Shear Strengthening of Reinforced Concrete Beams Using Externally Bonded Carbon Fibre Reinforced Plates
The fundamental aim of this study was to investigate the behaviour of reinforced concrete beams strengthened in shear by the addition of Carbon Fibre Reinforced Plastics (CFRP). An experimental programme was devised to investigate the shear behaviour of reinforced concrete beams strengthened by the attachment of different configurations and quantities of CFRP using epoxy adhesives. Finite Element Analysis (FEA) was used to model the behaviour of some of the strengthened beams. Thirty-eight reinforced concrete beams of 1.8 m length were constructed and tested. All the beams were designed to fail in shear. The following variables were taken into consideration: main reinforcement ratio, shear reinforcement and spacing between links in the shear span, and different configurations of CFRP plates on shear spans. The concrete strength was intended to be a consistent mixture but the cube strengths varied between 50 to 70 N/mm2 . The CFRP's role was to prevent significant opening of the shear cracks in the concrete. All beams tested showed a significant improvement in shear strength by the addition of plates. The increase varied between 20-120 % over the control beams. It was observed that there is a relationship between the increased ratio of the shear strength and the CFRP area, which covered the shear span. The best arrangement was to cover the whole shear span with horizontal unidirectional (UD) CFRP sheets. The optimum economic solution was to cover half of the shear span area using two horizontal CFRP strips, although the positioning of these strips may be critical. This solution increased the shear strength by about 50 %. One important outcome was the significant increase achieved by using wrapping fabric sheets. Despite the fact that wrapping materials have lower properties, they provided a considerable enhancement in shear strength of about 80% of what the pultruded plates are capable of developing. It should be noted that this contradicts the design guidelines provided by the Institution of Structural Engineers, where it is stated that the strength increase is highly dependent on the stiffness of the FRP, while in this research it was proved experimentally and numerically (by FEA modelling) that stiffness has no real effect. The role of the FRP materials is to prevent the shear crack from widening and opening up. Increasing the tensile strength or the Young's modulus for the FRP does not necessarily enhance the performance of the material in preventing shear failure. The results showed that shear strength is not affected by the number of fabric layers, nor by the thickness of the CFRP materials. It would appear that a cured composite fabric with very modest properties (almost one eighth that of pultrusions) can provide the same strengthening performance as the UD laminates. FEA techniques were used in this research to model the behaviour of certain cases of externally shear strengthened RC beams. A non-linear 2D plane stress with a cracked concrete model was adopted in this study. FEA results corresponded well to the experimental work and showed that using higher value of Young's modulus of concrete Ee, or using smaller values of concrete tensile strength cr 1 , reduced the failure load and the deflection. Results also showed that changing the properties of the CFRP materials or the thickness had little effect on the shear strength obtained. Results showed that FEA modelling in 2D can be used to predict the failure load for both pultruded and prepreg materials, but it underestimates the deflection at all stages due to the fact that the concrete model used in this study considers the tensile cracking, but not failure in compression. Finally guidelines for installation and design of shear strengthening with FRP are provided.
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