The influence of natural ageing on selected physical and mechanical properties of Norway spruce (Picea abies) and silver fir (Abies alba) structural timber was investigated. The wood was obtained from 10 buildings and dendrochronologically dated. The with ages were 95 to 391 years after felling. The chemical properties of the wood were analysed by FT-IR spectroscopy and the structural and histochemical properties by light and scanning electron microscopy (SEM), supported by energy dispersive X-ray spectroscopy (EDX). Colour was determined visually and instrumentally according to the CIELab standard. Physical properties included wood density, cell wall density, water vapour diffusivity, sorption properties, shrinkage and dimensional stability of the wood. The mechanical properties were determined statically using standard techniques and dynamically using ultrasound and vibration resonance method. The rheological properties of the wood were determined by static creep and mechanosorption tests. For aged wood, we confirmed a lower cell wall density compared to recently felled wood, a darker colour with a confirmed overall colour change (⠆E*), whereby the colour lightness (L*) decreased significantly with the age of the wood. An increase in colour chromaticity (+a*, +b*) was observed in the earlywood and a decrease in the yellow hue (-b*) in the latewood. The relative proportion of hemicelluloses (RDH) decreased with wood ageing, while the degree of cellulose crystallinity (IK) increased and the O/C-ratio in the late wood of the spruce also increased. Local delaminations and radial microcracks were found in the cell walls of the aged wood. We found that aged wood of both species exhibited lower equilibrium moisture content (EMC) and hygroscopicity (Δs = -0.04 %/%) and lower cell wall saturation moisture content (FSP), while dimensional stability and diffusivity remained the same. Parallel to the grain, higher sound and ultrasonic velocities, lower damping, higher specific dynamic and static stiffness as well as higher tensile and specific compressive strength and hardness of the wood were confirmed in the aged wood. Bending strength and impact toughness are unchanged in aged wood. In the transverse plane, the aged wood exhibited a lower specific normal and shear stiffness and a lower specific hardness, tensile and specific shear strength only in the tangential direction. Relative creep and mechanosorptive deformations were lower in aged wood than in recent wood.
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