Recycled Concrete Steel Tube Short Columns Under Axial Compression Performance

Literature Overview

This paper, published in the Chinese Journal of Applied Mechanics in 2014 by Dong Jiangfeng, Xue Chunqiang, Yuan Shucheng, and Wang Qingyuan from Sichuan University, investigates the mechanical behavior of recycled aggregate concrete (RAC) and its application in steel tube confined short columns. The study is particularly relevant in the context of post-earthquake reconstruction, where large volumes of demolished concrete debris can be repurposed as recycled coarse aggregate. The authors examined five replacement ratios of recycled coarse aggregate (0%, 25%, 50%, 75%, and 100%) for C30-grade concrete, evaluating both the base material properties and the composite structural performance.

Core Technical Content and Experimental Design

The experimental program was systematically designed to isolate the effect of recycled aggregate replacement ratio on multiple performance indicators. The base concrete was designed at C30 strength grade, with recycled coarse aggregate sourced from post-earthquake demolished buildings. The study measured compressive strength, splitting tensile strength, and the full stress-strain behavior of the concrete mixtures. Steel tube specimens were then fabricated and tested under axial compression to evaluate load-bearing capacity, deformation characteristics, and failure modes.

Key Findings and Technical Interpretation

The primary finding is that increasing the recycled aggregate replacement ratio leads to a progressive degradation in both compressive and tensile strength of the recycled concrete. This degradation is attributed to the presence of residual old mortar on the surface of recycled aggregate particles, which creates weaker interfacial transition zones (ITZ) compared to natural aggregate. The ITZ in recycled aggregate is inherently more porous and mechanically inferior, serving as a preferential path for crack propagation.

However, a critical and practically important observation is that the failure mode and deformation curve of steel tube confined recycled concrete columns show no significant difference from conventional steel tube concrete (STC) columns. This indicates that the steel tube confinement effectively compensates for the inherent weakness of recycled concrete, maintaining ductile failure behavior and stable load-deformation response. The steel tube provides lateral confinement that restrains the radial expansion of the concrete core, thereby enhancing its compressive performance even when the base material is weakened.

The authors established a stress-strain relationship for recycled concrete and used it to calculate the ultimate load-bearing capacity of steel tube recycled concrete columns. The calculated results showed approximately 10% deviation from experimental values, which is considered acceptable for engineering design purposes. This level of accuracy suggests that existing theoretical frameworks for conventional STC columns can be adapted with appropriate modifications to accommodate recycled concrete.

Engineering Practice Implications

From a practical standpoint, this research provides valuable justification for using recycled concrete in structural applications where steel tube confinement is employed. The 10% calculation error is within typical engineering tolerances, suggesting that design codes for conventional STC can be conservatively extended to recycled concrete applications. The fact that failure modes remain ductile is particularly significant for seismic design, as ductile behavior ensures energy dissipation capacity and provides warning before collapse.

For pipe manufacturers and structural engineers, this work highlights that recycled materials can be integrated into structural systems without compromising safety, provided appropriate confinement measures are implemented. The study supports the circular economy approach in construction, where waste materials are repurposed rather than disposed of in landfills.

Study Insights and Reflections

The research demonstrates that material degradation at the aggregate level does not necessarily translate to proportional degradation at the structural level when confinement mechanisms are employed. This is a fundamental principle in composite material engineering—the performance of the composite system depends not only on the individual constituent properties but also on the interaction between constituents and the confinement architecture. The 10% deviation between calculated and experimental results is encouraging, but further research with larger specimen dimensions and varied steel tube wall thicknesses would strengthen the generalizability of the findings.