Research On Seismic Behavior And Design Method Of High Ductile Concrete Jacketed Reinforced Concrete Columns

Reinforced concrete(RC)columns are prone to brittle failure under high axial load ratio or high shear stress due to the brittleness nature of concrete,which resulted in insufficient energy dissipation,serious damage or even collapse of the frame structure in strong seismic actions.In order to improve the seismic behavior of RC columns under high axial load ratio,high ductile concrete(HDC)jacketing was proposed to strengthen the model column specimens,and experimental study on seismic behavior of the specimens was carried out.Based on the test results,theoretical analysis was carried out,design method of the bearing capacity and deformation calculation model of HDC jacketed columns are proposed.The main research contents and achievements are as follows:(1)The four-point bending test and the double-side shear test of HDC were conducted.The effects of fiber content,water-binder ratio and fly ash content on the failure mode,bearing capacity and deformation capacity of the test specimens were studied.The test results indicated that the failure modes of HDC specimens exhibited ductile characteristics compared with the matrix(or concrete)specimens.Multiple cracking in HDC specimens was observed under bending and shear load,and the load-deformation curves of the test specimens exhibited plastic characteristics,which reflected good flexural toughness and shear toughness.Based on the evaluation method on flexural toughness of fiber reinforced concrete,the evaluation methods of flexural toughness and shear toughness of HDC were proposed.(2)The pseudo-static test of seven HDC jacketed columns,two ferrocement jacketed columns(employed as control specimens)and four non-strengthened columns was conducted,and the failure process,failure mode,bearing capacity,deformation capacity,energy dissipation capacity and stiffness degradation of the specimens were analyzed.The results indicated that the brittle failure mode of the specimens was effectively improved,meanwhile,the bearing capacity,deformation capacity and energy dissipation capacity were all significantly improved after jacketed by HDC.The external HDC jacketing significantly improved the seismic performance of RC columns,which can be used to strengthen the frame columns with high axial load ratio.(3)Based on the test results,the calculation formulas for flexural capacity of the HDC jacketed columns at crack state,yield state and peak state were proposed,and the calculated results agree well with the experimental results.Parameter analysis on the peak flexural capacity of the HDC jacketed columns was carried out,and design formula of the flexural bearing capacity of the jacketed columns,considering the effect of strain lag(“twice-loading”)of the HDC jacket,was developed.(4)According to the balanced condition of RC columns,the relative balanced compression height and limitation of axial load ratio of the HDC jacketed columns were derived.The results indicated that,the HDC jackets could still effectively improve the relative balanced compression height and the limitation of axial load ratio of the jacketed columns,even when the effect of strain lag of the strengthening jacket was considered.Design method of HDC jacketing for RC columns with exceed axial load ratio was proposed,which could ensure the HDC jacketed columns dominated by large eccentric compression.(5)Based on the truss-arch model and considering the strain-hardening behavior of HDC,the contribution of fiber bridging effect of HDC between the diagonal cracks to the shear strength is calculated by the soften-tensile strength of HDC.For HDC jacketed columns,the shear contribution of the HDC jacket is considered in the truss action and the shear contribution of axial load is considered in the arch action,thus,a modified truss-arch model for the HDC jacketed columns is obtained.And the calculation results verified the applicability of the modified model.(6)The flexural capacity of the specimens was improved after jacketing,the section curvature of the HDC jacketed columns was greatly improved and the longitudinal bars fully entered the plastic strengthening stage,thus the flexural deformation and slip deformation the HDC jacketed columns were significantly improved.The flexural deformation,shear deformation and slip deformation of the HDC jacketed columns at the crack state,yield state,peak state and ultimate state were calculated,respectively.The simplified skeleton curves were then obtained by calculating the deformation and load at each characteristic state,and the simplified skeleton curves showed good coincidence with the test skeleton curves.