Reference: D.P. Bentz, to be published in the Proceedings of the 28th International Thermal Conductivity Conference, New Brunswick, Canada (June 2005).

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Measurement and Microstructure-Based Modeling of the Thermal Conductivity of Fire Resistive Materials

Dale P. Bentz
Building and Fire Research Laboratory
National Institute of Standards and Technology
100 Bureau Drive, Stop 8615
Gaithersburg, MD 20899 USA



The thermal conductivity of fire resistive materials (FRMs) is one critical performance metric characterizing their ability to protect structural steel during a fire. Accurate values for this property are sorely needed for modeling the structural/fire performance of buildings and components, for example. Measurement of thermal conductivity for these materials is thus necessary over a large temperature range, from room temperature to beyond 1000 ºC. While high temperature measurements of thermal conductivity are difficult for any material, for FRMs, there are further complications due to the endothermic and exothermic reactions often occurring in the materials, possible mass transfer and subsequent condensation of steam and hot gases (generated from these reactions) through the materials, and radiation heat transfer at high temperatures. This paper will present a two-part approach to understanding the effective thermal conductivity of FRMs. First, a simple measurement technique based on the utilization of a steel plate slug calorimeter sandwich specimen is presented. This technique permits the assessment of effective thermal conductivities over a temperature range of 30 ºC to about 700 ºC. By using multiple heating and cooling cycles, information on the influence of chemical reactions, phase changes, and mass transfer of reaction gases is also obtained. Second, three-dimensional x-ray microtomography is utilized to obtain microstructural information, such as porosity and individual pore sizes, for the FRMs. The three-dimensional images of the FRM microstructures can then be input into a finite-difference computer program for directly computing the thermal conductivity of the FRM. To apply this microstructure-based modeling, accurate values for the thermal conductivities of the "solid" and "porous" regions of the microstructure are required. As an alternative to this detailed microstructure characterization and computation, previously developed theories for the thermal conductivity of porous materials as a function of specific gravity (porosity), pore size, and temperature can be applied. Application of these experimental and computational techniques to a variety of FRMs will be presented.

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