FRP shear strengthening of reinforced concrete beams

Abstract: The shear failure mechanisms of flexural reinforced concrete (RC) members is highly complex; its precise details cannot be explained with simple analytical relationships, and are the topic of considerable scientific debate. The studies described and examined the three most used shear theories in the world – the fixed angle truss model (45°TM), the variable angle truss model (VAT), and modified compression field theory (MCFT). These three theories rest on the assumption that a beam loaded in shear behaves as a truss. However, this assumption is applied in different ways in various codes. In this thesis, three major standards, each of which uses a different implementation of these theories (CEN, 2005; ACI-318, 2008; CSA-A23.3, 2009), were used to predict the shear force capacity of a RC railway bridge that was strengthened in flexure with near surface mounted (NSM) carbon fibre reinforced polymers (CFRP) and then tested to failure. The data obtained in this test indicated that the codes underestimated the real shear behaviour of the bridge. There are some accepted reasons for such inaccuracies, namely the use of empirically derived equations in the ACI (2008) and CSA (2009) standards and the omission of the concrete contribution in CEN (2005). Moreover, the NSM reinforcement material used exhibits elastic behaviour until the point of failure; it was found that the use of such materials introduces further decreases the accuracy of the models’ predictions. The strains that developed in the area of the bridge where shear failure was expected were monitored throughout the test using a specially-developed photographic method. The results obtained with this method were promising, especially for research purposes, since it generated reliable data using relatively affordable tools. The use of FRP for shear strengthening introduces further complications to the problem of shear in reinforced concrete members because introduces two new failure modes: debonding at the concrete interface and fibre rupture of the FRP. Extensiveresearch has been carried out on FRP shear strengthening around the world. Much of the data gathered in these studies has been compiled in a database. By analysing this large database, it was found that the effectiveness of FRP shear strengthening is influenced by many factors, including the properties of the FRPs, the FRP strengthening configuration used, the nature of the beam’s cross-section, the shear span to depth ratio, the presence of stirrups, and the nature of the tensile reinforcement. Analysis of this database also demonstrated that most of the studies reported in the literature had focused on investigating the influence of the properties of the FRPs and the different configuration systems, and that the other factors mentioned above have been sparsely investigated if not totally ignored. The strengthening configuration and the amount of fibres influence the failure mode of the FRP and the shear force that it can carry. It appears that the side-bonded and the U-wrapped configurations are most proneto failure by debonding. This is consistent with the findings of various small experimental programs, and was confirmed by analysis of the larger dataset. These findings are relevant because failure of the FRP by debonding is more complex mechanism than is the rupture of the fibres mechanism. As is shown in this thesis, the extent to which the FRP variables (properties and strengthening configuration) can affect the point at which failure occurs and the mode by which it happens is dependent on the quantity of stirrups and tensile reinforcement in the beam, to the position of the load in relation to the size of the cross section (shear span to depth ratio), the type of strengthening configuration, the concrete and FRP properties. For design purposes, it is important to predict the shear failure of FRP shear strengthened beams with as much accuracy as possible. Therefore, a design model for debonding of the shear strengthening of concrete beams with FRP was developed and the limitations of the truss model analogy were highlighted. The fracture mechanics approach was used to analyse the behaviour of the bond between the FRP composites and the concrete. In this model, of the parameters examined, the fracture energy of concrete and the axial rigidity of the FRP are considered to be the most important. The effective strain in the FRP when debonding occurs was determined and the limitations of the anchorage length over the cross section were analysed; ultimately, a simple iterative method for shear debonding was proposed. Since the model’s predictions were considered satisfactory but not really precise, an extensive review of the literature was conducted. All of the significant theoretical models for predicting the shear capacity of FRP strengthened RC beams that have been reported over the years were analysed and commented on, and their predictions were compared to the results recorded in a preliminary experimental database. The predictions of the models that are most widely used in design were compared to the experimental results reported in the database; the model developed by the author was evaluated alongside these more established models. All of the models, including that presented in this thesis, were found to generate inaccurate predictions, but two models have been calibrated so as to provide safe estimates of the FRP shear capacity. Finally a new model for FRP shear strengthening was proposed for use in engineering. The new model was developed on the basis of an analysis of the contents of the database of experimental findings. The model incorporates several design equations adopted from various models and is set up for engineering use. The predictions of the shear force carried by the FRP strengthening material are found to be conservative.