Mechanical Properties of CFST Members with Recycled Concrete

Overview of the Study

This paper by Xu Jinjun, Chen Zongping, and Chen Yuliang (2015), published in the Chinese Journal of Applied Mechanics, investigates the mechanical properties of concrete-filled steel tube (CFST) members using recycled concrete. The research was supported by multiple funding sources including the National Natural Science Foundation of China (Grants 50908057 and 51268004), Guangxi Natural Science Foundation (Grants 2012GXNSFAA053203 and 2013GXNSFDA019025), and Guangxi Science and Technology Project (Gui Ke Gong 12118023-3). The study is particularly significant in the context of sustainable construction, as the use of recycled coarse aggregate in structural concrete represents a viable pathway toward reducing construction waste and conserving natural resources.

Experimental Program and Key Parameters

The experimental program consisted of 16 specimens subjected to axial compression loading. The variation parameters included recycled coarse aggregate replacement rate, height-to-width ratio, and cross-sectional shape (circular and square steel tubes). The study provided the first systematic analysis of the effects of these parameters on energy dissipation, deformation ductility, and stiffness indicators of CFST members with recycled concrete.

The use of recycled concrete introduces inherent variability in material properties due to the irregular shape, higher water absorption, and lower density of recycled aggregates compared to natural aggregates. These material characteristics influence the stress-strain behavior of the concrete core and, consequently, the overall performance of the CFST member under axial loading.

Interpretation of Technical Findings

The study reveals that the load-displacement curves of square CFST specimens differ significantly from those of circular specimens, with the former exhibiting a distinct peak point. This observation is attributed to the different confinement mechanisms provided by the two cross-sectional shapes. Circular tubes provide uniform lateral confinement to the concrete core, resulting in a more gradual post-peak behavior. Square tubes, on the other hand, provide non-uniform confinement due to the varying stress distribution across the flat walls and corners, leading to a sharper peak and more abrupt post-peak degradation.

The effect of recycled aggregate replacement rate on energy dissipation capacity is nuanced and depends on the column slenderness. For short columns, increasing the replacement rate reduces the energy dissipation capacity, likely due to the reduced concrete strength and stiffness associated with higher recycled aggregate content. However, for medium and long columns, the trend is different: circular CFST members show improved energy dissipation with increasing replacement rate, while square CFST members exhibit a non-monotonic trend of first decreasing and then increasing. This complex behavior is attributed to the interaction between the reduced concrete stiffness and the enhanced ductility provided by the recycled concrete, which allows for greater deformation before failure.

The ductility coefficient generally increases with increasing recycled aggregate replacement rate, indicating that CFST members with recycled concrete can sustain larger deformations before failure. This is beneficial for seismic applications, where ductility is a critical performance parameter. The enhanced ductility is attributed to the more gradual degradation of the recycled concrete under compression, which allows the steel tube to undergo larger plastic deformations before the composite member fails.

The finding that the relative elastic stiffness increases with replacement rate is somewhat counterintuitive and warrants further investigation. This may be attributed to the different measurement methodology or the interaction between the steel tube and the recycled concrete core, where the reduced concrete stiffness leads to a more flexible composite system that exhibits higher apparent stiffness under certain loading conditions.

Impact of Height-to-Width Ratio on Performance

The height-to-width ratio, which characterizes the slenderness of the column, has a significant influence on the energy dissipation and ductility of CFST members. As the height-to-width ratio increases, the energy dissipation of circular CFST members exhibits a non-monotonic trend of decreasing, then increasing, and then decreasing again. This complex behavior is attributed to the transition between local buckling and overall buckling failure modes as the slenderness increases.

For square CFST members, the energy dissipation decreases steadily with increasing height-to-width ratio, indicating that the non-uniform confinement provided by square sections becomes increasingly ineffective as the column becomes more slender. The ductility coefficient of medium and long columns generally decreases with increasing height-to-width ratio, reflecting the reduced capacity for plastic deformation as the column becomes more susceptible to buckling failure.

Integration with Sustainable Construction Practice

The findings of this study have significant implications for sustainable construction practices. The use of recycled concrete in CFST members offers a viable pathway for incorporating construction waste into new structural applications while maintaining acceptable structural performance. The enhanced ductility of CFST members with recycled concrete is particularly beneficial for seismic-resistant structures, where the ability to sustain large deformations is critical for life safety.

Engineers considering the use of recycled concrete in CFST members should consider the following practical aspects:

• The replacement rate should be selected based on the specific structural requirements, with higher rates suitable for ductility-critical applications
• Circular cross-sections may be preferred over square sections for applications where energy dissipation is a primary concern
• The height-to-width ratio should be carefully controlled to avoid excessive slenderness that could compromise structural performance
• Quality control measures should be implemented to ensure consistent material properties of the recycled concrete

Key Questions and Reflections

The study raises important questions about the long-term durability of CFST members with recycled concrete. The higher water absorption of recycled aggregates may lead to increased moisture ingress, which could accelerate corrosion of the steel tube in aggressive environments. Additionally, the interface between the recycled concrete and the steel tube may be affected by the different thermal expansion coefficients of the recycled aggregate and the steel, potentially leading to cracking or debonding under temperature variations.

The study also does not address the effect of recycled concrete on the fire resistance of CFST members. Recycled concrete typically has lower thermal conductivity and heat resistance compared to natural concrete, which could affect the fire performance of the composite member. Future research should investigate the fire behavior of CFST members with recycled concrete to provide comprehensive design guidance for fire-resistant structural applications.

Summary and Implications

This study provides valuable insights into the mechanical properties of CFST members with recycled concrete, contributing to the advancement of sustainable structural engineering. The identification of recycled aggregate replacement rate, height-to-width ratio, and cross-sectional shape as the primary parameters influencing energy dissipation, ductility, and stiffness provides engineers with a practical framework for designing CFST members using recycled materials. The enhanced ductility and complex energy dissipation behavior of these members offer new opportunities for sustainable and resilient structural design. Further research should address long-term durability, fire resistance, and seismic performance to provide comprehensive design guidelines for the widespread adoption of recycled concrete in CFST applications.