Interfacial Bonding Performance of Square Steel Tube High-Strength Concrete After High-Temperature Water Spray Cooling

Literature Overview

This paper by Chen Zongping, Zhou Ji, Chen Jianjia, and Zhou Xingyu (2020), published in the Journal of South China University of Technology (Natural Science Edition), investigates the interfacial bonding performance between square steel tubes and high-strength concrete after exposure to high temperatures followed by water spray cooling. The study was supported by the National Natural Science Foundation (51268004), Guangxi Natural Science Foundation Key Project (2016GXNSFDA380032), and the Bagui Scholar Special Fund. The research involved 22 push-out specimens, including 18 water-cooled specimens and 4 natural-cooled specimens.

Core Technical Findings

The study systematically examines how multiple parameters affect the interfacial bond strength after fire exposure and cooling. The push-out test is the standard method for evaluating the steel-concrete interface bond in CFST members, as specified in relevant codes.

Key Experimental Parameters and Results

Bond Strength Behavior After Water Cooling

The study reveals a complex but important pattern: after high-temperature exposure followed by water spray cooling, the bond strength initially increases with temperature (up to a certain threshold) and then decreases at higher temperatures. This non-monotonic behavior is attributed to:

1. At moderate temperatures (up to approximately 400-500°C), thermal expansion of the steel tube may compress the interface, increasing frictional resistance.
2. At higher temperatures (above 500-600°C), the concrete undergoes significant strength degradation, thermal cracking, and potential spalling, which overwhelms any compressive benefit.
3. Water spray cooling introduces thermal shock, creating additional micro-cracks at the interface due to differential thermal contraction rates between steel and concrete.

Metallurgical and Material Science Analysis

From a materials science perspective, several mechanisms govern the post-fire interface behavior:

• Thermal expansion mismatch: Steel has a thermal expansion coefficient of approximately 12×10⁻⁶/°C, while concrete varies between 10-14×10⁻⁶/°C depending on aggregate type. During heating, this differential expansion creates complex interfacial stresses.
• Concrete decomposition: At temperatures above 400°C, concrete begins to lose strength due to dehydration of hydration products. Above 600°C, significant decomposition of calcium silicate hydrate (C-S-H) gel occurs, drastically reducing concrete strength.
• Water spray thermal shock: Rapid cooling by water spray creates steep thermal gradients within the composite member. The steel tube contracts more rapidly than the concrete, potentially creating tensile stresses at the interface that promote debonding.
• Steam pressure: At high temperatures, water within the concrete pores vaporizes. During water spray cooling, the sudden temperature drop can cause trapped steam to condense rapidly, creating localized pressure differentials.

Welding and Fabrication Implications

For square steel tube CFST members subjected to fire and subsequent cooling:

1. Corner weld integrity: Square tubes have corner welds (or formed corners) that are critical structural details. Under thermal cycling, these welds experience complex stress states that may lead to cracking or fatigue damage.
2. Post-fire inspection: After a fire event, the steel tube welds should be inspected using non-destructive testing (MT or PT) to detect any thermal-induced cracking.
3. Residual stress interaction: The residual stresses from welding interact with thermal stresses from fire exposure. In the heat-affected zone (HAZ) of welds, the material may have reduced toughness, making it more susceptible to thermal shock damage.
4. Repair considerations: If post-fire inspection reveals weld damage, repair welding must account for the altered material properties in the thermally affected zone. Preheating and controlled cooling rates are essential.

Quality Control Recommendations for Post-Fire Assessment

Engineering Practice Integration

The study's proposed formulas for ultimate bond strength and residual bond strength provide engineers with quantitative tools for post-fire structural assessment. The key engineering implications include:

• Water spray cooling, while effective for fire suppression, may compromise the interfacial bond in CFST members. This has implications for fire protection strategies in structures with CFST components.
• The finding that longer anchorage lengths reduce bond strength is consistent with established mechanics (longer interfaces have more potential for partial debonding), but the magnitude of this effect after fire exposure warrants careful consideration in design.
• The recovery of bond strength at 90 minutes compared to 60 minutes of isothermal exposure suggests that the interface may undergo some self-healing or stabilization at longer exposure durations, possibly due to complete decomposition of unstable phases.

Study Insights and Reflections

This research addresses a practically important scenario: the condition of CFST members after a fire has been extinguished by water spray, which is the standard fire suppression method in most buildings and industrial facilities. The finding that water-cooled specimens have lower ultimate and residual bond strength but greater energy dissipation capacity is counterintuitive yet physically reasonable. The reduced bond strength results from thermal shock damage, while the greater energy dissipation may reflect the creation of friction surfaces through partial debonding. For engineers responsible for post-fire structural assessment and repair, this paper provides the quantitative basis for evaluating whether a CFST member can remain in service or requires intervention. The proposed bond strength formulas should be validated against additional experimental data from different steel grades and concrete compositions before widespread application. Future research should also consider the effect of fire protection systems (such as intumescent coatings or water mist systems) on the interface behavior, as these systems modify the thermal exposure history experienced by the steel tube and concrete.