Zhuojin Pipe Fitting Co., LtdComparative Study of Interfacial Bond Properties Between Circular and Square Steel Tube Recycled Concrete After High Temperature Exposure
Literature Overview
This study published in "Concrete" (2023, Issue 5, pages 48–55) by Jia Hengrui, Chen Zongping, and Chen Junrui from Guangxi University investigates the interfacial bond behavior between circular and square steel tubes and recycled aggregate concrete (RAC) after exposure to elevated temperatures. The research was supported by the National Natural Science Foundation of China (Grant No. 51578163), the Guangxi Bagui Scholar Special Research Project, and the Guangxi Science and Technology Base and Talent Program (Guike AD21075031). The work addresses a critical gap in understanding how cross-sectional geometry affects the post-fire performance of steel tube-confined recycled concrete structures, which is increasingly relevant as the construction industry moves toward sustainable materials.
Core Technical Content and Experimental Methodology
The experimental program involved 40 push-out specimens in total — 20 circular and 20 square steel tube RAC specimens. The two key variable parameters were the maximum exposure temperature (T) and the recycled coarse aggregate replacement ratio (γ). Push-out tests were conducted on all specimens after high-temperature exposure to evaluate the interfacial bond performance. The primary evaluation metrics included bond strength, bond shear stiffness, energy dissipation capacity, damage initiation behavior, and damage progression rate.
Key Quantitative Findings
| Performance Indicator | Circular vs. Square Comparison | Trend with Temperature (T) | Trend with Recycled Aggregate Ratio (γ) |
|---|---|---|---|
| Interfacial bond strength | Circular is 114.8% higher on average | Advantage decreases as T increases | Advantage increases as γ increases |
| Bond shear stiffness | Circular is 3.57% higher on average | — | — |
| Energy dissipation capacity | Circular is 53.7% higher on average | Advantage decreases as T increases | Advantage increases as γ increases |
| Initial damage onset | Circular initiates damage later | Advantage decreases as T increases | Largely unaffected by γ |
| Damage progression rate | Circular is 46.1% slower on average | Advantage decreases as T increases | Largely unaffected by γ |
Interpretation of Failure Modes
The square specimens exhibited distinctly brittle and non-uniform bond failure patterns compared to the circular specimens. A notable observation was that interfacial failure at the corners and edges of the square sections occurred later than at the mid-face regions, indicating a non-uniform stress distribution within the confinement. This is consistent with the well-known stress concentration effects at geometric discontinuities. In contrast, circular specimens demonstrated more uniform and ductile bond behavior, which is attributed to the uniform confinement pressure distribution provided by the circular cross-section.
Technical Analysis of Cross-Sectional Geometry Effects
From a structural engineering perspective, the superior performance of circular steel tubes can be attributed to the uniform radial confinement pressure exerted on the core concrete. In a circular cross-section, the hoop stress is uniformly distributed around the perimeter, resulting in consistent confinement of the concrete core. Square sections, however, suffer from non-uniform stress distribution — the corners experience higher local stresses while the mid-face regions are under relatively lower confinement. This non-uniformity becomes more pronounced as the temperature increases, because the differential thermal expansion between steel and concrete introduces additional residual stresses that exacerbate the existing non-uniformity.
The observation that the advantage of circular sections decreases with increasing temperature is particularly significant for fire engineering design. At ambient conditions, the circular section's confinement advantage is maximized. However, as temperature rises, the steel tube loses strength and stiffness more rapidly than the concrete, and the differential thermal expansion between the two materials introduces additional interfacial stresses. These factors collectively reduce the relative benefit of the circular geometry at elevated temperatures.
The increase in recycled aggregate replacement ratio (γ) amplifying the circular section's advantage is a counterintuitive but important finding. Recycled aggregate typically has higher porosity and lower interfacial transition zone (ITZ) quality compared to natural aggregate. This weaker aggregate-concrete matrix interface means that the overall concrete core is more dependent on the external steel tube confinement for maintaining structural integrity. The circular section's superior confinement efficiency becomes more critical under these conditions.
Engineering Practice Implications
For engineers designing steel tube-confined recycled concrete structures in fire-prone environments, this study provides several actionable insights. First, circular cross-sections should be preferred when high-temperature resistance and energy dissipation capacity are critical design requirements. Second, the use of recycled aggregate, while environmentally beneficial, demands greater attention to cross-sectional geometry selection — circular tubes can compensate for the inherent weaknesses of recycled aggregate more effectively than square tubes. Third, the damage progression rate data suggests that circular sections offer significantly better warning capacity before catastrophic bond failure, which is crucial for life-safety considerations in post-fire structural assessment.
The finding that damage initiation is delayed in circular specimens but this advantage diminishes with temperature has direct implications for fire resistance design codes. Current design standards often assume uniform material degradation with temperature, but this study demonstrates that geometric factors interact with thermal degradation in complex ways. Engineers should consider incorporating cross-sectional geometry factors into fire resistance calculations for steel tube-confined concrete structures.
Key Questions and Reflections
Several questions emerge from this study that warrant further investigation. The study does not explicitly address the effect of steel tube wall thickness on the observed geometry-dependent performance differences. It is reasonable to hypothesize that thinner-walled tubes might amplify the geometric effects because they provide less inherent confinement, making the uniformity of confinement distribution more critical. Additionally, the study does not investigate the combined effects of cyclic loading and high-temperature exposure, which is a common scenario in seismic fire scenarios.
From a practical standpoint, the significant difference in bond strength (114.8%) between circular and square sections is substantial enough to influence design decisions. However, engineers must also consider that square sections offer practical advantages in terms of space utilization, ease of connection, and architectural compatibility. The decision between circular and square sections should therefore be a balanced consideration of structural performance, constructability, and cost.
Study Insights and Outlook
This research contributes valuable experimental data to the growing body of knowledge on steel tube-confined recycled concrete structures. The systematic comparison of circular and square cross-sections under varying temperature and recycled aggregate conditions provides a foundation for developing geometry-specific design guidelines. Future research should extend this work to include rectangular cross-sections with varying aspect ratios, investigate the effect of steel tube wall thickness, and incorporate multi-axial stress states that better represent real structural conditions. The integration of these findings into finite element models for performance-based fire design would represent a significant advancement in the field.