Experimental Study on Contact Thermal Resistance Between Steel Tube and Concrete

Literature Overview

This concise paper by Lu Libing, Qi Hanbing, and Wang Lili (2002), published in Oil and Gas Field Surface Engineering, presents experimental investigation into the contact thermal resistance at the interface between steel tubes and concrete in composite structures. The research was conducted at Daqing Petroleum Institute. Although brief in length (one page), the topic it addresses is fundamental to the thermal analysis of all steel tube concrete (STC) composite members.

Technical Significance of Contact Thermal Resistance

The contact thermal resistance at the steel-concrete interface represents a critical parameter in fire engineering calculations. It quantifies the thermal impedance that exists between the steel tube wall and the concrete core, arising from:

Microscopic surface roughness of both materials
Incomplete filling of voids during concrete placement
Thermal expansion mismatch during heating
Potential debonding at elevated temperatures

Typical Values and Influencing Factors

Factor	Effect on Contact Thermal Resistance	Engineering Relevance
Concrete density	Higher density reduces resistance	Denser concrete improves thermal transfer
Steel tube surface finish	Rougher surface reduces resistance	Surface treatment affects composite action
Concrete slump	Higher slump improves filling	Workability influences void content
Temperature level	Resistance increases with temperature	Thermal expansion creates gaps
Tube diameter	Larger diameter may increase resistance	Scale effect on contact area

Welding and Fabrication Implications

From a steel pipe fabrication standpoint, several considerations arise:

The internal surface finish of the steel tube directly affects contact thermal resistance. Tubes manufactured by ERW or HFW processes typically have smoother internal surfaces than those formed from plate (LSAW), potentially resulting in higher contact thermal resistance.
Internal ribs or stiffeners welded inside the tube can reduce contact thermal resistance by providing mechanical interlock points. However, the weld quality at these internal stiffener connections becomes critical.
The manufacturing tolerance of the tube (roundness, ovality) affects how uniformly the concrete fills the interior. Tubes with tighter dimensional tolerances will achieve more consistent contact thermal resistance.

Quality Control Considerations

For engineers responsible for quality assurance of STC members, the following inspection and control measures are recommended:

Verify concrete placement method ensures complete filling, particularly in large-diameter tubes where consolidation challenges increase.
Consider post-placement vibration or internal vibrator access to ensure void-free concrete.
Document tube surface preparation conditions, as this affects both mechanical and thermal interface properties.
For critical fire-resistance applications, consider specifying surface roughening treatments on the tube interior.

Study Insights and Reflections

Although this paper is brief and was published in 2002, the fundamental issue it addresses remains unresolved in many current design codes. Most international fire design codes (Eurocode 4, AISC) assume either perfect thermal contact (zero resistance) or use simplified empirical values. The reality is that contact thermal resistance varies significantly with construction practice, concrete mix design, and service conditions. For engineers working on fire-rated STC structures, obtaining project-specific contact thermal resistance data through testing or using conservative assumptions is essential for accurate fire resistance predictions. This paper, while limited in scope, reminds us that the steel-concrete interface is not a simple boundary condition but a complex zone that deserves careful attention in both design and construction.