Axial Compression Behavior of Steel Tube Recycled Concrete Long Columns

Literature Overview

This research by Wu Yanhai, Fang Yingping, Feng Wenxian, and Cai Yang was published in the Journal of Nanchang University (Engineering Technology) in 2015, Volume 37, Issue 1, pages 27-33. Funded by the National Natural Science Foundation of China (Grant 51278129), the study investigates the axial compression behavior of long columns composed of circular steel tubes filled with recycled concrete. This topic addresses the growing need for sustainable construction materials, as recycled concrete aggregates (RCA) are increasingly used to reduce construction waste and conserve natural resources.

Experimental Program and Test Setup

The study involved the static loading test of six circular steel tube recycled concrete (CFST-RCA) long columns under axial compression. The test specimens were designed to vary two key parameters: the slenderness ratio (L/D) and the confinement coefficient (kappa). The following table summarizes the typical test parameters:

The test setup included load cells for measuring axial load, displacement transducers for measuring axial deformation, and strain gauges for measuring the strain distribution in both the steel tube and the concrete core. The full loading process was recorded to capture the complete load-deformation and load-strain response.

Failure Modes and Loading Stages

The test results reveal that all CFST-RCA long columns failed by elastic-plastic buckling, with the failure process consisting of three distinct stages:

1. Elastic stage: The column behaves linearly, with load proportional to deformation. The stress-strain relationship for both steel and concrete is in the elastic range.
2. Elastic-plastic stage: The outer fibers of the steel tube yield, and the column begins to develop plastic deformation. The load continues to increase but at a reduced rate.
3. Plastic descending stage: After reaching the peak load, the column undergoes buckling instability, and the load decreases with further deformation. The failure is characterized by lateral bowing and local buckling of the steel tube.

The failure mode is consistent with the Euler buckling theory modified for material nonlinearity, where the effective modulus of the composite section determines the critical buckling load.

Parametric Effects on Structural Performance

The study identifies two primary parameters that influence the structural performance of CFST-RCA long columns:

The confinement coefficient kappa, defined as the ratio of the steel tube cross-sectional area to the concrete cross-sectional area (kappa = A_s / A_c), has a more significant influence on the structural performance than the slenderness ratio. This finding is consistent with the general understanding that steel confinement is the primary mechanism by which CFST columns achieve enhanced strength and ductility compared to plain concrete columns.

Comparison with Design Code Predictions

The authors compared the experimental ultimate load capacities with predictions from various design codes and standards:

The comparison shows that existing design codes provide reasonably accurate predictions for CFST-RCA long columns, with deviations generally within 10 to 15 percent. However, the codes were originally developed for CFST columns with natural aggregates, and the lower elastic modulus and higher variability of recycled concrete may introduce additional uncertainties.

Engineering Practice Considerations

For the practical application of CFST-RCA long columns in construction projects, the following considerations are important:

1. Recycled aggregate quality control: The recycled concrete aggregate (RCA) must be properly cleaned, graded, and tested to ensure consistent quality. The water absorption and strength of RCA are critical parameters that affect the concrete properties.
2. Steel tube fabrication: The steel tubes should be manufactured to maintain tight tolerances on diameter and wall thickness, as these directly affect the confinement coefficient and consequently the column capacity.
3. Concrete placement: The placement of recycled concrete inside the steel tube requires careful attention to avoid voids and ensure uniform compaction. The use of vibrators or pumping techniques should be carefully planned.
4. Slenderness limitations: The slenderness ratio should be limited to ensure that the column does not fail by elastic buckling before the composite section reaches its full strength capacity.

Welding and Connection Design

For CFST-RCA long columns used in multi-story buildings, the connections between columns are critical design elements. The typical connection types include:

• End plate connections: A steel end plate is welded to the top and bottom of the steel tube and bolted to the beam or slab.
• Internal diaphragm connections: Internal steel diaphragms are welded to the steel tube and bolted to the connected members.
• External jacket connections: A steel jacket is welded around the column end and bolted to the connected members.

The welding quality at these connection details is critical for the overall structural integrity. The welds should be designed and inspected in accordance with GB 50661-2011 or equivalent standards, with full penetration welds required at critical load paths.

Study Insights and Reflections

The research provides valuable experimental data on the axial compression behavior of CFST-RCA long columns, filling an important gap in the knowledge of sustainable structural systems. The finding that the confinement coefficient has a more significant influence than the slenderness ratio is particularly useful for design optimization.

However, several areas require further investigation:

1. The long-term durability of CFST-RCA columns, particularly the corrosion behavior of the steel tube in contact with recycled concrete that may contain higher chloride content.
2. The seismic performance of CFST-RCA columns, which is essential for their application in earthquake-prone regions.
3. The effect of recycled aggregate replacement ratio (0 percent to 100 percent) on the structural performance of CFST columns.

The study also highlights the importance of using appropriate design formulas for CFST-RCA columns. While existing codes provide reasonable predictions, the development of code-specific design provisions for CFST-RCA columns would provide greater confidence in their design and application.

Summary

This study provides comprehensive experimental evidence on the axial compression behavior of circular steel tube recycled concrete long columns, demonstrating that they fail by elastic-plastic buckling and that the confinement coefficient is the most influential parameter on structural performance. The comparison with existing design codes shows that current provisions can be applied to CFST-RCA columns with reasonable accuracy, though further research on long-term durability, seismic performance, and recycled aggregate replacement ratio effects is warranted. Engineers designing sustainable structures with CFST-RCA columns should ensure proper quality control of recycled aggregates, maintain tight tolerances on steel tube fabrication, and use appropriate design formulas validated against experimental data. The study contributes significantly to the advancement of sustainable construction practices using recycled materials in structural applications.