## 2 Experimental Work
### Test Results and Discussion
#### 2.2.3 Effect of the Steel Plate's Thickness on Flexural Strength and Energy Absorption  
The load and mid-span displacement responses of the over-reinforced beams vary significantly depending on their steel plate thickness. The characteristic bending response of the beams was considered in an attempt to describe the response of the tested specimens. The loading reading was carried out in five stages, as illustrated in Figure 7. The peak load that produced the center point of deflection, \(\Delta_{\text{spall}}\), was identified as the load at which horizontal cracking occurred in the compression  
Figure 6: Load-strain relationships for all the beam specimens’ applied strains until failure.  
zone, denoted as Pspall. The peak load, Ppeak, with deflection, \(\Delta_{\text{peak}}\), is the next highest load obtained after the spalling load. This second peak occurred due to the deformation-controlled testing system and the confinement effect.  
Figure 7 shows five stages of the response curve for each specimen. The first stage is the stage before the concrete cracks, represented by points O to A in the curve. The second stage represents a linear response for an already cracked cross section from A to B in the curves. The third stage shows the non-linear response, which is the stage before the beams reach their compressive capacity. This stage is the strain limit of the beams when the spalling of the compression zone is initiated, as indicated by segments B to C on the curves. The fourth stage, C to D, was observed for the BCSP beams but not for the CB. The CB exhibited a more brittle failure that made it fail after stage four (shown as C to F). The BCSP beams were more ductile and they withstood a longer post-peak strain (shown from D to F). This last stage developed due to the existing steel plates. Which increased stiffness and strength. The ultimate loads and maximum strains increased with an increase of the steel plate thickness, which suggests that the greater the steel plate thickness, the more improvement is achieved. The BCSP-15 beam was the stiffest and strongest.  
The CB failed immediately after reaching its maximum capacity. This shows its brittle nature (spalling at load). The responses for the beams show that the behaviors of the beams are greatly influenced by their different confinement configurations. The confinement configurations had no significant influence at the stages before the spalling load. However, the elastic bending stiffness increased when the confinements were introduced into the compression zone. At first cracking, the increase in the load capacity of the BCSP beams was 245.5%, 406.3% and 543.5% for BCSP-6, BCSP-10 and BCSP-15, respectively, in comparison with the CB. The increase in the spall loads was 46.3% and 78.4% for BCSP-6 and BCSP-10, respectively, whereas increases in the peak loads were 73.7%, 123% and 149% for BCSP-6, BCSP-10 and BCSP-15, respectively, in comparison with the CB, as shown in Table 4.  
The energy absorption (EA) capacities of the beams were estimated by calculating the areas under the load-displacement curves, as demonstrated in Figure 7[37; 38; 39]. The BCSP beams showed higher EA capacity values than the CB beam. They achieved higher load capacities and stiffness values than the control specimens. The CB beam had the lowest energy absorption capacity, at 4030 kN\(\cdot\)mm. The BCSP beam specimens' flexural capacity also, energy absorption increased with the steel plate thickness. The rates of increase were 247.5%, 317.5%, and 458.3% for the BCSP-6, BCSP-10, and BCSP-15 beams, respectively. The highest value of energy absorption recorded for the specimen had a BCSP with a steel plate thickness of 15 mm. Its energy absorption capacity was about 22,500 kN\(\cdot\)mm.  
Figure 7: Load-displacement plots for all of the over-reinforced concrete beam specimens.