# Intelligent Design of Pavement Concrete Based on RSM-NSGA-III-CRITIC-VIKOR
## 3 Results and Analysis
### Microstructural Analysis of Pore Distribution and Density Improvement in Optimal Mix Proportions  
To comprehensively evaluate the effects of optimized mix proportions on the microstructure of concrete, the section integrates the results of Scanning Electron Microscopy (SEM) analyses and LF-NMR, examining concrete performance from the perspectives of microstructural morphology and pore distribution. The unoptimized concrete (Figure 14a,b) contains a higher number of pores, characterized by larger, irregularly shaped voids. Larger pores reduce the mechanical strength and durability of the concrete, primarily affecting compressive strength and permeability resistance. They create weak zones within the concrete matrix, making the material more susceptible to changes in shrinkage and overall structural integrity, particularly over prolonged exposure to environmental conditions. In contrast, the concrete with optimized mix proportions (Figure 14c,d) demonstrates a significant reduction in pore volume. The reduced pore volume correlates with a denser and more uniform microstructure, which contributes to improved overall performance. According to the LF-NMR results presented in Figure 15, a significant change in the pore distribution is observed when comparing the optimized mix with the unoptimized control. The pore distribution curve for the optimized concrete shifts to the left, indicating a general reduction in pore size. The shift is primarily achieved by reducing the proportion of \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Number** & **W/C** & **Sand Ratio** & **AEA** & \(\mathbf{S_{i}}\) & \(\mathbf{R_{i}}\) & \(\mathbf{Q_{i}}\) \\ \hline
2 & 0.35 & 36.0 & 0.200 & 0.0412 & 0.0321 & 0.0000 \\
3 & 0.35 & 39.0 & 0.100 & 0.0523 & 0.0389 & 0.0124 \\
32 & 0.36 & 37.7 & 0.193 & 0.0587 & 0.0412 & 0.0189 \\
59 & 0.37 & 37.3 & 0.180 & 0.0631 & 0.0456 & 0.0253 \\
64 & 0.37 & 37.4 & 0.176 & 0.0654 & 0.0478 & 0.0287 \\
9 & 0.39 & 36.8 & 0.105 & 0.0672 & 0.0483 & 0.0301 \\
47 & 0.37 & 38.3 & 0.172 & 0.0689 & 0.0495 & 0.0329 \\
60 & 0.37 & 37.2 & 0.201 & 0.0701 & 0.0501 & 0.0344 \\
58 & 0.35 & 39.7 & 0.186 & 0.0715 & 0.0512 & 0.0367 \\
36 & 0.37 & 40.7 & 0.114 & 0.0728 & 0.0524 & 0.0385 \\ \hline \end{tabular} Note: only the top 10 VKOR-ranked solutions (out of 73 Pareto-optimal solutions) are presented in the table. The selection highlights the most relevant results for clarity and further discussion.  
\end{table} 
Table 10: VIKOR optimal solutions and conditions. larger pores and increasing the proportion of smaller pores, thus refining the pore structure. Such optimization enhances concrete density and overall performance by minimizing the presence of larger pores, which adversely affect concrete durability, particularly in the 10-100 \(\upmu\)m pore size range. Specifically, the optimized concrete shows a substantial increase in the proportion of pores in the smaller pore size range (such as 0.0001-0.1 \(\upmu\)m), while the proportion of pores in the larger size range (such as 0.1-10 \(\upmu\)m) decreases. The optimized mix effectively adjusted the pore distribution, resulting in a denser and more uniform pore structure. Figure 14: SEM characterization analysis of optimal mix rate. (**a**) Pre-optimization (200\(\times\)); (**b**) pre-optimization (500\(\times\)); (**c**) post-optimization (200\(\times\)); (**d**) post-optimization (500\(\times\)). Figure 15: Pore size distribution. The combined SEM and LF-NMR results provide a comprehensive understanding of the concrete microstructural improvements. While SEM offers clear observations of pore distribution, LF-NMR highlights the refinement of smaller pores that SEM analysis may not clearly capture. Integration improves the understanding of pore structure changes at different scales. In conclusion, SEM and LF-NMR results complement each other, demonstrating that optimizing the mix proportions significantly improves the concrete microstructure, particularly in terms of pore distribution.