Effect of a two-stage heat treatment process on the mechanical properties of full construction timber

Abstract

This study investigated the impact of an industrially used two-stage heat-treatment method on the mechanical properties of full construction timber. Bending tests of full-size Norway spruce posts demonstrated changes due to heat treatment, such as a decrease in the density and bending strength, as well as an increase in the modulus of elasticity (MOE). The variability in bending strength and MOE appeared to be higher for treated posts and as a result the 5% values decreased, especially for the bending strength. The MOE was dependent on density for both treated and untreated posts, but this was not so for the bending strength. There appeared to be a relationship between the bending strength and the MOE, although it was rather weak, especially for treated posts. It was difficult to correlate the occurrence of natural defects in treated Norway spruce posts with an effect on the mechanical properties. Only a combination of several defects, such as large knots, enclosed pith and a deviating slope of grain, appeared to decrease the bending strength and MOE of treated posts, at least more than for untreated posts. The effect of a three year period of outdoor exposure on the strength properties of heat-treated terrace planking was limited. In particular, for the 5% value low strength range, where wood defects strongly determine the mechanical properties of wood, a three-year outdoor exposure did not change the bending strength or MOE of heat-treated wood.