Omar Chaallal

The performance of steel elements retrofitted with carbon fiber-reinforced polymer (CFRP) relies on the bond behavior of the interface between CFRP and steel substrate, as the adhesive is typically the weakest link of the retrofitting system. Therefore, understanding the bond behavior between CFRP and steel substrate is crucial for retrofitting design of steel structures. This necessity has been the main impetus for conducting the present experimental study on the bond behavior of CFRP/steel double strap joints under static loadings. To that end, the adhesive type, CFRP-bonded length, and surface preparation are the parameters considered in this research. Additionally, the effect of CFRP elastic modulus on the bond behavior is examined by using Normal Modulus (NM) and Ultra-High Modulus (UHM) CFRP with approximately the same tensile capacity. Failure modes, bond strength, effective bond length, and bond–slip curves are some of the key issues that will be thoroughly discussed in this study.

In this study, the flexural behavior of notched steel beams retrofitted with CFRP is investigated. Two series of tests, including W200 × 22 and W14” wide-flange notched beams rehabilitated with externally bonded (EB) CFRP are evaluated under static loading. The W200 × 22 beams were received directly from a factory, whereas the W14” wide-flange beams were extracted from the Original Champlain Bridge after roughly 60 years in service. The parameters considered include the CFRP elastic modulus, CFRP configuration, notch depth, anchorage system, and adhesive type. The effect of the CFRP elastic modulus on the rehabilitation technique is examined by using Normal Modulus (NM) and Ultra-High Modulus (UHM) CFRP with approximately the same tensile capacity. Failure modes, load–deflection behavior, strain distributions along the CFRPs, and Crack Mouth Opening Displacement (CMOD) are thoroughly discussed in this study. The results reveal that both UHM and NM CFRP significantly enhance the load-carrying capacity. However, specimens retrofitted with UHM CFRP exhibit a brittle behavior, whereas those strengthened with NM CFRP show a more ductile behavior.

This paper presents the results of an experimental study involving 20 tests performed on elliptical concrete columns confined with externally bonded carbon fiber-reinforced polymer (EB-CFRP) laminates. The study aimed to evaluate the effects of elliptical aspect ratio (A/B) as well as confinement rigidity (number of EB-FRP layers) on confinement effectiveness. The experimental program consisted of one series of control concrete columns (unstrengthened) and three additional series, each one strengthened with one, two and three layers of EB-CFRP sheets, respectively. Furthermore, each series considered five elliptical aspect ratios (A/B) ranging from 1.0 to 1.6. Following compressive concentric tests until failure, the results were analyzed to characterize the confinement level with an increasing number of EB-CFRP layers as a function of the elliptical aspect ratio. The results show considerable enhancements in compressive strength and in the ductility of the confined columns. Furthermore, this improvement is amplified as the number of EB-CFRP layers increases, indicating a proportional relationship between the compressive strength and the number of CFRP layers. It is found that the ultimate strength of EB-CFRP-confined columns with three layers reached up to 130% compared to the control specimens. However, increasing the elliptical aspect ratio reduced the compressive strength and ductility of confined columns. This study investigated the relation between the CFRP hoop and axial strains and the elliptical aspect ratios. Moreover, through comparison, the results reveal that the prediction models proposed by the Canadian standards S806-12 and S6-19 do not capture the negative effect of the elliptical aspect ratio in confined concrete columns.

The application of the externally bonded (EB) carbon fiber-reinforced polymer (CFRP) technique for retrofitting steel elements offers significant advantages over the conventional method. The high strength-to-weight ratio and corrosion resistance of CFRP materials have made them a viable alternative for retrofitting steel structures. This paper covers a wide range of aspects discussed in the research investigations to date on CFRP bonded steel elements and provides a critical review of the topic under both static and fatigue loading conditions. In the end, research needs and recommendations are presented in this respect.

In medium- to high-rise buildings, single shear walls (SSWs) are often used to resist lateral force due to wind and earthquakes. They are designed to dissipate seismic energy mainly through plastic hinge zones at the base. However, they often display large post-earthquake deformations that can give rise to many economic and safety concerns within buildings. Hence, the primary objective of this research study is to minimize residual deformations in existing SSWs located in the Western and Eastern seismic zones of Canada, thereby enhancing their resilience and self-centering capacity. To that end, four SSWs of 20 and 15 stories, located in Vancouver and Montreal, were meticulously designed and detailed per the latest Canadian standards and codes. The study assessed the impact of three innovative strengthening schemes on the seismic response of these SSWs through 2D nonlinear time history (NLTH) analysis. All three strengthening schemes involved the application of Externally Bonded Fiber Reinforced Polymer (EB-FRP) to the shear walls. Accordingly, a total of 208 NLTH analyses were conducted to assess the effectiveness of all strengthening configurations. The findings unveiled that the most efficient technique for reducing residual drift in SSWs involved applying three layers of vertical FRP sheets to the extreme edges of the wall, full FRP wrapping the walls, and full FRP wrapping of the plastic hinge zone. Nevertheless, it is noteworthy that implementing these strengthening schemes may lead to an increase in bending moment and base shear force demands within the walls.

