*Architectural material – Wood 2
Student will learn what types of wood there are, their application in construction, mistakes that occur with wood, modern wood materials, and types of wood protection.
Students look at the image and learn about different uses of wood in construction
Photos from:
ДРВО И ПАПИР | Техника и технологија (wordpress.com)
https://www.naturalstoneandtimber.co.uk/news/canadian-oak-sleepers-buyer-guide
https://www.savokusic.com/blog/drvo/drvo-i-njegove-osobine
Students read the text and got the basic info about wood in its internal structure
Wooden construction
The wood in its internal structure on the cross-section forms concentric circles around the center of the cross-section, around the heart of the tree. Each of the rings is formed by one year’s growth of the tree and is called a year of the tree. On the cross-section of a tree that has been properly grown, the following can be distinguished: the core of the tree, the pith, the part of the tree where the growth process has already taken place, the sapwood, the part where the growth process of the tree is in progress. Wood tissue, according to the function it performs, occurs in two basic forms, such as vessels – conducting tissue, composed of thin-walled cells and large cavities, which extend the entire length of the tree. These are cells with thicker walls and smaller cavities, through which juices and water hardly pass. Wood consists of substances based on cellulose and lignin. Resin, starch, tannin, etc. are found in the juices and water that soak the cavities of the vessels.
A cut made normal to the axis of the tree is called a transverse surface.
A section made on the axis and one of the diameters of the tree is called a radial surface.
A section made parallel to the axis of the tree is called a tangential surface.
Students look at the presentation and get the basic info about various types of trees that are used in construction. Physical properties of wood.
Physical properties of wood
The appearance of the section and the color are the main characteristics for determining the type of wood. The value and purpose of the wood depend on the texture and color. The texture is different in transverse, longitudinal, and radial sections, so all three are used for the assessment of wood.
Volumetric mass depends on the structure, humidity, and specific mass of the wood. The specific mass of wood is about 1500 kg/m3. Due to high porosity, and volume the mass of dry wood is less than 1000 kg/m3, which is why it floats in water. Increasing humidity also increases the volume of wood.
Moisture is the most important property of wood because it changes with the amount of moisture changing the mechanical and most physical properties as well as the durability of the wood. Quantity moisture content in freshly cut wood can vary widely depending on of the species, i.e. wood porosity, and usually ranges from 40-200% in conifers and 35- 130% in deciduous tree species. By drying in natural conditions in the air, the humidity of the wood is reduced to about 20%, and the wood is artificially dried in the drying room contains 8-12% moisture. 15% is taken as standard humidity and wood properties are determined for such a level of humidity. Due to the loss of moisture, the dimensions are reduced volume, i.e. shrinkage of the wood occurs. Due to the inhomogeneous structure, shrinkage of the wood is not equal in all directions: linear shrinkage in the direction of the fibers amounts to 0.1-0.3%, in the radial direction 3-6%, and in the tangential direction 7-12%. Because of this uneven shrinkage can lead to various deformations. If the tree dries too quickly these volume changes can cause cracks to form. If dried wood is found in a humid environment, due to the hygroscopicity of cellulose and other ingredients, there is a re-increase in humidity and an increase in volume – swelling. The swelling of the wood is also uneven in various directions. Frequent and alternating changes in humidity lead to inevitable deformations of wood parts of the structure. Wood with a moisture content above 15% is susceptible to rotting, due to the effect of microorganisms that reproduce in wood in such conditions.
The standard distinguishes the following wood moisture levels:
- Watered wood (cavities in the wood are completely filled with water)
- Raw wood (moisture higher than fiber saturation point)
- Semi-dry wood (does not contain free water, humidity about 30%)
- Dried wood (moisture is at most 22%)
- Dried wood (moisture is 6-12%)
- Dry wood (humidity is around 0%)
Wood conducts heat poorly and has a low coefficient of thermal expansion so its thermal properties are good. The coefficient of thermal expansion in temperature in the area from – 50 to +50°C is 2.5 – 11‧ 10-6 in the fiber direction, and from 1.6 – 3.5‧ 10-5 in the radial direction. The coefficient of heat conduction mostly depends on of wood porosity and humidity and at 20°S in the direction of the fibers ranges from 0.1908 – 0.2844 W/m°C. In the radial direction, this coefficient amounts to 0.1044 – 0.1512 W/m°C. With increasing humidity, the heat conduction of wood increases significantly.
The acoustic properties of wood are also very good: the speed of sound conduction in wood in the fiber direction is 10-16 times higher than in air, and in the radial direction about 4 times larger. The speed of sound propagation depends on the type, density, humidity, and temperature tree.
Students do the quiz and check their learning achievements
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