Zhuojin Pipe Fitting Co., LtdExperimental Study on Creep Effects During Construction of Steel Tube Expansive and Recycled Concrete Structures
Literature Overview
The paper by Wang Haiyang, Zha Xiaoxiong, Huang Haochun, and Wang Xiaodong, published in Industrial Construction (2011, Vol. 41, No. 6, pp. 43-46), investigates the influence of creep on steel tube expansive concrete (STEC) and steel tube recycled concrete (STRC) structures during staged construction. The authors conducted multi-stage loading experiments to simulate the layered construction process and evaluated the effects of expansive agent dosage and loading interval on the ultimate load capacity and creep deformation of the composite columns. The study also examined the corresponding behaviour of recycled concrete variants. This work addresses a practical and often overlooked issue in the construction of large-scale steel tube concrete structures.
Experimental Design and Key Findings
The experimental programme involved circular steel tube columns filled with expansive concrete and recycled concrete, subjected to axial compression in multiple stages to simulate the layered pouring process used in actual construction. The steel tubes were fabricated from Q235B seamless steel pipes with a diameter of 150 mm and a wall thickness of 6 mm. The expansive concrete was produced by adding different dosages of magnesium oxide (MgO) expansive agent (0%, 3%, and 6% by cement weight), while the recycled concrete was produced by replacing natural aggregate with crushed recycled concrete aggregate at a 50% replacement rate.
The key findings of the study are summarised below. First, the use of expansive agent improved the ultimate load capacity of ordinary steel tube concrete columns by 5-15%, depending on the dosage. This improvement is attributed to the initial contact pressure generated by the expansive concrete, which pre-stresses the steel tube and enhances the composite action. Second, increasing the expansive agent dosage led to proportionally larger creep deformation, as the expansive reaction continued to develop over time, generating additional internal stresses that contributed to time-dependent deformation.
| Test Parameter | STEC (Ordinary Concrete) | STEC (Expansive Concrete) | STRC (Recycled Concrete) |
|---|---|---|---|
| Expansive agent dosage | 0% | 3% / 6% | 3% |
| Ultimate capacity improvement | Baseline | +5% to +15% | +3% to +8% |
| Creep deformation | Baseline | Increased proportionally | Significantly larger than STEC |
| Loading interval effect | Moderate reduction in creep | Moderate reduction in creep | Significant reduction in creep |
| Long-term stability | Good | Good with controlled dosage | Requires careful monitoring |
Third, when the loading interval between stages was increased, the creep deformation amplitude was significantly reduced, while the effect on ultimate load capacity was relatively minor. This finding has direct implications for construction scheduling: longer curing intervals between construction stages can substantially reduce long-term creep deformation without significantly compromising the ultimate structural capacity.
Material Science and Construction Process Analysis
The creep behaviour of expansive concrete is fundamentally different from that of ordinary concrete. In ordinary concrete, creep is driven by the sustained stress applied to the material and is governed by the viscoelastic properties of the cement paste and the aggregate skeleton. In expansive concrete, the ongoing chemical reaction of the expansive agent generates additional internal stresses that superimpose on the externally applied loads, leading to enhanced creep deformation. This phenomenon is particularly pronounced in recycled concrete, where the higher porosity and weaker interfacial transition zone (ITZ) between the recycled aggregate and the cement paste facilitate greater time-dependent deformation.
From a steel pipe perspective, the creep of the concrete core induces additional hoop stresses in the steel tube over time. If the creep deformation is excessive, it can lead to local buckling of the steel tube or progressive relaxation of the initial contact pressure, both of which reduce the long-term load-carrying capacity of the composite column. The steel tube material must therefore have adequate resistance to local buckling, which is governed by the t/D ratio and the material's strain-hardening behaviour.
Construction Practice Recommendations
Based on the experimental findings, the following recommendations are proposed for the construction of STEC and STRC structures. First, the expansive agent dosage should be optimised through trial mixes and laboratory testing to achieve the desired expansion rate without excessive creep deformation. A dosage of 3% by cement weight is generally recommended as a balance between expansion benefit and creep control. Second, the construction schedule should incorporate adequate curing intervals between pouring stages, with a minimum of 7 days at normal temperature (20 degrees Celsius) or equivalent maturity (200 degree-days). Third, for recycled concrete applications, additional monitoring of long-term deformation is recommended, as the creep behaviour of recycled concrete is more sensitive to construction conditions.
The quality control plan for STEC and STRC construction should include: dimensional and surface inspection of steel tubes before concrete pouring; chemical analysis of the expansive agent and recycled aggregate; in-situ strength testing of the concrete at 7 and 28 days; and periodic measurement of column deformation during and after construction. Any deviation from the predicted deformation should be investigated and documented to ensure the long-term structural integrity of the building.
Study Insights and Reflections
This paper addresses a practically important but often neglected aspect of steel tube concrete construction: the influence of staged construction and expansive agents on long-term creep behaviour. The findings have significant implications for the design and construction of large-scale steel tube concrete structures, particularly those involving expansive concrete or recycled concrete. For steel pipe engineers, the key takeaway is that the long-term performance of composite columns is not solely determined by the initial material properties and fabrication quality, but is also strongly influenced by the construction sequence and curing conditions. Engineers should integrate construction process planning into the structural design from the outset, ensuring that the construction schedule is compatible with the material properties and the long-term performance requirements of the structure. The study also highlights the potential of recycled concrete in steel tube concrete applications, provided that the enhanced creep behaviour is properly accounted for in the design and construction process.