Shear failure in reinforced concrete (RC) beams have always been a serious concern due to its brittle fracture mode. In addition, many questions are raised about the accuracy of current design guidelines for predicting the shear resistance contribution of externally bonded carbon fiber-reinforced polymers (EB-CFRPs) to the ultimate shear strength of strengthened RC beams, particularly with regard to the inverse interaction of the shear contribution between steel-stirrups and EB-CFRPs. The main objective of the present study is to implement experimental and numerical tests to develop analytical and numerical models for RC beams strengthened in shear using EB-CFRPs. Emphasis is placed on the negative inverse interaction between the steel-stirrups and the EB-CFRPs as the ratio of EB-CFRP-to-steel-stirrups increases with increasing the CFRP rigidity (CFRP thickness and CFRP width). The inverse interaction is not included in the design models proposed by most current guidelines, although it has a considerable effect on shear resistance as predicted by the guidelines. In fact, the shear contribution associated with EB-CFRP decreases as the ratio of EB-CFRP-to-steel-stirrups increases. Therefore, proposing reliable effective strains by including these parameters improve the calculated shear contribution of EB-CFRPs. First, an analytical model is proposed for CFRP effective strain considering the inverse interaction between EB-CFRPs and steel-stirrups. Afterward, a validation of the proposed model with experimental data is done by conducting a parametric study of the increasing trends with respect to the ratio of EB-CFRP-to-steel-stirrups. A numerical finite-element model for the reduction factor and the corresponding effective strain based on the inverse interactions between EB-CFRPs and steel-stirrups is also proposed, and the results are compared with various current guidelines. The results are presented in terms of shear crack patterns, load-midspan deflections, shear stresses, strain responses along the fibers, maximum strain profiles for all the CFRPs and specimens, applied shear forces and strains for all the steel-stirrups and EB-CFRPs, and interactions between steel-stirrups and EB-CFRPs based on their maximum strain contributions at the maximum shear forces and the maximum strain they experience after shear failure.

Coupled shear walls (CSWs) are structural elements used in reinforced concrete (RC) buildings to provide lateral stability and resistance against seismic and wind forces. When subjected to high levels of seismic loading, CSWs exhibit nonlinear deformation through cracking and crushing in concrete and yielding in reinforcements, thereby dissipating a significant amount of energy, leading to their permanent deformation. Externally bonded fiber-reinforced polymer (EB-FRP) sheets have proven to be effective in strengthening RC structures against various loading and environmental conditions. In addition, their high strength-to-weight ratio makes them an attractive solution as they can be easily applied without significantly increasing the structure’s weight. This study investigates the effectiveness of using EB-FRP sheets to reduce residual displacement in CSWs during severe earthquake loadings. Two series of 15-story and 20-story CSWs in Western and Eastern Canadian seismic zones, which serve as representative models for medium- and high-rise structures, were evaluated through nonlinear time history analysis. The numerical simulation of all CSWs and strengthened elements was carried out using the RUAUMOKO 2D software. The findings of this study provided evidence of the effectiveness of EB-FRP sheets in reducing residual deformation in CSWs. Additionally, significant reductions in the rotation of the coupling beams (CBs) and the inter-story drift ratio were observed. The results also revealed that bonding vertical FRP sheets to boundary elements and confining enhancement by wrapping CBs and wall piers is a very effective configuration in mitigating residual deformations.

The application of externally bonded (EB) carbon fiber reinforced polymer (CFRP) systems for strengthening existing structures, such as RC beams, has been widely approved in the construction industry worldwide for its numerous benefits. The CFRP-to-concrete bond has a governing role in the reliability and effectiveness of EB-CFRP systems. Indeed, failure of the CFRP-to-concrete bond can lead to rupture of CFRP-strengthened structures. Hence, ongoing research into assessment of bond behavior at the CFRP-to-concrete interface helps to bring more insightful clarity to the use of EB-CFRP strengthening techniques. The aim of this study is to evaluate the bond behavior between CFRP and concrete by conducting a series of pullout tests. The parameters considered include CFRP type (sheet versus laminate), bonded length, and bonded CFRP width. The results show that using CFRP fabric sheets can contribute to higher bond load-carrying capacity and ductility than CFRP laminates. Furthermore, through the analyses of databases in the literature, a bilinear bond–slip model is proposed that takes into account the CFRP width factor. Through a comparison, it is shown that the proposed model performs well in terms of predicting the maximum local bond stress and CFRP slippage. This research was aimed at investigating the use of CFRP composites for strengthening RC structures. The complexities regarding the behavior of CFRP-strengthened RC beams and columns, especially at the CFRP-to-concrete bond, provoked the authors to conduct this experimental study. The testing program was intended to evaluate the effect of certain parameters on the bond behavior of a CFRP-to-concrete interface. In addition, an analytical study was carried out to develop a model, capable of characterizing the interfacial behavior in terms of bond shear stress and bond slip. The proposed model proved to perform well at predicting the bond–slip behavior in a validation study. Findings obtained from this research can better familiarize civil engineers with the application of EB-CFRP strengthening techniques in concrete surfaces and possibly enhance the design of such retrofitting systems in deficient real-life RC structures.

The practice of externally bonded (EB) carbon fiber-reinforced polymer (CFRP) fabrics for structural strengthening increasingly shows evidence of effectiveness for the rehabilitation of existing structures. However, debonding of fiber-reinforced polymer (FRP), particularly in shear, is still a major premature failure mode when using FRP sheets to strengthen concrete structures. This paper presents the results of an experimental investigation of the performance of full-scale reinforced concrete (RC) beams strengthened in shear using EB-FRP U-wrapping (UW) and full wrapping (FW) with different fabric thicknesses. Full wrapping is used mostly on columns with all sides accessible and does not generally apply to beams. The main objectives of this study are to optimize FRP capacity and to evaluate the effect of confinement and the effectiveness of FRP using different strengthening configurations for RC beams, such as FW and UW. The results obtained in this study reveal that fully wrapped beams exhibit enhanced CFRP contribution to shear resistance compared to U-wrapped beams. In addition, the FW configuration eliminated the CFRP debonding failure mode and resulted in an increase in concrete strain.