International Concrete Abstracts Portal

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.

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 paper presents the residual fracture characteristics of concrete beams shear-strengthened with carbon fiber reinforced polymer (CFRP) sheets subjected to high temperature ranging from 25°C [77°F ] to 200°C [392°F] for three hours. The beams are intentionally unreinforced to focus on strengthening effects. Three levels of initial shear deficiency are simulated by notching the beams: a0/h ratios = 0.25, 0.38, and 0.50 where a0 is the notch depth and h is the beam height. Ancillary tests are conducted to examine the temperature-dependent residual material properties of concrete, CFRP, and epoxy-concrete interface. Empirical equations are then generated to predict such properties for a potential modeling purpose. The interfacial fracture capacity is significantly affected by the level of temperature exposure, including two different failure modes such as failures at the concrete substrate and within the adhesive layer. CFRP debonding, associated with diagonal tension cracking of the concrete, is the primary source of the failure of the strengthened beams. High temperature exposure above the glass transition temperature (Tg) of the adhesive accelerates the bond failure. Energy dissipation of the strengthened beams is influenced by the elevated temperatures. The contribution of the CFRP to compliance variation is, however, not significant.

The size effect in shear in reinforced concrete (RC) one-way members without shear reinforcement becomes more of concern when using glass fiber reinforced polymer (GFRP) reinforcement. In fact, the lower axial stiffness of GFRP reinforcement typically results in wider flexural cracks with respect to steel RC counterparts. This issue is especially relevant for the case of flexural members without stirrups, such as retaining walls and slab bridge superstructures. Little evidence has documented the extent of such effect. Cognizant of this knowledge gap, ACI Committee 440 (FRP Reinforcement) introduced the current nominal shear strength algorithm, which was calibrated in a conservative fashion based on test results from small beams. This algorithm assumes that the shear strength at the critical section is resisted predominantly through the uncracked concrete above the tip of the shear crack. Based on the same fundamental assumption, a fracture mechanics algorithm for steel RC beams was recently proposed by ACI Committee 446 (Fracture Mechanics of Concrete). In this paper, the ACI 440 and 446 algorithms are verified and discussed based on experimental evidence from tests on scaled GFRP RC beams without stirrups. The latter algorithm is modified to account for the smaller elastic modulus of GFRP, under the hypothesis that its relevant parameters and the shear failure mechanism are similar irrespective of the reinforcement material.