High-Temperature Damage Comparison of Circular and Square Steel Tube Recycled Concrete Members

Literature Overview

The study by Chen Zongping, Liang Houchan, and Wang Hanpeng, published in Industrial Construction in 2019 (Vol. 49, No. 3, pp. 160-166), investigates the high-temperature damage behavior of circular and square steel tube recycled concrete (RAC) specimens. The research was supported by the National Natural Science Foundation of China (Project No. 51578163), the Guangxi Natural Science Foundation Key Project (2016GXNSFDA380032), and the Guangxi High-Level Innovation Team Program (Gui Jiao Ren [2017] No. 38). Conducted at Guangxi University, the study uses temperature and recycled coarse aggregate replacement ratio as variables to analyze the apparent changes, failure modes, mass loss, load-carrying capacity, and stiffness degradation of both circular and square steel tube RAC specimens after high-temperature exposure.

Background and Significance

The use of recycled concrete aggregate in structural applications is an important strategy for promoting sustainability and reducing the environmental impact of the construction industry. However, recycled concrete aggregate (RCA) differs from natural aggregate in terms of physical properties, including higher porosity, lower density, and greater variability. When used in steel tube-concrete (SRC) members exposed to fire, the behavior of RCA may differ significantly from that of natural aggregate, particularly in terms of thermal degradation, mass loss, and mechanical property retention.

The comparison between circular and square steel tubes is also significant because the cross-sectional shape influences the confinement effectiveness, stress distribution, and failure mode under high-temperature conditions. Square steel tubes are more common in building construction due to their ease of connection with steel beams and columns, while circular tubes offer superior confinement efficiency.

Experimental Parameters and Test Program

The study varied two primary parameters:

The test program included:

1. Apparent observation: Visual inspection of surface changes, spalling, and cracking patterns.
2. Failure mode analysis: Documentation of the failure mechanism after high-temperature exposure and subsequent loading.
3. Mass loss measurement: Quantification of weight reduction due to moisture evaporation, decomposition, and spalling.
4. Mechanical testing: Determination of residual load-carrying capacity, stiffness, ductility, and energy dissipation capacity.

Key Findings

Temperature Effects

The results reveal distinct differences between circular and square steel tube specimens:

• Apparent changes and failure modes: Both circular and square specimens exhibit similar apparent changes and failure modes after high-temperature exposure, suggesting that the cross-sectional shape does not fundamentally alter the failure mechanism.
• Mass loss: The mass loss rate increases more rapidly for square specimens than for circular specimens as temperature rises. This may be attributed to the more complex stress distribution and potential for corner effects in square sections.
• Load-carrying capacity below 400°C: Circular specimens experience greater degradation in load-carrying capacity compared to square specimens at temperatures below 400°C.
• Stiffness, ductility, and energy dissipation below 400°C: Square specimens show more significant degradation in stiffness, ductility, and energy dissipation capacity compared to circular specimens at temperatures below 400°C.
• Load-carrying capacity above 400°C: Square specimens experience more significant degradation in load-carrying capacity compared to circular specimens at temperatures above 400°C.
• Stiffness, ductility, and energy dissipation above 400°C: Circular specimens show greater degradation in stiffness, ductility, and energy dissipation compared to square specimens at temperatures above 400°C.

Recycled Aggregate Replacement Ratio Effects

• Mass loss: The increase in mass loss is greater for circular specimens than for square specimens as the replacement ratio increases.
• Stiffness, ductility, and energy dissipation: Square specimens show more significant improvement in stiffness, ductility, and energy dissipation capacity with increasing replacement ratio, compared to circular specimens.
• Load-carrying capacity: Square specimens show slightly greater degradation in load-carrying capacity with increasing replacement ratio, compared to circular specimens.

Technical Analysis

The observed differences between circular and square specimens can be attributed to several factors:

1. Confinement effectiveness: Circular tubes provide more uniform confinement pressure around the concrete core, leading to more consistent material behavior. Square tubes have stress concentrations at the corners, which can lead to localized damage initiation.
2. Thermal gradient: The thermal gradient within the cross-section differs between circular and square sections, affecting the temperature distribution and consequently the mechanical behavior.
3. Stress distribution: The stress distribution under axial compression is more uniform in circular sections, while square sections experience non-uniform stress distribution, particularly near the corners.
4. Failure initiation: The location and mechanism of failure initiation differ between circular and square sections, with square sections more prone to corner cracking.

The behavior of recycled concrete aggregate under high temperatures is influenced by the presence of adhered mortar on the recycled aggregate particles. This adhered mortar has different thermal properties than the natural aggregate, leading to differential expansion and potential debonding at the aggregate-mortar interface. As the replacement ratio increases, the volume of this heterogeneous interface increases, affecting the overall thermal and mechanical behavior.

Engineering Practice Considerations

For engineers designing SRC members using recycled concrete aggregate in fire-exposed environments, the following considerations are important:

• The choice between circular and square steel tubes should consider not only structural efficiency but also the specific fire exposure conditions expected.
• The recycled aggregate replacement ratio should be carefully selected based on the required fire resistance performance and the expected temperature exposure.
• Quality control of the recycled aggregate is essential, as the properties of RCA can vary significantly depending on the source and processing method.
• Fire protection measures, such as intumescent coatings or fire-resistant board cladding, should be considered to mitigate the effects of high-temperature exposure.

Study Insights and Reflections

This research provides valuable comparative data on the high-temperature behavior of circular and square steel tube recycled concrete members. The findings highlight the complex interplay between cross-sectional shape, temperature, and recycled aggregate content, which is critical for rational design of sustainable composite structures in fire-exposed environments. The observation that the differences between circular and square specimens reverse at 400°C is particularly interesting and warrants further investigation. This temperature threshold likely corresponds to significant changes in the material properties of both the steel tube and the concrete, and understanding the underlying mechanisms could lead to improved design methodologies. However, the study could be strengthened by including additional temperature levels, particularly in the range of 600-800°C, which are relevant for severe fire scenarios. Furthermore, the long-term behavior of these members after cooling and re-exposure to service loads should be investigated to assess their post-fire usability. Overall, this research contributes to the growing body of knowledge on sustainable composite structures and provides a foundation for future investigations into the fire performance of recycled concrete-based SRC members.