## 4 Study on the Blast Resistance of Ultra-Wide Concrete Twin-Edge Box Girders Coated with Polyurea
### Parametric Analysis
#### 4.2.4 Analysis of the Impact of Different Polyurea Thicknesses  
The analysis in Section 4.2.2 shows that the central T-beam, quarter-span, and outer web are vulnerable points. To study the effect of the thickness of polyurea on blast resistance at these weak locations, simulations were performed with thicknesses ranging from 1.5 mm to 12 mm. The blast center was located 1 m above the mid-span, with a TNT equivalent of 500 kg. The different finite-element simulation cases for the weak regions with varying polyurea thicknesses are shown in Table 11. The vertical displacements of the baseplate at various points were recorded, starting from the initial point directly beneath the blast center and extending transversely across the bridge. The temporal evolution curves of the vertical shift and the peak shift at various points are depicted in Figure 41.  
As shown in Figure 41, the polyurea thickness significantly affects the vertical displacement of the baseplate beneath the blast center. When the polyurea coating thicknesses were 0 mm, 1.5 mm, 3 mm, 6 mm, 9 mm, and 12 mm, the corresponding vertical shift of the baseplate was 119.36 mm, 114.1 mm, 113.15 mm, 111.92 mm, 110.27 mm, and 110.23 mm, respectively. This indicates that increasing polyurea thickness reduces vertical displacement, enhancing blast resistance. At the center of the baseplate in the first box chamber, the vertical displacements for different polyurea thicknesses were 8.273 mm, 7.962 mm, 7.918 mm, 7.803 mm, 7.793 mm, and 7.603 mm, respectively. This trend further validates the positive correlation between polyurea thickness and the structure's blast resistance. At the baseplate center of box chamber 2, the vertical displacements for different polyurea thicknesses were 5.389 mm, 5.166 mm, 5.139 mm, 5.133 mm, 5.131 mm, and 5.129 mm, respectively. These findings confirm that increasing polyurea thickness reduces vertical displacement and boosts blast resistance. However, in the third box chamber, the vertical displacements for different polyurea thicknesses were 3.383 mm, 3.333 mm, 3.305 mm, 3.260 mm, 3.258 mm, and 3.241 mm, respectively. Although these data also show a correlation between polyurea thickness and vertical displacement, the trend is less pronounced in box chamber 3 than in the first two regions. In conclusion, increasing polyurea thickness enhances blast resistance, particularly in areas near the blast center. An optimal thickness of 9 mm is recommended.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Specimen**} & \multirow{2}{*}{**Condition**} & \multirow{2}{*}{**Polyurea Thickness/mm**} & \multicolumn{2}{c}{**Blast Charge Position**} & \multirow{2}{*}{**Blast Height/m**} & \multirow{2}{*}{**TNT**} & \multirow{2}{*}{**Scaled**} \\  & & & & & & & \\ \cline{5-8}  & & & **Transverse Direction** & & & & \\ \hline \multirow{3}{*}{PPCG} & 20 & 3 & & & & & \\  & 21 & 6 & T-beam section & & & & \\ \cline{1-1}  & 22 & 9 & center & Mid-span & 1 & 500 & 0.171 \\ \cline{1-1}  & 23 & 12 & & & & & \\ \hline \end{tabular}
\end{table}
Table 11: Different polyurea thickness conditions.