Axial Compression Behavior of Lightweight Square Steel Tube Recycled Concrete Columns

Literature Overview

This paper, published in Journal of Natural Disasters (Vol. 29, No. 4, 2020, pp. 142-148) by Liu Wenchao, Zhang Kesheng, Cao Wanlin, and Qiao Ziyun, presents experimental investigation on the axial compression mechanical properties of lightweight square steel tube (LWST) recycled concrete columns. The research was funded by the National Key R&D Program of China (2018YFD1100903), the Ministry of Housing and Urban-Rural Development Science and Technology Program (2019-K-100), and the China Postdoctoral Science Foundation (71001015201705). The study addresses a critical intersection of structural engineering and sustainable construction: combining lightweight steel profiles with recycled aggregate concrete to create efficient composite columns.

Experimental Design and Test Parameters

Six square cross-section lightweight steel tube recycled concrete columns were tested under axial compression. The study systematically varied three key parameters: concrete material type, steel tube wall thickness, and recycled aggregate replacement ratio. The test matrix provides valuable data for understanding the interaction between steel confinement and recycled concrete behavior.

Core Findings

The experimental results reveal several important engineering insights:

1. Failure mode: The lightweight square steel tube recycled concrete columns did not exhibit significant bending failure during axial compression testing. This indicates that the composite action between the steel tube and recycled concrete provides adequate lateral confinement to prevent local buckling, even with relatively thin steel walls.
2. Confinement effectiveness: Filling lightweight steel tubes with recycled concrete significantly improves both load-bearing capacity and ductility compared to empty steel tubes. The steel tube provides lateral confinement that prevents concrete spalling and enhances the compressive strength of the recycled concrete core.
3. Recycled aggregate effect: As the recycled aggregate replacement ratio increases, the improvement effect on structural performance decreases progressively. This is attributed to the higher porosity, lower interfacial transition zone (ITZ) strength, and greater water absorption of recycled aggregates compared to natural aggregates.
4. Overall performance: The lightweight square steel tube recycled concrete columns demonstrate good load-bearing capacity and ductility, making them viable for practical engineering applications.

Structural Behavior Analysis

The composite behavior of lightweight steel tube recycled concrete columns can be understood through the interaction mechanisms between the steel tube and the concrete core. Under axial compression, the concrete core tends to expand laterally due to Poisson's effect, and the steel tube provides reactive confinement pressure. This confinement pressure is particularly beneficial for recycled concrete, which typically exhibits lower tensile strength and more brittle failure characteristics than conventional concrete.

The absence of bending failure in the tests suggests that the slenderness ratio of the tested columns was within the stocky range, where axial compression governs the failure mechanism. For practical applications, the slenderness ratio must be carefully controlled to ensure that the beneficial composite action is maintained and that instability modes are avoided.

Engineering Practice Considerations

For engineers considering the application of lightweight square steel tube recycled concrete columns, the following practical aspects should be addressed:

1. Concrete pumping and filling: The lightweight steel tube geometry may present challenges for concrete placement. Proper vibrator selection and placement sequence are essential to ensure full concrete filling without voids.
2. Steel tube preparation: Surface treatment of the steel tube interior (e.g., shot blasting or epoxy coating) can improve the bond between steel and recycled concrete, enhancing composite action.
3. Recycled aggregate quality control: The quality of recycled aggregates varies significantly depending on the source material and processing. Consistent quality control through water absorption testing, particle size distribution analysis, and ITZ strength evaluation is essential.
4. Design codes and standards: Current design codes for steel tube concrete columns (e.g., GB 50934-2014, AISC 360, EN 1994-1-1) are primarily based on conventional concrete. Modifications or conservative adjustments may be necessary when using recycled aggregate concrete.

Key Questions and Reflections

Several important questions emerge from this study. First, the study focuses on short-term axial compression behavior, but long-term durability under sustained loads and environmental exposure (particularly for recycled concrete, which may have higher permeability) warrants investigation. Second, the seismic performance of these columns under cyclic loading should be evaluated, as the energy dissipation capacity is a critical parameter for earthquake-resistant design. Third, the economic viability of using lightweight steel tubes with recycled concrete compared to conventional reinforced concrete columns should be assessed through life-cycle cost analysis. The progressive reduction in performance improvement with increasing recycled aggregate content suggests an optimal replacement ratio that balances sustainability goals with structural performance requirements.

Study Insights and Implications

This research contributes meaningfully to the advancement of sustainable structural engineering by demonstrating that lightweight steel tube recycled concrete columns can achieve acceptable structural performance. The findings provide a foundation for developing design guidelines and code provisions for this emerging construction technology. For practicing engineers, the key takeaway is that recycled aggregate concrete can be effectively utilized in composite steel tube columns, provided that appropriate quality control measures are implemented and design parameters are conservatively calibrated. The combination of lightweight steel profiles with recycled concrete offers a promising pathway toward reducing the embodied carbon of structural systems while maintaining adequate safety margins.