Experimental Study on Eccentrically Loaded Long Columns of Square Steel Tube Recycled Concrete

Literature Overview

This 2017 paper by Yu Xiaolong, Wang Chenggang, Liu Bingkang, Hu Qiongfang, and Chen Jinbiao from the School of Civil and Hydraulic Engineering at Hefei University of Technology presents experimental research on eccentrically loaded long columns composed of square steel tubes filled with recycled concrete. Funded by the Ministry of Housing and Urban-Rural Development Science and Technology Program (Project No. 2013-K4-46), the study is published in Journal of Hefei University of Technology (Natural Science Edition) (Vol. 40, Issue 8, pp. 1110-1116). This research addresses the important topic of sustainable construction through the use of recycled aggregate concrete in composite structural members.

Core Technical Content

The study investigates eight square steel tube recycled concrete eccentrically loaded long columns through loading tests. The primary objective is to understand how recycled aggregate replacement ratios affect the structural performance of these composite columns under eccentric compression, considering the combined influence of multiple design parameters.

Test Specimen Parameters

Experimental Methodology

The loading tests provide comprehensive data including:

• Ultimate load capacity of each specimen
• Failure modes and crack patterns observed during testing
• Axial load-displacement curves characterizing the stiffness and ductility
• Axial load-lateral deflection curves at mid-column, indicating stability behavior

The specimens are designed as long columns to capture second-order effects (P-Δ effects), which are particularly important for slender members and can significantly reduce the effective load capacity compared to short column behavior.

Key Experimental Findings

Effect of Eccentricity Ratio and Slenderness Ratio

The ultimate load capacity decreases with increasing eccentricity ratio and slenderness ratio, consistent with fundamental structural mechanics. The eccentricity introduces a bending moment component that reduces the axial load capacity, while higher slenderness ratios amplify second-order effects and reduce the effective stiffness of the column.

Effect of Steel Tube Wall Thickness

Increasing the steel tube wall thickness enhances the ultimate load capacity through two mechanisms:

1. Direct contribution of additional steel area to load-bearing
2. Enhanced confinement effect on the recycled concrete core, improving concrete's compressive strength and ductility

Effect of Recycled Aggregate Replacement Ratio

The recycled coarse aggregate replacement ratio has a measurable but moderate effect on structural performance:

• Load capacity: Decreases with increasing replacement ratio, reflecting the generally lower strength and higher variability of recycled concrete compared to natural aggregate concrete
• Axial stiffness: Decreases with increasing replacement ratio, as recycled concrete typically has a lower elastic modulus
• Mid-column lateral deflection: Not significantly affected by replacement ratio, suggesting that the steel tube provides sufficient lateral restraint

Effect on Axial Stiffness

Effect on Lateral Deflection

The mid-column lateral deflection increases with larger steel tube wall thickness (due to increased overall member weight and moment arm), higher slenderness ratios (due to reduced lateral stiffness), and higher eccentricity ratios (due to increased bending moment). The recycled aggregate replacement ratio has no significant effect on lateral deflection, indicating that the steel tube dominates the lateral stability behavior.

Engineering Practice Implications

This research directly addresses the growing demand for sustainable construction materials while maintaining structural safety. The findings support several practical conclusions:

1. Recycled concrete is viable for composite columns: Even at 100% recycled aggregate replacement, the composite action between the steel tube and recycled concrete provides adequate structural performance for many applications.
2. Steel tube confinement is critical: The steel tube wall thickness is the most effective parameter for controlling structural performance, particularly when using lower-quality recycled concrete.
3. Design adjustments needed: Engineers using recycled concrete in composite columns should account for the reduced stiffness and capacity, potentially requiring thicker steel tubes or larger cross-sections compared to equivalent natural aggregate concrete designs.
4. Long column behavior: The study confirms that second-order effects remain important for recycled concrete composite columns, and design methods should include appropriate slenderness reduction factors.

Study Insights and Reflections

This research contributes meaningfully to the growing body of knowledge on recycled materials in structural engineering. The use of recycled aggregate concrete in steel tube composite columns represents a promising approach to reducing construction waste while maintaining structural integrity. The finding that the steel tube wall thickness is the dominant parameter for controlling performance is particularly encouraging, as it means that designers have a straightforward means of compensating for the reduced properties of recycled concrete. For the steel pipe and welding industry, this research highlights an important application niche: steel tubes serving as both structural elements and confinement members for recycled concrete, creating composite systems that combine sustainability with performance. As the construction industry increasingly adopts circular economy principles, research such as this provides the technical foundation for responsible and innovative material utilization.