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

Literature Overview

This paper by Chen Zongping, Liang Houren, and Wang Hanpeng, published in Industrial Construction (2019, Volume 49, Issue 3, pp. 160-166), provides a comprehensive comparison of high-temperature damage behavior between circular and square steel tube recycled concrete members. Funded by the National Natural Science Foundation (Grant No. 51578163), the Guangxi Natural Science Foundation Key Project (2016GXNSFDA380032), and the Guangxi High-Level Innovation Team Program, the research was conducted at Guangxi University. The study examines temperature and recycled coarse aggregate replacement rate as primary variables.

Core Technical Findings

The research reveals nuanced differences between circular and square steel tube configurations under high-temperature exposure:

• Post-high-temperature surface changes and failure modes are similar between circular and square steel tube specimens.
• Mass loss rate (burn loss) increases faster for square steel tube specimens as temperature rises.
• Below 400°C: circular steel tube specimens show greater load-bearing capacity degradation compared to square specimens, while square specimens show more significant degradation in stiffness, ductility, and energy dissipation capacity.
• Above 400°C: the trend reverses—square steel tube specimens show significantly greater load-bearing capacity degradation, while circular specimens show greater stiffness, ductility, and energy dissipation degradation.
• Increasing replacement rate: circular specimens show greater mass loss rate increase, while square specimens show more significant improvement in stiffness, ductility, and energy dissipation, though with slightly greater load-bearing capacity degradation.

Performance Comparison Matrix

Technical Analysis

The crossover behavior observed at approximately 400°C is particularly significant for engineering design. This temperature threshold corresponds to the onset of significant concrete degradation mechanisms, including the decomposition of calcium hydroxide and the release of free water from hydrated cement paste. The different confinement geometry creates distinct thermal and mechanical response patterns:

1. Circular steel tubes provide uniform radial confinement, distributing stresses evenly around the perimeter.
2. Square steel tubes create corner concentration effects and non-uniform confinement pressures, particularly at corners where the steel tube geometry provides less effective confinement.
3. The recycled coarse aggregate introduces additional complexity through its porous structure, which affects both thermal conductivity and high-temperature expansion behavior.

Recycled Aggregate Effects

The recycled coarse aggregate replacement rate introduces additional variability:

• Higher replacement rates increase the heterogeneous nature of the concrete matrix, affecting thermal stress development.
• The porous structure of recycled aggregate may provide some thermal buffering but also introduces additional weak interfaces.
• The effect on load-bearing capacity versus stiffness and ductility suggests that recycled aggregate primarily affects the elastic response while having a more complex influence on ultimate capacity.

Study Insights and Reflections

This research provides essential comparative data for engineers selecting between circular and square steel tube configurations for recycled concrete applications in fire-exposed environments. The temperature-dependent crossover in performance characteristics means that the optimal section shape depends on the expected fire exposure scenario and the critical performance metric being evaluated. For applications where load-bearing capacity is the primary concern at temperatures above 400°C, circular sections are preferable. For applications emphasizing stiffness and energy dissipation below 400°C, circular sections also perform better. However, the increased mass loss in square sections at higher temperatures raises concerns about long-term durability post-fire. The recycled aggregate replacement rate effect adds another dimension to the design decision, requiring engineers to balance sustainability goals with structural performance requirements.