# Strengthening of RC Beams with CFC Panels for Improving Impact Resistance
## 3 Experimental Results
### Impact Response  
Figure 8a presents the displacement-time history of each specimen under the first impact. The unstrengthened RC beam specimen (RC) experienced a larger midspan displacement than all the CFC-reinforced RC beams, demonstrating that reinforcing RC beams with CFC panels can improve their impact resistance. The maximum displacement of RC was 16.95 mm. Among the CFC-strengthened specimens, RC_F3S_CFC was the most effective and achieved the highest impact resistance, as shown as the lowest maximum displacement of 8.15 mm, followed by RC_FB_CFC 11.2 mm and RC_P3S_CFC 13.75 mm. Similarly, the residual displacement tended to decrease because of the strengthening of the CFC panel. According to Ref. [36], the authors determined the restitution coefficient of displacement, which was calculated as 1 minus the residual displacement divided by the maximum displacement, in order to illustrate the recovery performance. The conventional RC beam exhibited the lowest restitution coefficient of displacement (0.253), whereas the restitution coefficients of RC_P3S_CFC, RC_FB_CFC, and RC_F3S were 0.335, 0.635, and 0.822, respectively. Regarding the restitution coefficient of each specimen, the strengthened specimens exhibited a better recovery performance. Figure 8b presents the measurement results of the impact force-time history of each specimen under the first impact. RC_F3S_CFC clearly earned the highest maximum impact force of 144.11 kN in 0.0113 s, which is the shortest duration of the impact process, whereas the conventional RC beam earned the lowest impact force of 117.95 kN in 0.0193 s, which is the longest duration of the impact process. The maximum impact force of RC_FB_CFC and RC_P3S_CFC were 123.78 kN and 119.17 kN within the duration of the impact process of 0.0195 s and 0.0153 s, respectively. As mentioned for the failure mode and crack propagation, specimens RC and RC_P3S_CFC were considered as failed specimens after the first impact because their cumulative residual displacement reached 1% of the span; hence, no further impacts were applied. In contrast, RC_FB_CFC and RC_F3S_CFC were subjected to additional impacts until their residual displacement reached 1% of the span or delamination of the CFC panel was observed. Figure 6: Crack propagation at middle span of specimen after being subjected to each impact: (**a**) RC—1 st impact; (**b**) RC_FB_CFC—1 st impact; (**c**) RC_FB_CFC—2 nd impact; (**d**) RC_P3S_CFC—1 st impact; (**e**) RC_F3S_CFC—1 st impact; (**f**) RC_F3S_CFC—2 nd impact; (**g**) RC_F3S_CFC—3 rd impact. The response of each type of specimen that underwent the repeated impacts test is shown in Figure 9. The displacement tended to gradually increase as the number of repeated impacts increased. In Figure 9a,c, due to the second repeated impact, RC_FB_CFC reached 15.57 mm. of maximum displacement with a restitution coefficient of 0.247, although the maximum displacement of RC_F3S_CFC was only 9.14 mm. and the residual displacement increased slightly owing to a restitution coefficient of 0.821, which were slightly decreased when compared to the first impact that was applied. With the third repeated impact, the higher displacement of RC_F3S_CFC was generated for 12.84 mm with 0.708 as the restitution coefficient. The impact forces are shown in Figure 9b,d. The impact force continuously decreased under the repeated impact load, demonstrating that the impact force tended to decrease as the impact number increased while increasing the duration of the impact process. The relationship between the cumulative residual displacement and the number of repeated impacts was plotted for all the specimens, as shown in Figure 10. The RC_FB_CFC and RC_F3S_CFC specimens withstood an increasing number of repeated impacts before reaching the specified residual displacement. RC_FB_CFC failed at the second impact owing to CFC panel delamination, whereas RC_F3S_CFC failed at the third impact.