# Retrofitting of Steel Structures with CFRP: Literature Review and Research Needs
## 5 Research Needs and Recommendations  
A review of all the investigations regarding the application of CFRP materials for the retrofitting of steel members suggests that research in this area is rather limited. The number of parameters affecting the behavior of CFRP retrofitted steel members increases the complexity of their behavior. Accordingly, many experimental programs are needed to investigate the effect of these parameters. Furthermore, there are limited numerical investigations that can accurately predict the behavior of CFRP retrofitted steel components. Hence, special attention needs to be devoted to developing reliable numerical modeling. Although the applied CFRP in the literature includes various elastic moduli, investigations involving UHM CFRP have not been adequately documented. Therefore, further research is needed in this regard. Further investigations on the behavior of steel components retrofitted with CFRP could be conducted in the following recommended research areas:  
* Further investigation is needed to develop adhesives with enhanced mechanical properties to improve bond strength and durability.
* Further research should be conducted to investigate the effect of interrelated parameters on the bond behavior of the CFRP/steel interface to propose an optimal retrofitting system. The finite element modeling can be considered a cost-effective solution in this regard.
* To better analyze the impact of the CFRP elastic modulus on the performance of the retrofitted steel elements, it is advisable to use CFRPs with approximately the same tensile capacity but varying elastic modulus.
* More investigation is required to develop bond-slip models at the CFRP/steel interface under fatigue loading by considering the influencing variables that are representative of conditions in practice.
* More research is needed to investigate the effect of fatigue loading on the effective bond length. Also, the effect of shear combined with flexure on the bond length is to be clarified.
* More research could be conducted to investigate the effect of galvanic corrosion, especially in the CFRP retrofitting method utilizing a steel anchorage system. The long-term effect of galvanic corrosion has not been properly investigated.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Study** & **Software** & **Steel Element Type** & **CFRP Element Type** & **Adhesive Element Type** & **Interactions** \\ \hline
[39] & ANSYS (2009) & 8-node SOLID45 & 8-node SOLID45 & 8-node SOLID45 & Perfect interface \\ \hline
[60] & ABAQUS (V6.8) & CPE4R & CPE4R & COH2D4 & Quadratic traction damage initiation criterion \\ \hline
[46] & ABAQUS (V6.13) & C3D8R & SC8R & COH3D8 & Tie constraint \\ \hline
[18] & ANSYS (V17) & SOLID186 & SOLID186 & INTER204 & Mixed-mode bi-linear CZM \\ \hline
[77] & ABAQUS & T2D2 & T2D2 & COH2D4 & Bi-linear bond–slip derived from experimental data \\ \hline
[106] & ABAQUS & 2-node beam element & \begin{tabular}{c} 8-node plane stress \\ element \\ \end{tabular} & \
\begin{tabular}{c} 8-node plane stress \\ element \\ \end{tabular} &
\begin{tabular}{c} 8-node plane stress \\ element \\ \end{tabular} & Multipoint constraints \\ \hline
[122] & Not specified & \begin{tabular}{c} 8-node element with \\ 2 DOF per node \\ \end{tabular} & \
\begin{tabular}{c} 8-node element with \\ 2 DOF per node \\ \end{tabular} & Not specified &
\begin{tabular}{c} Reciprocal work contour \\ integral method \\ \end{tabular} \\ \hline
[59] & ABAQUS (2004) & C3D8 & S4R & COH3D8 & Tie constraint \\ \hline
[126] & ABAQUS (V6.4.1) & C3D20R & C3D20R & C3D20R & Common nodes merged \\ \hline
[16] & ANSYS & SOLID45 & LINK8 & COMBIN39 & Bi-linear bond–slip model \\ \hline
[127] & ABAQUS & C3D8I & COH3D8 & COH3D8 & Mixed-mode cohesive law \\ \hline
[128] & ABAQUS & C3D8I & C3D8R & COH3D8 & Trapezoidal mixed-mode CZM \\ \hline \hline \end{tabular}
\end{table}
Table 7: Summary of numerical studies.