Axial Compression Behavior of Square Steel Tube Recycled Concrete Long Columns

Literature Overview

The study by Hu Naidong, Du Xikai, Liu Puyuan, Niu Zhiqiang, and Yang Fan investigates the axial compression mechanical properties of long columns composed of square steel tubes filled with recycled aggregate concrete. Published in the Journal of Hebei Agricultural University (2017, Vol. 40, No. 3, pp. 124-127), this research addresses an important sustainability challenge in construction engineering: the utilization of recycled aggregates, particularly recycled brick fragments, in composite steel-concrete columns. The work is significant for steel pipe engineers because it examines how the quality and composition of the concrete infill affects the structural performance of the steel tube-concrete composite system, including the interaction between the steel tube and the concrete core.

Experimental Parameters and Design Matrix

The study adopted a parametric experimental approach with four key variables: recycled aggregate replacement rate, steel ratio (含钢率), slenderness ratio, and type of recycled aggregate. The experimental specimens were subjected to axial compression loading, and the following data were collected: load-displacement curves, load-strain curves, failure modes, and ultimate load capacities.

Key Experimental Findings

The most significant finding is the differential behavior between recycled brick aggregate columns and conventional recycled concrete columns. In the elastic stage, both types of columns exhibit no discernible difference in load-displacement response. However, in the descending stage after peak load, columns containing recycled brick aggregate show a significantly faster decline in axial compression stiffness. When the brick content reaches 50 percent, both load-bearing capacity and stiffness decrease substantially.

The steel ratio emerges as a critical parameter for controlling post-peak behavior. Higher steel ratios provide greater resistance to deformation and result in higher ultimate load capacities. This finding is directly relevant to steel pipe selection: the wall thickness of the square steel tube, which determines the steel ratio, plays a decisive role in compensating for the inferior mechanical properties of recycled brick aggregate concrete.

Failure Mode Analysis and Steel Tube-Concrete Interaction

The failure modes of the specimens reveal important insights into the composite behavior of square steel tube-recycled concrete columns:

• In specimens with high recycled brick content and low steel ratio, the concrete core crushes prematurely, leading to local buckling of the steel tube flanges.
• With higher steel ratios, the steel tube provides effective confinement to the concrete, delaying concrete crushing and promoting a more ductile failure mode.
• The long column specimens exhibit a tendency toward overall flexural buckling, with the failure point typically occurring near the mid-height of the column.

From a welding and fabrication standpoint, the local buckling of steel tube flanges in low steel ratio specimens highlights the importance of steel tube wall thickness and the quality of longitudinal and transverse welds. Local buckling initiates at stress concentrations, which may occur at weld toes or at locations where the steel tube has geometric imperfections.

Recycled Brick Aggregate Impact on Concrete Properties

The inclusion of recycled brick aggregate introduces several material-level challenges that affect the composite column performance:

The poor adhesion between recycled brick fragments and cement paste is attributed to the high water absorption capacity of fired brick, which reduces the effective water-cement ratio at the interface and creates weak zones in the concrete matrix. These weak zones serve as preferential crack initiation sites under compressive loading.

Implications for Steel Pipe Selection and Fabrication

For engineers specifying square steel tubes for recycled concrete columns, the following considerations are essential:

1. The steel ratio should be increased to compensate for the reduced concrete strength when using recycled brick aggregates. A minimum steel ratio threshold should be established based on the recycled brick content.
2. The steel tube wall thickness must be sufficient to provide effective confinement, particularly in the post-peak loading regime where the concrete has lost significant strength.
3. Weld quality is paramount, as weld defects can initiate local buckling at stress levels below the theoretical buckling load of a perfect tube.
4. Material certification for the steel tube should include verification of yield strength uniformity, as the study indicates that steel yield strength has a more significant influence on column performance than concrete compressive strength in certain parameter ranges.

Study Insights and Engineering Practice

This research contributes valuable data to the growing body of knowledge on sustainable construction using recycled materials. The finding that recycled brick aggregate columns behave identically to conventional recycled concrete columns in the elastic stage but diverge significantly in the post-peak stage has important implications for design philosophy. Engineers should not rely solely on elastic-stage behavior for design decisions when recycled brick aggregates are involved. Instead, the full load-displacement curve, including the descending branch, should inform the selection of steel tube dimensions and wall thickness. The practical recommendation is to limit recycled brick content to below 50 percent unless additional steel confinement is provided through increased tube wall thickness or supplementary internal reinforcement.