# Retrofitting of Steel Structures with CFRP: Literature Review and Research Needs
## 3 Flexural Retrofitting of Steel Beams
Furthermore, the CFRP laminates with higher elastic modulus have a better performance in enhancing the ductility of the strengthened beams [112]. Failure analysis and structural behavior of CFRP-strengthened steel beams show that the load-carrying capacity with thicker and longer CFRP plates increases, though excessive thickness could cause premature debonding [114]. In order to prevent debonding failure and use the full capacity of the CFRP, mechanical anchorage systems have been utilized. It was shown that the mechanically fastened CFRP technique outperformed CFRP sheets in flexural strength and ductility [115]. Moreover, the application of the anchorage system to hybrid FRP also resulted in the same results [17]. Furthermore, in this case, prestressing CFRP increases the efficiency of the retrofitting method [116]. In addition, this technique can delay crack propagation and also results in an increase in the fatigue life of specimens [117]. A comparison between the performance of different FRP techniques as well as using steel plates for retrofitting reveal that HM CFRP is superior to the other rehabilitation methods in terms of fatigue behavior [118]. CFRP can significantly extend the fatigue life of the steel specimens and increase the average number of load cycles to failure [4]. Furthermore, CFRP materials can enhance the flexural fatigue behavior of the steel beams in terms of elastic stiffness, yield load, and nominal capacity [119]. The CFRP retrofitting method can also contribute to a reduction in fatigue crack growth in damaged steel beams [20]. In this regard, the elastic modulus of the CFRP is also found an important parameter in enhancing the fatigue life of specimens. Specimens retrofitted with higher elastic modulus outperform those retrofitted with CFRP with lower elastic modulus [120]. Furthermore, the stress intensity factor can be affected by the number of CFRP layers. It was shown that the stress intensity factor in specimens with a double CFRP configuration is significantly less than of the specimens strengthened with a single CFRP layer [24]. Figure 4: Some of the failure modes of steel beams strengthened with FRP plate [111]. \begin{table}
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As for the type of adhesives, it was found that adhesives with relatively high ductility can effectively redistribute stress within the adhesive layer during increased loading [121]. Consequently, it is suggested to use nonlinear adhesives for retrofitting steel structures as the specimens bonded with linear adhesives are more susceptible to interfacial debonding [123]. Prestressing CFRP sheets can effectively increase the fatigue life of cracked steel elements as compressive forces are applied to the crack edges, thereby hindering crack growth in steel structures [21]. It was also shown that the prestressed CFRP can decrease the interfacial stress at the notch location and delay interfacial debonding [124]. Also, the level of the prestressing of the CFRP has a significant effect on extending the fatigue life of strengthened specimens [120]. The minimum level for prestressing the CFRP needed to increase the fatigue life of steel beams was determined in the literature [125].