Shearing Properties Of Low Strength Reinforced Concrete Beams Strengthened With Carbon Fiber Composite Sheets

The History of reinforce concrete structures is almost 150 years. Here comes the new century, large amount of old concrete bridges and protective buildings which were constructed in the former time using old design codes face the safety problems. Since replacement of such deficient structures incurs a huge amount of public money and time, strengthening has become the acceptable way to improve their load carrying capacities and extend their service lives. However low strength concrete is the general properties of these structures, how to rehabilitate safely and effectively is now needed to be solved by engineers.Recently, fiber reinforced polymers (FRP) have been used worldwidely since they are quite light, have high tensile strength and stiffness, good durability, easy to handle and apply etc. According to the past studies of experiments and analysis, external bonded FRP is found to be highly effective for strengthening of RC structures. Unfortunately, until now most of the tests used normal concrete (the cubic strength of concrete f_cu>15MPa), just few of them used low strength concrete (the cubic strength of concretef_cu＜15MPa).Therefore, low strength reinforced concrete beams (the cubic strength of concrete f_cu ≤12MPa) are strengthened by gluing carbon fiber composite U type strips across the section of the beam to enhance shearing properties of beams, tests of fourteen strengthened beams are done in the lab. The test results show that using effective anchoring measures, shearing capacity and deformation ability of a reinforced concrete beam whose cubic strength of the concrete f_cu is not smaller than 5 MPa, can be improved obviously by gluing carbon fiber composite U type strips on the surface of the beam. Most of the beams failed in shear as expectation, the failed modes are almost bonding failure, the splitting of concrete behind the sheets caused the CFRP sheets to be pushed outwards and debonding was observed in most of the beams. The average bond stress between concrete and CFRP sheets seems to be the concrete tensile strength. Generally, the shear cracks form in 40°～45° of angle to the...