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

Literature Overview

This paper, published in Progress in Steel Building Structures (2024, Vol. 26, No. 9), addresses a critical durability issue for composite steel-concrete structures in cold regions. The authors—Zeng Zaiping, Wang Yuhang, Wang Xiuli, and Ren Genli—investigate how repeated freeze-thaw cycles degrade the interfacial bond performance between steel tubes and infilled concrete, using push-out specimens subjected to varying freeze-thaw cycles, concrete grades, and diameter-to-thickness ratios. The work is funded by the Gansu Provincial Youth Doctor Support Program (2023QB-104) and the Gansu Provincial Department of Housing and Urban-Rural Development (JK2024-34).

Core Technical Findings

The study employs a systematic experimental program where cylindrical steel-concrete push-out specimens undergo freeze-thaw cycling followed by push-out testing to measure interfacial bond stress and slip. The key findings reveal three distinct degradation mechanisms:

1. The average bond strength decreases monotonically with increasing freeze-thaw cycle count, reflecting progressive microcracking at the steel-concrete interface.
2. The residual bond strength exhibits a linear reduction with freeze-thaw cycles, while the slip at the onset of the stable stage grows exponentially—indicating that damage accumulation accelerates nonlinearly.
3. Higher concrete strength increases both peak and residual bond strength, whereas larger diameter-to-thickness ratios reduce bond capacity due to increased internal restraint effects.

Interfacial Bond Stress Distribution

A significant contribution of this work is the determination that interfacial bond stress follows an exponential distribution along the bond length, confirmed by dense strain gauge measurements on the external steel tube. The authors derive an analytical 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 data.

Three-Stage Constitutive Model

The paper constructs a three-stage bond stress-slip constitutive model for the post-freeze-thaw interface:

• Stage 1 (Elastic): Linear relationship between bond stress and slip, governed by an initial bond stiffness that degrades with freeze-thaw damage.
• Stage 2 (Descending): After peak bond stress is reached, the bond stress drops as microcracks propagate at the interface.
• Stage 3 (Residual): A constant residual bond stress is maintained through frictional contact between the steel tube and cracked concrete.

The characteristic values—peak bond strength, residual bond strength, and corresponding slip values—are expressed as functions of freeze-thaw cycle count, concrete compressive strength, and diameter-to-thickness ratio.

Engineering Practice Implications

From a steel pipe manufacturing and structural engineering perspective, this research has several important implications:

• Material selection: In cold regions, selecting steel tubes with lower diameter-to-thickness ratios (thicker walls) provides better interfacial bond durability under freeze-thaw exposure. This aligns with the principle that thinner walls lead to higher internal restraint stresses that exacerbate interface cracking.
• Concrete mix design: Higher concrete grades improve interfacial bond resistance, but the benefits diminish with increasing freeze-thaw exposure. Engineers must balance economic considerations against long-term durability requirements.
• Design codes: Current design codes for composite structures (e.g., GB 50017, EC4) do not explicitly account for freeze-thaw degradation of interfacial bond. The proposed constitutive model and characteristic value formulas provide a basis for future code revisions.
• Inspection and maintenance: The exponential growth of stable-stage slip with freeze-thaw cycles suggests that periodic pull-out or push-out testing should be incorporated into structural health monitoring programs for composite bridges and buildings in cold climates.

Critical Reflection

The study's approach of combining experimental push-out tests with analytical bond stress distribution derivation is methodologically sound. However, the transition from laboratory-scale push-out specimens to full-scale structural behavior warrants caution. The bond stress distribution, while accurately modeled at the specimen level, may be affected by boundary conditions and load path complexities in actual structures. Additionally, the study does not address the combined effects of freeze-thaw cycling with chloride ingress or carbonation, which often occur simultaneously in real service environments. The proposed constitutive model should be validated against long-term field monitoring data before widespread adoption in design practice.

Summary

This paper provides a well-structured investigation into the degradation of steel-concrete interfacial bond under freeze-thaw cycling, offering a three-stage constitutive model with explicit characteristic value formulas. For structural engineers designing composite steel-concrete systems in cold regions, the key takeaway is that freeze-thaw damage is progressive and nonlinear, with residual bond strength declining linearly and slip capacity growing exponentially. The exponential bond stress distribution function derived via the energy method represents a valuable analytical tool for interface modeling in finite element analysis of composite structures.