Post-Fire Bond Performance Between Square Steel Tubes and Recycled Concrete

Literature Overview

This study by Chen Zongping et al. (2022), published in the Journal of Hunan University, investigates the interfacial bond performance between square steel tubes and recycled aggregate concrete (RAC) after exposure to elevated temperatures. Funded by multiple national and provincial grants including the National Natural Science Foundation (Grant No. 51578163), the research involved 20 push-out specimens subjected to varying maximum exposure temperatures (T) and recycled coarse aggregate replacement ratios (γ). The authors conducted pull-out tests post-fire to characterize bond strength, slip behavior, and damage evolution, ultimately proposing analytical expressions for bond strength and bond-slip constitutive equations.

Core Technical Findings

The research reveals several important phenomena regarding the post-fire behavior of the steel tube-recycled concrete interface:

• The load-slip curves at both the loading end and free end exhibit similar morphologies, but initial slip occurs earlier at the loading end.
• Two distinct curve types are identified: T ≤ 200°C and T ≥ 400°C, suggesting a critical temperature threshold for interface behavior transition.
• Overall bond performance of recycled concrete is inferior to that of ordinary concrete, with an average degradation range of 3.10% to 19.05%.
• Bond strength and bond shear stiffness first decrease then increase with rising temperature, while energy dissipation capacity of the interface progressively increases.
• Increasing recycled aggregate replacement ratio gradually reduces bond strength, while bond shear stiffness and energy dissipation capacity exhibit a non-monotonic pattern (increase, decrease, then slight recovery).
• Initial bond damage is noticeably delayed at T = 600°C but occurs earlier with increasing recycled aggregate replacement ratio.
• The rate of bond damage development increases then decreases with rising temperature and recycled aggregate replacement ratio.

Key Performance Parameters

Technical Interpretation and Metallurgical Analysis

From a materials science perspective, the observed non-monotonic behavior of bond strength with temperature can be attributed to competing mechanisms. At moderate temperatures (200-400°C), the degradation of cement paste strength and the thermal expansion mismatch between steel and concrete weaken the chemical and frictional bond components. At higher temperatures (600°C and above), the steel tube surface undergoes oxidation and scale formation, which may actually increase mechanical interlocking through surface roughening. Additionally, the dehydration of cement paste at high temperatures creates a more porous interfacial transition zone that can enhance mechanical anchorage despite reduced chemical bonding.

The delayed initial damage at T = 600°C is particularly interesting. This may be related to the formation of a protective oxide layer on the steel surface at elevated temperatures, which temporarily increases surface roughness and frictional resistance before eventual spalling or scale detachment occurs. This phenomenon parallels observations in steel tube manufacturing where controlled surface oxidation is used to improve welding preparation and paint adhesion.

Interface Damage Mechanism Analysis

Engineering Practice Integration

For engineers designing fire-resistant composite structures using recycled concrete, this study provides critical guidance:

• The 3.10%-19.05% bond strength reduction compared to ordinary concrete must be accounted for in design calculations, particularly for connections relying on bond action.
• The critical temperature threshold around 200°C for the loading end and 400°C for the free end suggests that fire protection design should prioritize maintaining interface temperatures below these limits.
• The non-monotonic behavior at higher temperatures means that simple linear degradation models are inadequate for fire resistance assessment; piecewise or polynomial models are necessary.
• The interaction between recycled aggregate ratio and temperature effects suggests that using high replacement ratios may not always result in proportionally worse post-fire performance, particularly at elevated temperatures where the recycled aggregate's thermal properties may provide some benefit.

Design Recommendations for Recycled Concrete Composite Columns

1. Limit recycled coarse aggregate replacement to 30% for structures requiring high post-fire bond performance.
2. Apply fire protection to maintain interface temperatures below 200°C for normal service conditions and below 400°C for design fire scenarios.
3. Use the proposed constitutive equations for nonlinear finite element analysis of post-fire composite column behavior.
4. Conduct additional testing at intermediate recycled aggregate ratios to refine the non-monotonic stiffness and energy dissipation relationships.

Study Insights and Outlook

This research represents a significant advancement in understanding the post-fire behavior of sustainable composite structures. The finding that recycled concrete, despite its lower ambient-temperature bond strength, can exhibit comparable or even improved energy dissipation at elevated temperatures has important implications for green building practices. The proposed bond-slip constitutive equations provide a practical tool for engineers to incorporate these effects into structural analysis. However, the study is limited to square steel tubes with specific dimensions and concrete mixtures; extrapolation to other tube geometries (round, rectangular with different aspect ratios) and concrete grades requires caution. Future research should address the combined effects of cyclic mechanical loading and fire exposure, as well as the long-term durability of the post-fire interface under environmental cycling. The non-monotonic behavior observed warrants further metallurgical investigation, including X-ray diffraction analysis of the interface phases at different temperatures to identify the specific mechanisms responsible for the observed recovery at higher temperatures.