Axial Compression Behavior of Square Steel Tube Recycled Concrete Long Columns

Literature Overview

The research by Zhang Zhaoqiang, Zhao Junhai, and Xu Yang, published in the Concrete journal in 2015, presents an experimental investigation into the axial compression performance of square steel tube recycled concrete long columns. Eight test specimens with varying slenderness ratios were subjected to monotonic axial compression loading. The study examines the influence of slenderness ratio on load-bearing capacity, identifies failure modes, and evaluates the applicability of existing Chinese design codes for calculating the axial compression capacity of square steel tube recycled concrete long columns.

Experimental Setup and Results

The eight specimens were designed with slenderness ratio as the primary variable, covering a range of short to long column behavior. The recycled concrete used in the specimens incorporates recycled aggregate, which typically has lower strength and higher water absorption compared to natural aggregate. The steel tubes provide confinement to the recycled concrete core, enhancing the compressive strength and ductility of the composite column.

Failure Mechanisms and Slenderness Ratio Effects

The experimental results reveal three distinct stages in the axial compression behavior: elastic stage, yield stage, and failure stage. For specimens with low slenderness ratios, the failure is characterized by material strength failure, where the recycled concrete core reaches its ultimate compressive strength and the steel tube provides confinement. For specimens with high slenderness ratios, the second-order effects become significant, and the failure mode shifts to overall flexural buckling.

A critical observation is that all specimens exhibited local buckling of the steel tube wall before final failure, regardless of the slenderness ratio. This local buckling phenomenon reduces the effective confinement pressure on the recycled concrete core and can lead to premature loss of load-bearing capacity. The load-bearing capacity decreases gradually with increasing slenderness ratio, which is consistent with classical column behavior theory.

Code Applicability Assessment

The authors evaluated the applicability of several Chinese design codes for calculating the axial compression capacity of square steel tube recycled concrete long columns:

The recommendation to use DBJ 13-51-2003 and CECS 159:2004 for axial compression capacity calculation is significant, as these codes were originally developed for conventional concrete-filled steel tube columns. The applicability to recycled concrete columns suggests that the fundamental mechanics of the composite action are similar enough that the code formulas remain valid, provided that the material properties of the recycled concrete are properly characterized.

Welding and Fabrication Implications

From a steel pipe and welding engineering perspective, this research has several important implications. The use of recycled concrete in steel tube columns is an environmentally sustainable practice that reduces the consumption of natural resources and construction waste. However, the recycled concrete may have different workability and setting characteristics compared to conventional concrete, which affects the concrete-filling process in the steel tube.

The local buckling of the steel tube wall observed in all specimens highlights the importance of steel tube fabrication quality. The wall thickness uniformity, corner weld quality of square tubes, and overall dimensional accuracy of the steel tube directly influence the local buckling resistance. Engineers should ensure that the steel tubes are fabricated to tight tolerances, with particular attention to the corner regions where stress concentrations are highest.

The welding of end plates and connection details for these long columns should be designed to accommodate the expected axial compression forces and any potential eccentricity. The weld details should be inspected using appropriate non-destructive testing methods, with acceptance criteria aligned with the structural importance of the column.

Study Insights and Recommendations

This research contributes to the growing body of knowledge on recycled concrete-filled steel tube structures, which are important for sustainable construction. The confirmation that existing Chinese codes can be applied to recycled concrete columns, with appropriate material property adjustments, provides engineers with a practical design basis. However, the observation of local buckling in all specimens underscores the need for careful steel tube fabrication and quality control. As a steel pipe and welding professional, I recommend that engineers involved in the fabrication of recycled concrete-filled steel tube columns pay particular attention to steel tube dimensional accuracy, wall thickness uniformity, and weld quality at the corners and end connections. The sustainability benefits of recycled concrete should not come at the expense of structural safety, and rigorous quality control throughout the fabrication and construction process is essential.