Constant effective thermal conductivity of intumescent coatings: Analysis of experimental results

This article presents the results of an investigation to obtain the constant effective thermal conductivities of intumescent coatings under the influence of different intumescent coating factors (type of intumescent coating, coating thickness, steel section factor, fire condition), based on the analysis of an extensive collection of fire test data. The constant effective thermal conductivity is not a fundamental property of the intumescent coating, but is a desired quantity for simplified practical fire resistance design. It is defined as the temperature-averaged value of the temperature-dependent effective thermal conductivity within the temperature range of interest for fire resistance design of steel structures. The results indicate that for each of the intumescent coating types examined, a consistent constant effective thermal conductivity exists. The constant effective thermal conductivity tends to increase with decreasing steel section factor and to decrease with increasing coating thickness. For intumescent coating–protected steel I-sections, incorporating the shadow effect gives more consistent values of constant thermal conductivity compared to those without accounting for the effect. The same constant effective thermal conductivity obtained from the ISO fire tests may be used for different fire conditions as long as the steel temperature is higher than 400 °C. The results of this research make it possible to develop a simple method to calculate temperatures of intumescent coating–protected steel sections under different fire conditions.