Generally, wood for building construction is not tested in the laboratory. It is a practice to classify them by their species and by visually examining them for defects. However, the following tests are conducted for important works:
- Tensile Strength Test
- Compressive Strength test
- Shear Strength Test
- Bending Strength Test
- Moisture Content Test
Tensile Strength Test
The specimen used for tensile strength test is of size 5 cm × 5 cm × 20 cm long. The testing is done in a wood testing machine. The load may be applied either parallel to the grains or perpendicular to the grains. The tested result is compared with the standard value.
Compressive Strength Test
For this test also the specimen size is as that of tensile test. The load is applied parallel to the grains. The specimen shall not have a slope of grain more than 1 in 20 parallel to its longitudinal edges. The test result is compared with the standard value.
Shear Strength Test
The size of the specimen is 5 cm × 5 cm × 6.25 cm and notched in a corner so as to produce a failure on 5 cm × 5 cm surface in the radial or tangential surface as desired. Shear strength is determined only parallel to the grain. Shear strength is important when the timber is used as beams or slabs.
Bending Strength Test
The Bending Test is performed on specimens of size 5 cm × 5 cm × 7.5 cm. The slope of the grain should not be more than 1 in 20 parallel to the longitudinal edges. This test is performed to determine the Modulus of Rupture and Modulus of Elasticity of timber.
Moisture Content Test
The Moisture Content Test is an important test that has to be performed for all types of works. The presence of high moisture produces shrinkage of wood work, attack by organisms, causes warping, etc. Even blistering of paints occurs if the moisture content is high.
In this test, the weight of moist wood and the weight of oven dry wood are determined. Then
10.11 INDUSTRIAL TIMBER PRODUCTS
To meet the minor needs of wood works, natural wood can be made into many other industrial products by industrial processes for building construction. These industrial-made products are also called composite boards. The common industrial timber products are:
- Plywood
- Particle board or chip board
- Hard board
- Fibre board
- Block board
- Decorative laminates
10.11.1 Plywood
Plywoods are boards which are prepared from thin layers of wood or veneers. Veneers are thin sheets of slices of wood of superior quality. The thickness of veneers varies from 0.4 to 6 mm.
To make plywood, round logs of 1.5–2.5 m lengths are first cut. They are then steamed and veneers are cut from these logs with a wide sharp knife. About three or more veneers in multiples of odd numbers are glued one over the other. Care is taken to place the direction of the grains in one sheet at right angles to the other. This arrangement is needed to provide adequate longitudinal and transverse strengths. The arranged veneers with adhesives in between are pressed under a pressure of 0.7–1.40 N/mm2 under hot or cold conditions (IS: 303–1989).
Three-ply plywood is made using only three plies. The thickness varies from 6 to 25 mm (Fig. 10.10). Multiply plywood is made using more than three plies but keeping the number of veneers used as odd in number.
Figure 10.10 Typical three-ply plywood
Particle Board or Chip Board
These boards are made of particles of wood embedded in synthetic resins, which are then pressed under heat. They are made by extrusion pressing or by pressing between parallel plates. In the extrusion pressing process, the particles are oriented in the direction perpendicular to the plane of the board, whereas in the other case the particles are oriented parallel to the plane of the board.
Particle board is heavier than solid wood. Further, it provides broad and stable panels of reasonable strength. They can be sawn just like regular wood. Particle boards are preferred for furniture works than for construction works.
10.11.3 Hard Board
Hard Board is made as per the Indian Standard (IS: 1658–1966). It is made out of wood pulp that is compressed to make sheets usually of 3-mm thickness. Its top surface is smooth and hard and the rear side is rough with patterns or cross lines.
As per the CPWD specification, they are classified as given in Table 10.2.
Table 10.2 CPWD specification of hard board
For door shutters only tempered hard board should be used.
Fibre Board
The technique adopted for the manufacture of fibre board is the combination of the process used is making particle board and hard board. In this case, wood chips are steamed to separate the fibres from each other. These fibres are blended with resin and wax. These are turned into sheets by pressing under controlled heat and pressure. The boards are available in thicknesses of 25–32 mm and as sheets of 2.44 m × 1.22 m size. They are suitable for flush doors, cabinets, etc.
Block Board
Block board is also known as batten-board or solid-core board. Batten board consists of a core solid block of thin wood sawn from natural wood. The central block is then glued between two or more outer veneers on either side (Fig. 10.11). Thicknesses are from about 19 mm upwards. They are suitable for door panels, partitions, table tops, etc.
Figure 10.11 Typical batten board and lamin board
Lamin board is similar to that of batten board with the difference being the core is made up of multiply veneers. The thickness of veneers is about 6 mm, and the total thickness of the board is about 50 mm.
Another type of block board called the metal-faced plywood is the one where the core is sandwiched between thin sheets of aluminium, steel, copper, etc. This is a rigid type of plywood (Fig. 10.12).
Figure 10.12 Metal-faced block board
10.11.6 Decorative Laminates
Laminates are the products made by bonding together two or more layers of materials. High-pressure decorative laminates are pasted on plywood sheets, which are used for large areas like cabinets, wide door shutters, tables, etc.
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