Exploring the Influence of Thin Intumescent Coatings on the Onset and Rate of Charring for Mass Timber During Fire

This experimental fire study investigates the application of two thin intumescent coatings on mass timber (CLT), to assess their capacity as a means of passive fire protection to delay the onset of charring and reduce the charring rate of timber. Coated and exposed timber samples were subjected to a heat flux of 50 kW/m² for 60 min. The bare timber samples experienced rapid ignition and flaming upon exposure to heat, resulting in char fall-off during testing, while the surface of the coated timber samples intumesced rapidly, developing a thick, insulating layer. In addition, sporadic cracking was noted on the surface of the intumesced coating – generally along the edges of the samples and between timber boards – caused by separation of the timber surface underneath due to charring and regression. Where cracking was more pronounced for the coated timber samples, localised flaming was witnessed of the timber underneath. The experimental findings demonstrated that thin intumescent coatings can effectively delay, but not completely prevent, the onset of timber charring (assumed in this study to occur when timber reaches 300 °C). Outcomes of this study also showed that the rate of charring was reduced to about half for timber coated with thin intumescent coatings. It is worth mentioning that the change in the (1) onset of charring and (2) rate of charring is specific to the timber samples and test conditions chosen for these experiments; more specifically, the thickness of the applied coating and severity of the heating condition (i.e., incident radiant heat flux).