## 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  
This study examines the impact of close-range explosion shock waves on polyurea-coated ultra-wide concrete twin-edge box girders. The explosions considered typically involve car bombs or the detonation of fireworks and chemical hazardous materials transported on bridge decks, with their blast power equivalent to TNT charges applied to the bridge structure. According to the FEMA 426 manual [39], based on common vehicle types and capacities, a large truck is typically capable of transporting at least 10,000 pounds (around 4536 kg) of TNT-equivalent explosives. Vans and smaller passenger vehicles are able to transport between 500 and 4000 pounds (roughly 227 to 1814 kg) of TNT equivalent. Drawing on the \"Terminology and Definitions of Automobile and Trailer Types\", [PERSON] [40] classified common vehicles into five categories based on loading capacities: standard passenger cars, crossover vehicles, multi-purpose vehicles, minibuses, and box trucks. Due to strict national military security regulations on explosives, the equivalent quantity of common industrial explosives is reduced by a factor of 0.5. Thus, the TNT equivalent range for car bombs is set between 100 and 1000 kg. Figure 23: Finite-element model of the prototype ultra-wide concrete twin-edge box girder segment. In this study, five TNT-equivalent charge weights were selected for finite element simulations: 100 kg, 200 kg, 300 kg, 400 kg, and 500 kg. The specific experimental conditions are shown in Table 6.  
The damage patterns of the top plate of the prototype box girder segment before and after polyurea coating, under different scaled blast distances, are shown in Figure 24, with three measured dimensions of the top plate breach provided in Table 7. When the blast load impacts the top plate of the box girder, stress and strain within the concrete elements increase rapidly. As the blast shock wave persists, stress and strain in the concrete progressively reach their limits, leading to localized perforations in the top plate. As the TNT equivalent increases from 100 kg up to 500 kg, the length of the breach along the long edge of the uncoated PG specimen increased from 1.210 m to 2.429 m, and along the short edge from 1.361 m to 2.540 m. For the polyurea-coated PPCG specimen, the breach length along the long edge grew from 1.613 m to 2.829 m, and along the short edge from 1.755 m to 2.928 m. This indicates that the PPCG specimen with the polyurea coating exhibited relatively larger breach dimensions under the same blast load. The reason for this is that the polyurea-coated structure effectively absorbs and disperses the energy generated by the blast, converting more kinetic energy into fracturing energy on the top plate, thus resulting in larger breaches [30]. In the effective plastic strain contour diagrams shown in Figure 20, the area immediately below the blast center for both the PG and PPCG specimens is colored red, indicating a region of localized high strain, which suggests that damage is more severe in areas closer to the blast center. As the TNT charge increases, the breach area in the concrete enlarges, damage to the top plate worsens, and minor spalling occurs in the baseplate and web concrete. Additionally, the extent of the high-strain region immediately below the blast center expands. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multirow{2}{*}{**Condition**} & \multirow{2}{*}{**TNT Equivalent/kg**} & \multicolumn{2}{c}{**Blast Position**} & \multirow{2}{*}{**Blast Height/m**} & \multirow{2}{*}{**Scaled Distance/m/kg\({}^{1/3}\)**} \\ \cline{5-6}  & & & **Transverse Direction** & & & \\ \hline \multirow{5}{*}{PG/PPCG} & 1 & 100 & & & 0.215 \\  & 2 & 200 & Center of the top & & 0.171 \\ \cline{1-1}  & 3 & 300 & plate of box & Mid-span & 1 & 0.149 \\ \cline{1-1}  & 4 & 400 & chamber 2 & & & 0.136 \\ \cline{1-1}  & 5 & 500 & & & & 0.126 \\ \hline \hline \end{tabular}
\end{table}