Axial Compressive Performance of Steel Tube Self-Compacting Lightweight Aggregate Concrete Columns

Literature Overview

The research by Hu Qiang, Yang Huanwei, Wu Huiqin, Kuang Zheyang, and Zhang Jun from Guangxi University of Science and Technology, published in the Journal of Guangxi University of Science and Technology in 2020, investigates the axial compressive behavior of steel tube columns filled with self-compacting lightweight aggregate concrete (SCLAC) using unsintered fly ash ceramsite as the coarse aggregate. Funded by the Guangxi Science and Technology Program and related institutional grants, this work addresses the dual challenges of reducing structural self-weight while maintaining adequate load-bearing capacity and ductility in composite columns.

Material Design and Mix Optimization

The self-compacting lightweight aggregate concrete mix design was optimized through systematic variation of three key parameters:

Parameter	Range Tested	Optimal Value	Rationale
Water-binder ratio	0.30-0.45	0.35	Balances workability and strength
Ceramsite content (by volume)	30%-70%	50%	Balances lightweight property and strength
Fly ash content (by binder mass)	10%-40%	25%	Improves workability without excessive strength loss

The use of unsintered fly ash ceramsite as the lightweight aggregate is particularly significant because it utilizes industrial waste (fly ash) and requires no high-temperature sintering, reducing the carbon footprint of the concrete production. The self-compacting property eliminates the need for vibration, which is important in confined spaces where traditional concrete placement is difficult.

Axial Compression Test Results

Seven steel tube SCLAC columns and three steel tube self-compacting normal concrete (SCNC) comparison columns were tested under axial compression. The key findings are summarized below:

Test Parameter	SCLAC Columns	SCNC Comparison Columns
Ultimate axial load	Lower than SCNC by 15-25%	Higher reference values
Failure mode	All ductile failure	All ductile failure
Concrete failure pattern	Curvature failure, strength failure, or shear failure depending on confinement ratio and slenderness	Primarily strength failure
Steel tube yielding	Occurs before peak load in all specimens	Occurs before peak load
Average concrete strain at failure	Significantly higher than peak strain due to confinement	Similar confinement effect
Ductility index	Comparable to SCNC columns	Baseline

The confinement ratio (ratio of steel tube constraint pressure to concrete compressive strength) was identified as the primary factor determining the concrete failure pattern. Higher confinement ratios promote curvature failure (more ductile), while lower confinement ratios with higher slenderness ratios promote shear failure (less ductile).

Load-Bearing Capacity Calculation Method

Based on ultimate equilibrium theory, the authors developed a calculation method for the axial compressive capacity of steel tube SCLAC columns that accounts for:

The reduced compressive strength of the lightweight concrete core
The enhanced confinement effect from the steel tube on the lightweight concrete
The interaction between steel tube hoop stress and concrete radial pressure
The slenderness effect through an appropriate reduction factor

The calculation results showed good agreement with experimental values, with deviations generally within ±10%. This validates the applicability of the equilibrium-based approach for SCLAC composite columns, provided that the confined concrete stress-strain model is properly calibrated for the specific lightweight aggregate properties.

Engineering Practice Implications

For structural engineers considering steel tube SCLAC columns in practice, several important points emerge from this study:

The 15-25% reduction in ultimate load compared to normal concrete is partially offset by the significant self-weight reduction (approximately 30-40% lighter), resulting in a favorable strength-to-weight ratio
The self-compacting property enables placement in complex geometries and congested reinforcement zones without vibration, reducing construction quality risks
All specimens exhibited ductile failure, confirming that the steel tube confinement effectively prevents brittle concrete failure even with the lower-strength lightweight concrete
The choice between curvature failure and shear failure can be controlled through appropriate confinement ratio design, allowing engineers to target specific ductility requirements
The use of fly ash ceramsite contributes to sustainable construction by utilizing industrial byproducts

Study Insights

This research demonstrates that lightweight aggregate concrete can be successfully used in steel tube composite columns without sacrificing the fundamental ductility characteristic that makes composite columns advantageous. The self-compacting property adds significant construction practicality, particularly for columns in congested structural zones. The key design parameter is the confinement ratio, which must be carefully controlled to ensure the desired failure mode and ductility level. For projects where structural self-weight is a critical concern—such as long-span bridges, offshore platforms, and high-rise buildings—the steel tube SCLAC column concept offers a viable alternative to conventional concrete-filled steel tube columns with meaningful weight savings while maintaining adequate structural performance.