Contact Thermal Resistance Between Steel Tube and Concrete in Concrete Structures

Literature Overview and Research Context

The paper by Lu Libing, Qi Hanbing, and Wang Lili, published in Oil and Gas Field Surface Engineering in 2002 (Vol. 21, No. 5, p. 90), presents experimental research on the contact thermal resistance between steel tubes and concrete in composite structures. This work was conducted at the Daqing Petroleum Institute, reflecting the practical engineering needs of the oil and gas industry where steel tube concrete structures are commonly used in platforms, pipelines, and wellhead structures.

Significance of Contact Thermal Resistance

The contact thermal resistance at the steel-concrete interface is a critical parameter that governs the heat transfer efficiency between the two materials in composite members. In fire conditions, the steel tube heats up rapidly due to its high thermal conductivity, while the concrete core heats more slowly. The contact thermal resistance determines how effectively heat is transferred from the steel tube to the concrete core, which directly affects the temperature distribution within the composite section and, consequently, the fire endurance of the member.

The existence of contact thermal resistance is attributed to several physical factors:

Surface roughness of both the steel tube inner wall and the concrete surface creates micro-gaps filled with air or voids.
The coefficient of thermal expansion mismatch between steel and concrete causes micro-separation at elevated temperatures.
The concrete surface layer near the steel tube may have a different density and porosity compared to the bulk concrete, creating a transition zone with distinct thermal properties.

Experimental Approach and Key Findings

The experimental methodology involved instrumenting steel tube concrete specimens with thermocouples at various locations to measure the temperature distribution during heating. By comparing the measured temperatures with theoretical predictions based on assumed perfect thermal contact, the researchers were able to quantify the contact thermal resistance.

Test Parameter	Typical Range	Effect on Contact Resistance
Concrete strength grade	C30 to C60	Higher strength may increase resistance due to lower permeability
Steel tube surface condition	As-welded, polished	Surface roughness directly affects contact area
Concrete placement method	Pumped, vibrated, free-flow	Compaction quality affects interface density
Temperature range	20°C to 800°C	Resistance increases with temperature due to differential expansion

The study found that the contact thermal resistance is not a constant value but varies significantly with temperature, concrete quality, and steel tube surface condition. At ambient temperature, the resistance may be relatively low, but as temperatures rise above 200°C, the differential thermal expansion between steel and concrete creates micro-cracks and voids at the interface, increasing the thermal resistance substantially.

Engineering Practice Implications

For engineers designing steel tube concrete structures for fire resistance:

The contact thermal resistance should not be assumed to be zero in thermal analysis; neglecting it leads to underestimation of steel tube temperatures and overestimation of fire endurance.
The quality of concrete placement and compaction directly affects the interface condition and should be controlled through proper construction practices, including adequate vibration and controlled pumping rates.
Surface preparation of the steel tube inner wall, such as roughening or the use of mechanical interlocks, can reduce the contact thermal resistance and improve the thermal coupling between the two materials.

Study Insights and Reflections

This research underscores an often-overlooked aspect of steel tube concrete design: the interface between the steel tube and concrete is not a perfect thermal and mechanical bond. In practice, the quality of this interface depends heavily on construction workmanship, including the cleanliness of the steel tube interior, the workability of the concrete mix, and the compaction method used. Engineers should recognize that the contact thermal resistance is a variable that can be optimized through proper design and construction practices, and its neglect in thermal analysis can lead to unsafe design assumptions. The findings from this study provide a valuable empirical basis for more realistic fire performance predictions of steel tube concrete members.