Study On Seismic Behavior Of Rectangular Section Compression Members Of Ultra-high Performance Concrete With Mixed Reinforcement

In order to improve the seismic performance of the structure and enhance the recovery of the structure after earthquake,the UHPC(ultra-high performance concrete)column with high-strength reinforcement and CFRP(carbon fiber reinforced polymer)reinforcement is proposed.The ductility of high-strength steel bars and the ductility of UHPC can improve the energy dissipation capacity of members,and the linear elastic properties of FRP bars can achieve smaller residual deformation of members.The anchorage performance of CFRP bars in UHPC determines whether they can work together effectively.Although the existence of embossing on the surface of CFRP bars can effectively improve the bond strength between CFRP bars and the bonding medium,the longitudinal fibers in the reinforcement will be partially squeezed during the embossing process,which will weaken the cross-section of the reinforcement,thus reducing the strength of the reinforcement.At the same time,the strength of reinforcement material is reduced within the acceptable range.Based on this,the anchorage performance of CFRP bars with different surface structures and the seismic performance of UHPC columns with mixed reinforcement are studied in this paper.The main research contents are as follows:1.Experimental study on anchorage performance of CFRP bars with different surface structures in UHPCThe experimental study on the anchorage performance of CFRP bars in UHPC was carried out with the surface structure and anchorage length as parameters.The results show that:(1)The surface embossing structure has obvious influence on the tensile strength of CFRP bars.Compared with plain CFRP bars,the tensile strength of shallow,medium and deep embossed CFRP bars is reduced by 14%,20% and 50% respectively.(2)The surface structure of CFRP bars has significant influence on its anchorage performance.The peak load and bond strength of plain CFRP bars are only 14%～17%of those of embossed CFRP bars.Although the bond strength,ductility coefficient and energy consumption of deep embossed specimens are significantly higher than those of shallow and medium pressure specimens,the tensile strength is reduced by 30%.Considering the influence of the surface structure of CFRP bars on the tensile strength and bond strength,the embossing depth should be 1.5%～2% of the reinforcement diameter,and the rib height width ratio should be 3%～4%.(3)There are two failure modes in the test,namely pullout failure and tensile failure of CFRP bars;for the specimens with pullout failure,the peak load and energy consumption of CFRP bars increase with the increase of anchorage length,while the bond strength and ductility coefficient decrease with the increase of anchorage length.(4)According to the test results,the prediction formula of bond strength and critical anchorage length of CFRP bars in UHPC is established,and the effect of rib height width ratio is considered.The calculated results are in good agreement with the experimental results.2.Experimental study on seismic behavior of UHPC rectangular section compression members with mixed reinforcementThe seismic behavior of UHPC columns is studied by means of reinforcement and axial compression ratio.The results show that:(1)The failure mode of all specimens is normal section failure with good ductility and energy dissipation capacity.(2)The results show that the larger the axial compression ratio is,the greater the peak load and the smaller the ultimate displacement,and the worse the ductility,energy dissipation capacity and recoverable performance are;the mixed reinforcement can significantly improve the recoverable performance of UHPC columns,and the residual deformation of hybrid reinforced columns is only 40% of that of high-strength reinforced columns.(3)When the axial compression ratio is low(n=0.1),compared with the highstrength reinforced column,the peak load increases by 41.4%,the ultimate displacement increases by 24.9%,the ductility coefficient increases by 24%,and the cumulative energy consumption at failure is close to that of the high-strength reinforced column;however,when the axial compression ratio is high(n=0.3),the reinforcement form has no obvious influence on the peak load,ultimate displacement and ductility coefficient of the specimen,and the cumulative energy consumption of the composite reinforced column is about high 65% of the reinforced column.The results show that the hybrid reinforced UHPC columns can give full play to the respective properties of materials and improve the seismic performance of members when the test axial compression ratio less than 0.3.(4)In the Open Sees analysis model,the equivalent stirrup ratio is used to consider the constraint of steel fiber in UHPC,and the corresponding calculation method is proposed.The simulation results are in good agreement with the experimental results.(5)The finite element software Open Sees was used to analyze the parameters.The results show that the mixed reinforcement is more suitable for the specimen with the design axial compression ratio less than 0.6,and the proportion of CFRP bars in the longitudinal reinforcement of UHPC columns should not be greater than 1/3;increasing the axial compression ratio will significantly reduce the energy dissipation capacity and ductility of the specimens;if the displacement ductility coefficient is not less than3,the design axial compression ratio limit value of UHPC column can be taken as 0.8;increasing the ratio of longitudinal reinforcement can significantly improve the bearing capacity and ductility of UHPC columns Increasing the strength of concrete can improve the bearing capacity of the specimen and reduce the ductility of the specimen,but it has little effect on the energy dissipation capacity and self reset performance.