Axial Compression Behavior of Square Steel Tube Recycled Concrete Long Columns

Literature Overview

This paper by Hu Naidong, Du Xikai, Liu Puyuan, Niu Zhiqiang, and Yang Fan, published in the Journal of Hebei Agricultural University (2017, Vol. 40, Issue 3, pp. 124–127), investigates the axial compression mechanical properties of square steel tube recycled concrete long columns. The research was funded by the Hebei Agricultural University Science and Technology Foundation Project (ZD201403).

The study adopts a parametric experimental approach, varying four key parameters: recycled aggregate replacement rate, steel ratio, slenderness ratio, and type of recycled aggregate (specifically recycled fired brick aggregate). The specimens are tested under axial compression loading, and the resulting load-displacement curves, load-strain curves, and failure modes are recorded and analyzed.

Experimental Parameters and Test Setup

The experimental matrix is designed to isolate the influence of each parameter on the structural response. The recycled aggregate replacement rate controls the proportion of recycled material substituted for natural aggregate in the concrete mix. The steel ratio represents the volume of steel tube relative to the total cross-sectional area. The slenderness ratio (L/D) governs the susceptibility of the column to buckling instability. The type of recycled aggregate introduces variability in the concrete's internal bonding and elastic properties.

The square steel tube cross-section is chosen for its ease of fabrication and connection to other structural elements. Compared with circular steel tubes, square sections provide flat surfaces for bolted or welded connections but may exhibit more complex buckling behavior due to corner effects and non-uniform stress distribution.

Key Findings and Technical Interpretation

The experimental results reveal several important observations regarding the structural behavior of these composite columns:

1. In the elastic loading range, the square steel tube recycled concrete columns with recycled brick aggregate show no significant difference from those with conventional recycled concrete aggregate. This indicates that the initial stiffness is governed primarily by the steel tube and the overall concrete mix design rather than the specific recycled aggregate type.
2. In the post-peak descending branch, the columns incorporating recycled brick aggregate exhibit a more rapid degradation of axial compression stiffness. This behavior is attributed to the weaker interfacial transition zone between recycled brick aggregate particles and the cement paste, which leads to earlier micro-cracking and progressive loss of load-bearing capacity.
3. When the brick content reaches 50%, both the load-bearing capacity and stiffness decline significantly. This suggests a threshold beyond which the quality of recycled brick aggregate becomes detrimental to the overall structural performance.
4. Higher steel ratios result in greater resistance to deformation and higher load-bearing capacity. The increased steel ratio enhances the confinement effect on the concrete core, delaying the onset of concrete crushing and providing a more ductile failure response.

Welding and Fabrication Considerations for Square Steel Tubes

From a steel pipe fabrication standpoint, square steel tubes present distinct challenges compared to circular pipes. The fabrication of square steel tubes typically involves cold bending or roll forming of flat steel strips, followed by welding of the longitudinal seam. The corner regions experience higher plastic deformation during the forming process, which can lead to work hardening, reduced ductility, and potential micro-cracking in the heat-affected zone of the longitudinal weld.

For the long columns studied in this paper, the steel tubes are likely fabricated through one of the following processes:

• Cold-formed square tubes with a single longitudinal weld, suitable for smaller dimensions and lower wall thicknesses.
• Hot-rolled square hollow sections (SHS) with four butt welds joining four flat plates, used for larger dimensions and higher wall thicknesses.
• Bent and welded square tubes fabricated from flat steel plates, where the corners are formed by bending and the longitudinal welds are applied.

The welding quality of the longitudinal seams is critical because these welds carry the majority of the hoop stress in the steel tube under axial compression. Any defects in the weld, such as lack of fusion, porosity, or undercut, can serve as crack initiation sites and lead to premature failure of the column.

Quality control measures for square steel tube fabrication should include:

• Visual inspection of all welds for surface defects.
• Magnetic particle testing (MT) or dye penetrant testing (PT) of longitudinal welds.
• Ultrasonic testing (UT) for internal defects in thicker sections.
• Mechanical property testing of the base steel to ensure compliance with the specified grade (e.g., Q235, Q355, or equivalent).

Study Insights and Implications for Engineering Practice

This research contributes valuable experimental data on the structural behavior of steel tube recycled concrete columns, which are increasingly relevant in sustainable construction practices. The finding that recycled brick aggregate can be used without affecting elastic-stage performance but degrades post-peak behavior at higher replacement rates provides a practical design guideline: recycled brick aggregate is acceptable for columns designed to remain in the elastic range under service loads, but caution is warranted for columns expected to undergo significant inelastic deformation.

The emphasis on steel ratio as a key parameter reinforces the importance of selecting appropriate steel tube dimensions and wall thicknesses in composite column design. Engineers should not compromise on steel tube quality to reduce costs, as the steel tube provides both direct load-carrying capacity and confinement to the concrete core.

In conclusion, this study demonstrates that steel tube recycled concrete long columns can achieve satisfactory structural performance when properly designed, but the quality of the steel tube fabrication and welding must be rigorously controlled to ensure the predicted load-bearing capacity and deformation resistance are realized in practice.