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

Literature Overview

Xiang Xingyun, Zhao Renda, Jia Yi, and Wang Yongbao (2017), from the Southwest Jiaotong University, published a study in the Journal of Shenyang Jianzhu University examining the mechanical behavior of square steel tube self-consolidating recycled concrete (SCRC) eccentrically loaded short columns. The research was funded by the National Key R&D Program of China (2016YFB1200401). The study investigated how recycled coarse aggregate replacement ratio, eccentricity ratio, and steel ratio influence the load-bearing capacity and peak deflection of these composite columns.

Experimental Design and Test Parameters

The experimental program focused on square steel tube columns filled with self-consolidating recycled concrete, subjected to eccentric compression loading. The test matrix included variations in three key parameters:

Self-consolidating concrete eliminates the need for mechanical vibration during placement, which is particularly advantageous for densely reinforced sections and thin-walled steel tubes where conventional concrete may experience segregation or incomplete filling.

Core Technical Findings

Failure Mode and Load-Deflection Behavior

The columns failed through lateral flexural buckling, which is characteristic of eccentrically loaded steel tube concrete columns. The load-deflection curves exhibited three distinct stages:

1. Elastic stage, where the load-deflection relationship is linear and both the steel tube and concrete core deform elastically.
2. Elastic-plastic stage, where yielding begins in the steel tube and microcracking initiates in the concrete core, causing the curve to deviate from linearity.
3. Descending stage, where progressive buckling of the steel tube and crushing of the concrete core lead to a reduction in load-carrying capacity.

Influence of Recycled Aggregate Replacement Ratio

A significant finding is that the recycled coarse aggregate replacement ratio has no significant effect on the ultimate load-bearing capacity of the columns. However, the peak deflection is notably affected by the replacement ratio, with higher replacement ratios leading to greater peak deflections. This indicates that while the overall strength is maintained due to the steel tube confinement, the stiffness characteristics are influenced by the recycled aggregate properties.

The load-bearing capacity remains stable because the steel tube provides sufficient confinement to prevent premature concrete failure, and the composite action ensures that the steel tube carries a substantial portion of the axial load regardless of the concrete quality.

Influence of Eccentricity and Steel Ratio

Both the eccentricity ratio and the steel ratio have significant effects on both the load-bearing capacity and peak deflection. Higher eccentricity ratios reduce the load-bearing capacity and increase the peak deflection, as expected from classical beam-column theory. Similarly, higher steel ratios increase the load-bearing capacity and reduce the peak deflection due to greater confinement and higher section stiffness.

Engineering Practice Implications

Feasibility of Self-Consolidating Recycled Concrete in Steel Tubes

The study confirms that self-consolidating recycled concrete is feasible for use in square steel tube columns. The self-consolidating property ensures complete filling of the steel tube without mechanical vibration, which is important for maintaining the composite action between the steel tube and concrete core. The recycled aggregate content does not significantly compromise the structural performance when the steel tube provides adequate confinement.

Design Recommendations

Study Insights and Reflections

The research provides important evidence that sustainable materials can be integrated into structural steel tube concrete systems without significant compromise in load-bearing performance. The distinction between load-bearing capacity and peak deflection is particularly relevant for design engineers. While the former may be maintained through confinement, the latter is more sensitive to material properties and should be considered in serviceability limit state design.

The use of square steel tubes introduces additional complexity compared to circular sections, as the corner regions experience less effective confinement. This is consistent with established knowledge in steel tube concrete design, where circular sections generally provide superior confinement efficiency. However, the square section's practical advantages in construction and connection detailing may justify its use in certain applications.

Summary

This study demonstrates that square steel tube self-consolidating recycled concrete columns are a viable structural solution for eccentrically loaded applications. The recycled aggregate replacement ratio does not significantly affect load-bearing capacity but does influence peak deflection, while eccentricity and steel ratio remain the dominant design parameters. The findings support the practical application of recycled materials in steel tube concrete systems and provide a theoretical basis for future engineering use. Engineers should carefully consider deflection limits when specifying recycled concrete content, and the self-consolidating property should be maintained to ensure reliable composite action throughout the column length.