Axial Compression Bearing Capacity of Square Steel Tube Reinforced Recycled Aggregate Concrete Short Columns

Literature Overview

This study note addresses a 2021 publication in the Chinese Journal of Applied Mechanics by Liu Jian, Tian Yong, Zhang Pengcheng, and colleagues from Guangzhou University and related institutions. The paper investigates the axial compression bearing capacity of square steel tube reinforced recycled aggregate concrete (RRACFSST) short columns, considering the dual confinement effects of both the steel tube and the internal rebar cage. The research was supported by the National Natural Science Foundation of China (Grant No. 51678168) and the Guangdong Provincial Natural Science Foundation.

As a steel pipe and welding engineer, I appreciate the intersection of pipe manufacturing technology with structural engineering applications. The use of square steel tubes as structural members in composite columns represents a significant application area where pipe forming technology, welding quality, and material properties directly influence structural performance.

Core Technical Methodology

The researchers employed limit analysis methods combined with confinement theory to derive a comprehensive bearing capacity formula for RRACFSST short columns. The formula incorporates several critical parameters that reflect both the geometric configuration and the material characteristics of the composite member.

The dual confinement mechanism is the central technical contribution of this paper. In a conventional steel tube concrete column, the steel tube provides lateral confinement to the concrete core, enhancing its compressive strength and ductility. In the RRACFSST configuration, an additional rebar cage is introduced within the concrete core, providing a second level of confinement. This dual mechanism is particularly important when recycled aggregate is used, because the recycled concrete typically exhibits lower strength and more variable mechanical properties compared to conventional concrete.

Technical Points and Engineering Analysis

The limit analysis approach assumes that the column fails when the internal work rate equals the external work rate at the ultimate load. This is a rigorous theoretical framework that avoids the empirical fitting commonly found in design codes. The confinement theory component models the interaction between the steel tube and the concrete core, accounting for the Poisson effect that generates radial pressure on the steel tube walls as the concrete is compressed axially.

The recycled coarse aggregate replacement ratio is a critical variable. Recycled concrete made from construction and demolition waste typically exhibits 10-25% lower compressive strength compared to conventional concrete due to the higher water absorption of recycled aggregates and the presence of residual mortar. The paper demonstrates that the dual confinement mechanism effectively compensates for this strength reduction, making recycled aggregate concrete a viable option for composite column applications.

The width-thickness ratio of the square steel tube is particularly relevant from a manufacturing perspective. Thin-walled tubes provide greater confinement effectiveness but are more susceptible to local buckling under compressive loads. The optimal width-thickness ratio represents a balance between confinement effectiveness and structural stability, and this balance is directly influenced by the pipe forming process, welding quality, and material grade.

Validation and Engineering Practice

The theoretical predictions were validated against experimental test data, and the agreement was found to be satisfactory. This validation is essential because limit analysis methods, while theoretically rigorous, can deviate significantly from actual behavior when the assumptions of the model are not fully satisfied. The satisfactory agreement indicates that the dual confinement model accurately captures the essential mechanics of the RRACFSST column.

From a manufacturing quality control perspective, several implications emerge:

1. The square steel tube must be manufactured with tight dimensional tolerances to ensure uniform confinement pressure distribution around the concrete core.
2. The welding joints in the square tube must be of high quality, as weld defects can initiate local buckling under compressive loading.
3. The recycled aggregate concrete must be carefully mixed and placed to ensure adequate compaction within the steel tube, as voids and honeycombing would compromise the confinement effectiveness.

Study Insights and Implications

This research contributes to the growing body of knowledge on recycled materials in structural applications. As the global construction industry faces increasing pressure to reduce waste and carbon emissions, the use of recycled aggregate concrete in composite columns represents a practical and technically sound solution. The dual confinement mechanism provides a robust engineering approach that accommodates the variable properties of recycled concrete.

For the steel pipe industry, this research highlights an important application market. Square steel tubes for composite columns require high-quality forming and welding processes, and the structural performance depends critically on the quality of the tube manufacturing. Engineers in the pipe manufacturing sector should be aware of these application requirements when developing process specifications and quality assurance procedures.

The broader implication is that structural engineers and pipe manufacturers must collaborate more closely. The performance of composite columns depends on the synergy between the steel tube and the concrete core, and this synergy can only be optimized when both parties understand the interaction mechanisms and the quality requirements that govern them.