# Retrofitting of Steel Structures with CFRP: Literature Review and Research Needs
## 3 Flexural Retrofitting of Steel Beams  
This section reviews the studies related to the flexural behavior of the steel structures retrofitted with CFRP. Various modes of failure in CFRP retrofitted beams were reported, as shown in Figure 4, such as (a) CFRP end-debonding and intermediate-debonding; (b) CFRP delamination or rupture; (c) bending failure in which yielding occurs in beam flanges and web; (d) local bucking, which occurs in compression flange or web; (e) lateral-torsional buckling [18]. To investigate CFRP retrofitting technique efficiency, the lateral and local buckling of steel beams should be controlled or eliminated [105]. Among the aforementioned failure modes, the debonding of the CFRP has been found as the dominant failure mode for the specimens retrofitted with NM CFRP [18; 106; 107; 108; 109]. CFRP end-debonding is the common failure mode for beams that are retrofitted in flexural yielding [18]. It should be noted that if a longer CFRP plate is applied, the failure mode could turn into a different failure mode, in particular, intermediate-debonding [109]. On the other hand, the CFRP rupture was reported as the common failure mode for HM and UHM CFRP [5; 13]. This phenomenon occurs due to the lower rupture strain of CFRP featuring a higher elastic modulus and the reduced stresses of the adhesives at the ends [39]. However, strengthened steel beams using UHM CFRP strips showed debonding failure at strip panel finger joints due to high shear stresses [110]. The most studied parameters in existing research investigations of CFRP retrofitted structures in flexure include adhesive thickness, adhesive type, CFRP configurations, CFRP thickness, and CFRP bond length. Details of previous studies on the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Type of Model** & **Bond–Slip Model** & **Bond–Slip Curves** \\ \hline
[98] & Tri-linear & \(\begin{cases}\tau=\tau_{f}\frac{\delta}{\tau_{i}}&\delta\leq\delta_{i}\\ \tau=\tau_{f}\frac{\delta_{f}-\delta}{\tau_{f}-\delta}&\delta_{i}<\delta \leq\delta_{f}\\ \tau=0&\delta>\delta_{f}\end{cases}\) & \(\begin{cases}\tau=\tau_{f}\frac{\delta}{\tau_{i}}&\delta\leq\delta_{i}\\ \tau=\tau_{f}\frac{\delta_{f}-\delta}{\tau_{f}-\delta}&\delta_{i}<\delta \leq\delta_{f}\\ \tau=0&\delta>\delta_{f}\end{cases}\) & \(\begin{cases}\tau_{f}\\ 0.125+\frac{\delta-\delta_{i}}{\tau_{f}}&\delta\leq\delta_{i}\\ \tau_{f}\end{cases}\) & \(\begin{cases}\tau_{f}\\ 0.125+\frac{\delta-\delta_{i}}{\tau_{f}}&\delta_{i}\\ \tau_{f}\end{cases}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ref. ** & **Type of Model** & \(\tau_{f}\) & \(\delta_{1}\) & \(\delta_{2}\) & \(\delta_{f}\) \\ \hline
[63] & Bi-linear & \(0.8f_{s}\) & \(0.8\frac{\delta_{f}}{\tau_{f}}/{\tau_{f}}\) & N/A & \(\frac{\chi_{c}}{\tau_{f}}\) \\
[67] & Bi-linear & \(f_{s}\) & \(\frac{\delta_{s}}{\tau_{f}}\) & N/A & \(\left\{\begin{array}{ll}\frac{\delta_{s}}{\tau_{f}}&f_{s}=0.1-0.5\;\text{mm} \\ 0.125+\frac{\delta-\delta_{i}}{\tau_{f}}&f_{s}=0.5-1\;\text{mm} \\ \hline \text{ }\textflexural behavior of the CFRP-strengthened and rehabilitated steel beams are presented in Table 6.  
Experimental observations on the effect of CFRP retrofitting systems on steel-concrete composite beams indicated a significant strength increase in specimens retrofitted with CFRP [13; 112]. In these cases, concrete crushing and CFRP rupture were reported as the main failure modes. In addition, it was suggested to attach CFRP laminates to beam webs which can lead to a decrease in the interfacial shear stress of the adhesive between the bottom CFRP plate and the steel due to shifting the neutral axis to a lower level [112]. The application of small-diameter strands of CFRP was also found effective in flexural strengthening of steel bridge girder, especially for NM CFRP [113]. The flexural capacity of the steel beams can also be enhanced by using CFRP laminates with higher tensile strength or increased thickness [15].