Comparative Study of Post-Fire Interfacial Bond Performance Between Circular and Square Steel Tube Recycled Concrete

Literature Overview

This paper, authored by Jia Hengrui, Chen Zongping, and Chen Junrui from Guangxi University, investigates the interfacial bond behavior between steel tubes and recycled concrete (RC) after exposure to elevated temperatures. The study employs a push-out test methodology on 40 specimens—20 circular and 20 square steel tube recycled concrete (STC) assemblies—varying the maximum exposure temperature (T) and recycled coarse aggregate replacement ratio (γ). Published in the journal Concrete (2023, Issue 5), this work is funded by the National Natural Science Foundation of China (51578163) and several Guangxi provincial programs. The research addresses a critical gap in understanding how cross-sectional geometry influences the post-fire durability of steel tube concrete composite structures, particularly when recycled aggregates are incorporated to promote sustainable construction practices.

Core Technical Findings and Analysis

The key quantitative results reveal significant performance differences between circular and square cross-sections. The interfacial bond strength of circular specimens exceeds that of square specimens by an average of 114.8%, the bond shear stiffness is higher by 3.57%, and the energy dissipation capacity is greater by 53.7%. These advantages tend to diminish with increasing temperature T but grow with increasing recycled aggregate replacement ratio γ. Regarding damage evolution, circular specimens exhibit later initial interfacial damage onset and a damage progression rate that is on average 46.1% slower than square specimens. This advantage also decreases with rising temperature but is largely insensitive to changes in γ.

Mechanistic Interpretation

The superior performance of circular cross-sections can be attributed to several metallurgical and mechanical factors. First, the uniform curvature of a circular tube provides isotropic confinement pressure on the surrounding concrete, promoting a more homogeneous stress distribution at the steel-concrete interface. In contrast, square tubes concentrate stresses at the corners and edges, leading to non-uniform bond degradation and premature localized failure. The authors note that corner regions in square specimens experience delayed interfacial failure compared to flat face regions, indicating a geometrically dependent damage initiation sequence.

Influence of Temperature and Recycled Aggregate Ratio

The temperature parameter T introduces additional complexity through thermal degradation mechanisms. At elevated temperatures, the steel tube undergoes strength reduction (typically beginning above 400°C per Eurocode 4 guidelines), and the concrete experiences dehydration of ettringite and C-S-H gel decomposition. These phenomena reduce the effective confinement pressure and weaken the chemical adhesion at the interface. The observation that the circular advantage diminishes with temperature suggests that at sufficiently high temperatures, the differential confinement effect becomes less pronounced as both geometries suffer proportional bond degradation.

The recycled aggregate replacement ratio γ introduces heterogeneity through the presence of interfacial transition zones (ITZ) between the recycled aggregate particles and the cement paste. Higher γ values increase the number of weak ITZ regions within the concrete matrix, which paradoxically enhances the relative performance of circular tubes because the uniform confinement from circular geometry better compensates for internal concrete heterogeneity.

Engineering Practice Implications

For fire-resistance design of steel tube concrete composite members using recycled aggregates, the following practical recommendations emerge:

1. Section selection: Circular cross-sections should be preferred for applications involving fire exposure combined with recycled aggregate use, as they provide substantially superior post-fire interfacial integrity.
2. Temperature thresholds: Designers should pay particular attention to specimens exposed above 600°C, where the performance gap between geometries narrows and both configurations face significant bond degradation.
3. Recycled aggregate limits: When high replacement ratios (γ > 50%) are specified for sustainability targets, the circular cross-section advantage becomes more pronounced, providing additional justification for its selection.
4. Quality control: Push-out testing at representative temperatures should be incorporated into qualification testing for composite members designed to withstand fire exposure, particularly when recycled aggregates are involved.

Key Questions and Reflections

A critical question remains regarding the scalability of these findings. The laboratory push-out specimens represent a simplified boundary condition compared to full-scale structural members, where axial compression, bending moments, and shear forces act simultaneously. The authors acknowledge this limitation, and I believe future work should extend these findings to full-scale beam-column specimens subjected to combined fire and mechanical loading.

Another reflection concerns the welding and fabrication implications. Circular steel tubes are typically produced by ERW or HFW processes, while square tubes require additional forming operations that may introduce residual stresses and microstructural variations at the corners. These fabrication-induced differences could further contribute to the observed performance gap, and a comprehensive study should decouple the geometric effect from the fabrication effect.

Study Insights and Outlook

This research provides valuable quantitative data for the design of sustainable steel tube concrete structures in fire-exposed environments. The clear superiority of circular cross-sections in post-fire interfacial performance, combined with the positive interaction with recycled aggregate content, offers a compelling argument for the use of circular steel tubes in green building applications requiring fire resistance. The methodology employed—systematic parametric variation with push-out testing—follows sound experimental design principles and the results are reproducible. Future research should integrate these findings with finite element modeling to develop simplified design expressions for post-fire interfacial bond capacity, which would be directly applicable in practical engineering design per standards such as GB 51249 or Eurocode 4.