Experimental Study And Mechanical Analysis On Seismic Behavior Of Concrete Columns Reinforced With Carbon Fiber Reinforced Polymer Bars

Reinforced concrete column is the main component bearing vertical load in civil engineering structures such as high-rise residential buildings and industrial plants.Under the action of the earthquake,the reinforced concrete columns will produce large lateral residual deformation,which is difficult to repair.After the earthquake,it can only be pushed down for reconstruction,resulting in huge economic losses.Compared with ordinary reinforcement,carbon fiber reinforced polymer(CFRP)bars have the advantages of high strength and linear elasticity.It can effectively reduce the residual deformation of concrete columns after earthquake,and well meet the requirements of recoverable structure design concept.At present,there are few experimental and theoretical studies on the seismic performance of CFRP bar reinforced concrete columns in China.In this paper,the seismic performance of CFRP bar reinforced concrete column is studied by the combination of experimental research,numerical simulation and theoretical research.The main research contents and results are as follows:(1)A total of eight concrete columns reinforced with CFRP bars and steel bars were fabricated and tested under low cyclic repeated load.The crack development pattern,failure mode,hysteretic curve,skeleton curve,residual displacement,ductility,relative rotation,energy dissipation,strain distribution of longitudinal rebar,and stiffness degradation were analyzed.Three variables were set in the test,which were CFRP bars replacement ratio,CFRP bars position and axial compression ratio.There were three kinds of CFRP bars replacement ratios,0%,50% and 100%,two kinds of CFRP bars positions,the corner of cross-section and the middle of cross-section,and two kinds of axial compression ratios,0.15 and 0.30.The test results show that,the peak lateral force and corresponding drift ratio increase with the increase of CFRP bars replacement ratio,residual displacement and energy dissipation capacity decrease with the increase of CFRP bars replacement ratio.Comparing the two kinds of CFRP bar positions,specimens with CFRP bars placed at the corner of cross-section have a better lateral bearing capacity and smaller residual deformation.With the increase of the axial compression ratio,the peak lateral force of the specimen increases,but after the specimen reaches the peak lateral force,the lateral force decreases more rapidly and the residual deformation becomes larger.(2)In the plastic hinge region,reducing the bond strength between CFRP bars and concrete can effectively reduce the residual deformation,and the reduction range is proportional to the height of the low bond region;MRD can enhance the energy dissipation capacity and peak lateral force of CFRP bars reinforced concrete columns.The peak lateral force and energy dissipation capacity increase with the increase of current.(3)At the same time of low cyclic repeated load test,digital image correlation method is used to measure the deformation on the back of the specimens,and the relative angle curve,lateral displacement curve and crack development of the columns are analyzed,which verifies the feasibility and effectiveness of measuring specimen deformation by digital image correlation method in low cycle repeated load test.The results show that at the same drift ratio,the relative rotation angle in the height range of 50 mm to 350 mm decreases with the increase of CFRP bar replacement ratio and increases with the increase of axial compression ratio.The relative rotation angle in the height range of 350 mm to 650 mm increases with the increase of CFRP reinforcement replacement ratio and decreases with the increase of axial compression ratio.The lateral displacement at 350 mm height decreases with the increase of CFRP bar replacement ratio and increases with the increase of axial compression ratio.(4)Based on test results,an analysis model of concrete columns reinforced with CFRP bars and steel bars is established by using Open Sees software,the influence of CFRP bar replacement ratio,CFRP bar replacement positions and axial compression ratios on the seismic performance were taken into account.Comparing the numerical simulation results with the experimental results,it is found that the numerical simulation results are in good agreement with the experimental results.After verifying the effectiveness of the model,taking the specimens with 100% CFRP bar replacement ratio and 0.15 axial compression ratio as the control,the effects of specimen effective height,concrete strength,CFRP bars ratio and section shape on the seismic performance of CFRP bar reinforced concrete columns are studied.The results show that the peak lateral force increases with the increase of CFRP bars ratio and concrete strength,and decreases with the increase of effective height of specimens;the residual displacment decreases with the increase of CFRP bars ratio and concrete strength,and decreases with the increase of effective height of specimens;the cumulative energy dissipation of specimens increases with the increase of CFRP bar ratio,effective height and concrete strength;the peak lateral force of circular section specimen is lower than that of square section specimen,and the energy dissipation capacity of circular section specimen is weaker than that of square section specimen;When the drift ratio is the same,the residual drift ratio of circular section specimen is larger than that of square section specimen.(5)Based on the research results of our research group on the bond slip behavior of CFRP bars,a method for calculating the hysteretic performance of concrete columns that takes into account the bond-slip behavior of CFRP bars has been proposed.In this method,the column section is divided into finite fibers.On the basis of considering the bond-slip behavior of CFRP bars,the force balance equations of longitudinal reinforcement in the length direction are established,the stress-strain distribution of column section is analyzed,and the cross-sectional moment is finally obtained.The calculation results were compared with the experimental results and the two were found to be in good agreement.(6)Based on the low cyclic loading test results of concrete columns reinforced with CFRP bars and steel bars,a resilience model of concrete columns reinforced with all CFRP bars is proposed.In the resilience model,a calculation method of the peak lateral force of all CFRP bars reinforced concrete columns is proposed,which takes the compressive rupture of CFRP bars as the judgment condition.The hysteretic rules of all CFRP bars reinforced concrete columns are proposed,which are modified by using the test data.