Bond-Slip Constitutive Model of Steel-Concrete Interface Under Freeze-Thaw Cycles

Overview of the Study

This paper by Zeng Zai-Ping et al. (2024), published in Progress in Steel Building Structures, addresses a critical durability issue for steel-concrete composite structures in cold regions. The authors investigate how repeated freeze-thaw cycles degrade the interfacial bond between the steel tube and concrete infill, which is a fundamental mechanism governing the composite action of steel tube concrete (STC) members. The study employs pull-out tests on STC specimens subjected to varying numbers of freeze-thaw cycles, concrete grades, and steel tube diameter-to-thickness ratios. The work is supported by the Gansu Provincial Youth Doctor Support Program (2023QB-104) and the Gansu Housing and Urban-Rural Construction Department Science and Technology Project (JK2024-34).

Core Technical Findings

The research establishes three principal observations regarding the degradation of interfacial bond performance:

1. The average bond strength decreases with increasing freeze-thaw cycles, while the slip at ultimate load gradually increases.
2. The residual bond strength exhibits a linear reduction with cycle count, whereas the onset slip of the stable stage increases exponentially.
3. Peak bond strength and residual bond strength increase with concrete strength but decrease with increasing diameter-to-thickness ratio of the steel tube.

Quantitative Degradation Trends

The authors propose a three-stage bond stress-slip constitutive model for the post-freeze-thaw interface, consisting of the elastic ascending stage, the descending stage after peak, and the residual friction stage. The characteristic values—peak bond strength, residual bond strength, and corresponding slip values—are correlated with freeze-thaw cycle number, concrete compressive strength, and steel tube geometry through empirical formulas.

Bond Stress Distribution Analysis

An important methodological contribution is the identification of the interfacial bond stress distribution. Strain gauges densely arranged on the outer surface of the steel tube reveal that the bond stress follows an exponential distribution along the member length. The authors derive a theoretical bond stress distribution function using the energy method combined with load-slip relationships, and demonstrate that this analytical approach achieves high accuracy compared to experimental measurements. This provides a new pathway for analytical modeling of interface behavior in STC members.

Engineering Practice Implications

From a metallurgical and welding perspective, several practical considerations emerge:

• Steel tube selection for cold regions: The diameter-to-thickness ratio has a direct influence on bond performance. Thicker-walled tubes (lower D/t ratio) maintain better composite action after freeze-thaw degradation. For critical structures in cold regions, designers should consider reducing the D/t ratio to improve interface durability, even if this increases material consumption.
• Welding quality at connections: In STC columns and beams, the circumferential welds or bolted connections at joints are critical locations where interface bond forces are transferred. The degradation of bond strength under freeze-thaw cycles means that joint design must account for reduced composite action over the service life, potentially requiring additional reinforcement or connection detailing.
• Concrete mix design: Higher concrete strength provides greater resistance to freeze-thaw degradation of the interface. This aligns with the practice of using air-entrained concrete and appropriate water-cement ratios for cold-region applications, as specified in GB/T 50476 and related standards.
• Corrosion considerations: The freeze-thaw process also accelerates corrosion at the steel-concrete interface. For STC members exposed to chloride environments (e.g., coastal cold regions), the combined effect of chloride ingress and freeze-thaw cycling can be synergistically damaging. The interfacial zone is particularly vulnerable because microcracks formed during freeze-thaw cycles provide pathways for aggressive agents.

Key Questions and Reflections

The study raises several important questions for further investigation:

1. How does the type of steel tube (ERW, HFW, seamless, LSAW) affect the bond degradation rate? The manufacturing process influences the surface roughness, residual stress state, and microstructural characteristics of the steel tube, all of which may interact with the freeze-thaw mechanism.
2. What is the role of welding defects at the tube-concrete interface? In welded STC members, the heat-affected zone (HAZ) near circumferential welds may have altered microstructures that are more susceptible to freeze-thaw cracking. This deserves dedicated investigation using metallographic analysis of HAZ regions after freeze-thaw cycling.
3. The exponential increase in stable-stage onset slip suggests a progressive degradation mechanism. Engineers should consider whether existing design codes adequately account for this time-dependent deterioration in cold-region STC structures.

Study Insights and Practical Recommendations

The proposed three-stage constitutive model provides a practical tool for structural analysis of STC members in cold regions. However, engineers should be cautious about direct application without considering the specific steel grade, welding procedure, and environmental exposure conditions. The model parameters should be calibrated for local materials and conditions through supplementary testing. For design practice, I recommend incorporating a time-dependent bond degradation factor into the composite action calculation, with the rate of degradation estimated from the proposed empirical relationships. Future research should integrate the effects of steel grade, welding procedure, and corrosion inhibitors on the freeze-thaw bond degradation to provide a more comprehensive design framework.