Axial Compression Behavior of Steel Tube Recycled Concrete Short Columns

Literature Overview and Research Background

This paper by Zhi Zhengdong, Zhang Dachang, Xu Enxiang, and Chen Kai (2012, Industrial Construction, Vol. 42, No. 12) investigates the axial compressive performance of short columns composed of thin-walled steel tubes filled with recycled concrete. The research was motivated by the dual pressures of construction waste disposal and the need for sustainable structural materials. With over nine million tonnes of construction demolition waste generated annually in China alone, the reuse of recycled aggregates in structural concrete has become an urgent engineering priority. The authors conducted axial compression tests on six steel tube recycled concrete (STRC) short columns and three conventional steel tube concrete (STC) short columns to systematically evaluate the influence of recycled aggregate replacement ratio and slenderness ratio on load-bearing capacity and deformation characteristics.

Core Technical Findings

The experimental programme examined two key variables: the recycled aggregate replacement ratio (ranging from 0% to 100%) and the slenderness ratio (L/D) of the columns. The deformation and failure patterns observed in STRC columns were found to be broadly similar to those of conventional STC columns, with progressive outward bulging of the steel tube wall as the primary failure mode. However, the ultimate load-bearing capacity decreased systematically with increasing recycled aggregate replacement ratio and slenderness ratio.

A particularly significant finding is that the conventional design formula for calculating the ultimate load-bearing capacity of STC columns remains applicable to STRC columns, with calculated values showing good agreement with experimental results. This validates the use of existing design codes without modification for recycled aggregate-filled CFST columns, provided appropriate material property adjustments are made for the recycled concrete mix.

Interpretation of Key Technical Points

From a materials science perspective, the recycled aggregates introduce surface porosity, attached mortar residue, and higher water absorption capacity into the concrete matrix. These factors reduce the interfacial transition zone (ITZ) quality between aggregate and cement paste, leading to lower compressive strength and elastic modulus of the recycled concrete compared to natural aggregate concrete. The steel tube confinement partially compensates for this strength reduction by providing lateral restraint that confines the concrete core, preventing premature crushing and maintaining ductile behaviour.

The fact that the conventional STC design formula remains valid is noteworthy. This suggests that the confinement mechanism of the steel tube is robust enough to accommodate the variability introduced by recycled aggregates. However, engineers should exercise caution when applying this formula to columns with very high replacement ratios (above 80%) combined with high slenderness ratios, as the combined effects may push the system beyond the validated range.

Engineering Practice Implications

For practical applications, the following recommendations emerge from this research:

1. Recycled aggregate replacement ratios up to 70% can be adopted for STRC columns without significant compromise in structural performance, provided the concrete mix is properly designed to account for the higher water demand of recycled aggregates.
2. Quality control of recycled aggregates is critical: thorough cleaning, proper grading, and moisture conditioning of recycled aggregates before use can substantially improve the performance of the resulting concrete.
3. The slenderness ratio should be kept within code-specified limits to ensure adequate confinement effectiveness, as the confining pressure from the steel tube becomes less efficient at higher slenderness ratios.
4. When using existing design formulas, the compressive strength of recycled concrete should be determined from cylinder tests on the actual mix, not assumed from natural aggregate concrete specifications.

Study Insights and Reflections

Having worked extensively with steel pipe and pipe fitting fabrication for over six decades, I find this research particularly relevant to the broader question of material sustainability in structural engineering. The steel tube itself—whether seamless, ERW, or HFW welded—serves as both a structural element and a formwork for concrete placement. The paper demonstrates that the confinement mechanism is forgiving of moderate reductions in concrete quality, which has important implications for the use of locally available or recycled materials in construction.

One area where further investigation would be valuable is the long-term behaviour of STRC columns under sustained loads and environmental exposure. Recycled aggregates may have different creep and shrinkage characteristics compared to natural aggregates, which could affect the long-term load redistribution between the steel tube and concrete core. Additionally, the corrosion resistance of the steel tube in the presence of recycled concrete—potentially containing higher chloride and sulphate content—is a concern that deserves separate study.

Reference Value and Outlook

This paper provides a solid experimental foundation for the use of recycled aggregates in CFST columns. The validation of existing design formulas gives engineers confidence in adopting recycled concrete without requiring new design methodologies. Future research should extend to eccentric loading, cyclic loading, and fire resistance of STRC columns to provide a comprehensive design basis for practical applications in sustainable construction.