Study On Ductility Of Reinforced High Strength Concrete Beams Confined By CFRP

With the advantages of high strength, light weight, corrosion resistance, temperature stability, convenience and other merits, FRP has got a variety of applications and research progress in structural reinforcement. Accompanied with the national achievements in science and technology, and the development of construction industry, large span & high-rise structures are continuously processing. Meanwhile, high strength concrete is applied extensively; nevertheless, the property of fragileness and poor ductility appeared in destruction hinder its applications. Traditional FRP-strengthening concrete beams can increase the flexural strength to 160%; however, the significant ductility loss cannot be ignored when damaged. If the bearing capacity of beams is enhanced excessively in designing, not only will brittle failure happen, but also higher requirements of the anchoring should be satisfied, which is not economical. As a consequence, when facing the higher ductility demand of high strength RC beams reinforced by FRP, it is necessary to develop a simple and effective method to improve the ductility. Based on this background and funded by the Natural Science Foundation of China (50878035), the ductility performances of reinforced high strength concrete beams strengthened by CFRP have been studied.1) Through the four points bending static load test, the research experiments on reinforced high strength concrete beams with plastic hinge at mid-span CFRP-Wrapped have been carried. The number of wrapped layers, wrapped length, and reinforcement ratio were selected as variables. Studies found that the displacement ductility coefficient increases when the wrapped layer increases, but the scale improved declines. And the coefficient rises when wrapped length grows, with an uptrend of the scale improved; the improvement of beams with high reinforcement ratio is obvious than that with low reinforcement ratio; the capacity of rotation of plastic hinge shows the same uptrend. This indicates that this approach of ductility strengthening is effective.2) Through the four points bending static load test, the research experiments on the reinforced high strength concrete beams with its tensile face CFRP-Bonded and plastic hinge CFRP-Wrapped have been carried. Studies found that the CFRP wrapping the plastic hinge can not only improve the ductility obviously, but also can effectively constrain the slippage and debonding of bonding CFRP after it fractures, improving the bearing capacity and the ductility after fractures. It indicates that the wrapping CFRP can not only enhance the ductility of beams before bonding CFRP fails, but can also improve the ductility and the bearing capacity after bonding CFRP fails.3) The stress-strain relationship of CFRP-confined model suggested by ACI440.2R-08 is chosen and a methodology of full-procedures section analysis is proposed. Effective constraint coefficients are selected to represent the number of wrapped layers. Moreover, wrapped length ratio is utilized to represent wrapped length. Specially, nominal pressure area coefficient is used to indicate the reinforcement ratio and bonding CFRP. Then, the relationship between the displacement ductility coefficients is analyzed, and an interrelated formula is proposed; Based on the above two methods, design suggestions are proposed and compared; and they can all result in safety-orientated calculation designs.