Study On Ductility Performance Of Reinforced High Concrete Columns Confined By CFRP

In recent years, with the development of structures toward long-span, heavy load, high-rise and super high-rise, high-strength concrete is frequently used in civil engineering However, as concrete strength increases, its brittleness becomes notable and its ductility reduces remarkably, which is harmful for anti-seismic. The strengthening method of being externally wrapped fiber-reinforced polymers (FRP) can significantly improve the seismic performance of concrete structures. Because of its low density, high-strength, anticorrosion, fatigue resistance, good temperature stability, convenient construction and no changes in structure rigidity, the method has attracted popular attention in civil engineering. It is proved that research on FRP improving the behavior of the high-strength concrete has been examined in many literatures, but researcher rarely consider FRP about improving the ductility of the high-strength RC columns specifically, or a relatively small consideration, or not systematic. Besides, the calculation method of strengthening index based on ductility coefficient has not been established. Therefore, a cyclic horizontal loading test was conducted combining with the research programs of National Natural Science Foundation of China (50878035) in order to study the ductility performance of CFRP strengthening high-strength RC columns mainly from being wrapped externally CFRP in transverse and anchoring measures. The main contents are following:(1) Through a cyclic horizontal loading test, CFRP improving the seismic behavior of the high-strength RC columns was researched and the influence of axial load ratio, aspect ratio and strengthening volume on the ductility and energy absorption capacity of the high-strength RC columns was discussed. Experimental results showed that the ductility, energy absorption capacity and strength can be significantly reduced as the increase of axial load ratio and the decrease of aspect ratio. However, the seismic behavior of columns with high axial load ratio or low shear span ratio was improved obviously after being wrapped CFRP in transverse. And the ductility, energy absorption capacity increased with the increase of strengthening volume, but the improvement extent reduced gradually.(2) Through a cyclic horizontal loading test, the high-strength RC columns with only being wrapped CFRP and being wrapped CFRP plus anchoring measures were studied. Experimental results showed that hybridized retrofit method not only supplied the corner concrete with confinement, but also supplied confinement for the core concrete, which increased the effective confinement areas and improved the ductility of the columns more than the strengthening technique of only being wrapped CFRP.(3) Using two different anchoring techniques (embedding bolt bar and clamping plates), to confine the non-confinement areas of the high-strength RC square columns, the effect of the two techniques on CFRP improving the seismic behavior of the high-strength RC columns was studied. Because the hybridized retrofit technique of combining clamping plates and wrapping CFRP sheets was an active confinement method compared with the one of combining embedding bolt bars and wrapping CFRP that a passive confinement method, the effective confinement areas were increased and the ductility was enhanced.(4) Based on the test results of CFRP strain, comprehensively analyzing the influence factors about effective constraint coefficient, the calculation method of CFRP effective constraint coefficient was proposed.(5) According to the research results of the high-strength RC columns strengthened by CFRP, the calculation formula of displacement ductility coefficient, comprehensively consider shear span ratio, axial load ratio, volumetric ratio eigenvalue of transverse reinforcement, volumetric ratio eigenvalue of CFRP, tie configuration, effective confinement of CFRP and anchoring measures, was presented.