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Class 5 wood: not durable (CEN EN)

Wood grows ever during the lifetime in length and in thickness. In tropical regions the growth is continuous whereas in other areas the growth is discontuously leaving year markings. Wood is the hard, tough substance that forms the trunk of trees. Wood has been used for thousands of years as fuel and construction material. The markings, called grain, found on all types of wood, are caused by the structure of wood. Wood consists essentially of fine cellular tubes, which carry water and dissolved minerals from the roots to the leaves and which are arranged more or less vertically within the trunk. This usually forms straight-grained lumber. Many types of wood have prominent growth rings. Only a thin layer entirely surrounding the trunk grows, this called the cambium. In most trees, the wood formed early in season is lighter of colour than wood growing later in the year. New concentric sheaths are formed around the trunk of a tree each year, the year markings. As a tree grows older the central portion of the trunk, called heartwood, dies completely. Knots are areas of the trunk in which the base of a branch is embedded. When the wood is sawed into planks, knots become clear as somewhat circular discontinuities or irregularities in the grain structure. Knots are generally undesirable in lumber from the standpoint of appearance and their negative influence on the strength of the wood. The principle physical properties of wood are strength, hardness, stiffness and density. Dense types of wood are usually hard and strong. The term strength covers a number of different properties. Strength varies greatly with seasoning and with the direction of the grain; wood is much stronger when cut along the grain than when cut across it. Toughness is a measure of strength against sudden, repeated stress. Wood is naturally very durable. If not attacked by living things, it can last for hundreds or even for thousands of years. The most important threat for wood are fungi that cause so-called dry rot. The heartwood of a few species is naturally resistant to these fungi. Other types of natural resistance to various of other types of attack, have been discovered in other species. These types are usually very aromatic. It is suspected that they are protected by the resins and other chemicals they contain. Wood may need to be preserved by protecting it chemically against deterioration.


Idemat 2003