# Bond Strength Properties of GFRP and CFRP according to Concrete Strength
## 1 Introduction  
Reinforced concrete is the most commonly used construction material because the coefficient of thermal expansion values of the rebar and concrete are similar, and because the high tensile strength of rebar complements the limitations of concrete [1]. In Korea, the number of reinforced structures has increased since the 1980s due to rapid urbanization. The demand for coarse aggregate and fine aggregate has also increased, and the country faced a shortage of river gravel and river sand supply. Therefore, rubble and washed sea sand were substituted for coarse and fine aggregate. However, the salt content of sea sand accelerates the corrosion of rebar. Simultaneously, concrete inhibits corrosion in reinforced structures through its strong alkaline properties; however, cracks on its surface caused by drying, shrinkage, and other factors are inevitable. Various deterioration factors permeate the concrete through these cracks and accelerate rebar corrosion [2, 3]. Rebar corrosion causes the structure's load carrying capacity to diminish. Corrosion damages the rebar's rib, lowers bond strength, and eventually shortens its service life [4]. The bond strength reduction is affected by the strength of concrete, surface conditions of rebar, fiber reinforcement, cracks in concrete and anchorage length, cover thickness, and stress conditions around the concrete. The uncertainty caused by these factors increases the maintenance costs when addressing corrosion damage, which inhibits economic and social prospects [5]. To solve these problems, previous studies have suggested methods such as epoxy coating the surface of deformed reinforcing bars and increasing the concrete coating thickness [6]. However, difficulties have occurred during construction because of the cracking of the epoxy coating on therebar's surface. Further, the increase in cover thickness leads to a heavier load. Subsequently, fiber-reinforced plastics (FRP) were developed to solve this problem [6]. FRP is a fiber-reinforced material used for specialized structures in many industries, such as the aerospace industry since the 1960s. It became a commonly used material in the construction industry in the 1990s. In the construction industry, short fibers are mixed in the concrete, whereas long fibers wrap the outside of the structure to reinforce it. FRP benefits from the fiber's high tensile strength and possesses noncorrosive and nonconductive properties [7]. Because of its high corrosion resistance, FRP is used as a replacement for rebar in structures that will potentially be exposed to salt damage. It is also a lightweight material, exhibiting one fourth the weight of steel, which effectively reduces the weight of the structure. This allows FRP to create a lightweight structure that possesses a high strength. However, FRP possesses engineering disadvantages such as low modulus of elasticity and brittle fracture. Furthermore, there is a limit on the increase of the diameter of reinforcing bars [8]. Glass fiber-reinforced plastics (GFRP) and carbon-reinforced plastics (CFRP) were developed to solve the problems of FRP; they have several independent benefits. First, GFRP is inexpensive; however, it is made of glass fiber and contains thorn-shaped protrusions on the surface [9], which pose a risk when used on site. Moreover, since GFRP has a small elastic modulus--approximately 0.2 times that of that of rebar--there is a possibility that a relatively large deflection may occur compared to a rebar with the same reinforcing ratio [10]. CFRP possesses a higher modulus of elasticity and tensile strength than GFRP. It is also lightweight and makes construction convenient. Moreover, CFRP exhibits a high corrosion resistance and economic feasibility due to advanced technology and increased demand [11; 12]. Bond strength is a crucial factor for the bearing strength of reinforced concrete structures. When bond strength is reduced, it affects the monolithic behavior of rebar and concrete, resulting in a decline in structural capacity [13].