## 5 Conclusions  
This paper presents a new economical and effective method for the recovery and utilization of old asphalt binder. In this paper, the preparation method of a carbon-nanotube-modified, waste engine oil, recycled asphalt mixture was studied, and the road performance of the high-temperature stability, low-temperature cracking resistance and Marshall stability of a carbon-nanotube-modified, waste engine oil, recycled asphalt mixture was studied through a high-temperature rutting test, a low-temperature bending test and the Marshall stability test. By adding waste engine oil and carbon nanotubes to the old asphalt binder, the high-temperature rutting resistance, low-temperature bending resistance and Marshall stability of the waste-engine-oil-modified RAP could be greatly improved, and it could be restored to the level close to or even better than the new asphalt mixture. The conclusions are as follows:  
Carbon nanotubes can improve the high-temperature stability of waste-engine-oil-modified RAP. The results of the high-temperature rutting test show that RAP could increase the hardness of asphalt binder in recycled asphalt mixtures, improve the softening point and then increase the high-temperature performance of 40%HMA. The waste engine oil could restore some of the physical properties of the old asphalt binder, which reduced the high-temperature performance of 40%RHMA to close with that of NHMA. Carbon nanotubes play a role of \"reinforcing\" between waste engine oil regenerant and asphalt binder and effectively enhance the high-temperature stability of 40%RHMA. Among them, 40%RHMA-1.5C showed the greatest improvement in high-temperature rutting resistance. However, too many carbon nanotubes had adverse effects on the high-temperature stability of the waste-engine-oil-modified RAP. Carbon nanotubes can improve the low-temperature crack resistance of waste-engine-oil-modified RAP. The results of the low-temperature bending test show that RAP could \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\boldsymbol{MS_{1}}\) **(kN)** & \(\boldsymbol{MS_{2}}\) **(kN)** & \(\boldsymbol{MS}\) **(\%)** \\ \hline NHMA & 14.4 & 13.6 & 94.4 \\
40\%HMA & 17.53 & 12.8 & 73.2 \\
40\%RHMA & 11.83 & 10.3 & 87.1 \\
40\%RHMA-1C & 13.3 & 12.19 & 91.6 \\
40\%RHMA-1.5C & 14.63 & 13.85 & 94.4 \\
40\%RHMA-2.5C & 15.05 & 13.51 & 89.7 \\ Specification & - & - & \(\geq\)80 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of Marshall stability test. greatly reduce the low-temperature crack resistance of 40%HMA. The addition of waste engine oil regenerant could restore the fluidity of part of the old asphalt binder and make 40%RHMA not prone to brittle failure, thus improving its low-temperature cracking resistance. However, it still could not reach the level of NHMA. When carbon nanotubes were added into 40%RHMA, the low-temperature cracking resistance first increased and then decreased with the amount of carbon nanotubes. The 40%RHMA-1.5C sample had excellent high-temperature deformation resistance and low-temperature crack resistance, even better than NHMA. Carbon nanotubes could improve the Marshall stability of the waste-engine-oil-modified RAP. The addition of waste engine oil regenerant could significantly improve the Marshall stability of 40%HMA. However, there was still a gap between 40%RHMA and NHMA. After adding carbon nanotubes to 40%RHMA, the MS of 40%RHMA-1.5C was restored to the level of the new asphalt mixture. Data curation, D.Y. ; formal analysis, H.W. ; investigation, W.L. and H.Y. ; methodology, H.W. and Z.C. ; project administration, C.P. ; supervision, C.P. ; writing--original draft, C.P. and D.Y. ; writing--review and editing, Y.Z. All authors have read and agreed to the published version of the manuscript.