Zhuojin Pipe Fitting Co., LtdEffect of High-Strength Concrete Self-Shrinkage on Axial Compression Mechanical Properties of Steel Tube Concrete Members
Literature Overview
This 2007 paper by Song Bing and Wang Zhan, published in the Journal of Architecture and Civil Engineering by the Guangzhou Institute of Building Science and Shantou University, investigates the influence of self-shrinkage of high-strength concrete on the axial compression mechanical properties of steel tube concrete (STC) members. The study employs nonlinear finite element analysis, representing the bond initial stress between the steel tube and concrete as a proxy for the shrinkage effect of the core concrete. The research was supported by the National Natural Science Foundation of China (Grant No. 59978084).
The Problem of Self-Shrinkage in High-Strength Concrete
Self-shrinkage is a chemical shrinkage phenomenon that occurs in high-strength concrete during the hardening process, particularly when the water-to-binder ratio is low and the concrete is sealed. Unlike drying shrinkage, which is caused by moisture evaporation, self-shrinkage results from the chemical binding of water during cement hydration, leading to a reduction in the internal water content and a corresponding volume decrease. In steel tube concrete members, the sealed environment created by the steel tube prevents moisture exchange with the external environment, making self-shrinkage particularly significant.
Self-Shrinkage Characteristics of High-Strength Concrete
| Concrete Strength Grade | Typical Self-Shrinkage Strain (μɛ) | Time to Reach 80% of Total Shrinkage |
|---|---|---|
| C40 | 200–300 | 14–28 days |
| C50 | 300–400 | 14–28 days |
| C60 | 400–500 | 7–21 days |
| C80 | 500–700 | 7–14 days |
| C100 | 600–900 | 7–14 days |
Numerical Modelling Approach
The nonlinear finite element model represents the self-shrinkage effect through the introduction of bond initial stresses at the steel tube-concrete interface. This approach captures the mechanical consequence of shrinkage without explicitly modelling the time-dependent shrinkage process, providing a practical and computationally efficient method for assessing the impact of self-shrinkage on the load-bearing behaviour of STC members.
Modelling Strategy for Shrinkage Effects
| Shrinkage Direction | Modelling Approach | Mechanical Effect |
|---|---|---|
| Longitudinal shrinkage | Axial bond initial stress (compressive) | Pre-compression of concrete core, reduced confinement effectiveness |
| Radial shrinkage | Radial bond initial stress (tensile) | Reduced interfacial bond, potential debonding |
| Combined shrinkage | Both axial and radial initial stresses | Combined effects on load-deflection behaviour |
Key Findings and Results
The numerical analysis revealed several important conclusions regarding the influence of self-shrinkage on the mechanical properties of STC members:
- Reduction in proportional limit: Self-shrinkage reduces the proportional limit of the load-strain curve, indicating that the member enters the nonlinear regime at a lower applied load compared to the non-shrinkage case.
- Decrease in yield strength: The apparent yield strength of the STC member is reduced by self-shrinkage, with the magnitude of reduction depending on the degree of shrinkage and the concrete strength grade.
- Modulus of elasticity reduction: Self-shrinkage reduces the effective modulus of elasticity of the member, leading to larger deformations under the same applied load.
- Adverse effect on ultimate load capacity: The ultimate load-bearing capacity of the STC member is negatively affected by self-shrinkage, with the reduction becoming more pronounced for higher shrinkage strains.
Comparison of Shrinkage Scenarios
| Scenario | Proportional Limit Change | Yield Strength Change | Ultimate Load Change |
|---|---|---|---|
| No shrinkage (baseline) | Reference | Reference | Reference |
| Longitudinal shrinkage only | −5% to −10% | −3% to −8% | −2% to −5% |
| Radial shrinkage only | −3% to −7% | −5% to −10% | −4% to −8% |
| Combined shrinkage | −8% to −15% | −8% to −15% | −6% to −12% |
Engineering Practice Implications
The findings of this study have significant implications for the design of steel tube concrete members using high-strength concrete:
- The design of STC members with high-strength concrete (C60 and above) should account for the self-shrinkage effect, particularly when the member is subjected to sustained axial compression loads.
- The bond initial stress approach provides a practical method for incorporating shrinkage effects into finite element analysis, and this approach can be extended to other composite structural members.
- The combined effect of longitudinal and radial shrinkage is more severe than either direction alone, suggesting that comprehensive shrinkage modelling is necessary for accurate structural assessment.
- Mitigation measures such as the use of shrinkage-reducing admixtures, controlled concrete curing, or the incorporation of expansive agents can be considered to reduce the self-shrinkage effect in high-strength concrete.
Study Insights and Reflections
This paper addresses a critical but often overlooked aspect of steel tube concrete design — the influence of concrete self-shrinkage on the structural performance of STC members. The use of bond initial stress as a proxy for shrinkage effects is an elegant and practical approach that bridges the gap between material-level shrinkage phenomena and structural-level mechanical response. The finding that self-shrinkage reduces the proportional limit, yield strength, modulus of elasticity, and ultimate load capacity of STC members provides clear guidance for design engineers. The magnitude of the effect is particularly significant for high-strength concrete grades (C60 and above), where self-shrinkage strains can reach 600–900 μɛ. Engineers designing STC structures with high-strength concrete should incorporate shrinkage effects into their structural analysis and consider appropriate mitigation measures to ensure the long-term performance and safety of the structure. The bond initial stress modelling approach presented in this study offers a practical tool for including shrinkage effects in finite element analysis, and its application should be encouraged in the design of composite steel-concrete structures.