Flexural Performance of Steel Tube Recycled Large Aggregate Self-Consolidating Concrete Members

Literature Overview

The research by Wang Jianchao, He Wentao, and Li Mingxiang from Shenyang Jianzhu University (published in 2021, Volume 53, Issue 5, pages 643–651) investigates the flexural behavior of steel tube recycled large aggregate self-consolidating concrete (SCC) members. The study was funded by the National Natural Science Foundation of China (51678374), the Liaoning Provincial Department of Education (LJZ2017028), and the Liaoning Provincial Science and Technology Department (20180550696). The work is significant because it addresses the sustainable construction challenge of incorporating recycled aggregates into high-performance structural concrete, while leveraging the advantages of self-consolidating concrete technology and steel tube confinement.

Experimental Program

The study designed and tested 8 flexural members with three key parameters:

The steel ratio refers to the ratio of steel tube cross-sectional area to the total cross-sectional area of the member. Higher steel ratios provide greater confinement and contribution to flexural capacity.

Key Experimental Results

The study found that:

1. Recycled aggregate strength and size have minimal effect on flexural capacity: This is a counterintuitive but important finding. The steel tube confinement and the self-consolidating nature of the concrete appear to compensate for the lower quality of recycled aggregates. The recycled aggregate strength and size affect the concrete's compressive strength and stiffness, but the flexural capacity of the composite member is dominated by the steel tube contribution.
2. Steel ratio has a significant effect on flexural capacity:

• Increasing the steel ratio by 20.3% increased the ultimate flexural capacity by 13.15%.
• Increasing the steel ratio by 56.1% increased the ultimate flexural capacity by 31.21%.

This non-linear relationship suggests that the steel tube contribution to flexural capacity is not simply proportional to the steel ratio, likely due to the interaction between the steel tube and the concrete core.

Design Formula Development

The authors developed a design formula for the ultimate flexural capacity of steel tube recycled large aggregate SCC members. The formula is based on the principle of internal force equilibrium and the assumption of a plastic stress distribution in the steel tube and concrete core. The formula was validated against the experimental data, showing good agreement.

The formula likely takes the form:

The design formula should account for the confinement effect of the steel tube on the concrete core, which increases the concrete's compressive strength and ductility. The confinement effect is more pronounced for recycled aggregate concrete because the recycled aggregate has a higher water absorption and lower interfacial transition zone strength, making it more susceptible to confinement.

Engineering Practice Implications

The findings of this study have several important implications for engineering practice:

1. Sustainable construction: The use of recycled large aggregate in steel tube concrete members is feasible and does not significantly compromise the flexural capacity. This supports the use of recycled materials in structural applications, reducing the demand for natural aggregates and diverting construction waste from landfills.
2. Self-consolidating concrete technology: SCC eliminates the need for vibration, which is particularly beneficial for steel tube concrete members where vibration is difficult to apply. The self-consolidating property ensures that the concrete fills the steel tube completely, including around any internal reinforcement.
3. Steel ratio optimization: The study demonstrates that increasing the steel ratio is an effective way to increase flexural capacity. However, the non-linear relationship suggests that there is an optimal steel ratio beyond which the incremental benefit decreases. Engineers should optimize the steel ratio based on the specific loading conditions and cost considerations.
4. Recycled aggregate quality control: Although the study found that recycled aggregate strength and size have minimal effect on flexural capacity, this does not mean that recycled aggregate quality is unimportant. The recycled aggregate affects the concrete's compressive strength, durability, and long-term performance. Engineers should still ensure that the recycled aggregate meets the relevant quality standards (e.g., GB/T 25177 for recycled concrete aggregate).

Quality Control Recommendations

Summary

This study makes a significant contribution to the sustainable construction field by demonstrating that steel tube recycled large aggregate self-consolidating concrete members can achieve adequate flexural capacity despite the use of recycled aggregates. The finding that recycled aggregate strength and size have minimal effect on flexural capacity is particularly encouraging for the widespread adoption of recycled materials in structural applications. The developed design formula provides a practical tool for engineers to design these members. However, engineers should note that the study focuses on flexural capacity and does not address other important aspects such as shear capacity, fatigue performance, and long-term durability. Future research should extend the investigation to these areas to provide a comprehensive design basis for steel tube recycled aggregate SCC members.