Mechanical Behavior of Square Steel Tube Self-Consolidating Recycled Concrete Eccentrically Loaded Short Columns

Literature Overview

This study by Xiang Xingyun, Zhao Renda, Jia Yi, and Wang Yongbao from Southwest Jiaotong University, published in the Journal of Shenyang Jianzhu University (Natural Science) in 2017, investigates the mechanical behaviour of square steel tube self-consolidating recycled aggregate concrete (SCRC) eccentrically loaded short columns. The research was funded by the National Key R&D Program of China (2016YFB1200401). The study addresses the combined challenge of using self-consolidating concrete technology with recycled aggregates in steel tube concrete structures subjected to eccentric compression, a common loading condition in building and bridge columns.

Test Programme and Parameter Matrix

The experimental programme involved eccentrically loaded short columns with square steel tube cross-sections filled with self-consolidating recycled aggregate concrete. The key parameters investigated included the recycled coarse aggregate replacement ratio, the eccentricity ratio, and the steel ratio (confinement ratio).

Failure Modes and Load-Deflection Behaviour

The authors report that all square steel tube SCRC eccentrically loaded short columns failed due to lateral flexural buckling. The load-deflection relationship curves were divided into three distinct stages:

1. Elastic stage: Linear relationship between load and deflection, with the column behaving as a composite section.
2. Elastic-plastic stage: Nonlinear behaviour begins as concrete cracking initiates on the tension side and the steel tube on the compression side begins to yield.
3. Descending stage: Post-peak strength degradation as lateral buckling progresses and the steel tube loses local stability.

The load-deflection curves exhibited the characteristic shape of eccentrically loaded columns with progressive stiffness degradation and a gradual descending branch, indicating a degree of ductility provided by the steel tube confinement.

Influence of Recycled Aggregate Replacement Ratio

A key finding of this study is the differential influence of recycled aggregate replacement ratio on bearing capacity versus peak deflection. The authors found that:

• The bearing capacity of the columns was not significantly affected by the recycled coarse aggregate replacement ratio.
• The peak deflection was notably influenced by the replacement ratio, with higher replacement ratios leading to greater peak deflections.

This differential effect can be explained by the lower elastic modulus of recycled aggregate concrete compared to virgin aggregate concrete. The recycled aggregate concrete core deforms more under the same load, leading to greater lateral deflections, but the steel tube confinement maintains the overall composite action and preserves bearing capacity. This is an important insight for structural design, as it suggests that recycled aggregate concrete can be used in eccentrically loaded SRC columns with appropriate adjustments to serviceability limit state criteria (deflection limits) rather than ultimate limit state criteria (bearing capacity).

Influence of Eccentricity Ratio and Steel Ratio

Both the eccentricity ratio and the steel ratio were found to have significant effects on both the bearing capacity and peak deflection:

• Increasing eccentricity ratio reduces the bearing capacity and increases the peak deflection, as expected from classical column theory.
• Increasing the steel ratio (thicker steel tube walls or smaller concrete core) increases the bearing capacity and reduces the peak deflection, due to enhanced confinement and direct load-bearing contribution of the steel tube.

Engineering Practice Implications

The findings have direct relevance to the design and construction of steel tube concrete structures using sustainable materials:

1. Self-consolidating recycled aggregate concrete is feasible for use in square steel tube concrete columns, reducing the need for vibration during construction and improving fill density in complex geometries.
2. The use of recycled aggregates does not compromise ultimate bearing capacity, supporting sustainable construction practices.
3. Serviceability design should account for the increased deflection tendency of recycled aggregate concrete cores, particularly at higher replacement ratios.
4. The steel ratio and eccentricity ratio are the primary design levers for controlling both strength and deformation performance.

Key Questions and Reflections

The study is limited to short columns, which means slenderness effects and second-order P-delta effects are not captured. In practice, many structural columns have moderate to high slenderness ratios, and the interaction between recycled aggregate concrete properties and slenderness warrants further investigation. Additionally, the study does not address the long-term creep and shrinkage behaviour of self-consolidating recycled aggregate concrete, which can be significant in eccentrically loaded columns where sustained eccentric loads develop additional deflections over time.

Summary

This paper provides valuable experimental evidence that self-consolidating recycled aggregate concrete can be successfully used in square steel tube concrete eccentrically loaded short columns. The most important engineering insight is that while recycled aggregates increase deflection, they do not significantly reduce bearing capacity, making them a viable option for sustainable structural applications. The steel ratio and eccentricity ratio remain the dominant design parameters for controlling structural performance. Engineers should note that the study's scope is limited to short columns and does not address slenderness effects or long-term deformation, which are critical considerations for real-world structural design.