## 3 Explosion Test Program
### Blast-Resistance Performance  
After explosion tests, the steel fibers were recovered from damaged panels for further scanning electron microscope (SEM) tests. As shown in Figure 12, the steel fiber was magnified by 500, 1000 and 2000 times. The steel fiber pullout in the cement matrix is a non-destructive process since the adhesion is not strong enough.  
As pointed out by [PERSON] et al. [44], the concrete (with reinforcement) panels may exhibit four typical damage modes under detonations. They are cratering, collapse, perforation and punching, as illustrated in Figure 13. The cratering damage is only featured with front surface damage but no scabbing on the rear surface. The collapse damage has both front and rear surface cratering while the panel is not perforated. The more severe damage is perforation, whereby the front crater meets the  
\begin{table}
\begin{tabular}{l c c c} \hline
**No.** & **Front Crater Diameter** & **Front Crater Depth** & **Rear Crater Diameter** \\ \hline S9-1 & 205/205 & 35 & 207/273 \\ S9-2 & 185/185 & 24 & 267/275 \\ S10-1 & 143/143 & 38 & 249/334 \\ S10-2 & 153/186 & 33 & 249/268 \\ S11-1 & 120/170 & 32 & 222/270 \\ S11-2 & 140/155 & 32 & 254/290 \\ S12-1 & 149/181 & 32 & 260/260 \\ S rear crater. When the detonation shock is fierce enough, the panel may suffer shear plugging where the whole damaged zone is totally punched outward.  
For this work with contact detonations, the post-test concrete panels showed two different damage modes, i.e., the plain concrete (No. S1), PP FRC (No. S2) and PVA FRC (No. S3) panels were perforation damaged while the rest were collapse damaged. The Steel FRC together with the PP-Steel HFRC and PVA-Steel HFRC panels were characterized with less damage whereby both front and rear craters occurred but were not perforated yet. To quantitively describe the damage level, damage coefficient \(K_{a}\) and blast-resistant coefficient \(K_{b}\) were introduced with reference to [44]:  
\[d=K_{a}W^{(1/3)} \tag{1}\]  
\[h=K_{b}W^{(1/3)}-0.5e \tag{2}\]  
where \(d\) is the crater diameter, \(h\) is the crater depth, \(W\) is the TNT charge weight, and \(e\) is the standoff.  
With Equations (1) and (2), the damage coefficient \(K_{a}\) and blast-resistant coefficient \(K_{b}\) were calculated for both PP-Steel and PVA-Steel HFRC, respectively. Since the contact detonation test was performed with 44 mm height TNT, the standoff distance \(e\) was estimated as 22 mm.  
The results were clearly depicted in Figure 14 for explosion damage level analyses. For both \(K_{a}\) and \(K_{b}\), the smaller value reveals better blast-resistant performance. It was demonstrated that HFRC with 1.5% steel + 0.5% PP-fiber goes moderately ahead of 2% steel fiber content concrete. Also 1.33% steel + 0.67% PP HFRC showed similar blast-resistant level than Steel FRC control specimen. Unfortunately, PVA-Steel HFRC had much higher \(K_{b}\) than Steel FRC and \(K_{a}\) was not improved either. The blast-resistant performance improvement was only achieved by 1.5% steel + 0.5% PP hybrid fiber reinforcement case.  
Figure 12: SEM images for steel fiber surface after complete pullout.  
Figure 13: Typical damage mode of concrete panels caused by contact explosion [44].  
This interesting phenomenon may result from the fact that the relatively flexible PP or PVA-fiber leads to improved toughness and strain capacity in post-cracking response while the steel fiber with high modulus and high tensile strength could effectively increase the strength of concrete. The hybrid mix with 1.5% steel fiber + 0.5% PP or PVA-fiber reveals a synergetic response which significantly increases the concrete strength comparable to S4, and simultaneously improves the ductility. The other mixes used herein are stronger than the control batch S4, thus exhibiting weaker blast resistance.