Zhuojin Pipe Fitting Co., LtdEccentric Compression Performance of Square Steel Tube Recycled Concrete Long Columns
Literature Overview
Wang Chenggang, Yuan Quan, and Zhang Zeyang (2022), published in the Journal of Hefei University of Technology (Natural Science Edition), report on an experimental study of 15 square steel tube recycled concrete long columns subjected to uniaxial eccentric compression. The study was funded by the Anhui Provincial Natural Science Foundation (Grant No. JZ2019AKZR0220) and the Anhui Provincial Housing and Construction Science and Technology Plan (Grant No. 2016YF-12). The research systematically investigates the effects of recycled coarse aggregate replacement ratio, steel tube wall thickness, slenderness ratio, and eccentricity ratio on the load-bearing capacity and initial axial stiffness of the composite columns.
Core Technical Findings
The experimental results demonstrate that square steel tube recycled concrete long columns exhibit two primary failure modes under eccentric compression:
- End-buckle failure: Local buckling of the steel tube at the column end, followed by progressive concrete crushing.
- Global flexural instability: Overall bending failure of the column, typically initiated at the mid-span region.
The square steel tube provides effective lateral confinement to the recycled concrete, which tends to exhibit greater compressive expansion than natural aggregate concrete due to the weaker aggregate-matrix interface. Despite the use of recycled aggregates, the composite columns maintain good load-bearing capacity and deformation capacity, comparable to conventional steel tube concrete columns.
| Variable Parameter | Effect on Load-Bearing Capacity | Effect on Initial Axial Stiffness |
|---|---|---|
| Recycled aggregate replacement ratio ↑ | Capacity decreases | Stiffness decreases |
| Steel tube wall thickness ↑ | Capacity increases | Stiffness increases |
| Slenderness ratio ↑ | Capacity decreases | Stiffness decreases |
| Eccentricity ratio ↑ | Capacity decreases | Stiffness decreases |
| Fly ash addition | Capacity generally increases | Modest improvement |
| Expansive agent addition | Capacity moderately increases | Modest improvement |
Interpretation of Technical Points
The confinement effectiveness of square steel tubes on recycled concrete is a critical finding. Recycled coarse aggregates possess a weaker interfacial transition zone (ITZ) between the old mortar adhered to the aggregate surface and the new cement paste. Under compressive loading, this weak ITZ leads to greater lateral expansion of the concrete core compared to natural aggregate concrete. The square steel tube, through its geometric configuration, provides uniform lateral restraint that effectively counteracts this expansion, maintaining the concrete in a triaxial compressive state that enhances both strength and ductility.
The beneficial effects of fly ash and expansive agents are particularly noteworthy. Fly ash, as a pozzolanic material, reacts with calcium hydroxide released during cement hydration to form additional calcium silicate hydrate (C-S-H) gel, which densifies the ITZ and improves the bond between recycled aggregates and the cement matrix. The expansive agent compensates for the autogenous shrinkage of concrete and can partially repair micro-cracks that develop during drying shrinkage, thereby maintaining the integrity of the composite section.
The finding that both load-bearing capacity and initial axial stiffness decrease with increasing eccentricity ratio is consistent with the fundamental mechanics of eccentrically loaded members. The eccentricity introduces a bending moment that superimposes on the axial load, creating a combined stress state that reduces the effective load-carrying area and accelerates the onset of non-linear behaviour.
Engineering Practice Implications
For the practical design of square steel tube recycled concrete columns, the following recommendations are offered:
- The use of recycled coarse aggregates is structurally viable in square steel tube concrete columns, provided that the replacement ratio is controlled and supplementary measures (such as fly ash and expansive agents) are employed.
- The slenderness ratio and eccentricity ratio should be limited to prevent premature flexural instability, which is a stability-sensitive failure mode.
- Wall thickness should be optimized to provide adequate confinement without excessive material usage, considering the cost-benefit balance between recycled aggregate savings and steel tube material costs.
- The inclusion of fly ash and expansive agents is recommended as a low-cost measure to partially compensate for the performance degradation caused by recycled aggregates.
The study contributes to the growing body of evidence supporting the use of recycled aggregates in structural concrete applications, particularly in composite construction systems where confinement effects can be effectively utilized.
Key Questions and Reflections
Several aspects of the study warrant further investigation. The long-term durability of recycled concrete within steel tubes under cyclic loading and corrosion exposure is not addressed. The effect of recycled aggregate gradation and shape on the confinement effectiveness of square steel tubes remains unclear. Additionally, the study does not consider the influence of concrete strength grade on the interaction between recycled aggregates and steel tube confinement. Engineers applying these findings should conduct additional verification testing specific to their project conditions, particularly when high replacement ratios or aggressive environmental conditions are involved. The integration of recycled materials into composite structural systems represents a promising pathway toward sustainable construction, and this study provides a solid experimental foundation for further research and development.