Eccentric Compression Behaviour of Steel Tube Recycled Concrete Columns

Literature Overview

The paper by Jia Lifu, Luo Suibing, and Du Jinhui (2020), published in the journal Concrete, presents a systematic experimental investigation on the eccentric compression performance of steel tube recycled concrete (RCA) columns. Seven groups of specimens were tested under uniaxial eccentric compression, with recycled aggregate replacement ratio, slenderness ratio, and steel tube wall thickness serving as the primary design variables. The work is supported by the National Natural Science Foundation of China (Grant No. 90915003) and the Central Universities Fundamental Research Funds (Grant No. 2572016EBC1). This study is particularly relevant to engineers working on sustainable structural design where recycled aggregates are increasingly mandated to reduce environmental impact.

Core Technical Findings

The experimental results reveal that all specimens, regardless of replacement ratio, slenderness ratio, or wall thickness, follow a progressive failure sequence during loading: slight bulging, bulging-buckling deformation, severe deformation, and ultimately global failure. Two distinct failure patterns were identified:

End-buckle failure: Local buckling occurs at a position 120–220 mm from the top of the specimen, followed by progressive bending until complete failure.
Mid-span flexural instability: Global bending instability initiates at the mid-span region, leading to overall flexural collapse.

The influence hierarchy of design parameters on load-bearing capacity is clearly established: slenderness ratio and wall thickness exert significantly greater effects than the recycled aggregate replacement ratio. Specifically, under constant other parameters, capacity increases with decreasing replacement ratio, decreases with increasing slenderness ratio, and improves with increasing wall thickness.

Design Parameter	Effect on Load-Bearing Capacity	Effect on Lateral Stiffness	Effect on Lateral Deflection
Recycled aggregate replacement ratio ↑	Capacity decreases	Stiffness decreases	Deflection increases
Slenderness ratio ↑	Capacity decreases significantly	Stiffness decreases	Deflection increases markedly
Wall thickness ↑	Capacity increases	Stiffness slightly increases	Deflection increases noticeably

Interpretation of Technical Points

From a metallurgical and structural engineering perspective, the confinement effect of the steel tube on recycled concrete is the governing mechanism that enables these composite columns to maintain acceptable structural performance despite the inferior mechanical properties of recycled aggregates compared to natural aggregates. Recycled coarse aggregates typically exhibit higher water absorption, lower crushing strength, and a rougher surface texture with adhered old mortar, all of which degrade the concrete matrix quality. However, the steel tube provides lateral confinement that compensates for this degradation to a considerable extent.

The finding that wall thickness increases cause lateral stiffness to slightly increase while lateral deflection increases noticeably deserves careful attention. This apparent paradox can be explained by the fact that thicker steel tubes provide greater axial resistance and thus higher ultimate loads, but the increased axial load drives greater second-order P-Δ effects, amplifying lateral deflections even though the initial stiffness is marginally improved. Engineers should be cautious not to over-rely on wall thickness alone for deflection control in slender columns.

The slenderness ratio emerges as the most critical design variable, which is consistent with classical column stability theory. The transition from end-buckle failure to mid-span flexural instability as slenderness increases reflects the shift from material-dominated failure to stability-dominated failure. This transition has direct implications for the selection of appropriate design codes and the determination of effective length factors in practical engineering.

Engineering Practice Implications

For practical design of steel tube recycled concrete columns, the following guidelines can be derived:

The recycled aggregate replacement ratio can be increased to moderate levels (typically up to 50–70%) with only modest reductions in load-bearing capacity, provided that slenderness ratio and wall thickness are appropriately controlled.
Slenderness ratio should be limited to prevent the onset of global flexural instability, which is a brittle and potentially catastrophic failure mode.
Wall thickness optimization should consider both load capacity and deflection performance, as simply increasing thickness may not proportionally improve serviceability.
The two identified failure patterns should be considered in the development of capacity design and ductility requirements, as end-buckle failure may be more ductile than mid-span flexural instability.

Key Questions and Reflections

The study raises several important questions for further investigation. First, the interaction between recycled aggregate properties (such as particle size distribution, water absorption, and surface texture) and the steel tube confinement effect warrants deeper exploration. Second, the long-term behaviour under sustained eccentric loading, including creep and shrinkage effects on recycled concrete, remains unaddressed. Third, the study does not consider the effect of loading eccentricity variation, which is a critical design parameter in practical structures where moment-to-axial-force ratios vary along the column height. Engineers should supplement the findings of this study with additional testing or simulation data when applying the results to specific project conditions.

This study provides a valuable experimental foundation for the rational use of recycled aggregates in steel tube concrete columns, demonstrating that sustainable structural design is achievable without significant compromise in structural performance, provided that geometric design parameters are appropriately controlled.