Axial Compressive Bearing Capacity of Square Steel Tube Recycled Concrete Short Columns with Embedded Steel Sections

Literature Overview and Research Background

The 2021 study by Liu Jian, Zhang Pengcheng, Liu Changjiang, Tian Yong, Ren Da, Zeng Rongsen, Bai Yuxiang, and Chen Panpan from Guangzhou University addresses an important topic in sustainable structural engineering: the design of composite columns that combine recycled concrete with steel tubes and embedded steel sections. As the construction industry increasingly turns to recycled materials to reduce environmental impact, there is a pressing need for reliable design methods that account for the unique mechanical behavior of recycled concrete in composite members. This study proposes a theoretical framework for calculating the axial compressive bearing capacity of square steel tube recycled concrete short columns with internal steel sections, providing engineers with a practical tool for designing such members.

The research is supported by the National Natural Science Foundation of China and the Guangdong Provincial Natural Science Foundation, reflecting the national priority placed on sustainable construction technologies. The combination of recycled coarse aggregate concrete with steel tube confinement and embedded steel sections represents a promising approach to achieving both structural performance and environmental sustainability.

Theoretical Framework and Key Parameters

The analytical approach developed in this study is based on several key simplifications and assumptions that enable tractable calculation while maintaining engineering accuracy. The primary simplification is the equivalence of the square steel tube recycled concrete section to an equivalent circular steel tube recycled concrete section with the same cross-sectional area and steel ratio. This equivalence allows the application of well-established design formulas for circular sections to the more complex square geometry.

Two reduction coefficients are introduced to account for the differences between idealized and actual behavior. The equivalent confinement reduction coefficient (psi) accounts for the non-uniform distribution of lateral confinement provided by the square steel tube compared to a circular tube. The recycled concrete strength reduction coefficient (gamma_u) accounts for the reduced mechanical properties of recycled concrete compared to virgin concrete, primarily due to the presence of residual mortar on the recycled aggregate surface that weakens the aggregate-paste interface.

The theoretical derivation employs the limit analysis method combined with the double-shear unified strength theory, which provides a more accurate representation of the multiaxial stress state within the confined concrete than traditional Mohr-Coulomb or von Mises criteria. The resulting capacity formula incorporates the effects of all key parameters and is validated against experimental test data, showing good agreement between calculated and measured values.

The influence of each parameter on the axial compressive capacity is clearly delineated in the study. Increasing the inner diameter to thickness ratio reduces the confinement effectiveness and therefore decreases capacity, as the steel tube becomes more slender and susceptible to local buckling. Increasing the confinement coefficient enhances the lateral restraint on the concrete and increases capacity, as the concrete is subjected to higher confining pressure that promotes triaxial compression. Increasing the steel section ratio increases capacity in a near-linear fashion because the steel section directly contributes to the load-bearing capacity. Increasing the recycled aggregate replacement rate decreases capacity because the recycled concrete has lower strength and stiffness than virgin concrete.

Engineering Practice and Design Implications

The practical value of this study lies in providing a validated design formula that can be directly applied in the design of composite columns using recycled concrete. The formula accounts for the key geometric and material parameters that influence capacity, and the validation against experimental data gives confidence in its accuracy. Engineers can use this formula to design columns that meet strength requirements while maximizing the use of recycled materials.

From a sustainability perspective, the ability to design reliable composite columns using recycled concrete is significant. The construction industry generates substantial volumes of concrete waste, and the use of recycled aggregates in structural applications helps divert this waste from landfills and reduces the demand for virgin aggregate extraction. The composite action of the steel tube and embedded steel section compensates for the reduced strength of recycled concrete, enabling the use of higher replacement rates without compromising structural safety.

The design formula developed in this study can be integrated into structural design software and incorporated into design codes as the technology gains wider acceptance. The introduction of reduction coefficients for confinement and recycled concrete strength provides a transparent and rational basis for design adjustments, making it easier for regulatory authorities to approve the use of recycled materials in structural applications.

Study Insights and Future Directions

This study represents an important step toward the practical adoption of recycled concrete in structural composite members. The theoretical framework is rigorous, the validation is thorough, and the resulting design formula is practical. However, several areas for future development are apparent. First, the study focuses on short columns, and the behavior of slender columns with recycled concrete under axial compression involves additional considerations such as buckling and second-order effects. Second, the long-term behavior of recycled concrete under sustained loading, including creep and shrinkage, should be investigated to ensure durability of the composite members. Third, the effect of seismic loading on these columns should be examined, as recycled concrete may exhibit different ductility and energy dissipation characteristics compared to virgin concrete.

The equivalence approach used in this study, while practical, introduces some uncertainty because the stress distribution in a square section differs fundamentally from that in a circular section. Future research could develop direct analytical models for square sections that avoid the equivalence simplification and provide more accurate predictions. Nevertheless, the current approach provides a reliable engineering tool that can be used with confidence for preliminary design and capacity verification of recycled concrete composite columns.