Push-Out Test Investigation of Interfacial Shear Bond Strength in Concrete-Filled Steel Tubes

Literature Overview

The study by Liu Yongjian and Chi Jianjun, published in Industrial Construction in 2006, presents experimental results from push-out tests conducted on 11 specimens comprising 6 square CFST and 5 circular CFST specimens. The research investigates the interfacial shear bond performance between the steel tube and the infilled concrete, examining how the cross-sectional shape of the steel tube influences the bond behavior. This work is of direct relevance to steel pipe engineers because the interfacial bond strength governs the composite action between the steel pipe shell and the concrete core, which in turn determines the load-bearing capacity, ductility, and fatigue resistance of CFST members.

Experimental Design and Methodology

The push-out test is the standard experimental method for evaluating the bond strength at the steel-concrete interface in CFST members. In this test, a steel tube is partially filled with concrete, and a steel rod anchored in the concrete is pulled out, causing the steel tube to move relative to the concrete. The load-slip relationship at the interface is recorded to determine the bond strength characteristics.

The test program was designed to capture the full load-slip curve, which includes the initial elastic phase, the transition phase, and the residual strength phase. Cyclic loading was applied to simulate the repeated stress conditions that CFST members may experience in seismic or fatigue scenarios.

Key Experimental Findings

The research establishes that square and circular CFST interfaces exhibit similar load-slip curve shapes, indicating that the fundamental bond mechanism is consistent regardless of cross-sectional geometry. However, the circular CFST interface demonstrates higher shear bond strength compared to the square CFST interface. This difference is attributed to the more uniform radial confinement provided by the circular steel tube, which generates a more consistent confining pressure on the concrete core.

The study also reveals that concrete strength has a relatively minor influence on the interfacial shear bond strength of circular CFST members. This finding is somewhat counterintuitive and suggests that the bond strength is more governed by the mechanical interlock and friction between the steel tube inner surface and the concrete, rather than by the tensile strength of the concrete itself. The surface roughness of the steel tube inner wall, the quality of concrete placement (particularly the avoidance of voids and honeycombing), and the degree of concrete compaction are likely more significant factors.

Implications for Steel Pipe Manufacturing and Quality Control

For steel pipe manufacturers supplying tubes for CFST applications, this research carries several important quality implications. The internal surface quality of the steel tube becomes a critical quality parameter, as it directly affects the bond strength. Surface defects such as scale, oxide inclusions, and internal weld bead protrusions can either enhance or degrade the bond performance. The internal weld bead height in ERW and HFW pipes must be controlled within tight tolerances to ensure consistent bond performance around the full circumference.

From a welding perspective, the internal weld seam in longitudinally welded pipes creates an asymmetry in the bond strength distribution. The weld bead protrusion on the inner surface can act as a mechanical key, potentially increasing local bond strength, but it can also create stress concentrations that may initiate debonding under cyclic loading. Engineers should consider the orientation of the weld seam relative to the primary loading direction when specifying pipe for CFST applications.

Study Insights and Engineering Reflections

This research provides a valuable experimental basis for understanding the interfacial behavior in CFST members. The finding that circular tubes outperform square tubes in bond strength is a strong argument for preferring circular cross-sections in critical CFST applications, particularly where seismic performance is a primary design concern. However, the similar load-slip curve shapes suggest that the differences can be accounted for through appropriate design modifications, such as increased concrete cover or additional mechanical anchorage at corners in square sections.

The observation that concrete strength has limited influence on bond strength challenges the conventional assumption that higher-strength concrete always leads to better composite performance. This insight encourages engineers to focus on the quality of the interface itself — surface preparation, concrete placement methodology, and compaction quality — rather than relying solely on material strength upgrades. For steel pipe suppliers, this means that maintaining consistent internal surface quality and dimensional accuracy is more important than achieving marginal gains in steel grade.