# Strengthening of RC Beams with CFC Panels for Improving Impact Resistance
## 1 Introduction  
Reinforced concrete (RC) is a versatile composite material widely used in structures owing to its variety of applications and properties. Owing to their widespread utilization, RC structures can be subjected to different types of loads during their service, including impact loads. During the last few decades, many researchers have investigated the structural component behavior when subjected to impact loads, such as RC beams, and this has been a major concern in the case of RC beams in civil engineering structures [1, 2]. Many researchers have conducted studies on structural members subjected to impact loads. RC beams suffer from two main failure modes: flexural failure [3, 4] and shear failure [5, 6]. The failure mode of RC beams with stirrups under low-to-high impact velocities was investigated by [PERSON] et al. [7] and [PERSON] [8]. The authors investigated the relationship between the impact velocity and the failure mode of RC beams, and the results demonstrate that the failure mode of RC beams tends to change from bending to shear when the impact velocity increases. This failure mode phenomenon was also demonstrated by adopting a numerical analysis [9]. [PERSON] et al. [10] conducted a drop-weight impact test and provided a numerical method that mainly focused on the failure mode of an RC beam under low-velocity impact. The results confirm the literature that the RC beam encountered the flexural failure mode while subjected to low-impact velocity. Besides that, concrete crushing was observed in the impact region and tended to increase while the impact velocity increased. To improve the impact resistance of RC beams in order to guarantee safety and durability, external strengthening materials and methods were studied and introduced. Bars or sheets made of fiber-reinforced polymers (FRP) [11; 12; 13] and carbon fiber-reinforced polymers (CFRP) [14; 15; 16] have been commonly used to improve the load-bearing performance of RC structural members. The results prove that the external strengthening of the FRP and CFRP improved the stiffness and load resistance of the conventional RC beams' either flexure or shear, indicating that a larger load capacity and lower displacement were acquired, as well as the crack propagation was controlled. The improvement of the shear capacity was clearly proved by wrapping CFRP around the RC beams without the stirrup; the beams were advantageous for shear strengthening [17]. In addition, aramid fiber-reinforced polymer (AFRP) [18] sheets and ultra-high-performance concrete (UHPC) [19] have been employed to strengthen RC beams against impact loads. The impact resistance capacity of the RC beam was significantly increased, but it still suffered from either sheet debonding or rupturing, which is likely to occur because of insufficient adhesive strength, particularly when impact loads are applied. Although many previous studies have been conducted on FRP-reinforced RC beams using either sheets or rods to improve the impact resistance of RC beams, they might practically suffer from unsuitable workability and long-term behavior when exposed to a different and severe environment [20]. Therefore, further research is required to precisely evaluate the retrofitting effectiveness of the reinforcing materials on the impact response of RC beams. The CFC panels were fabricated as a sandwich structure comprising a carbon fiber core dovetailed by flexible boards that allow for easy installation on existing concrete structures, whereas the dovetail layer provides weather-resistance properties. The load-bearing capacity of RC beams reinforced with CFC panels, which TIASEI Corporation or TIASEI Construction Company, Ltd., has already adopted for seismic reinforcement of RC columns, was studied for impact loads and the applicability as reinforcement material for impact resistance in the current research. In various studies, the behavior of the structural members was investigated through experimental tests and numerical analyses.