Experimental Study on Bond Performance Between Internal Steel Reinforcement and Concrete in Concrete-Filled Steel Tubes

Literature Overview

This paper, published in the Journal of Harbin Institute of Technology (2020, Vol. 52, No. 8, pp. 121–131) by Zhao Weiping and colleagues from China University of Mining and Technology (Beijing) and China Electric Power Research Institute, presents a systematic push-out test investigation on the bond behavior between internal steel reinforcement and concrete within concrete-filled steel tube (CFST) columns. The study was supported by the National Natural Science Foundation of China (Grant No. 51474218). Eighteen push-out specimens were tested, covering two types of internal reinforcement: I-shaped steel sections and inner steel tubes. The research addresses a critical interface mechanism that governs the composite action in hybrid CFST members, a topic of growing importance in modern high-rise and infrastructure engineering where composite construction is increasingly adopted to maximize structural efficiency.

Core Technical Findings

The authors identified three distinct stages in the bond stress-slip curves for both reinforcement types:

1. Cohesive stage — characterized by linear elastic behavior where the bond stress increases proportionally with slip.
2. Nonlinear initial slip stage — where micro-cracking initiates at the steel-concrete interface and the relationship becomes nonlinear.
3. Slip stage — where the bond stress reaches its peak and then degrades as slippage continues.

A notable finding is that the bond stress distribution at peak load follows a negative exponential distribution for both reinforcement types. For I-shaped steel sections, the average bond stress on the outer face of the flange is approximately 1.77 times that on the web, indicating a significant non-uniformity in stress transfer that must be accounted for in design calculations.

Factor Influence Analysis

The orthogonal experimental design revealed distinct factor hierarchies for each reinforcement type:

Key trends observed:

• For I-shaped steel: ultimate bond strength increases with cross-sectional dimensions and concrete strength but decreases with increasing bond length.
• For inner steel tubes: ultimate bond strength increases with cross-sectional dimensions and expansion agent dosage but shows a decreasing trend with increasing bond length.

The inverse relationship between bond length and ultimate bond strength is particularly significant from a design perspective. This phenomenon can be attributed to the non-uniform stress distribution along the bond length — longer embedment lengths lead to lower average bond stresses due to the cumulative effect of stress redistribution. This finding has direct implications for specifying minimum embedment lengths in design codes and for evaluating splice details in composite columns.

Technical Interpretation and Standards Relevance

The empirical bond strength formula proposed by the authors is validated against results from other researchers, demonstrating good applicability. This is important because existing design codes such as GB 51248-2017 (Code for Design of Concrete-Filled Steel Tubular Structures) and EC4 (Eurocode 4) provide simplified bond strength values that may not adequately capture the interaction between internal reinforcement and concrete in hybrid CFST members.

From a materials science perspective, the role of expansion agents in enhancing bond strength for inner steel tubes is noteworthy. Expansion agents introduce micro-cracking and internal pressure that improve the interfacial friction and mechanical interlock between steel and concrete. This mechanism is analogous to the use of shrinkage-compensating admixtures in post-tensioned anchorage zones, where controlled expansion is used to enhance prestress transfer efficiency.

The negative exponential distribution of bond stress along the embedment length is consistent with Bondi's theoretical model (1961) and subsequent refinements. This distribution pattern means that the first 2–3 times the bar diameter contributes the majority of the bond resistance, which has practical implications for anchorage design and for evaluating the effectiveness of partially filled CFST columns where the concrete fill height may be limited.

Engineering Practice Implications

In practical engineering applications, the findings of this study have several direct consequences:

• Connection design: The 1.77 ratio between flange and web bond stresses suggests that I-shaped steel reinforcement may require additional anchorage measures (such as mechanical anchors or turn-up bars) at the flange tips to prevent premature bond failure.
• Material selection: For inner steel tube reinforcement, the use of expansion agents is recommended to enhance composite action, particularly in applications where the steel tube diameter is relatively small compared to the outer tube diameter.
• Quality control: The push-out test method provides a reliable means to verify the bond performance of composite columns in the field. Engineers should consider incorporating representative push-out tests into the quality assurance program for critical composite structures.

Key Reflections

This study contributes valuable experimental data to the understanding of internal reinforcement-concrete interaction in CFST columns. However, several questions remain open for future investigation: the long-term bond behavior under sustained loading and environmental degradation, the effect of seismic cyclic loading on bond degradation, and the scalability of these findings to full-scale columns with different geometric proportions. The empirical formula derived should be further validated with a larger database before being adopted into design codes. The authors' systematic approach using orthogonal experimental design is commendable and provides a clear framework for identifying the most influential parameters in composite interface behavior.