Bond strength between steel and self-compacting lower expansion concrete in composite columns

In order to study the bond behavior between the steel tube and the concrete infill, a total of 17 self-compacting lower expansion CFST columns were prepared and tested. Cross-sectional dimension, dosage of concrete expansive agent, concrete compressive strength, steel tube fabrication method and interface condition were varied to assess their effect on the ultimate average bond strength. The push-out load-slip curves for each specimen are obtained and the distribution of the interface bond stress along the member axial direction and around the cross-section for various load levels are reported. The test results indicate that the steel tube interface roughness, influenced by both the steel tube fabrication method and the interface condition, the maximum flat width-to-thickness ratio (D/t), the concrete compressive strength, and the dosage of expansion agent are the main influence factors for interface bond strength. Then, regression analysis was performed to examine the bond strength of the test results of 19 square and 13 rectangular CFST columns with cold-formed steel tubes from this investigation and other additional references. Considering the influences of the dosage of concrete expansion agent, concrete cube compressive strength and D/t ratio to the bond stress, two empirical equations have been proposed for predicting the ultimate average bond strength for square and rectangular self-compacting lower expansion CFST columns with cold-formed steel tube respectively. Finally, by the analysis of the test data and the previous four segments bond stress-slip curve model, a new bond strength-slip model fitting equation has been proposed to predict the bond stress development.