Experimental Investigation of Creep Behavior in Steel Tube Concrete Under Varying Strength Grades and Temperature Conditions

Literature Overview

This paper, authored by Ma Jiaojiao and colleagues from Lanzhou Jiaotong University, published in the journal "Concrete" (2019, Vol. 3, pp. 23-26), presents a systematic experimental study on the creep performance of steel tube concrete (SRC) specimens under different concrete strength grades and temperature regimes. The research addresses a critical gap in long-term structural performance prediction for composite steel-concrete systems, which are increasingly used in high-rise buildings, bridges, and industrial structures where long-term deformation control is essential.

Core Experimental Configuration

The test matrix is carefully designed to isolate the effects of two primary variables: concrete compressive strength and ambient temperature. The following table summarizes the key experimental parameters:

The use of an 8% expansive agent is a notable design choice, intended to compensate for shrinkage cracking at the steel-concrete interface and ensure intimate bonding between the two materials under sustained loading. This is particularly relevant for SRC members where interface debonding can significantly degrade structural performance.

Key Findings and Technical Interpretation

Effect of Concrete Strength Grade on Creep

The study demonstrates that higher concrete strength grades exhibit lower creep coefficients. This is consistent with fundamental concrete mechanics: high-strength concrete possesses a denser microstructure with a lower water-to-cement ratio, resulting in reduced pore connectivity and slower diffusion-driven deformation mechanisms. However, the authors emphasize an important nuance — the creep coefficient does not decrease proportionally with increasing strength. The reduction from C45 to C80 is more pronounced than from C80 to C120, indicating a diminishing returns effect. This non-linear relationship is critical for engineers who may erroneously assume that upgrading concrete grade will yield proportional improvements in long-term deformation control.

From a microstructural perspective, this behavior can be attributed to the fact that at very high strength grades (C120), the cement paste becomes extremely dense, and the creep mechanism shifts from primarily water diffusion and viscous flow in the gel pores to more complex processes involving aggregate-paste interface debonding and microcracking. These mechanisms are less sensitive to further increases in paste density.

Effect of Temperature on Creep

The temperature influence ranking is: variable temperature > high temperature > room temperature. This finding has significant implications for SRC structures in regions with large diurnal or seasonal temperature swings. The variable temperature condition (10-50°C cycling) produces the largest creep coefficient because repeated thermal cycling induces differential expansion between the steel tube and the concrete core, generating cyclic interfacial shear stresses that progressively loosen the bond and accelerate time-dependent deformation. This is a particularly important consideration for SRC columns in regions with extreme climates or near heat-generating equipment.

The high temperature condition (60°C constant) also significantly increases creep compared to room temperature, primarily because elevated temperature accelerates the hydration and microstructural evolution processes within the cement paste, reducing the effective viscosity of the gel phase and facilitating faster creep deformation.

Governing Factors: Water-Cement Ratio and Elastic Modulus

The authors identify water-cement ratio (w/c) and elastic modulus (E) as the two principal factors governing creep behavior. This conclusion aligns with established concrete creep theory, where the w/c ratio directly controls the porosity and connectivity of the capillary pore system, and the elastic modulus reflects the overall stiffness of the cement paste matrix. For SRC members, the steel tube provides additional confinement that can partially mitigate creep, but the concrete core still dominates the long-term deformation behavior under sustained loading.

Engineering Practice Implications

For engineers designing SRC structures, several practical recommendations emerge from this study:

1. When long-term creep control is a primary design objective, upgrading concrete grade beyond C80 provides diminishing returns. A more effective strategy may involve optimizing the w/c ratio within a given strength grade or incorporating supplementary cementitious materials (silica fume, fly ash) to further densify the microstructure.
2. In environments subject to significant temperature variation, SRC members should be designed with additional creep allowances. The variable temperature effect can increase creep coefficients substantially, and this must be accounted for in serviceability limit state (SLS) checks according to relevant codes such as GB 50010 or Eurocode 2.
3. The fitted creep coefficient formula validated by the authors provides a practical tool for preliminary design calculations. However, engineers should exercise caution when extrapolating this formula beyond the tested parameter ranges, particularly for very high temperature or very long duration conditions.

Reflections and Limitations

While the study provides valuable experimental data, several limitations should be noted. The specimen dimensions (140 mm diameter, 350 mm length) represent small-scale laboratory specimens, and the size effect on creep behavior — where larger specimens often exhibit lower creep coefficients due to reduced surface-to-volume ratios — may not be captured. Additionally, the steel tube wall thickness of 3.0 mm is relatively thin, and the confinement effect may be less pronounced than in full-scale SRC columns with thicker walls. The study also does not address the combined effects of cyclic loading and temperature, which is relevant for seismic regions. Future research should incorporate larger specimens with thicker steel tubes and combined loading conditions to better represent real-world structural behavior.

Summary

This study provides a solid experimental foundation for understanding the creep behavior of steel tube concrete under varying strength and temperature conditions. The key takeaway for practicing engineers is that both concrete strength grade and temperature regime significantly influence long-term deformation, with variable temperature cycling being the most detrimental condition. The non-proportional relationship between strength and creep reduction, along with the identification of water-cement ratio and elastic modulus as governing parameters, offers practical guidance for optimizing SRC design for long-term serviceability. Engineers should incorporate these findings into their design methodology, particularly when SRC members are subjected to elevated or fluctuating temperatures, to ensure adequate long-term performance and structural safety.