## 5 Conclusions  
The molding process of a cement-stabilized macadam base directly affects its performance. In order to explore the influence of heavy compaction molding and vibration molding on the composition and structure of a CSM base at the meso level, compaction molding and vibration molding tests were simulated based on the three-dimensional discrete element method. Laboratory tests verified numerical simulation model results. CSM's structural changes were captured and analyzed during the molding process, and quantitative indexes of porosity and coordination number were proposed to evaluate the molding effects of the simulation tests. The main conclusions drawn are:  
1. The DEM numerical simulation of the cement-stabilized macadam forming test effectively simulated CSM's porosity and specimen height variation trends during the forming process.
2. The specimen height and porosity changes in the compaction molding process illustrate that the first 10 compactions had the most significant influence on the compaction effect. During the first 10 compactions specimen height and porosity fell fastest, accounting for approximately 80% of the compaction effect, indicating that real-world construction must pay attention to the initial compaction link.
3. The discrete element method simulation can explain the CSM forming process from the perspective of particle motion and interaction. The DEM simulation illustrates that during the initial compaction molding action, impact force was transmitted slowly from top to bottom and the compaction load's transmission speed increased as the mixture was continuously squeezed and compacted. During its initial state, the vibration molding simulation's force showed a bottom-to-top transmission, presented a trend of transfer from the top and bottom ends towards the middle at approximately 30 s, and finally reached an equilibrium state.
4. Particle distribution in the vibration molding specimen was more uniform, whereas particles in the compaction molding specimens showed a dense distribution at the top of the specimen and sparse distribution at the bottom. The porosity of the vibration molding specimen was 2.5% lower than that of the compaction molding specimen, which showed that vibration molding was more effective.
5. In the simulation experiments, the average coordination number of particles in the vibration molding specimen was 2.3% higher than that of the compaction molding specimen, implying that the vibration forming specimen had higher load-bearing capacity and stability.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Moding Method** & **Upper Porosity (\%)** & **Lower Porosity (\%)** \\ \hline Compaction molding & 8.23 & 8.36 \\ Vibration molding & 8.08 & 8.11 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Porosity difference between upper and lower parts of specimens.  
**Author Contributions:** Conceptualization, C.L.; Data curation, X.Y.; Formal analysis, F.L.; Investigation, F.L.; Methodology, H.Z.; Project administration, H.B.; Software, H.Z.; Supervision, H.B.; Validation, C.L.; Visualization, X.Y.; Writing--original draft, H.Z.; Writing--review & editing, C.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Transportation Science and Technology Program of Jilin Province (2018 ZDGC-9).  
**Acknowledgments:** We gratefully acknowledge the researchers of all reports cited herein and the financial support of the Science and Technology Project of the Jilin Provincial High Class Highway Construction Bureau.  
**Conflicts of Interest:** The authors declare no conflict of interest.