Fire Resistance of One-Sided, Surface-Charred Silver Fir and European Ash Timber

The aim of this work was to investigate the fire resistance of silver fir (Abies alba L.) and European ash (Fraxinus excelsior) boards charred using the traditional yakisugi method and to compare the results with the fire resistance of non-charred boards as a reference and exploit its potential as a material with fire protection properties. After the boards were surface-charred on one side, specimens with different char thicknesses, resulting from their different position in the chimney, were selected from each wood species and subjected to analysis. Specimens with dimensions of 250 × 90 mm underwent a small flame test, those of 220 × 170 mm received indirect flame exposure by constant heat flux radiation from an infra-red emitter and those of 600 × 600 mm were subjected to a fire resistance test according to EN 1363-1:2020. The results of the small flame tests showed statistically significant fire resistance enhancement of specimens with 6 and 3 mm char-layer thickness in fir and ash wood, respectively, and a 110% and 75% improvement when compared to reference specimens. The constant heat flux radiation tests did not reveal any significant differences between the reference and charred specimens. The up-scaled fire resistance test, in which an assembled panel was exposed to flame, also indicated significant improvement. The reference burn-through time of fir and ash specimens was improved significantly with increasing char layer thickness, resulting in 10%–26% of fire resistance improvement for fir and 5%–12% for ash wood specimens. These results, based on the tests performed, suggest that the one-sided surface-charring of wood can enhance its fire resistance; however, this was mostly achieved in boards with the thickest char layer in both wood species studied and not all fire resistance indicators were considered. Further in-depth studies are required to better understand the complex behaviour of charred wood in response to fire.