The effect of relative humidity and moisture to the durability of spruce and laminated timber

Wood is a hygroscopic material. Material properties are affected by the hygro-scopicity of wood; for example, the strength value decreases with increasing mois-ture content of wood. Wood in the living tree generally has a moisture content (MC) of 30% or greater. The cell wall is fully saturated. After the tree is felled, the green wood is exposed to atmospheric conditions. It loses water until it comes to an adequately low moisture content to be at equilibrium with the surrounding atmosphere. This is called equilibrium moisture content (EMC) which is approxi-mately proportional to the relative atmospheric humidity (RH). Also, EMC varies with the kind and distribution of cell-wall constituents, different wood species, af-fected by temperature, between heartwood and sapwood, extractives, previous ex-posure history and mechanical stress. The EMC decreases with decreasing relative humidity, also, increases with the increasing relative humidity of the surrounding air at a constant temperature. The important point that the EMC at a given rel-ative humidity is not constant. It is increased or decreased to reach equilibrium depends on the level of moisture in the timber. The paper presents experimentally and theoretically approach to the effect of relative humidity and moisture to the durability of massive spruce and spruce laminated timber during to drying and wetting exposure.

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