Creep Behavior of Steel Tube Concrete Under Varying Temperature Conditions

Literature Overview

The paper by Zhang Rongling, Hao Zhaofeng, and colleagues from Lanzhou Jiaotong University, published in Materials Reports (2021, Vol. 35, No. 20), addresses a critical but often overlooked aspect of steel tube concrete (SRC) design: the influence of temperature variations on creep behavior. The authors developed a temperature-dependent creep prediction model based on the ACI 209 model framework, the age-adjusted effective modulus method, and Arrhenius theory, then validated it through laboratory tests conducted at constant 20°C and under seasonal temperature variation conditions.

Core Technical Content

Theoretical Framework

The development of the temperature-dependent creep model involves several key theoretical steps:

1. Base model selection: The ACI 209 creep model provides the fundamental mathematical form for describing concrete creep as a function of age, loading history, and environmental conditions.
2. Age-adjusted effective modulus method: This approach accounts for the time-dependent stiffness of concrete by introducing an aging coefficient that modifies the elastic modulus over time.
3. Arrhenius temperature function: The temperature dependence of creep is introduced through the Arrhenius equation, which relates the rate of a thermally activated process to temperature.

The resulting model can be expressed conceptually as:

ε_creep(t, T) = ε_creep(t, T_ref) × f(T)

where f(T) is the Arrhenius-based temperature correction function and T_ref is the reference temperature (typically 20°C or 23°C).

Experimental Program

Key Quantitative Findings

The experimental results provide critical quantitative data on temperature effects:

• Temperature increase from 20°C to 40°C: Creep strain increased by 57.8% relative to the constant 20°C condition.
• Temperature increase from 40°C to 60°C: Additional creep strain increase of 34.1% relative to the 40°C condition.
• Model accuracy: The theoretical predictions based on the developed model showed good agreement with experimental results, validating the Arrhenius-based temperature correction approach.

Temperature Effect Analysis

Technical Interpretation and Engineering Practice Insights

Physical Mechanism of Temperature-Enhanced Creep

The substantial increase in creep strain with temperature can be attributed to several physical mechanisms:

1. Accelerated hydration: Higher temperatures accelerate the ongoing hydration reactions in concrete, producing additional calcium silicate hydrate (C-S-H) gel that can undergo further creep deformation.
2. Reduced viscosity of pore solution: Elevated temperatures decrease the viscosity of the capillary pore solution, facilitating viscous flow of water within the concrete microstructure.
3. Thermal expansion mismatch: The coefficient of thermal expansion of steel (approximately 12 × 10⁻⁶/°C) differs from that of concrete (approximately 10–14 × 10⁻⁶/°C), creating additional interface stresses during temperature cycling.
4. Microcrack propagation: Temperature gradients can induce microcracking in the concrete core, which accelerates creep by providing additional deformation pathways.

Implications for Steel Tube Concrete Design

For engineers designing SRC structures in environments with significant temperature variations—such as bridges in cold regions, industrial facilities with process heat, or structures in hot desert climates—the following design considerations are essential:

1. Creep deflection calculations: Standard creep prediction models (ACI 209, CEB-FIP, Eurocode 2) typically assume isothermal conditions. The 57.8% increase in creep strain at 40°C indicates that ignoring temperature effects can lead to severe underestimation of long-term deflections.
2. Prestress loss: In prestressed SRC members, temperature-induced creep acceleration can lead to additional prestress losses beyond those predicted by standard models. This is particularly critical for long-span bridges and large industrial structures.
3. Second-order effects: Increased creep strains amplify P-Δ effects in slender SRC columns, potentially reducing the effective buckling resistance. For columns with slenderness ratios exceeding 30, this effect can be significant.
4. Interface behavior: The steel tube-concrete interface is a critical load-transfer mechanism in SRC members. Temperature-induced differential expansion can weaken the bond interface, reducing the effectiveness of composite action.

Comparison with Existing Standards

Practical Recommendations for Engineers

Based on the findings of this study, the following recommendations are appropriate for engineering practice:

1. For SRC structures in environments with maximum temperatures exceeding 35°C, apply a temperature correction factor of 1.5–1.6 to the predicted creep strain when using standard models.
2. For structures exposed to temperatures exceeding 45°C, develop a project-specific creep model incorporating the Arrhenius temperature function as proposed in this paper.
3. During construction, monitor the temperature history of SRC members and use the actual temperature record for creep predictions rather than assuming standard curing conditions.
4. For critical structures, conduct supplementary creep tests at representative service temperatures to validate the predicted long-term behavior.

Key Questions and Reflections

A significant question that arises is the extrapolation validity of the model beyond the tested temperature range of 20–60°C. The Arrhenius equation is well-established for thermally activated processes, but concrete creep involves multiple concurrent mechanisms that may not all follow simple Arrhenius kinetics. At temperatures above 80°C, the concrete microstructure undergoes significant changes (dehydration of C-S-H gel, ettringite decomposition), and the Arrhenius-based model may lose predictive accuracy.

Another important consideration is the interaction between temperature-induced creep and the confining effect of the steel tube. The steel tube provides lateral confinement to the concrete core, which generally reduces creep. However, at elevated temperatures, the steel tube itself undergoes thermal expansion, potentially reducing the effective confining pressure. The study does not explicitly address this interaction, which could be significant for thin-walled steel tubes with high slenderness ratios.

Study Insights and Implications

This research makes a valuable contribution to the understanding of SRC long-term behavior under realistic thermal conditions. The quantitative data on creep enhancement (57.8% at 40°C, 34.1% additional at 60°C) provides engineers with concrete numbers for assessing the adequacy of existing design practices. The validated prediction model offers a practical tool for incorporating temperature effects into routine design calculations.

For the steel pipe and welding industry, this study highlights an important consideration: the steel tube in SRC members is not merely a passive confining element but actively participates in the time-dependent behavior of the composite section. Engineers specifying steel tubes for SRC applications should consider the long-term thermal environment and select materials with appropriate creep resistance properties. The welding quality of any steel tube joints in SRC members is also relevant, as weld defects can become crack initiation sites under sustained thermal and mechanical loading.

In conclusion, this study demonstrates that temperature effects on SRC creep are not merely academic curiosities but have direct and significant implications for structural performance, particularly in non-isothermal environments. Engineers should incorporate temperature-dependent creep considerations into their design methodology for SRC structures, using the validated model presented in this paper as a starting point for more refined analyses.