II. Answer the following questions. 1. How is timber classified ?
1. How is timber classified ?
2. What timber is used for structural purposes?
3. What timber is used where resistance to abrasion is required?
4. How does a tree grow?
5. What is the structure of the wood?
6 .Is the structure of the wood constant or different in different types of trees?
MATERIAL FOR HOME READING.
Heat Insulation and fire resistance
Wood has a thermal conductivity of about 0,96, which places it among the range of moderately good heat-insulating material. Its chief use as an insulator is in the form of boarding or sheathing of some kind, and it must be remembered that if the full value of its insulating properties is to be obtained, there must not be air gape between individual boards. In addition to having a useful value as insulation, timber also has a low thermal capacity. .
It therefore feels warm to the touch and will heat up quickly. Although some timbers ignite and burn quite readily when they are in thin sections, a heavy bulk of timber is very difficult to burn completely. It is possible to improve the fire resistance of timber by fire impregnation and also by some types of proprietary paints.
Seasoning.
When timber is felled it contains a large amount of moisture. As this dries out the timber shrinks - though not equally in all directions. If after considerable drying, timber is placed in a damp atmosphere, it reabsorbs moisture and in doing so, -ехpands. It is, therefore, always liable to reversible moisture, movement according to the humidity of its surroundings. A less important point is that in seasoning in normal conditions the full strength of timber is developed. There are two chief methods of seasoning 1) natural seasoning, by drying under exposure to ordinary atmospheric conditions; and 2) kiln seasoning, where the wood is dried in artificially heated drying rooms, visually with humidity control by the admission of steam. A third method of seasoning is chemical seasoning. This is hardly a method of seasoning in itself but rather an adjunct to kiln seasoning.
Preservation
Timber used under unsuitable conditions is very liable to fungus attack. Often the only protection needed is a proper degree of seasoning and a proper choice of constructional method. Sometimes, however, the timber has to be used in such a way that attack is liable to occur. For such cases some form of preservative treatment is used. The following methods are in fairly соmmon use.
Treatment with Creosotes
This, if properly carried out, is probably the most effective method, though it has certain, disadvantages such as smell and difficulty in subsequent painting either of the timber or of material in close contact with it. It should be noted that for proper protection the timber must be saturated with creosote - not merely placed on the surface. Two methods are used : a factory process in which the timber is treated under pressure, and a rather less effective method of hot and cold steeping, which gives fair protection without the use of en elaborate plant, but which cannot deal with very long length of wood.
Treatment with water: Soluble chemicals such as sodium fluoride or others. These are best applied under pressure.
Treatment with solvent type preservatives- i.e. substances dissolved in avolatile solvent. The latter ultimately evaporates and leaves the toxical chemicals in the wood. They can be applied bу brush or spray.
Ordinary Paint Treatment: this will not give sufficiant protection to withstand conditions as it is not strictly a preservative, but is very widelу used for protection from conditions such as normal exterior exposure.
SHEET MATERIALS
General Properties.
Weight: Weight in itself is not likely to be of great importance in the choice of thin sheet materials, but it gives an indication of their probable performance as heat insulating material (remembering that this is roughly inversely proportional to density).
Thermal properties
Sheet materials follow the general rule that the lighter they are in weight the higher their heat insulation value is likely to be. In addition to the value of insulation obtained directly from thinness of material it is often possible to use a method of fixing that provides an air space which will add appreciably to the overall insulation of a structure. A low thermal capacity for interior lining materials is also important. Most thin sheet materials are good in this respect especially if they are not in direct contact with a heavy structure. Two points should bе watched. First, that the figures given for insulation value on dry specimens of absorbent materials will be reduced if those are used in damp situations. Second, that owing to their nature most of the thin sheet materials are fixed to some form of studding, and this is very liable to lead to differences in heat transmission at the stud positions.
Sound Insulation.
Sound insulation depends chiefly upon weight, so that the value for all thin sheet materials is small, it is worth noting that the use of a staggered stud partition gives an appreciable improvement, and also that the sand filling of the hollow plastic type raises the sound insulation value considerably.
Sound Absorptions
Unless used as panel absorbents, most of the sheet materials give low absorption values. Special acoustic types of fibre board are made and these can give a useful absorption.
Values for panel absorbents will vary considerably according to fixing, depth of air space behind, the type of decoration, and whether the air space itself is lined with an absorbent material. Panels usually give a good absorption at low frequencies.
EXERCISES
I. Find out sentences with the word "should" and define the functions of this word.
II. Find out the words with the suffixes "ly, ion, ance, ty" end name the parts of speech they form, translate both.
III. Answer the following questions:
1. Is weight in itself of great importance in sheet materials ?
2. When is the insulation value of an absorbent materiel, reduced?
3. To what are thin sheet materials usually fixed?
4. What does sound insulation depend upon?
5. Do fibre boards give useful absorption of the sound?
6. What factors affect the values for panel absorption
7. At what frequencies do panels give good sound absorption?