Axial Compression Behaviour of Square Steel Tube Recycled Aggregate Concrete Short Columns

Literature Overview

This study by Zhang Jicheng and colleagues, published in the Journal of Guangxi University (Natural Science Edition) in 2016, investigates the influence of steel tube width-to-thickness ratio and recycled aggregate replacement rate on the axial compressive behaviour of square steel tube recycled aggregate concrete (STRAC) short columns. Nine specimens were designed and tested under monotonic axial compression. The research was funded by the National Natural Science Foundation of China (Project 51378077) and Hubei Provincial Department of Education Science and Technology Research Project (D20151304). This work is significant because it addresses the growing need for sustainable construction materials while maintaining structural reliability through composite steel tube concrete systems.

Core Technical Parameters and Test Configuration

The experimental programme varied two principal parameters: the width-to-thickness ratio of the square steel tube and the replacement rate of recycled coarse aggregate in the concrete matrix. The specimens were short columns subjected to axial compression until failure. Key observations included the load-displacement curves, load-strain curves, and failure morphology.

Interpretation of Key Technical Findings

The most noteworthy finding is that the width-to-thickness ratio and recycled aggregate replacement rate have minimal influence on the final failure morphology. All specimens exhibited shear-type failure, consistent with conventional steel tube concrete (SRC) columns. This indicates that the steel tube confinement mechanism remains dominant regardless of the concrete quality degradation caused by recycled aggregates. From a steel tube manufacturing perspective, this finding has important implications for the selection of tube wall thickness in recycled concrete applications.

The load-displacement curves reveal that as the recycled aggregate replacement rate increases, the descending branch becomes steeper, indicating reduced post-peak ductility. However, a critical threshold effect was observed: when the width-to-thickness ratio is sufficiently large, specimens with recycled aggregate replacement rates exceeding 30% show converging ultimate bearing capacities. This suggests that the steel tube confinement effect becomes the governing factor at higher wall thicknesses, effectively compensating for the reduced concrete strength associated with recycled aggregates.

Connection with Steel Pipe Manufacturing and Welding Practice

From a steel pipe fabrication standpoint, this research has several practical implications. First, the finding that shear failure dominates regardless of concrete quality means that the tube wall thickness design should prioritize confinement efficiency rather than worrying about differential failure modes caused by recycled concrete. Second, the convergence of bearing capacity at higher width-to-thickness ratios implies that for recycled aggregate concrete applications, designers can be more liberal with tube dimensions, potentially simplifying fabrication.

The width-to-thickness ratio directly relates to local buckling resistance of the steel tube. For square steel tubes fabricated by ERW or HFW processes, the width-to-thickness ratio is a critical quality parameter governed by standards such as GB/T 3093 and API 5L. Tubes with higher width-to-thickness ratios are more susceptible to local buckling, which requires careful control of the welding process to avoid residual stresses that could initiate premature buckling. The longitudinal deformation exceeding 30 mm at failure indicates significant plastic deformation of the steel tube, which places demands on the ductility of the steel grade used.

Capacity Calculation Method and Standards Comparison

The authors proposed a bearing capacity calculation method for STRAC short columns and compared results with relevant codes. The approach likely builds upon the conventional SRC column design formulas found in GB 50936 and CECS 230, with modifications to account for the reduced concrete strength due to recycled aggregates. The key insight is that considering the adverse effect of recycled aggregate replacement rate in the calculation is more rational than ignoring it, particularly when the width-to-thickness ratio is not sufficiently high to compensate.

Study Insights and Engineering Implications

This research contributes valuable data for the practical application of recycled aggregate concrete in steel tube concrete structures. The finding that the steel tube confinement mechanism is robust enough to accommodate recycled aggregate degradation up to certain thresholds provides engineering confidence for sustainable construction practices. However, the steeper descending branch at higher replacement rates signals a reduction in energy absorption capacity, which is a concern for seismic design. Engineers should consider that while the ultimate bearing capacity may be acceptable, the post-peak behaviour warrants additional attention in ductility-critical applications. The proposed calculation method offers a practical tool for preliminary design, but should be validated against more extensive test data covering a wider range of parameters before widespread adoption in practice.