Abstract: The dilation effect induced by the interfacial slip of steel tube and infilled concrete was investigated. The goal is to uncover the mechanism behind the formation of interfacial pressure during the push-out test process. Estimations based on the mass conservation principle suggest that the dilation due to the crushing of concrete-steel interface materials ranges from 20 μm to 40 μm. The profile and size deviations of the steel tubes result in equivalent dilation or shrinkage effects at the interface during sliding. The amount of equivalent dilation depends on the interfacial slippage. Satisfying the interface deformation compatibility requirements, a series of equations were derived to calculate the amount of dilation from the push-out test data. Analysis of three push-out test specimens previously reported in the literature reveals an average interfacial dilation in the range of 23 μm to 45 μm. To highlight the dilation effect, a constitutive model for the steel-concrete interface was developed. This model combines the cohesive zone model and Coulomb's friction law, linking the evolution of dilation to energy dissipation along the sliding interface. The mean value of the test data was adopted as the dilation parameter for performing a push-out test simulation. The study demonstrates that the interface pressure is primarily influenced by the dilation effect, while the Poisson effect of longitudinally compressed core concrete exerts a relatively minor impact. The interface model presented in this paper provides reasonable projections regarding the average interface pressure and the average strain of the steel tube. The research findings deepen the understanding of the bond-slip behavior at the steel-concrete interface and provide a new approach for studying the bond-slip performance.