Introduction

Fire resistive materials (FRMs) are currently generally evaluated using the ASTM standard test method E119.¹ This test provides a time "rating" for which the FRM will adequately protect a specific element or subsystem of a structure. Two of the major criteria determining the performance of a FRM are the measured average and maximum temperatures of a series of thermocouples placed on the (steel) substrate. While useful as practical failure criteria, these data alone provide little insight into the key thermal properties of the FRM that would allow a better understanding of its performance. The thermal performance of the FRM is controlled by its heat capacity, density, thermal conductivity, and any heat released, absorbed, or transported due to chemical reactions (dehydrations, etc.) and phase changes. ² The goal of this paper is to present an experimental setup that maintains the "spirit" of the ASTM E119 test setup while providing detailed data on the fundamental thermophysical properties and thermal performance of the FRM.

The key components extracted from the ASTM E119 testing are that a steel substrate is protected by a specific thickness of FRM material and is exposed to a controlled temperature-time environment in a furnace. The test specimen size is reduced from the ASTM E119 testing to a square 152 mm by 152 mm specimen that is nominally 25 mm in thickness. Thermocouples are placed in the steel substrate and also at the "exposed" surfaces of the FRMs to monitor dynamically the temperature gradient that exists across the sample. Additionally, by using a "sandwich" specimen configuration and a central stainless steel plate of known mass and thermal properties, the heat flow through the FRM specimens can be easily estimated from a simple energy balance, taking advantage of the adiabatic boundary condition that exists at the central axis of the steel plate slug. Knowing the heat flow and the temperature gradient, an "effective" thermal conductivity for the FRM as a function of temperature can be determined. This effective thermal conductivity will be influenced by the true thermal conductivity of the material, any endothermic or exothermic reactions occurring in the FRM, and any additional energy/mass transport due to vaporization of water (steam) and other reaction products formed from dehydration, decarbonation, etc. of the FRM material. The influence of these reactions can be conveniently explored by exposing the sandwich specimen to multiple heating/cooling cycles, as the reactions will likely be present only during the first heating cycle. This approach is presented in more detail in the sections that follow. A somewhat similar approach for determining an effective thermal conductivity of intumescent coatings using a cone calorimeter and numerical analysis has been presented recently.³