Effect of Circumferential Uniform Debonding on Axial Compression Performance of Square CFST Columns

Literature Overview

Published in the journal Experimental Mechanics (Vol. 40, No. 1, 2025, pp. 80-90), this study by Hu Mengyao, Ji Yunpeng, Hu Hongsong, and Gao Yichao from Huaqiao University investigates the impact of circumferential uniform debonding between the steel tube and the infilled concrete on the axial compression performance of square concrete-filled steel tube (CFST) columns. Funded by the National Natural Science Foundation of China (Grants 52278183 and 51878303), the research employs both experimental testing and analytical analysis to quantify the structural consequences of this specific defect type.

Experimental Design and Parameters

The study designed six test specimens divided into two groups: three specimens with circumferential uniform debonding defects and three without debonding. The key variables studied include the presence or absence of debonding, steel tube yield strength, and steel tube thickness.

The experimental program followed a systematic approach where each specimen was subjected to monotonic axial compression loading until failure, with full-field strain measurement to capture the deformation and failure mechanisms.

Key Technical Findings

The research yielded several important conclusions regarding the structural behavior of CFST columns with debonding defects:

1. Initial stiffness: Circumferential uniform debonding has a relatively small effect on the initial stiffness of short square CFST columns, meaning the elastic response remains largely unaffected.
2. Ultimate bearing capacity: Debonding significantly reduces the axial compression bearing capacity, which is the most critical structural property for safety assessment.
3. Failure mode: The failure morphology is not dramatically altered by debonding, but the load-carrying capacity at failure is substantially diminished.
4. Confinement ratio effect: Increasing the confinement ratio (k) has limited effect on the peak stress of the infilled concrete but effectively slows the stress degradation rate and mitigates the negative impact of debonding on bearing capacity.
5. Quantitative impact: At a debonding rate of 0.2%, the circumferential uniform defect causes a reduction of approximately 12% in the peak stress of the infilled concrete.

Mechanism Analysis

The mechanism behind these observations can be understood through the interaction between the steel tube and the concrete core. In a properly constructed CFST column, the steel tube provides lateral confinement to the concrete, which in turn prevents outward buckling of the tube. When circumferential debonding occurs, this composite action is disrupted at the interface. The concrete core loses part of its lateral confinement, leading to earlier stress degradation under axial compression. The steel tube, no longer fully supported by the concrete, may experience earlier local buckling.

The finding that the confinement ratio mitigates debonding effects is particularly relevant to engineering practice. A higher confinement ratio means a thicker steel tube or higher steel content relative to concrete, which provides a more robust confining action that can partially compensate for the loss of interface bond. This suggests that for applications where debonding risk is elevated, designing with a higher confinement ratio provides a useful safety margin.

Engineering Practice Implications

This research carries significant implications for the design, construction, and assessment of CFST columns in practice:

• Construction quality control: Debonding between the steel tube and concrete can arise from inadequate concrete compaction, poor interface preparation, or construction defects. The 12% reduction in peak stress at only 0.2% debonding rate underscores the sensitivity of CFST performance to interface integrity.
• Non-destructive assessment: Current standards for CFST columns (such as GB 50936 and EN 1993-1-1) do not explicitly address debonding as a defect category. This study provides the technical basis for incorporating debonding assessment into structural evaluation procedures.
• Design conservatism: For critical structures such as high-rise buildings or bridges where CFST columns are used, the design should account for the possibility of partial debonding. A design margin that incorporates potential interface defects can improve structural reliability.
• Repair and retrofit: For existing CFST columns where debonding has been detected, the study supports the effectiveness of increasing confinement through external steel wrapping or additional stiffening rings.

Study Insights and Reflections

This paper contributes to a growing body of research on the real-world behavior of CFST structures, which often deviates from the idealized composite action assumed in design codes. The finding that even a small debonding rate of 0.2% can cause a 12% reduction in concrete peak stress is particularly striking. In my professional experience, CFST columns are often assumed to perform as perfectly bonded composites, but field inspections routinely reveal partial debonding due to construction imperfections.

The practical significance of this study is amplified by the widespread use of CFST columns in modern construction. The results suggest that engineers should not take interface integrity for granted and should incorporate debonding considerations into both design and inspection protocols. The recommendation to increase the confinement ratio as a mitigation strategy is straightforward and cost-effective, making it an attractive solution for critical applications.

Reference Value and Outlook

This study provides essential experimental data on the structural consequences of debonding in CFST columns, which was previously lacking in the literature. Future research should extend these findings to long columns where global buckling may be influenced by debonding, and to cyclic loading conditions relevant to seismic performance. The quantitative relationship between debonding rate and capacity reduction established in this study could serve as a basis for developing assessment methods and design recommendations in future revisions of CFST design codes.