The strengthening of concrete structures by means of externally bonded (EB) fiber-reinforced polymers (FRP) is now routinely considered to be an effective method for enhancing the loading capacity of existing structures. However, the debonding failure often governs the behavior of FRP shear-strengthened beams and prevents them from attaining their full load capacity. This paper presents a finite element model that was developed to investigate the FRP/concrete interfacial properties on the performance of FRP shear-strengthened beams. Nonlinear behavior of the plain concrete, steel reinforcing bars, FRP composites and FRP/concrete interface are simulated with appropriate models. Once the accuracy of the numerical model is established, the numerical analysis is carried out to investigate the parameters responsible for characterizing the initiation and propagation of debonding. These are the interfacial stiffness, the interfacial bond strength and the interfacial fracture energy. In this study, the variation of load–deflection relations is considered as a basis for the comparison. Results show that the interfacial stiffness and the bond strength have neglected influence on the behavior of FRP shear-strengthened beams. Furthermore, the interfacial fracture energy is the main parameter among the bond stress–slip model parameters influencing the strengthening performance of FRP shear-strengthened beams in terms of load–deflection relations and ductility.

This study focuses on the characterization of the fracture properties of the concrete-epoxy interface (CEI) under FRP sheets and FRP Uwraps. In particular, the Mode I and Mode II fracture energies are obtained with and without the effect of the FRP Uwrap anchor. Results indicate that fracture energy of the CEI increases due to the confining effect of Uwrap. These material properties, along with a proposed bond-slip model, are used in numerical simulations of concrete elements strengthened with externally bonded FRP sheets and anchored with FRP Uwraps. Results show that the characterization of the fracture properties of the CEI is needed to accurately predict the complex behavior of the CEI under the FRP Uwrap.

In this paper, the problem of debonding in flexural members strengthened with FRP layers bonded on their tensed and compressed faces is investigated using the fracture mechanics theory. This problem is particularly relevant to double sided FRP applications for the strengthening of masonry or reinforced concrete walls to resist cyclic or dynamic loading. The paper adopts an analytical methodology and compares between two fracture mechanics based approaches for the assessment of the initiation, evolution, and stability of the debonding process. The first approach uses the nonlinear fracture concept of the cohesive interface. The second approach adopts the classical fracture mechanics concept of the energy release rate. In both models, the effect of geometrical nonlinearity and buckling of the compressed layer and its role as the driving force for the debonding process are considered. The two approaches are compared and emphasis is placed on the stability of the debonding process and the post-debonding behavior. These aspects are illustrated through a numerical study that focuses on a masonry specimen strengthened with double-sided FRP systems and subjected to flexure. Conclusions on the behavior of the unique structural system, its stability, and its handling using the fracture mechanics approaches close the paper.

A major research program was carried out to analyze the mechanism of FRP debonding from concrete beams using global-energy-balance approach (GEBA). The key findings are that the fracture process zone is small so there is no R-curve to consider, failure is dominated by Mode I behavior, and the theory agrees well with tests. The analyses developed in the study provide an essential tool that will enable fracture mechanics to be used to determine the load at which FRP plates will debond from concrete beams. This obviates the need for finite element (FE) analyses in situations where reliable details of the interface geometry and crack-tip stress fields are not attainable for an accurate analysis. This paper presents an overview of the GEBA analyses that is described in detail elsewhere, and explains the slightly unconventional assumptions made in the analyses, such as the revised moment-curvature model, the location of an effective centroid, the separate consideration of the FRP and the RC beam for the purposes of the analysis, the use of Mode I fracture energies and the absence of an R-curve in the fracture mechanics analysis.

Externally-bonded fiber reinforced polymer (FRP) laminates are commonly used to strengthen or repair reinforced concrete members. The behavior of strengthened members is influenced not only by the properties of reinforced concrete and FRP laminates but also by their interface properties such as bond strength. Conventional strength based approaches lack the accuracy required to predict the interface behavior because they do not account for energy release during debonding. A fracture mechanics approach can provide a better alternative because it can account for post-cracking behavior, debonding propagation, and flaws and defects at the interface and within the materials. This paper presents a review on recent fracture mechanics approaches used to understand the debonding behavior of reinforced concrete members with externally-bonded FRP laminates. With a brief description of failure modes of FRP-strengthened beams, the methodology and limitations of strength based models of debonding are summarized and discussed. As the debonding of FRP from beams can be categorized as Mode-I, Mode-II, or Mixed Mode, test methods to attain the critical energy release rate of the FRP-concrete interfaces in specific modes are also presented. Analytical and numerical fracture mechanics based models are reviewed with respect to fracture energy and energy calculation methods.