Trä utomhus : årsringarnas betydelse för sprickbildning

När trä används utomhus, spelar det någon roll för åldrandet av virket hur det har sågats i förhållande till årsringarna? Undersökningar av furu- och granytor, som exponerats utomhus under flera år, visar att den exponerade ytans förhållande till årsringarna har betydelse för sprickbildningen. Virke med stående årsringar spricker betydligt mindre än virke med liggande årsringar.Atstandard saw pattern tangential surfaces are exposed, while the star-sawingmethod gives radial surfaces. The development of cracks and changes inappearance have been investigated on radial and tangential surfaces of pine(Pinus silvestris L) and spruce (Picea abies Karst) which have been exposed foroutdoor climates for 33 months. The degradation of the surfaces has also beenstudied at the micro-level. The annual ring orientation is the most importantfactor for crack development on weathering. The type of wood and impregnationtreatment has only a marginal effect on the crack development. After 33 monthsof outdoor exposure, tangential surfaces of pine have 13 times more total cracklength per unit area than the corresponding radial surfaces. In spruce, thetotal crack length on the tangential surfaces is 6 times greater than on theradial surfaces. Tangential surfaces of both pine and spruce have a greaternumber of cracks per unit area and wider cracks than the corresponding radialsurfaces. Tangential and radial surfaces show the same colour change in thesurface as a result of weathering. On the micro-level, tangential surfaces havemore and deeper cracks than radial surfaces. The cracks on the tangentialsurfaces occur frequently in both earlywood and latewood. On radial surfaces,cracks occur primarily at the annual ring borders, but to a certain extent alsoin the earlywood. The radial cell wall of the earlywood has a large number ofpits, which are degraded at an early stage. Decomposition of the cell walltakes place on both radial and tangential surfaces. Delamination in the middlelamella is especially noticeable in the latewood on tangential surfaces