# Experimental Research on Bond Behavior between GFR Bars and Stirrups-Confined Concrete
## 1 Introduction  
The use of fibre reinforced polymers (FRP) as reinforcement in concrete structures is considered to be a possible alternative to steel in situations where corrosion is present. It is imperative that the bond behavior between FRP bars and concrete be given top priority prior to studying the performance of FRP-reinforced concrete (RC) structures. This is due to the crucial role that bond plays in terms of the coaction between FRP bars and concrete. An adequate level of bond is required between an FRP bar and concrete in order to ensure the successful transmission of forces from one to another. Unlike steel bars, the FRP bars behave anisotropic, non-homogeneous, and linear elastic properties, which may result in different force transfer mechanisms between bars and concrete. Some research on the performance of FRP-RC members [1; 2] or structures [3; 4] purport that the bond stress (\(\tau\))-slip (s) relationship differs greatly between that of FRP bars and that of steel bars, notably influencing the usability of the concrete structure. Contrarily, unlike steel bars, no uniform manufacturing standards have been established for FRP bars, resulting in divergences between the performance of FPR bars across different countries and manufacturers. Research into the bond behavior of different FRPbars will culminate in a standardization of the application and manufacturing regulations for the FRP-RC structures. When compared with carbon fibre-reinforced polymer (CFRP) and aramid fibre-reinforced polymer (AFRP) bars, the glass fibre-reinforced polymer (GFRP) bars have received elevated attention due to its high chemical resistance, inexpensive price, easy processability, and significant real-world applicability. Many studies have been conducted to investigate the interfacial bond behavior of GFRP bars in concrete. The bond strength of the GFRP bars is typically within 40-80% of that corresponding to steel bars [5; 6; 7]. Smooth bars only develop 40-70% of bond strength of deformed bars [6; 8; 9]. Surface deformations with a height of at least 6% of the bar diameter are necessary to develop adequate bond behavior to concrete [10]. Larger diameter and embedment lengths of bars develop less average bond stress [6; 9; 11; 12; 13]. Moreover, some effective nondestructive evaluation techniques [14; 15; 16; 17] were developed to monitor the initial installation quality and the long-term efficiency of the interfacial bonding between FRP and concrete. Theoretical efforts have also been made to analyze the bond mechanism of FRP to concrete. Based on the experimental results, three analytical \(\tau\)-s models, namely, the Malvar model, which was the first one to be proposed for FRP bars by [PERSON]. [18]; the Bertero-Popov-Eligehausen (BPE) model, which was initial proposed for steel bars by [PERSON] et al. [19]; and, the Cosenza-Manfredi-Realfonzo (CMR) model, which was proposed for FRP bars by [PERSON] et al. [20], have been reported in literature. The three models were compared by fitting experimental results and using finite element method [21; 22]. Moreover, some scholars have updated these three models to suit the bond properties of FRP bars in different materials and environments [23; 24; 25; 26; 27]. Correspondingly, design codes for GFRP bars in the United States (U.S.) [28], Canada [29; 30], and Japan [31] have stipulated guidelines that are associated with bond mechanism in terms of both embedment length and bond strength. Research has indicated that stirrups ostensible effect on the bond behavior of a steel bar. Test results that were obtained by [PERSON] and [PERSON] [32; 33] have shown that, with an increased level of confinement provided by stirrups, bond strength increases; the bond strength of stirrup-containing specimens is more closely aligned to the deformation pattern that is evident in the surface of steel bars as compared to specimens without stirrups. The research by [PERSON] et al.