Axial Compressive Capacity of Circular Steel Tube Steel-Reinforced Recycled Concrete Short Columns Based on Unified Strength Theory

Literature Overview

The paper by Ma Hui, Hu Guangbin, Li Zhe, and Guo Tingting (2017), published in the Chinese Journal of Applied Mechanics, presents a comprehensive investigation into the axial compressive behavior of circular steel tube steel-reinforced recycled concrete (CSTSRC) short columns. Nine specimens were tested under axial compression, with the primary variables being recycled coarse aggregate replacement rate, diameter-to-thickness ratio, and steel ratio. The theoretical analysis employs the Unified Strength Theory (UST), which provides a more general framework than classical yield criteria by incorporating material strength parameters that account for the asymmetry between tensile and compressive behavior.

Experimental Results and Parametric Analysis

The experimental data demonstrate that the axial compressive capacity decreases with increasing recycled coarse aggregate replacement rate, as expected due to the higher porosity and lower interfacial transition zone strength of recycled aggregates. Conversely, reducing the diameter-to-thickness ratio and increasing the steel ratio both enhance the load-bearing capacity, confirming the effectiveness of steel tube confinement and internal steel reinforcement in compensating for the reduced concrete quality.

A notable finding is that despite the degradation in concrete quality due to recycled aggregates, the CSTSRC columns retain good ductility and deformation capacity. This is attributed to the combined confinement effect of the circular steel tube and the internal steel section, which effectively restrains the lateral expansion of the recycled concrete core.

Theoretical Analysis Using Unified Strength Theory

The theoretical framework adopted in this study applies the thick-walled cylinder UST to calculate the confining stress exerted by the steel tube on the core steel-reinforced recycled concrete. The unified strength theory introduces two material parameters: the tension-compression ratio α (where α = 1 for isotropic materials and α ≠ 1 for anisotropic materials) and the material strength parameter b (ranging from 0 to 1). The derived formula for axial compressive capacity incorporates these parameters to provide a more accurate prediction than traditional approaches that assume linear-elastic or perfectly plastic material behavior.

The validation results are excellent: the mean ratio of calculated to experimental values is 0.99, with a variance of only 0.001, indicating high accuracy and consistency. This level of agreement is remarkable and validates the applicability of the UST framework for composite column design.

Key theoretical insights include:

• When α is constant, the axial capacity increases with the material strength parameter b.
• When b is constant and α ≠ 1, the axial capacity increases with α.
• The capacity decreases monotonically with increasing diameter-to-thickness ratio.
• The capacity enhancement coefficient decreases as the recycled concrete strength increases, suggesting that the confinement benefit is more pronounced for lower-strength concrete cores.

Engineering Practice Considerations

From a fabrication standpoint, CSTSRC columns require careful attention to the welding of the internal steel section to the steel tube, as well as the quality of the concrete pour. The recycled concrete used in these columns typically exhibits higher water demand and lower workability, which can lead to voids and honeycombing if not properly compacted. The steel tube diameter-to-thickness ratio should be selected to balance structural efficiency with fabrication practicality, as thinner walls are more susceptible to local buckling during concrete placement and under load.

The UST-based design formula provides a valuable tool for engineers working with recycled materials, as it explicitly accounts for the anisotropic behavior of recycled concrete. However, the formula assumes uniform confinement pressure around the circumference, which may not hold for partially filled sections or columns with eccentric loading. Future research should extend the UST framework to eccentric and flexural loading conditions to broaden its applicability.

Study Insights and Reflections

This research contributes significantly to the sustainable construction field by demonstrating that recycled concrete can be effectively used in composite columns without sacrificing structural performance. The excellent agreement between the UST-based theoretical predictions and experimental results is particularly encouraging, as it suggests that the unified strength theory can be reliably applied to design CSTSRC columns in practice. The finding that the capacity enhancement coefficient decreases with increasing concrete strength is an important design consideration: it implies that the economic benefit of using recycled concrete is maximized when the concrete strength is moderate, as the confinement effect provides a greater relative improvement. Engineers should also be aware that the long-term durability of recycled concrete in corrosive environments may differ from that of natural aggregate concrete, and appropriate protective measures should be specified.