Hysteretic Behavior of Square Steel Tube Recycled Concrete Columns

Literature Overview

This paper by Zhang Xiangang et al., published in Journal of Earthquake Engineering and Engineering Vibration in 2016 (Vol. 36, No. 2, pp. 158-164), investigates the hysteretic behavior of square steel tube recycled concrete (SRC) columns through pseudo-static testing. The research is funded by the Henan Provincial Key Research Project (15A560008) and the Henan Polytechnic University Doctoral Fund (B2015-72). Six SRC column specimens were tested with recycled coarse aggregate replacement rate and axial compression ratio as design parameters. The study examines failure modes, hysteresis curves, and the influence of design parameters on displacement ductility coefficient, strength, stiffness, and energy dissipation coefficient.

Core Technical Points

Recycled aggregate concrete (RAC) has gained increasing attention as a sustainable construction material, but concerns about its structural performance under seismic loading persist. The use of square steel tubes as external confinement provides a solution to enhance the ductility and energy dissipation capacity of RAC columns. The key findings from this study are summarized as follows:

Test Specimen Configuration

Hysteretic Performance Indicators

Failure Mode Analysis

The failure modes of the SRC recycled concrete columns were found to be similar to those of conventional square steel tube concrete columns, which is a significant positive finding for the engineering applicability of recycled aggregate concrete. The typical failure progression follows:

1. Elastic stage: Both the steel tube and concrete work in the elastic range. The load-displacement relationship is linear, and no visible cracks appear.
2. Crack initiation stage: Concrete cracks begin to appear on the lateral surfaces. The steel tube starts to provide confinement, and the load-displacement curve begins to deviate from linearity.
3. Yield stage: The steel tube yields, and concrete cracks propagate rapidly. The bottom of the column begins to bulge outward.
4. Post-yield stage: The steel tube bottom experiences significant outward bulging (local buckling). The recycled concrete at the bottom is crushed and spalls. The load capacity gradually decreases but maintains a residual strength due to the steel tube confinement.
5. Failure stage: The steel tube bottom develops pronounced local buckling with concrete spalling. The column loses its load-bearing capacity.

Influence of Design Parameters

The key finding is that except for stiffness, which decreases with decreasing axial compression ratio, the other hysteretic performance indicators are not significantly sensitive to the tested range of axial compression ratio. This suggests that the square steel tube confinement effectively stabilizes the structural response across different axial load levels.

Engineering Practice Integration

From a practical engineering standpoint, the findings of this study have several important implications:

• Feasibility of recycled aggregate use: The study demonstrates that square steel tube recycled concrete columns can achieve satisfactory hysteretic performance, making them viable for use in load-bearing structural members, particularly in seismic regions.
• Design recommendations: For practical engineering applications, a recycled coarse aggregate replacement rate of up to 50% can be considered acceptable for SRC columns, provided that the concrete mix is properly designed and the steel tube confinement is adequate.
• Seismic design considerations: The relatively stable hysteretic performance across different axial compression ratios suggests that SRC recycled concrete columns can maintain consistent seismic performance under varying gravity loads, which is advantageous for seismic-resistant design.

Quality Control Considerations for Recycled Aggregate

Key Questions and Reflections

The study raises several important considerations for future research and engineering practice. First, the long-term durability of recycled aggregate concrete under cyclic loading, particularly regarding fatigue behavior, needs further investigation. Second, the interface behavior between the steel tube and recycled concrete under seismic loading warrants more detailed study, as the bond characteristics of recycled aggregate concrete may differ from natural aggregate concrete. Third, the study used a replacement rate of up to 50%, but practical applications may require higher replacement rates (75% or 100%) in regions with abundant construction waste. The hysteretic performance at higher replacement rates remains to be established.

Additionally, the study does not address the welding quality of the square steel tube connections, which is critical for the overall structural integrity. In practice, the welding joints of square steel tubes must be inspected using non-destructive testing methods (MT, UT, or PT) to ensure weld integrity, particularly at the column ends where plastic hinges are expected to form.

Summary

This study provides experimental evidence that square steel tube recycled concrete columns exhibit satisfactory hysteretic behavior, with failure modes similar to conventional SRC columns and stable performance across different axial compression ratios. The findings support the feasibility of using recycled aggregate concrete in seismic-resistant structural applications, contributing to sustainable construction practices. However, further research is needed on higher replacement rates, long-term fatigue behavior, and detailed interface mechanics to fully establish the design basis for widespread engineering adoption.