| Ultra high toughness cementitious composite(UHTCC)is a kind of short fiber reinforced cement-based composite with tensile strain hardening characteristics and good crack control ability under tensile load.The application of UHTCC in structural components can greatly improve its deformation ability.However,in view of its high price,the strategic use of UHTCC in structural components is also one of the focuses in recent years.In this paper,the reinforced UHTCC is used as the core concrete of sheath to make RUHTCC-concrete composite columns.The seismic performance of eight composite columns with different shear span ratio,stirrup ratio and axial compression ratio is studied by monotonic loading and quasi-static low cycle reciprocating test,and compared with the RC columns.Finally,the seismic damage assessment is carried out.The main conclusions are as follows:1)Compared with ordinary RC short columns,reinforced UHTCC sheath can restrain the internal concrete well,and can change the failure mode of RC short columns from brittle inclined section failure to inclined section failure with certain ductility,which can show better resistance to lateral displacement and ability to maintain high load under large deformation.It not only has high ductility and energy consumption,but also has good performance.It is proved that UHTCC encased confined concrete is an economic and effective method to improve the seismic performance of RC short columns.2)The amount of sheath hoop has little effect on the bearing capacity,but has great effect on the deformation of composite columns.With the increase of stirrup ratio,the yield of the specimen is delayed,the deformation,ductility and energy consumption corresponding to the peak value and limit are significantly increased,the degradation of bearing capacity is slowed down,showing better hysteretic performance,and it is suggested that the minimum stirrup ratio should be configured according to the specification in order to play the constraint role of UHTCC sheath more effectively.The results also show that there is an upper limit for the hooping of UHTCC sheath.3)With the increase of axial compression ratio,the initial stiffness and bearing capacity increase,but the deformation and ductility of the specimen are poor,the bearing capacity decreases rapidly,and the energy consumption capacity decreases.The advantages ofusing RUHTCC concrete composite columns are more obvious under the condition of low axial compression ratio(n ≤ 0.2).Special attention should be paid to the interface bond between sheath and core concrete when using ruhtcc concrete composite structure under high axial compression ratio.4)With the increase of shear span ratio,the bearing capacity of composite columns decreases,but the deformation increase,ductility and energy consumption increase.5)Combined with the damage process of the composite columns,five performance grades are divided into "intact","slight damage","moderate damage","relatively serious damage" and "damage",and the corresponding angular displacement of different performance levels is determined.Kratzig and Park-Ang damage models are selected to calculate the damage index of the composite columns.The results show that Kratzig damage index converges to 1.0,but the damage index is large in the early stage,while the damage value of Park-Ang damage model is divergent,which is greater than 1.0.Finally,the Kratzig modified damage model is given,and the damage index interval of each state is determined,which is verified by experiments. |
