Behaviour and strength of RC beams shear-strengthened with externally bonded FRP reinforcement

Abstract

Strengthening of RC beams using externally bonded (EB) fiber-reinforced polymer (FRP) composites has become a popular structural strengthening technique over the past two decades due to the well-known advantages of FRP composites including their high strength-to-weight ratio and excellent corrosion resistance. This thesis presents a systematic study into the analysis, behavior, and design of RC beams shear-strengthened with EB FRP reinforcement. In particular, the study is focused on the effects of shear interaction between the three components that contribute to the shear capacity of a shear-strengthened RC beam: the concrete, the internal steel reinforcement and the external FRP reinforcement, aiming to correct deficiencies in this area in existing knowledge and shear strength models. The work presented in this thesis is limited to shear failures associated with the development of shear tension cracks (or diagonal tension cracks). For such beams, the two typical shear failure modes are FRP rupture failure and FRP debonding failure. Both failure modes are studied in this thesis. For ease of discussion, only FRP reinforcement in the form of discrete strips is explicitly considered in the thesis unless otherwise stated. An experimental study on RC beams shear-strengthened with FRP strips wrapped around the entire beam cross-section (i.e. complete FRP wraps), with the beam sides either bonded to the FRP strips or intentionally left unbonded to the FRP strips, is presented to gain a better understanding of the shear resistance mechanism of such beams. The effect of bonding between the FRP strips and the beam sides and the interaction between the external FRP strips, concrete and internal steel stirrups are given particular attention. Using strain values measured in side-unbonded FRP strips, the effect of adverse shear interaction between the concrete and the FRP strips is examined. The effective (average) strain in the FRP strips when the shear resistance of concrete reaches its peak value is clarified. The test results also show that no significant adverse interaction exists between the steel stirrups and the FRP strips. Attention is next shifted to various aspects of the FRP debonding failure mode, particularly the effect of adverse shear interaction between external FRP strips and internal steel stirrups. In this part of the work, it is assumed that the shear failure of a shear-strengthened RC beam is dominated by the development of a single critical shear crack. A closed-form solution for the development of shear resistance contributed by FRP with the shear crack width is presented and validated using predictions from a simple computational model. This is followed by the presentation of a numerical investigation into the shear interaction between external FRP strips and internal steel stirrups using a similar computational model; a number of issues affecting shear interaction are clarified through this investigation. Based on both the closed-form solution and the results from the numerical investigation, a shear strength model considering shear interaction is proposed and is shown to predict the test results well. The final part of the thesis is concerned with the development and applications of an advanced finite element (FE) model capable of accurate modelling of the behaviour of shear-strengthened RC beams. The present FE model overcomes various limitations of the existing FE models for such beams, particularly in the modelling of localised cracking. Accurate modelling of localised cracking is crucially important for the accurate prediction of debonding failure in FRP-strengthened RC beams. Numerical results obtained from the FE model are used to provide a more accurate assessment of the effects of shear interaction and to provide additional support for the proposed shear strength model for the FRP debonding failure mode. The effect of pre-loading on the effectiveness of shear strengthening is also explored using results obtained with the FE model. The proposed FE model is the most advanced model available for debonding problems in FRP-strengthened RC structures and can be deployed to study many issues in debonding failures of such beams in the future.