Stem or branch failure is a recurrent problem in silviculture and arboriculture. The risk of rupture varies with species in relation to the inherent mechanical properties of the species and the presence of defects. In general, calculations of critical loads for breakage are based on mechanical properties determined from defect-free samples and adjustment factors that try to scale up to full trees that include defects. This study aims at developing an objective method to scale up mechanical resistance to breakage from defect-free samples to full trees, including different types of defects. It combines two approaches. In the first one, a correction factor is determined from a meta-analysis of various tree-pulling studies involving balsam fir [Abies balsamea (L.) Mill.], white spruce [Picea glauca (Moench) Voss], jack pine [Pinus banksiana Lamb.], and black spruce [Picea mariana (Mill.) B.S.P.]. The second approach consists in obtaining empirical data from threepoint bending tests using 8-foot (2.44 m) logs with various amounts of decay. Results show that the correction required varies according to the species and the presence of some defects. For balsam fir, which was the species showing the most important difference between whole log and small sample values, differences in correction factors were found between tree-pulling tests and three-point bending tests. Data from winching tests tend to underestimate the stem's resistance to breakage since they likely represent the weakest trees among those tested. No relationship was found between the adjustment factors and different indices used in arboriculture to account for decay, showing the complexity of mechanical resistance at the stem level.
Severe windthrows often require salvage operations that can lead to increased costs. Given these extra costs, it is of paramount importance to make sure that wood degradation does not become so advanced that significant value loss is incurred. The rate at which wood deteriorates is a function of many factors, including species and climate. The study was conducted in a northern area affected by two partial windthrows. Logs from the damaged area were collected for two species, balsam fir (Abies balsamea) and black spruce (Picea mariana). Logs were classified into one of three degradation classes based on visual assessments. A sample of logs from standing trees was also collected. In total, 167 logs were sampled. Each log was sawn and one piece of lumber was selected from each to determine the bending strength and stiffness and the visual grade. The time since tree death, as determined from dendrochronology, ranged from 1 to 31 years. The visual grade of the lumber was not affected after 1 year but severe downgrades were observed after 4 years. Moisture content decreased rapidly for both species during the first year and continued to decrease until 4 years after mortality. No clear decrease in bending stiffness was identified even though such a tendency was noticed for older black spruce windthrows. Bending strength became variable after 4 years for balsam fir and was reduced after 4 years for black spruce. Windthrows older than 7 years will produce low visual grade timber of reduced bending strength and possibly of lower bending stiffness.
In order to determine the feasibility of manufacturing wood-gypsum and wood-gypsum-cement particleboards, the hydration reactions of four Eastern Canadian wood species with gypsum and Portland cement were studied. The compatibility of hot water extracted and raw wood particles with both gypsum and a gypsum-Portland cement mixture were determined for jack pine, balsam fi r, aspen and white birch by isothermal calorimetry. The four species were found to be compatible and suitable for the manufacturing of inorganic-bonded wood composite boards on the basis of the compatibility factor. The hot water extractive content was low in the four species and did not have a consistent impact on the compatibility factor. Nevertheless, hot water soluble extractives did have an impact on the time required to reach the maximum heat fl ux, delaying it to undesirable levels particularly in the case of balsam fi r mixed with neat gypsum. However, the effect of balsam fi r on neat gypsum hydration was markedly reduced by the addition of 30% Portland cement. Hot water extraction of the wood particles and the addition of Portland cement to the wood-gypsum mixture signifi cantly reduced the time required to reach the maximum heat fl ux. The gypsum-cement mixture was less sensitive to the inhibitory effect of wood than neat gypsum.Keywords: Wood compatibility, hot-water soluble extractives, isothermal calorimetry, hydration, gypsum, Portland cement. RESUMENLa viabilidad de cuatro especies de madera del este de Canadá para la elaboración de tableros de partículas aglomeradas mezcladas con yeso y con una mezcla de yeso-cemento fue estudiada por calorimetría isotérmica siguiendo las reacciones de hidratación de las mezclas yeso-madera y yesocemento-madera. La compatibilidad fue determinada sobre dos tipos de partículas de madera, unas sometidas a un pre-tratamiento de extracción con agua caliente y otras sin pre-tratamiento. Se utilizaron las siguientes especies: Pinus banksiana, Abies balsamea, Populus tremuloides y Betula papyrifera. De acuerdo al factor de compatibilidad, las cuatro especies fueron consideradas compatibles con el yeso y la mezcla yeso-cemento, siendo aptas para la fabricación de tableros compuestos. El contenido de extractivos solubles en agua caliente fue bajo en las cuatro especies y no infl uyó en la determinación del factor de compatibilidad. Sin embargo, los extractivos afectaron los tiempos de hidratación y en el caso del Abies balsamea retrasaron la hidratación del yeso a un nivel indeseable. Tanto la adición de 30% de cemento al yeso como la extracción de las partículas de madera con agua caliente redujeron Maderas. Ciencia y tecnología 110(3): [275][276][277][278][279][280][281][282][283][284][285][286][287][288] 2008 276 Universidad del Bío -Bío visiblemente el tiempo requerido para alcanzar el máximo fl ujo de calor, además de eliminar el efecto causado por el Abies balsamea en la hidratación del yeso. En general, la mezcla yeso-cemento fue menos sensible a los efectos inhibitorios de la madera que ...
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