Temperature Field Analysis of Steel Tube Concrete Members in Large Temperature Difference Cold Regions

Literature Overview

This paper by Qi Qiang, Zhang Rongling, Hu Ruipeng, Xue Yanjin, and Wang Bingzhong, published in the journal "Railway Engineering" (Volume 57, Issue 5, 2017, pages 16-19), investigates the temperature field distribution and steel tube-concrete debonding behavior of steel tube concrete (CFST) members under large temperature difference conditions typical of cold regions in Northwest China. The research was conducted by the Key Laboratory of Road and Bridge and Underground Engineering of Gansu Province at Lanzhou Jiaotong University and was supported by multiple funding sources including the National Youth Talent Support Program (2015QNRC001) and the Gansu Province Basic Research Innovation Group (145RJIA332).

Core Technical Findings

The authors conducted temperature field and debonding tests on CFST members in an atmospheric simulation chamber, simulating the daily average air temperature variation of January in a typical cold region. The key findings are as follows:

The temperature distribution across the cross-section of the CFST member under atmospheric temperature action is nonlinear.
The presence of the steel tube causes the stress and strain at the concrete surface to be larger than those in plain concrete.
Debonding between the steel tube and concrete can occur when the temperature difference stress generated by atmospheric temperature decrease exceeds the bond strength between the steel tube and concrete.

Temperature Field Distribution and Thermal Stress Analysis

The nonlinear temperature distribution across the CFST cross-section is a critical finding, as it has direct implications for the thermal stress state and potential debonding of the composite member. The steel tube, with its relatively high thermal conductivity compared to concrete, experiences temperature changes more rapidly than the core concrete. This differential thermal response creates thermal gradients and associated thermal stresses at the steel tube-concrete interface.

The following table summarizes the key thermal properties and their implications for CFST member performance in cold regions:

Property	Steel Tube	Concrete	Implication for CFST
Thermal conductivity	High (~50 W/m·K)	Low (~1.5-2.5 W/m·K)	Differential temperature response
Coefficient of thermal expansion	~12×10⁻⁶/°C	~10×10⁻⁶/°C	Thermal stress at interface
Temperature response time	Fast	Slow	Nonlinear temperature gradient
Thermal stress magnitude	Moderate	Moderate	Potential debonding risk

Debonding Mechanism and Critical Temperature Difference

The debonding of the steel tube from the concrete fill is a critical failure mode for CFST members in cold regions. The bond strength between the steel tube and concrete is influenced by several factors, including the surface roughness of the steel tube, the concrete mix design, the curing conditions, and the construction sequence. Under large temperature differences, the thermal contraction of the steel tube relative to the concrete can generate interface stresses that exceed the bond strength, leading to partial or complete debonding.

The critical temperature difference at which debonding occurs depends on the thermal properties of both materials, the geometry of the member, and the bond strength at the interface. The authors' experimental approach, using an atmospheric simulation chamber to replicate real-world temperature conditions, provides valuable empirical data for validating analytical models and informing design practices.

Engineering Practice Integration

The findings of this study have important implications for the design and construction of CFST members in cold regions, particularly in Northwest China where annual temperature variations can exceed 50°C. The nonlinear temperature distribution and the associated thermal stresses must be accounted for in the structural design to ensure long-term performance and durability.

From a steel pipe engineering perspective, the selection of the steel tube material and geometry is critical for minimizing thermal stress and debonding risk. The following considerations are important for steel tube selection in cold-region CFST applications:

The steel tube material should have a thermal expansion coefficient that is as close as possible to that of the concrete to minimize differential thermal strains.
The diameter-to-thickness ratio of the steel tube should be optimized to balance structural efficiency with thermal stress resistance.
The surface finish of the steel tube should be designed to maximize bond strength with the concrete fill.

Construction and Quality Control Measures

To mitigate the risk of thermal debonding in cold regions, the following construction and quality control measures are recommended:

The concrete should be placed in stages to minimize temperature gradients during construction.
Thermal insulation should be applied to the exterior of the CFST member to reduce the rate of temperature change.
The bond strength at the steel tube-concrete interface should be verified through pull-out tests or other appropriate methods.
The steel tube should be inspected for surface defects and contamination prior to concrete placement to ensure adequate bond development.

Study Insights and Reflections

This study addresses a critical but often overlooked aspect of CFST member performance in cold regions: the effects of large temperature differences on the temperature field distribution and the steel tube-concrete interface. The nonlinear temperature distribution and the potential for thermal debonding represent significant challenges for the design and durability of CFST members in these environments.

The experimental approach adopted by the authors, using an atmospheric simulation chamber to replicate real-world temperature conditions, provides a valuable empirical basis for understanding the thermal behavior of CFST members. This approach bridges the gap between theoretical analysis and field performance, offering practical insights that can be directly applied to engineering design.

From a materials engineering perspective, the study highlights the importance of understanding the thermal compatibility of the steel tube and concrete materials. The selection of appropriate material combinations, considering both mechanical and thermal properties, is essential for ensuring the long-term performance of CFST members in cold regions. The findings also underscore the need for comprehensive quality control procedures during construction to ensure adequate bond development and thermal performance.

The implications of this study extend beyond CFST members to other composite structures used in cold regions, including steel-concrete composite bridges and buildings. The principles of thermal compatibility and interface integrity are universally applicable to composite structures subjected to large temperature variations, making the findings of this study broadly relevant to the structural engineering community.