## 4 Study on the Blast Resistance of Ultra-Wide Concrete Twin-Edge Box Girders Coated with Polyurea
### Parametric Analysis
#### 4.2.3 Analysis of the Impact of Different Repeated Explosion Patterns  
(1) Single-point repeated detonation  
As discussed in Section 2.3.2, the initial blast load causes localized damage to the reinforced concrete box girder and leads to material degradation focused on the explosion center. Under repeated blast loading, the box girder sustains progressively more severe damage. A comparative analysis was performed to investigate the failure characteristics of ultra-wide box-girder segments under both single and repeated explosion scenarios. Details of the calculation conditions are provided in Table 9.  
Figure 34: Temporal evolution curves of vertical shift at the baseplate center of the box chamber for condition 12. Figure 35: Temporal evolution curves at the bottom of the diaphragm beam near the area directly below the explosion center. The comparison of damage patterns between repeated explosions and a single explosion on the ultra-wide box girder coated with polyurea is shown in Figure 36. As depicted in Figure 36a, after the initial 100 kg TNT blast, a through-hole appeared in the top plate center of chamber 2 of specimen PG, measuring 1.205 m along the long edge and 0.964 m along the short edge. After the second 100 kg TNT blast, the area of the through-hole increased to 2.220 m along the long edge and 2.544 m along the short edge. Similarly, in specimen PPCG, a through-hole appeared after the first 100 kg TNT blast, measuring 1.613 m along the long edge and 1.755 m along the short edge, and it was enlarged to 1.990 m by 1.734 m after the second blast. The shockwave passed through the hole, hitting the baseplate directly and causing concrete spalling on the rear face. The bending of both the transverse and longitudinal reinforcement bars around the perforation in the box girder increased, as did the permanent deformation of the bars. As shown in Figure 36b, under the condition described in Table 9 (200 kg single explosion), a through-hole formed at the center of the top plate of chamber 2 in specimen PG, measuring 1.609 m along the long edge and 1.756 m along the short edge, while the hole in specimen PPCG measured 2.014 m by 2.151 m. It can be concluded that compared to the 200 kg single explosion, both specimens PG and PPCG exhibited more severe damage under repeated explosions with two 100 kg charges, despite the same total charge. The top surface of the hole in specimen PPCG was smaller than that in PG, but the bottom surface hole in PPCG along the short edge increased significantly and was larger than in PG. This is because the energy-absorbing mechanism of the polyurea coating converts part of the internal energy into the crushing of the concrete. From the comparison in Figure 36c,d, it can be observed that the same pattern holds when comparing two 200 kg repeated explosions with a single 400 kg explosion. Figure 37 compares the temporal evolution curves of vertical shift at the center of the baseplate in the super-wide box girder subjected to repeated and single explosions. As shown in Figure 37a, under the first explosion of 100 kg on specimen PG, the peak vertical shift of the baseplate in chamber 2 was 6.83 mm, and under the second explosion of 100 kg, the peak displacement was 3.92 mm, resulting in a total displacement of 10.75 mm for both explosions. Under a single 200 kg explosion, the peak vertical displacement was 8.67 mm. For specimen PPCG, the peak vertical displacement was 5.66 mm under the first 100 kg explosion and 4.64 mm under the second explosion, with a total displacement of 10.30 mm. In contrast, the peak displacement under a single 200 kg explosion was 8.45 mm. Therefore, the increase in displacement under two 200 kg repeated explosions, compared to a single 400 kg explosion, was 19.34% for PG and 17.96% for PPCG. Under two 100 kg repeated explosions, the displacement decreased by 4.19% when comparing PG and PPCG specimens.