The effect of time on the mechanical properties of wood is of interest for structural engineers, wood technologists and conservators; for the old timber structure assessment, for the potential reuse of salvaged timbers and poles and for the conservation of wooden artefacts as well. The topic was investigated since the 50's, but the results reported in literature are not always concordant. This is a consequence of the fact that this kind of research works are quite difficult, as a consequence of the material characteristics itself: mechanical properties variability, low availability of material, uncertainty about the "history" of the tested material, unknown original mechanical properties. Another source of uncertainty between the research works is a consequence of the different research approaches: some have investigated only the effect of the time passing (therefore, aging), others consider the aging effect together with other effects, like the state of conservation 2 and the duration of load. The main interest of the researchers was in the bending properties variation, while for other mechanical properties less information is available. In this paper, the results of several research works are presented and analysed regarding the differences in the mechanical properties for elements with different age levels. Moreover, recommendations for future research are included attending to the conclusions drawn from the analysed literature.

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ABSTRACT:The potential use of beech (Fagus sylvatica Linnaeus) sawn timber for structures has been investigated. Beech stands in transition from coppice to high forest after thinning interventions have been sampled from different Italian sources. A sample of 160 beams of two different cross sections was extracted and tested, according to the normalized procedures for the characterization of wood for structures. The specimens were visually graded considering the strength relevant defects, according to the Italian standard. To determine the mechanical properties of the timber (namely strength and stiffness), four-point bending tests were performed and the main characteristic values were derived. The results of the study reveal that knot ratio was the principal defect that influenced both resistance and yields. The potential strength class D30 was achieved with the resulting characteristic values, although, due to the small number of samples tested, the reached class could not be formally assigned. To improve the effectiveness of the grading rule in the Italian standard, some modifications and a new grade were proposed for the beech sawn timber. Finally the D40 strength class could be theoretically reached, but with reduced yields.

The influence of the thermal modification process on wood machinability was investigated with four minor species of low economic importance. A set of representative experimental samples was machined to the form of disks with sharp and dull tools. The resulting surface quality was visually evaluated by a team of experts according to the American standard procedure ASTM D-1666-87. The objective quantification of the surface quality was also done by means of a three dimensions (3D) surface scanner for the whole range of grain orientations. Visual assessment and 3D surface analysis showed a good agreement in terms of conclusions. The best quality of the wood surface was obtained when machining thermally modified samples. The positive effect of the material modification was apparent when cutting deodar cedar, black pine and black poplar in unfavorable conditions (i.e., against the grain). The difference was much smaller for an easy-machinability specie such as Italian alder. The use of dull tools resulted in the worst surface quality. Thermal modification has shown a very positive effect when machining with dull tools, leading to a relevant increment of the final surface smoothness.

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