Changes in Weight and Strength of Sitka Spruce Associated with Decay By a Brown-Rot Fungus, Poria Monticola

The effects of early decay by the brown-rot fungus Poria monticola on the strength properties of Sitka spruce in static bending were studied by measuring the extent of decay as the loss in dry weight of the decayed specimen expressed as a percentage of the adjusted initial weight. One hundred test specimens, 1/4 by 1/4 by 4-1/2 inches in size, were tested in five groups each consisting of 10 test specimens and 10 end-matched controls. After six days’ exposure, there was a highly significant increase in equilibrium moisture content, which was reflected in an average weight increase of the test specimen of 0.26 percent over that of the control at equilibrium in an oven maintained at 40?C. Another factor contributing to the observed “dry” weight increase was the increased chemically combined water of constitution resulting from hydrolysis. Total weight loss averaged 2.03 percent at the end of 2 weeks. After 2 weeks, the decayed specimens attained an equilibrium moisture content of 10.96 percent under the same conditions for which undecayed control specimens reached an equilibrium moisture content of 9-83 Percent. No significant loss was found in fiber stress at proportional limit, modulus of elasticity, or work to proportional limit, but losses of 26 percent in work to maximum load and 13 percent in modulus of rupture were noted at the end of the 14-day period. This highly significant loss in strength after a loss of only about 2 percent of the dry weight indicates the importance of very small weight losses in relation to strength reduction when brown rots are involved. It also points up the danger of using too broad a durability classification when rating wood to be used in places where strength is important.

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