & 3018 & 3426.11 \\ Specification & - & \(\geq\)2000 & - \\ \hline \hline \end{tabular}
\end{table}
Table 6: Results of low-temperature bending test.