Wood properties influencing surface cracking and moisture dynamics of untreated Norway spruce exposed outdoors

Abstract

Untreated wood has excellent environmental benefits due to the lack of treatments; however, its durability needs to be great enough to provide a sufficient service life to not override the environmental benefits. The aim of this study was to investigate some wood properties of untreated, unfinished Norway spruce and their influence on moisture dynamics and crack development under natural exposure. Three field-trials were carried out, all under natural exposure during various exposure times. The specimens differed in their exposure direction (north/south), composition (heartwood/ sapwood), density, and thickness. Moisture measurements were carried out either by use of sensors or weighing the specimens, while the crack formation was measured using digital calipers. Generally, high-density spruce exhibited more rapid moisture fluctuations than low-density; this agreed well with the increased crack development observed in the field-trials. More cracks were observed for specimens containing sapwood rather than heartwood. This was likely caused by an increase in moisture uptake, generating greater moisture gradients. The results also showed that the crack tendency was greater in specimens within the high-density group placed facing south, which is likely due to an increase in moisture variation, and perhaps also faster UV-deterioration. No clear correlation between crack tendency and thickness was found.