Zhuojin Pipe Fitting Co., LtdCreep Characteristics of High-Strength Steel Tube Confined High-Strength Concrete
Literature Overview
The paper by Huang Yonghui, Liu Airong, Fu Jiyang, Zhu Shuhan, and Rao Rui from Guangzhou University, published in Engineering Mechanics (2021, Vol. 38, Issue 8, pp. 204-212), presents a comprehensive experimental investigation into the shrinkage and creep behavior of high-strength steel tube confined high-strength concrete (HSCFST). Funded by multiple National Natural Science Foundation grants (Nos. 11972123, 51878188) and the Discipline Innovation Introduction Program (D21021), the study tests 15 axial compression short columns with varying steel ratios over a 365-day duration.
Core Technical Content
Experimental Program
The experimental matrix is designed to isolate the effect of steel ratio (the ratio of steel tube cross-sectional area to total cross-sectional area) on creep behavior. Fifteen specimens were loaded axially and monitored for 365 days, measuring both free shrinkage and loaded creep deformation. The concrete core employs high-strength concrete, while the steel tubes are of high-strength grade, making this a study of the interaction between two high-performance materials.
| Specimen Parameter | Range | Purpose |
|---|---|---|
| Number of specimens | 15 | Statistical significance and parametric coverage |
| Steel ratio range | Multiple values | Quantify confinement effect on creep |
| Test duration | 365 days | Capture long-term creep development |
| Loading type | Axial compression | Simulate column behavior |
| Comparison models | MC90, ACI 209, MC2010 | Benchmark prediction accuracy |
Key Experimental Findings
The results demonstrate several important relationships:
- Steel confinement dramatically reduces creep: The creep coefficient of plain concrete exceeds that of HSCFST by more than a factor of two at 365 days. This is attributed to the lateral constraint imposed by the steel tube, which suppresses the microcracking and moisture migration mechanisms that drive concrete creep.
- Steel ratio effect: As the steel ratio increases, the creep coefficient decreases monotonically. This confirms that the confinement pressure from the steel tube is directly proportional to the effective restraint on concrete deformation.
- Concrete strength effect: When compared with conventional CFST (ordinary strength concrete core), the creep of HSCFST is lower, indicating that higher concrete compressive strength inherently reduces creep susceptibility due to the denser microstructure and reduced porosity.
Creep Prediction Model Comparison
| Model | Year | Fit to HSCFST Data | Recommendation |
|---|---|---|---|
| MC90 | 1990 | Moderate deviation | Not recommended for HSCFST |
| ACI 209 | 2008 | Acceptable but biased | Usable with caution |
| MC2010 | 2010 | Best agreement with test data | Recommended for HSCFST design |
The MC2010 model's superior performance is attributed to its more refined treatment of concrete age at loading, environmental humidity, and the nonlinear time function, which better captures the behavior of high-strength concrete under confinement.
Engineering Practice Implications
Design of Tall Building Columns
For high-rise buildings employing HSCFST columns, the reduced creep coefficient has direct implications for long-term deflection and prestress loss in prestressed members. Engineers can confidently use the MC2010 model for serviceability limit state verification, knowing it provides accurate predictions for this specific composite system. The finding that creep is reduced by more than 50% compared to plain concrete means that secondary creep deflections in HSCFST columns will be significantly smaller than those predicted by conventional design codes that do not account for the steel tube confinement effect.
Material Selection Strategy
The study provides quantitative justification for the use of both high-strength steel tubes and high-strength concrete cores. The dual benefit—reduced creep from both the confinement effect (high-strength steel) and the intrinsic material property (high-strength concrete)—makes HSCFST an attractive option for applications where long-term deformation control is critical, such as:
- Tall building core columns in seismic regions where P-Δ effects are amplified by creep deflection
- Long-span bridge piers where differential settlement due to creep must be minimized
- Nuclear containment structures where dimensional stability is paramount
Steel Pipe Specification Considerations
From a steel pipe manufacturing perspective, the study reinforces the value of high-strength steel grades (Q460, Q550, or higher per GB/T 1591) for CFST applications. The higher yield strength allows thinner wall thicknesses while maintaining confinement effectiveness, which reduces material cost and weight. However, the steel ratio must be optimized—not merely maximized—because excessive steel ratio increases cost without proportional creep reduction benefits due to diminishing returns in confinement effectiveness.
Reflections on Methodology and Limitations
The 365-day test duration, while substantial, does not capture the full creep development curve for high-strength concrete, which is known to exhibit continued creep beyond one year. The study would benefit from extension to 730 or 1000 days to validate the long-term extrapolation of the MC2010 model. Additionally, the study focuses exclusively on axial compression, whereas in practice, CFST columns are subjected to combined axial and bending loads, which may alter the creep behavior due to non-uniform stress distribution.
The comparison with conventional CFST provides valuable context but should be interpreted cautiously, as the conventional CFST data may come from different test series with different environmental conditions and concrete mix designs. A controlled comparison with matched environmental conditions would strengthen the conclusions.
Study Insights and Implications
This research provides the structural engineering community with reliable creep data for HSCFST, a system that is increasingly specified in modern high-rise construction but has historically lacked dedicated experimental databases. The recommendation of the MC2010 model for HSCFST creep prediction is a practical contribution that can be directly incorporated into design calculations. For steel pipe manufacturers, the findings validate the market demand for high-strength steel tubes in structural applications and provide quantitative performance data that can be used in technical marketing and engineering support. The work also highlights an important principle: in composite systems, the interaction between constituent materials can dramatically alter the behavior of individual components, and design models must account for this interaction rather than treating the components independently.