## 4 Conclusions  
The compaction characteristics of an OGFC-5 UPFC specimen designed using the MCM and simulated field cores were studied at multiple scales using laboratory compaction tests, DEM, and X-ray CT techniques. On the basis of the results, some conclusions were drawn, as follows:  
1. The compaction-test data demonstrate that the simulated field specimens with thicknesses of 15 and 20 mm barely achieved the target compaction thicknesses. All the UPFC specimens of different thicknesses failed to meet the AV requirements of the UPFC mixes designed via the MCM.
2. As the compaction thickness decreased, the values of parameters \(a\) and \(k\) also decreased. The initial compaction density of the UPFC specimen decreased, which resulted in compaction difficulties.
3. According to the results of the SGC test based on the DEM, the smaller the height of the virtual specimen, the earlier its displacement and stress reached a stable state. Moreover, as the model decreased in height, the D\({}_{\rm H}\) value; that is, the compacted thickness per unit height of the model, decreased, and the model was more difficult to compact.
4. DEM models with different thicknesses exhibited several force concentration regions. Compared to that of the 63.5 mm model, the maximum contact normal force of the 20 mm and 40 mm models increased by 266% and 800%, respectively. The decrease in the UPFC thickness strengthened the strong force chain in quantity and strength inside the specimen, resulting in difficulty in evenly diffusing and transferring pressure inside the specimen.
5. Based on the results of the UPFC specimens obtained using CT scanning, a small compaction thickness was conducive to uniform compaction. The AV distributions of the Marshall specimens were quite different in the vertical and horizontal directions from those of the UPFC specimens that simulated the field compaction. The MCM-designed UPFCs showed poor consistency with the field compaction.
6. According to the investigation results, the MCM-designed UPFC is not in compliance with on-site construction. In order to better align with actual construction and ensure the performance of the UPFC, potential paths to improve UPFC designs from the perspective of mixture design include reducing the height of the MCM design specimens and changing the UPFC design methods.  
These research results provide feasible references for achieving more durable UPFCs. However, this study did not take into account the influence of some factors, such as mineral gradation and aggregate characteristics, and it lacked on-site experimental validation. Future research will focus on the above aspects, particularly regarding the effects of aggregate morphology and crushing on the compaction behavior of UPFCs.  
Figure 13: The horizontal distribution: (**a**) mean AV content; (**b**) horizontal variability.  
**Author Contributions:** Conceptualization, X.D.; methodology, X.D. and H.L.; validation, M.S.; formal analysis, X.D., H.L. and W.L.; investigation, X.D., H.L., M.S. and W.L.; data curation, X.D., H.L., M.S. and W.L.; writing--original draft preparation, H.L.; writing--review and editing, X.D. and H.L.; supervision, H.Z. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the commercial privacy.  
**Conflicts of Interest:** Author [PERSON] was employed by Guangzhou Baiyun International Airport Co., Ltd. Author [PERSON] was employed by Shanghai Fengxian Construction Development Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.