Quantifying fire performance and minimum char thickness of pre-charred wood : effect of density and moisture content

Abstract

Wood is experiencing renewed interest in sustainable construction due to its low carbon footprint, yet its high flammability remains a major obstacle to widespread use. Pre-charring has been proposed as a method to enhance durability by protecting inner wood from moisture and biological degradation, but its effects on fire performance have not been fully quantified. In this study, a uniform pre-charred layer was formed on three wood species of varying densities: fir (300 kg/m3), beech (650 kg/m3), and merbau (900 kg/m3), under a radiative heat flux of 20 kW/m2. The charring rate was found to be inversely proportional to wood density. A minimum effective char-layer thickness was identified, beyond which piloted ignition delay time significantly increased with the increasing char-layer thickness under 40 and 50 kW/m2. As the wood density increases from 300 kg/m3 to 900 kg/m3, this minimum thickness of the effective pre-charred layer increased from 2.6 mm to 6.0 mm, which was comparable to the thermal penetration depth of the specified wood. A one-dimensional computational model was also developed to investigate the fire performance of pre-charred wood and the degradation of hemicellulose, cellulose, and lignin using 5-step chemical kinetic mechanism, accurately reproducing mass-loss evolution and predicting the influence of moisture content. These findings offer new insights into the fire performance of pre-charred wood and support its potential as a fire-safe, sustainable material in building applications.