Experimental Study And Finite Element Analysis On Seismic Performance Of Recycled Concrete Column With Circular Steel Tube Under High Axial Compression Ratio

Recycled aggregate concrete（RAC）is a new concrete material which is made by processing waste concrete into recycled coarse aggregate and then replacing natural aggregate with it in a certain proportion.The mechanical properties and durability of RAC are both inferior to that of natural aggregate concrete,which limits the application of recycled concrete in concrete structures.RACFST is a feasible scheme to apply RAC in actual load-bearing structures.At present,domestic and foreign scholars have carried out a certain amount of research on the compressive performance and seismic performance of RACFST,but the research on the seismic performance of RACFST under high axial compression ratio has not been reported.This topic through the pillar of round steel tube recycled concrete with a constant axial force under horizontal low cyclic loading test,this paper discusses the axial compression ratio,slenderness ratio,steel tube wall thickness,the factors,such as recycled aggregate the influence law of seismic performance of the component,and on the basis of experimental study on the finite element simulation analysis of the components and the restoring force model of theoretical analysis,The main conclusions are as follows.（1）Under the combined action of constant axial force and horizontal low-cycle reciprocating load,the recycled concrete column of round steel tube will experience three typical stress stages:elastic rise,elasto-plastic rise and elasto-plastic fall.The final failure form of the specimen is generally the compression bending failure at the bottom of the column.The steel pipe wall at the bottom of the column buckles and the concrete inside the bottom of the column is crushed and the bottom of the column forms a plastic hinge that can rotate.Along with the steel tube wall thickness,axial compression ratio,slenderness ratio and other parameters change,the round steel tube failure process and failure characteristics of recycled concrete column there exist certain differences.（2）Under the condition of low axial compression ratio,the hysteresis curve of recycled concrete column with circular steel tube is smooth and full,and it has good ductility and energy dissipation capacity.When the axial compression ratio is high or the slenderness ratio is large,the hysteresis curve of the specimen has obvious phenomenon of sudden drop of bearing capacity and stiffness,and poor energy dissipation capacity and ductility when it reaches the peak load.（3）In the case of low axial compression ratio,the descending section of the skeleton curve of the recycled concrete column with circular steel tube is relatively gentle,indicating that the specimen has a good deformation ability after reaching the peak load.When the axial compression ratio is high or the slenderness ratio is large,the skeleton curve of the specimen drops sharply at the peak load,and the deformation ability is poor at the later stage.（4）With the increase of axial compression ratio,the horizontal bearing capacity of the specimens increases,the ductility coefficient decreases obviously,and the ductility of the specimens becomes worse.With the increase of slenderness ratio,the horizontal bearing capacity and ductility coefficient of the specimens decreased obviously.The replacement rate of recycled aggregate has no obvious effect on the horizontal bearing capacity and ductility coefficient of the specimens.The increase of wall thickness will enhance the horizontal bearing capacity and ductility coefficient of the specimen.（5）When the axial compression ratio of the specimen is 0.4,the average failure displacement Angle of the specimen is between 1/27 and 1/26,and when the axial compression ratio is 0.7,the average failure displacement Angle of the specimen is between 1/67 and 1/37.This indicates that the specimen with high axial compression ratio has a small displacement Angle and poor deformation ability when it is damaged.Therefore,the axial compression ratio should be limited in the design of recycled concrete columns filled with steel tubes to ensure their adequate ductility.（6）The stiffness degradation curve of all specimens is basically the same,which is fast in the early stage and slow in the late stage.The increase of axial compression ratio and wall thickness can increase the stiffness of specimens at the initial loading stage,but the stiffness degradation rate of specimens with higher axial compression is faster.The increase of slenderness ratio will decrease the stiffness of the specimen.The addition of recycled aggregate does not affect the stiffness degradation of the specimen.（7）The equivalent viscous damping coefficient of the specimen,ξ_eq,remained within the range of 0.32～0.42 at the end of the test,indicating that the recycled concrete columns filled with steel tubes have better energy dissipation capacity under low cyclic load than the reinforced concrete columns.The increase of axial compression ratio will increase the equivalent viscous damping coefficient of the specimen and enhance the energy dissipation capacity of the specimen.（8）In this paper,the finite element model of circular steel tube recycled concrete column under constant axial force and horizontal low cyclic load was established by using the finite element analysis software ABAQUS,and the model analysis results were compared with the test results to verify the rationality and effectiveness of the finite element model established.Then,based on the reasonable and effective finite element model,the influences of axial compression ratio,steel strength and steel pipe wall thickness on the seismic performance of this type of specimen were studied by changing the model parameters.The results show that when the axial compression ratio of the specimen is greater than or equal to 0.7,the bearing capacity and ductility of the specimen decrease obviously with the increase of the axial compression ratio.Under the condition of constant axial compression ratio of 0.7,the bearing capacity and ductility of the specimens can be improved by increasing steel strength and steel tube wall thickness.（9）In this paper,the theoretical restoring force model of recycled concrete column with round steel tube under high axial compression ratio is established and verified by experimental fitting method.The model can well reflect the hysteretic relationship between load and displacement of specimens and can be used to analyze the elastic-plastic response of recycled concrete structures with circular steel tubes.