Experimental and theoretical research on bond behavior of concrete filled steel tube with inner steel tube

In order to research the bond strength of CFST with inner steel tube, the bond tests of 12 pull-out specimens and 16 push-out specimens were conducted to obtain the bond strength and its influencing factors, so as to make clear the relationship between the influencing factors and the bond strength and its failure process. Most specimens failed from the loss of bond connection between inner steel tube and concrete, only a few specimens from the loss of bond connection between outer steel tube and concrete. The load-slip curve of each specimen was drawn, which generally first rises and then fails. The test results show that the bond strength of pull-out specimens is smaller than that of push-out specimens, the average bond strength of pull-out specimens is 0.63 MPa, and the average bond strength of push-out specimens is 0.76 MPa. The influence of steel tube diameter-thickness ratio on bond strength is most significant. A fitting formula of the relationship between the tube diameter-thickness ratio and the bond strength was proposed. Comparing fitting values, code values and test values, it is found that the ratio between fitting values and test values are from 0.833 to 1.262 and the ratio between code values and test values are from 1.045 to 1.258, which mean these three groups of values are very close.