We investigated the structural, sorption and diffusion properties of charred spruce wood (Picea abies), which are important, among other things, for understanding the post-fire behaviour of structural timber and its further use. The 35 mm thick samples were burnt in a cone calorimeter at a constant heat flux of 50 kW/m2 in accordance with ISO 5660-1:2015. The hygroscopicity and sorption properties of the charred samples were determined by dynamic water vapour sorption (DVS) at 20 °C in several depth layers (0 mm, 2 mm, 4 mm and 20 mm). The diffusivity of water vapour through the charred surface of the samples was determined in a non-stationary manner by measuring the equilibrium half-time of the samples between dry (T = 20 °C, φ = 30 %) and humid climates (T = 20 °C, φ = 75 %). The surface of the charred samples proved to be corrugated, with a layer of charred wood (⡈ 2 to 3 mm) and an underlying layer of thermally modified wood (⡈ 1 mm). The outer layers of the charred samples were more thermally degraded and had a lower equilibrium moisture content over the entire hygroscopic range (φ = 0...90 %). The saturation of the primary sorption sites was lower in the charred area of the samples (0...4 mm; u0 = 3.6 %) than in the control wood (20 mm; u0 = 5.8 %) and the binding energy of the water molecules bound to the primary sorption sites was high (ΔG1 < –4000 J/mol). The diffusion coefficient of the charred samples was three times lower (D = 8.33×10–11 m2/s) than that of the control samples (D = 2.60×10–10 m2/s). The charred surface of the samples represents a diffusion barrier for the transport of bound water.
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