## 4 Study on the Blast Resistance of Ultra-Wide Concrete Twin-Edge Box Girders Coated with Polyurea
### Parametric Analysis
#### 4.2.1 Analysis of the Impact of Different Scaled Blast Distances

Table 6: Blast conditions for varying charge weights. Figure 24: Damage patterns of prototype box girder segments before and after polyurea coating under different scaled blast distances. Figure 25 shows the stress-time history curves for concrete elements at four locations under condition 1: the T-beam quarter-span, the middle of the top plate in box chamber 1, box chamber 3, and the outer cantilever section. The figure illustrates that the stress variation trend remains consistent at each point regardless of polyurea application, suggesting that polyurea affects only the peak stress, not the overall trend. For the uncoated specimen (PG), peak stresses are 8.73 MPa, 13.77 MPa, 13.28 MPa, and 6.61 MPa, while for the polyurea-coated specimen (PPCG), they are 4.75 MPa, 9.91 MPa, 10.68 MPa, and 5.62 MPa. These results show that polyurea significantly reduces peak stress at all locations. Peak stress reductions are 45.59%, 28.03%, 19.58%, and 14.98%, respectively. These findings confirm that the polyurea coating effectively reduces peak stress in concrete structures under blast loading, especially at the T-beam quarter-point, where the stress reduction is most pronounced. Moreover, peak stresses increase with proximity to the blast center. This suggests that the blast shockwave impact diminishes as the distance from the blast center increases. As the shockwave propagates outward, it sequentially reaches different points, producing peak stresses at each location. In summary, polyurea significantly reduces peak stress in concrete structures subjected to blast loading, especially at locations near the blast center. The time-history curves of concrete stress at the top-plate center of box chamber 1 under different scaled distances are shown in Figure 26. When the blast height is constant, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multirow{2}{*}{**TNT Equivalent/kg**} & \multirow{2}{*}{**Charge Position**} & \multirow{2}{*}{**Blast Height/m**} & \multicolumn{2}{c}{**Breach Length/m**} \\ \cline{5-6}  & & & & **Transverse Direction** & **Longitudinal Direction** \\ \hline \multirow{4}{*}{PG} & 100 & & & 1.205 & 0.964 \\  & 200 & & & 1.609 & 1.756 \\  & 300 & Middle of the top plate & 1 & 2.015 & 2.141 \\  & 400 & in box chamber 2 & & 2.026 & 2.146 \\  & 500 & & & 2.412 & 2.149 \\ \hline \multirow{4}{*}{PPCG} & 100 & & & 1.613 & 1.755 \\  & 200 & & & 2.014 & 2.151 \\ \cline{1-1}  & 300 & Middle of the top plate & 1 & 2.620 & 2.535 \\ \cline{1-1}  & 400 & in box chamber 2 & & 2.819 & 2.546 \\ \cline{1-1}  & 500 & & & 2.829 & 2.928 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Length of concrete spall on the top plate of box girder coated with polyurea segments under different charge amounts. Figure 25: Temporal evolution curve of concrete stress of the ultra-wide box girder (condition 1). the peak stress in the top plate of box chamber 1 increases with the explosive charge, resulting in a reduced scaled distance. This demonstrates the concrete structure's sensitivity to explosive charges, where higher charges lead to increased peak stress. For the five different test conditions, the maximum stress values for specimen PG were 13.28 MPa, 18.22 MPa, 19.42 MPa, 22.77 MPa, and 24.29 MPa, respectively. In contrast, the maximum stress values for specimen PPCG were 9.91 MPa, 17.03 MPa, 18.61 MPa, 21.83 MPa, and 22.51 MPa, showing consistently lower values than those of specimen PG. The reduction in peak stress due to the polyurea coating was 25.43%, 6.53%, 4.17%, 4.13%, and 7.33%, respectively, indicating that polyurea effectively mitigates the peak stress in concrete structures under blast impact. Notably, stress reduction is more significant under lower explosive charges.