During two years of natural weathering the course of the moisture content of test samples of pine sapwood (Pinus sylvestris L.), Norway spruce (Picea abies L.), oak heartwood (Quercus robur L.) and Douglas fir heartwood (Pseudotsuga menziesii Franco.) was recorded by means of 12,000 individual gravimetric readings. The samples were untreated, treated with a weather protection varnish, or impregnated with melamine resin. The data were evaluated with regard to the number of days per year on which the moisture content of wood exceeded the limit of 25%. From the results of subsequent laboratory investigations into the adsorption of liquid water and moisture vapour desorption using the same test material, it was possible to work out a mathematical relationship (MRI = moisture induced risk index) which correlates closely with the number of days of more than 25% moisture content during the second year of natural weathering. The MRI is proposed as a parameter for the assessment of the protective effect of wood treatments which are intended to reduce moisture content and also for durability prediction of untreated wood in out of ground contact situations.
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