Interface Bonding Performance of Steel Tube Concrete at Elevated Temperatures

Literature Overview

The research by Chen Jun, Xue Yuanyuan, Tan Qinghua, and Wu Jinliang (2018, Journal of Civil Engineering) investigates the interface bonding performance of steel tube concrete specimens subjected to sustained elevated temperatures. Push-out slip tests were conducted on steel tube concrete specimens with varying temperatures, length-to-diameter ratios, and diameter-to-thickness ratios. The study examines the temperature-time and load-slip relationship curves at the interface, providing critical data for understanding the degradation of composite action under fire conditions. This study is directly relevant to steel pipe engineers because it addresses the thermal performance of the steel tube-concrete interface, which is fundamental to the design of steel tube concrete structures in fire-exposed environments and has implications for welding residual stress and heat-affected zone behavior.

Core Technical Findings

Temperature Effect on Interface Bonding

The study examined the effect of sustained temperature on the average bond strength of the steel tube concrete interface across a temperature range of 20 to 900 degrees Celsius. The results revealed a complex non-monotonic behavior:

The maximum reduction in average bond strength reached 90 percent compared to ambient temperature conditions. This dramatic degradation highlights the vulnerability of the steel tube concrete interface under fire exposure and underscores the importance of fire protection measures in composite steel tube concrete structures.

Geometric Parameter Effects

The study also investigated the influence of geometric parameters on the interface bond strength at elevated temperatures:

• Length-to-diameter ratio: Increasing the length-to-diameter ratio led to a decrease in average bond strength, with a maximum reduction of 50 percent. This is attributed to the increased temperature gradient along the specimen length, which creates differential thermal expansion and reduces the effective bond area.
• Diameter-to-thickness ratio: The diameter-to-thickness ratio had a relatively minor effect on the average bond strength, with reductions within 10 percent. This suggests that the tube wall thickness has limited influence on the interface bonding behavior under thermal exposure.

Interface Failure Modes

The push-out slip tests revealed distinct failure modes at different temperature levels. At lower temperatures, the failure mode was predominantly cohesive failure within the concrete interface zone, indicating that the bond strength was governed by the properties of the interfacial transition zone. At higher temperatures, the failure mode shifted toward adhesive failure at the steel tube surface, suggesting that the bond between the steel tube and the surrounding concrete was the weaker link.

Process and Standards Analysis

Welding and Thermal Effects

The interface bonding performance at elevated temperatures has direct implications for the welding quality of steel tubes used in composite concrete structures. The heat-affected zone (HAZ) of a welded steel tube is already a region of altered microstructure and potentially reduced toughness. When subjected to elevated temperatures, the HAZ may experience additional microstructural changes that could affect the bond performance at the interface.

For welded steel tubes, the following considerations arise:

• The longitudinal weld seam creates a region of non-uniform material properties along the tube circumference.
• Under thermal exposure, the weld zone may expand differently from the base metal, creating localized stresses at the interface.
• The weld cap or weld groove geometry may create stress concentrations that promote crack initiation at the interface under thermal loading.

Quality control measures should include:

• Ultrasonic testing of all weld seams to ensure full penetration and absence of defects.
• Hardness testing of the HAZ to verify that the microstructure has not been adversely affected by the welding process.
• Consideration of post-weld heat treatment for critical applications where fire exposure is anticipated.

Fire Protection Design Implications

The study's findings on the significant degradation of interface bond strength at elevated temperatures have important implications for the fire protection design of steel tube concrete structures:

• Fire protection coatings or encasements should be specified to maintain the interface temperature below the critical degradation threshold.
• The length-to-diameter ratio of steel tube concrete members should be considered in fire protection design, as longer members are more susceptible to bond degradation due to thermal gradients.
• The diameter-to-thickness ratio, while having a minor effect on bond strength, should still be optimized for overall structural performance, as it influences the fire resistance of the steel tube itself.

Engineering Practice Integration

Application to Steel Pipe Quality Control

For steel pipe manufacturers supplying tubes for composite concrete structures, the following quality control recommendations emerge from this study:

• Surface treatment: The inner surface of the tube should be cleaned and prepared to ensure optimal bonding with the concrete. Any mill scale, rust, or contamination should be removed, as these can act as weak interfaces under thermal exposure.
• Dimensional consistency: The tube diameter and wall thickness should be maintained within tight tolerances to ensure uniform concrete placement and consistent interface geometry.
• Material certification: The steel grade and chemical composition should be verified to ensure that the tube material has adequate fire resistance properties. Certain alloying elements may improve the high-temperature strength retention of the steel tube.

Design Recommendations

Based on the experimental findings, the following design recommendations can be made:

• The interface bond strength at elevated temperatures should be reduced by appropriate factors in fire design calculations, reflecting the significant degradation observed in the study.
• The length-to-diameter ratio of steel tube concrete members should be limited in fire-exposed applications to minimize the effect of thermal gradients on bond performance.
• Fire protection measures should be designed to maintain the interface temperature below 600 degrees Celsius for as long as possible, as this is the temperature range where bond degradation becomes severe.
• The diameter-to-thickness ratio can be selected based on structural requirements without significant concern for interface bonding performance, as the effect is relatively minor.

Key Questions and Reflections

A significant question arising from this study is the effect of heating rate and cooling rate on the interface bonding performance. The study focuses on sustained elevated temperatures, but in real fire scenarios, the heating rate may be rapid and the cooling rate may be influenced by fire suppression measures. Rapid heating could create thermal shock effects that further degrade the interface, while rapid cooling could induce additional residual stresses.

Another reflection concerns the interaction between the interface bonding degradation and the overall structural behavior of the steel tube concrete member. The study focuses on the interface bond strength in isolation, but in practice, the interface bond works in conjunction with the confinement effect, the shear transfer mechanism, and the overall composite action. The degradation of interface bonding may have cascading effects on the structural performance that are not captured by the push-out slip test alone.

Study Insights and Implications

This research provides essential data on the thermal degradation of the steel tube concrete interface, which is a critical aspect of fire design for composite steel tube concrete structures. The identification of the complex non-monotonic temperature effect and the quantification of geometric parameter influences offer valuable guidance for engineers. For steel pipe engineers, the study reinforces the importance of surface quality, dimensional accuracy, and material consistency in tube manufacturing, as these factors directly influence the fire performance of the composite system. The findings also highlight the need for continued research on the interaction between welding-induced microstructural changes and thermal degradation of the interface, as this intersection of welding metallurgy and fire engineering remains an area of active development.