Study On Interface Performance Of Concrete-Encased Concrete-Filled Steel Tubular Column

Concrete-encased Concrete-Filled Steel Tube(CE-CFST)is a highperformance structure composed of outer concrete and inner Concrete-Filled Steel Tube(CFST).CE-CFST as the main structure is widely applied in long-span bridges in mountainous areas of Southwest China.The multiple steel-concrete interfaces with outer concrete-steel tube-core concrete are existing in CE-CFST.The performance of multiple steel-concrete interfaces is the key to ensure the synergistic force of each component.However,the research on the interface performance between outer concrete and steel tube is rarely reported.Therefore,the method combing with typical experiment,numerical simulation and theoretical analysis is adopted to study the interface performance between outer concrete and steel tube of CE-CFST.The contents of whole process of loading,bond strength and bond-slip relationship are reseached.The main conclusions are as follows:(1)The 27 push-out tests of CE-CFST CFST specimens were carried out.The main test parameters include: steel tube diameter,steel tube strength and concrete strength.According to the test results,the failure mode of the concretesteel tube interface of CE-CFST short columns is analyzed,the typical slip curves of the concrete-steel tube interface of CE-CFST are summarized,the bond stress composition of the concrete-steel tube interface of CE-CFST is determined,and the bond slip mechanism is clarified.The interface is different from the CFST interface,and the influence of various parameters on the interface performance and its action law are discussed.(2)The 12 push-out tests of CE-CFST with stud conncector were carried out.The test parameters included concrete strength and number of studs.The influence of studs on the interface performance is analyzed.According to the test results,it is concluded that studs can improve the bearing capacity of the concrete-steel interface of CE-CFST columns.The average peak load of stud specimens with UHPC and C60 is 13.3 % and 9.4 % higher than that of stud-free specimens,respectively.(3)A refined finite element model was established and verified by test results,which including the ultimate load and slip.The expansion analysis was carried out with the main parameters of stud diameter,strength and load ratio θ.Taking1.8 as the critical load ratio,the bearing capacity calculation method of stud control and interface bonding control are established respectively.The average ratio of the calculated value to the test value are 0.969 and 0.949 with variances of 0.001 and 0.002,respectively.(4)The calculation methods for bond strength of interfaces between outer concrete and core concrete to steel tube established by combing with numerical calculation and theoretical analysis.According to the test result of this paper and the test data of other relevant literatures,the average ratio of the calculated value to the test value are 0.951 and 0.983 with variances of 0.015 and 0.183 respectively.The simplified slip models of interfaces between outer concrete and core concrete to steel tube established by the method of bond strength proposed and the slip result tested in this paper,respectively.At the same time,the simplified slip models are verifed and have a good agreement with other test results.