# Retrofitting of Steel Structures with CFRP: Literature Review and Research Needs
## 2 Bond Behavior between CFRP and Steel
### Adhesive  
The mechanical properties of adhesive have a notable effect on the bond behavior of steel members retrofitted with CFRP [45; 46; 47; 48]. As the adhesive is the weakest link in the CFRP and steel joint, selecting a proper adhesive can ensure the maximum capacity of the retrofitting system. In most research studies, commercial adhesives are utilized for retrofitting structures [34; 49; 50]. Figure 3 shows the stress-strain curves for these adhesives. It should be noted that the mechanical properties of the adhesive may decrease with increasing temperature [43]. Commercial adhesives have been found to possess insufficient strength, often resulting in brittle failure. Consequently, research efforts have been directed towards enhancing adhesive performance, particularly in terms of mechanical properties [45; 51; 52; 53; 54; 55]. Utilizing enhanced adhesive can change the failure mode of the CFRP/steel joints [56; 57].  
Figure 1: Schematic view of the (**a**) single strap joint; (**b**) double strap joint test setups.  
Figure 2: Possible failure modes of CFRP/steel joints under tensile loading [5].  
Among the commercial adhesives, Sikadur and Araldite have been commonly utilized. It was shown that specimens with Araldite 2015 adhesive exhibited a much higher interfacial fracture energy compared to the specimens with Sikadur 30 adhesive [49]. Indeed, specimens retrofitted with nonlinear adhesives having higher strain capacity but lower elastic modulus exhibit higher interfacial fracture energy and thereby a higher bond strength compared to the ones with linear adhesives having similar or even higher elastic modulus [34, 58]. The type of adhesive can also significantly influence the bond-slip relationship. Bond-slip can be estimated as a triangular shape for linear adhesive and a trapezoidal shape for nonlinear adhesive [59]. The average effective bond length of specimens bonded with nonlinear adhesive was also found to be greater than that of specimens with linear adhesive, which could be attributable to the higher ductile behavior of nonlinear adhesive compared to the linear one [60]. It is also found that the effective bond length, interfacial fracture energy, and ultimate load increase with the increase in adhesive thickness [49, 61]. In addition, increasing the adhesive thickness can change the failure mode from cohesive failure to CFRP delamination [34, 62, 63]. However, specimens with lower adhesive thickness exhibited better performance in terms of the average peel stresses and failure load [64]. It is worth mentioning that assessing the effect of adhesive thickness on the failure mode is difficult due to the non-uniform thickness of the bond-line [65]. It is suggested to increase the adhesive thickness up to 2 mm to avoid cohesive failure [63, 66]. Non-cohesive failure in the bond test has been reported in the literature [67, 60, 68, 69, 34, 60]. Increasing the adhesive thickness to 3 mm may result in lower bond strength compared to a thickness of 2 mm [34]. Consequently, it can be concluded that the adhesive's mechanical properties and thickness are the key factors in the bond behavior of CFRP retrofitted steel components [49, 55]. Furthermore, the adhesive layer, regardless of the type of adhesive, can help prevent potential galvanic corrosion if the bond quality is strictly controlled [70]. Furthermore, investigations on the impact of bond-line defects on the bond behavior of CFRP steel joints suggest that the size of the defect plays a more significant role in determining the failure mode than the number of defects [71].  
Figure 3: Tensile stress–strain curve of commercial adhesives used in bonding CFRP to steel substrate [45, 49].  
\begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**References Parameters**} & \multicolumn{1}{c}{**References Parameters**} & \multicolumn{1}{c}{**References Parameters**} \\ \hline
**ac** & \(\hat{d}_{00}\) & \(\hat{e}_{00}\) & \(\hat{e}_{00}\) & \(\hat{e}_{00}\) & \