Axial Compression Behaviour of CFST Columns with Recycled Brick Aggregate Concrete

Literature Overview

This paper, published in the Journal of Rail and Transportation Engineering (Vol. 17, No. 3, 2020, pp. 699-706) by Huang Jing, Lin Mingming, Gao Chang, and Deng Peng from Hunan University, presents an experimental investigation into the axial compression behaviour of concrete-filled steel tube (CFST) columns using recycled brick aggregate concrete. The research was funded by the National Key R&D Program of China (2017YFC0703305). Twenty circular CFST specimens were tested under axial compression to examine the effects of recycled coarse aggregate replacement rate and confinement ratio on load-bearing capacity, stress-strain behaviour, and failure modes.

Experimental Programme and Key Findings

The experimental programme was designed to systematically investigate the influence of two primary parameters: the recycled coarse aggregate replacement rate and the confinement ratio (defined as the ratio of steel tube cross-sectional area to concrete cross-sectional area). The following table summarises the key experimental parameters:

The study found that CFST columns with recycled brick aggregate concrete exhibit a failure process similar to that of conventional CFST columns, which is an important finding because it suggests that the fundamental structural behaviour of the composite system is preserved even when recycled materials are used. However, both the recycled aggregate replacement rate and the confinement ratio were found to have measurable effects on the load-bearing capacity and deformation properties of the specimens.

Code Modification and Calculation Method Comparison

A significant contribution of this study is the systematic comparison of twelve different calculation formulas from three theoretical frameworks: confinement concrete theory, unified strength theory, and superposition calculation theory. The following table summarises the comparison results:

Based on this comparison, the authors proposed a modification to the calculation formula in the Chinese Code for Design of Concrete-Filled Steel Tubular Structures (GB 50936-2014) to account for the effect of recycled coarse aggregate replacement rate. A modification coefficient θ was introduced and validated against the experimental data, providing engineers with a practical tool for the design of CFST columns using recycled brick aggregate concrete.

Engineering Practice Implications

The use of recycled brick aggregate in CFST columns represents a significant step toward sustainable construction practices. By incorporating recycled materials from demolished brick structures into new construction, the industry can reduce waste, conserve natural resources, and lower the environmental footprint of construction projects. The findings of this study provide the technical basis for engineers to confidently specify recycled brick aggregate concrete in CFST column applications.

However, several practical considerations must be addressed. First, the quality and consistency of recycled brick aggregate must be carefully controlled, as variations in brick type, mortar content, and weathering condition can significantly affect the mechanical properties of the recycled aggregate concrete. Engineers should implement rigorous quality control procedures, including aggregate grading, moisture content control, and compressive strength testing of the recycled aggregate concrete.

Second, the modification coefficient θ proposed in the study should be validated against additional experimental data from different sources before being widely adopted in design practice. Engineers should be cautious about extrapolating the results of a single experimental programme to a wide range of applications without additional verification.

Third, the long-term performance of CFST columns with recycled brick aggregate concrete under sustained loading, fatigue, and environmental exposure has not been addressed in this study. Engineers should consider conducting additional tests or relying on existing research on the long-term behaviour of recycled aggregate concrete before specifying such columns for critical applications.

Key Questions and Reflections

Several important questions arise from this research that warrant further investigation. First, the study focuses on brick aggregate, but other types of recycled aggregates, such as recycled concrete aggregate and recycled glass aggregate, may exhibit different behaviour in CFST columns. Second, the study does not address the effect of recycled aggregate on the fire resistance of CFST columns, which is a critical design parameter for structural applications.

Third, the study examines only axial compression loading, but CFST columns in real structures are subjected to combined axial and bending loads, as well as shear forces. The interaction between recycled aggregate effects and combined loading conditions requires further research to provide engineers with comprehensive design guidance.

Study Insights and Implications

This research makes a meaningful contribution to the field of sustainable structural engineering by demonstrating that CFST columns using recycled brick aggregate concrete can achieve acceptable structural performance with appropriate design modifications. The proposed modification coefficient θ provides a practical tool for engineers to incorporate recycled aggregate effects into their design calculations, bridging the gap between experimental findings and code-based design practice.

For the steel pipe and welding industry, this study highlights the growing importance of sustainable construction practices and the role that composite structural systems can play in reducing the environmental impact of construction. Engineers should be aware that the use of recycled materials may introduce additional variability in material properties, which must be accounted for in both the design and fabrication phases of a project.

In conclusion, this study provides valuable experimental data and design guidance for the application of recycled brick aggregate concrete in CFST columns, and the proposed modification to the Chinese design code offers a practical pathway for the wider adoption of sustainable construction practices in the steel pipe and composite structural engineering community.