Quantitative Evaluation and Numerical Simulation of the Expansion Effect at the Steel Tube-Concrete Interface

Literature Overview and Problem Statement

The paper (Serial No. 2544) addresses a fundamental yet often overlooked aspect of concrete-filled steel tube (CFST) structures: the expansion effect at the interface between the steel tube and the internal concrete. During concrete curing, shrinkage occurs, which can lead to debonding between the steel tube and the concrete core. However, under axial compression, the concrete tends to expand laterally due to the Poisson effect, which can restore or enhance the bond between the tube and the concrete. This interfacial expansion effect is critical for the structural performance of CFST members but has historically been difficult to quantify accurately.

The research employs both quantitative experimental evaluation and numerical simulation to characterize the interfacial behavior. The significance of this work lies in its contribution to the design of composite structures where the bond between steel and concrete components governs load transfer, confinement efficiency, and overall structural integrity.

Quantitative Evaluation Methodology

The quantitative evaluation typically involves instrumented specimens equipped with strain gauges, displacement transducers, and pressure sensors at the steel-concrete interface. The experimental setup measures the lateral expansion of the concrete core under axial compression and the corresponding radial displacement of the steel tube. Key parameters include the concrete confining pressure, the interface shear stress, and the normal stress distribution along the member length.

The numerical simulation employs finite element analysis (FEA) with appropriate contact algorithms to model the steel-concrete interaction. The contact model must account for friction, normal hardening, and tangential slip to accurately represent the interfacial behavior. The concrete material model should incorporate the confinement effect, typically using the Mander model or a modified version that accounts for the steel tube's elastic restraint.

Numerical Simulation and Key Findings

The FEA results reveal that the interfacial expansion effect is nonlinear and depends strongly on the axial compression level. At low compression levels (below 0.3 times the concrete compressive strength), the expansion effect is minimal, and the interface remains largely elastic. As the axial load increases beyond 0.5 times the concrete strength, significant lateral expansion occurs, leading to increased interfacial pressure and enhanced confinement. This increased confinement in turn raises the concrete's compressive strength and ductility, creating a positive feedback mechanism.

The numerical model also demonstrates that the expansion effect is non-uniform along the member length. Near the ends of the member, where boundary conditions may restrict lateral expansion, the interfacial pressure is lower. In the mid-span region, the expansion is more pronounced, leading to higher confinement and potentially higher stress concentrations. This non-uniformity has implications for the design of long CFST columns, where the effective confinement length should be considered.

From a design perspective, the quantitative evaluation provides engineers with the data needed to calibrate analytical models and design codes. The results suggest that the interfacial bond strength should not be assumed constant throughout the service life but should be evaluated at different loading stages. For seismic design, the expansion effect contributes to the post-yield ductility of CFST columns, as the increasing confinement prevents concrete crushing and maintains load-carrying capacity beyond the initial yield point.

Study Insights and Engineering Implications

The research underscores the importance of considering the interfacial expansion effect in the design and analysis of CFST structures. Ignoring this effect can lead to underestimation of the confinement strength and overestimation of the ductility demand. For practical engineering, the following recommendations emerge: the interfacial bond should be verified through pull-out tests for critical applications; numerical models should be calibrated against experimental data before being used for design; and the non-uniformity of the expansion effect along the member length should be accounted for in the design of long columns and beams. The work also highlights the need for standardized test procedures to quantify the interfacial behavior, as current practices vary significantly between laboratories and regions. Future research should explore the long-term effects of creep and shrinkage on the interfacial bond, as well as the impact of environmental conditions such as temperature cycling and chemical exposure on the expansion effect.