Charring rate and temperature distribution of timber in fire: The role of fire-retardant treatments, moisture, and tree species

Abstract

The fire performance of timber structures is a significant concern in both the preservation of ancient buildings and the broader application of timber in construction. The charring rate, a key indicator of fire performance, varies with wood species, environmental conditions, and treatment methods. This study used ISO834 standard fire tests to examine the fire performance of three commonly used wood species in the China’s market: Douglas fir (Pseudotsuga menziesii), China fir (Cunninghamia lanceolata), and larch (Larix gemelinii). The research evaluated the effects of different cross-sectional dimensions (100, 140, and 180 mm), density (300–700 kg/m³), moisture content (8%–24%), and fire-retardant treatments on the fire performance of timber. The applied phosphorus-nitrogen-based fire-retardant treatments involved brushed coatings and impregnation. Results show significant differences in the charring rates among three species. The average charring rates of Douglas fir (0.66 mm/min) and China fir (0.64 mm/min) are similar, while larch (0.47 mm/min) has the lowest rate. Within the scope of this study, density shows a negative correlation ($r=-0.42$) with the charring rate, while moisture content shows no correlation. Although fire-retardant coatings do not significantly affect the overall charring rate, they help to reduce internal temperatures in the early stages of fire exposure.