Strength reduction of spruce wood through slow freezing

Some selected mechanical properties of spruce wood (Picea abies L.) were determined after freezing green timber boards under different conditions. The influence of the freezing rate and the time of exposure to negative temperatures were evaluated by applying three different freezing conditions which may occur in winter when green timber is stored in an open yard. It was found that a high freezing rate (−10 °C/h) does not affect wood strengths at all, while slow freezing (by −1 °C/h) significantly reduces all mechanical wood properties, especially MOR, MOE and the compressive strength (by 20…30 %), the Janka hardness (by 18 %), and also the tensile strength (by 10 %). A longer time of exposure, involving repeated freezing and thawing due to natural temperature variations led to further reduction of MOR, MOE and the compressive strength (up to 37 %), but without further affecting the tensile strength, shear strength and hardness. The obtained results may be useful to industrials with respect to a more careful planning of green timber purchase and storage in wintertime.