Zhuojin Pipe Fitting Co., LtdFinite Element Study of Square Steel Tube Recycled Brick-Concrete Aggregate Concrete Short Columns under Axial Compression
Literature Overview and Research Context
The paper by Liu Xiao, Shen Shen, Hou Dongxu, and Li Yingchun (2024, Journal of Shenyang Jianzhu University, Vol. 40, No. 5, pp. 857-866) addresses a timely and practically important problem: the structural behaviour of square steel tube confined concrete short columns in which recycled brick-concrete aggregate replaces a portion of natural aggregate. The research is supported by the National Natural Science Foundation of China (Grant 52078306), the Shenyang Science and Technology Plan Project (21-108-9-21), and the Liaoning Provincial Department of Education Basic Research Project (LJKQZ2021165). The work employs ABAQUS finite element modelling to systematically investigate the influence of recycled aggregate replacement ratio, chamfer ratio, steel tube yield strength, and steel tube wall thickness on the axial compressive behaviour of these composite short columns.
Core Technical Findings and Parameter Analysis
The study validates its finite element model against experimental data before proceeding with parametric analysis. The key findings are summarised below.
| Parameter | Variation Range | Effect on Ultimate Bearing Capacity |
|---|---|---|
| Recycled brick-concrete aggregate replacement ratio | 0% to 50% (and above) | Less than 10% reduction when replacement ratio is below 50% |
| Chamfer ratio | Up to 0.16 | Slight improvement in ultimate bearing capacity at 0.16 |
| Steel tube yield strength | Increasing | Positive correlation with bearing capacity |
| Steel tube wall thickness | Increasing | Positive correlation with bearing capacity |
The most significant engineering insight from this work is that recycled brick-concrete aggregate replacement ratios up to 50% introduce less than a 10% reduction in ultimate bearing capacity. This finding is directly relevant to engineers seeking to incorporate construction and demolition waste into structural applications without compromising safety margins. The chamfer ratio analysis reveals a nuanced behaviour: a moderate chamfer ratio (up to 0.16) improves confinement effectiveness by allowing the steel tube to engage the corner regions more uniformly, but excessive chamfering weakens the cross-sectional area to the point where bearing capacity declines. This represents a classic engineering trade-off between geometric optimisation and material loss.
Interpretation of the Chamfer Effect on Confinement Mechanics
The chamfer ratio is defined as the ratio of chamfer depth to the side length of the square cross-section. In a conventional square steel tube, the sharp internal corners create stress concentration zones that can initiate early local buckling of the steel tube wall under compressive loading. By introducing a controlled chamfer at the internal corners, the stress distribution becomes more uniform, and the steel tube can exert more effective lateral confinement on the core concrete, particularly in the corner regions. However, the chamfer necessarily reduces the total cross-sectional area of the composite member. The paper demonstrates that at a chamfer ratio of 0.16, the improvement in confinement efficiency marginally outweighs the area loss, resulting in a net positive effect on ultimate capacity. Beyond this threshold, the geometric weakening dominates.
This finding has practical implications for the design of recycled-aggregate confined concrete columns in buildings. For square sections, a chamfer ratio in the range of 0.10 to 0.16 may be considered as an optimisation strategy, provided that the reduction in cross-sectional area is accounted for in the overall structural design.
Integration with Engineering Practice and Quality Control Considerations
From a quality assurance perspective, the use of recycled brick-concrete aggregate introduces additional variability compared to conventional natural aggregate. Recycled aggregates typically exhibit higher water absorption, lower specific gravity, and more irregular particle shapes, all of which affect the workability and strength development of the concrete mix. In a steel tube confined column, the confined concrete is expected to achieve higher strength and ductility than unconfined concrete, but the base properties of the concrete still govern the overall behaviour.
For engineers specifying recycled-aggregate concrete in confined columns, the following quality control measures are recommended:
- Aggregate characterisation: Conduct sieve analysis, water absorption tests, and Los Angeles abrasion tests on the recycled aggregate batch before use.
- Concrete mix design adjustment: Increase the water-reducing admixture dosage to compensate for the higher water demand of recycled aggregate, and verify slump and air content at the point of placement.
- Steel tube welding inspection: The longitudinal and transverse welds of the square steel tube must be inspected using magnetic particle testing (MT) or ultrasonic testing (UT) in accordance with GB/T 3323 or NB/T 47013, as appropriate for the service conditions.
- Compressive strength verification: Core specimens or cube specimens cast alongside the column should be tested at 7, 28, and 90 days to monitor strength development, particularly given the potential for slower hydration in recycled-aggregate concrete.
Key Questions and Reflective Insights
One question that arises from this study is the long-term durability of recycled-aggregate confined concrete columns under cyclic or fatigue loading, which is not addressed in the present work. Brick-concrete recycled aggregates may contain residual mortar and clay particles that could be susceptible to freeze-thaw degradation or chloride-induced corrosion of the embedded steel tube. For structures in corrosive environments, such as coastal regions or industrial zones, additional durability testing would be warranted before widespread adoption.
Another point of reflection concerns the scalability of the findings. The study focuses on short columns, where buckling is not a governing failure mode. For slender columns, the reduced stiffness of recycled-aggregate concrete could amplify second-order effects and lead to earlier buckling. Engineers should therefore exercise caution when extrapolating these results to columns with high slenderness ratios.
Summary and Implications
This study provides valuable quantitative evidence that recycled brick-concrete aggregate can be used in square steel tube confined concrete short columns with replacement ratios up to 50% without significant loss of bearing capacity. The chamfer ratio analysis offers a practical geometric optimisation strategy for improving confinement efficiency. The steel tube properties remain the dominant design parameter, reinforcing the importance of proper steel tube selection, welding quality, and corrosion protection. For engineers involved in sustainable construction, this work supports the feasibility of incorporating construction waste into load-bearing structural elements, provided that appropriate quality control measures are implemented throughout the material supply chain and construction process.