Modelling the moisture induced risk of decay for treated and untreated wood above ground

During two years of natural weathering the course of the moisture content o f test samples of pine sap wood (Pinus sylvestris L.), Norway spruce (Picea abies L.), oak heartwood (Quercus robur L.) and Douglas fir heartwood (Pse udotsuga menziesii France.) was recorded by means of 12,000 individual grav imetric readings. The samples were untreated, treated with a weather protec tion varnish, or impregnated with melamine resin. The data were evaluated w ith 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 deso rption using the same test material, it was possible to work out a mathemat ical relationship (MRI = moisture induced risk index) which correlates clos ely with the number of days of more than 25 % moisture content during the s econd year of natural weathering. The MRI is proposed as a parameter for th e 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.