Experimental Study on Eccentric Compression Performance of Steel Tube Recycled Concrete Composite Columns

Literature Overview

The paper by Ke Xiaojun, Yang Chunhui, Su Yisheng, and Li Juntao, published in the Journal of Basic Science and Engineering in 2020, presents an experimental investigation into the mechanical behavior of steel tube recycled concrete (RRC) composite columns under eccentric compression. The study was funded by the National Natural Science Foundation of China and Guangxi Provincial Natural Science Foundation, reflecting its significance in the field of sustainable structural engineering. Sixteen composite column specimens were designed and tested with varying recycled coarse aggregate replacement ratios, eccentricities, steel ratios, and slenderness ratios. This work addresses an increasingly important engineering challenge: how to incorporate recycled construction waste into structural components without compromising safety and performance.

Core Technical Parameters and Test Design

The experimental matrix covered four key variables systematically:

The specimens were fabricated using seamless steel tubes filled with recycled concrete, representing a composite column configuration where the steel tube provides lateral confinement to the core concrete. The test program included both axial compression and eccentric compression loading protocols, enabling a comprehensive understanding of the column behavior under combined bending and compression.

Key Experimental Findings

Influence of Recycled Aggregate Replacement Ratio

The most significant finding is that the replacement of natural coarse aggregate with recycled coarse aggregate has a relatively minor effect on the ultimate bearing capacity and failure mode of the composite columns. The failure process under eccentric compression remains similar to that of conventional concrete-filled steel tube columns, characterized by progressive local buckling of the steel tube followed by core concrete crushing. This observation is reassuring from a practical standpoint, as it suggests that recycled concrete can be used in composite columns with confidence, provided that appropriate quality control measures are maintained for the recycled aggregates.

Influence of Eccentricity

Compared to axially loaded specimens, eccentrically loaded specimens exhibited reduced initial stiffness, lower ultimate bearing capacity, and diminished deformation capacity. The eccentricity ratio directly affects the moment arm and thus the bending moment component, leading to asymmetric stress distribution and earlier initiation of local buckling on the tension side. The reduction in initial stiffness is attributed to the non-linear stress-strain behavior of the composite section under combined loading, where the tension zone contributes less to the overall flexural rigidity.

Influence of Steel Ratio and Slenderness Ratio

Increasing the steel ratio improved both the flexural stiffness and ultimate bearing capacity of the columns, which is consistent with the enhanced confinement effect and increased axial load capacity provided by the steel tube. Similarly, reducing the slenderness ratio led to higher flexural stiffness and ultimate bearing capacity, as shorter columns are less susceptible to overall buckling and can develop their full material strength. The interaction between steel ratio and slenderness ratio is particularly important for design purposes, as the combined effect can be leveraged to optimize column performance.

Interpretation of Technical Points

The confinement mechanism in composite columns is central to understanding their behavior. The steel tube provides lateral restraint to the core concrete, which in turn enhances the compressive strength and ductility of the concrete. Under eccentric compression, this confinement effect becomes even more critical, as the asymmetric stress distribution can lead to localized failure if the confinement is insufficient. The recycled concrete, despite its potentially lower strength and more heterogeneous nature due to the presence of adhered mortar on recycled aggregates, still benefits from the steel tube confinement, which effectively compensates for the reduced material quality.

The proposed calculation method for bending-compression bearing capacity accounts for both the contribution of the steel tube and the confinement effect on the core concrete. This is a notable advancement over conventional design methods that may not adequately consider the composite action in recycled concrete-filled steel tube columns. The method provides a more rational basis for design, particularly for eccentrically loaded columns where the interaction between axial force and bending moment is complex.

Engineering Practice Implications

From a practical engineering perspective, this study supports the use of recycled concrete in composite column applications, which aligns with sustainable construction goals and waste reduction initiatives. However, several considerations must be addressed in real-world applications:

• The quality of recycled aggregates must be controlled, particularly regarding the amount of adhered mortar and the presence of contaminants, as these factors can affect the bond between the recycled aggregates and the new cement paste.
• The interface between the steel tube and the recycled concrete is critical for ensuring composite action. Proper surface treatment of the steel tube, such as roughening or the use of shear connectors, may be necessary to ensure adequate bond.
• The welding quality of the steel tube components is paramount, as weld defects can initiate local buckling and compromise the structural integrity of the column. Non-destructive testing methods such as ultrasonic testing (UT) and magnetic particle testing (MT) should be employed to verify weld integrity.
• The slenderness ratio must be carefully controlled in design to avoid overall buckling, which can be particularly detrimental in composite columns with recycled concrete cores due to the potentially lower elastic modulus of the composite section.

The study also highlights the importance of considering the composite action in design calculations. Traditional approaches that treat the steel tube and concrete as independent components may underestimate the bearing capacity of composite columns, leading to overly conservative designs. The proposed calculation method, which accounts for the confinement effect, provides a more accurate and efficient basis for design.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the long-term durability of recycled concrete-filled steel tube columns under cyclic loading or environmental exposure is not addressed in this experimental study. Given that recycled concrete may have higher permeability than conventional concrete, the potential for corrosion of the steel tube and degradation of the recycled concrete under long-term service conditions is a concern. Second, the study focuses on short-term static loading, and the behavior under dynamic or seismic loading conditions remains to be explored. Third, the effect of different types of recycled aggregates, such as those derived from demolished concrete versus those from construction and demolition waste, on the composite column performance is not investigated.

The study also raises questions about the optimal replacement ratio for recycled aggregates in composite columns. While the experimental results suggest that the bearing capacity is not significantly affected by the replacement ratio, the long-term performance and durability may be more sensitive to this parameter. Further research is needed to establish guidelines for the maximum allowable replacement ratio in structural applications.

Study Insights and Implications

The findings of this study are encouraging for the adoption of recycled concrete in structural applications, particularly in composite columns where the steel tube provides beneficial confinement. The relatively minor impact of recycled aggregate replacement on the eccentric compression performance suggests that recycled concrete can be used with confidence, provided that appropriate quality control and design considerations are applied. The proposed calculation method for bending-compression bearing capacity represents a valuable contribution to the design of recycled concrete-filled steel tube columns, as it accounts for the composite action and confinement effect in a rational manner.

From a welding and fabrication perspective, the quality of the steel tube components and their welds is critical to ensuring the structural integrity of composite columns. Any welding defects, such as porosity, lack of fusion, or cracks, can initiate local buckling and compromise the composite action between the steel tube and the core concrete. Therefore, rigorous quality control measures, including visual inspection, non-destructive testing, and mechanical property verification, should be implemented during the fabrication process. The selection of appropriate welding processes, such as gas metal arc welding (GMAW) or flux-cored arc welding (FCAW), depends on the steel grade and wall thickness of the tube, and should be determined based on applicable standards such as GB/T 985 or ISO 13919.

This study contributes to the growing body of knowledge on sustainable structural engineering and provides a practical basis for the design of recycled concrete-filled steel tube columns. Future research should focus on long-term durability, dynamic loading behavior, and the development of comprehensive design guidelines that account for the unique characteristics of recycled concrete in composite columns.