General:

The choice of a material for certain purpose is influenced by its economic value, aesthetic appearance, durability and its properties. Therefore it is essential to have the knowledge about materials for better selection to suite the intended need.

Natural materials:

Stones:

The history of mankind is supposed to have begun with the Stone Age, marked by the use of implements and weapons made of stone. Stones used for buildings are obtained from rocks. The various types of rocks from which building stones are usually derived are granite, basalt, trap, marble, slate, sandstone and limestone. Different stones have different hardness and texture, based on which the use of particular type of stone is decided. Hardness values of standard materials on Moh’s hardness scale are given in Table 3.1.

For example, a given substance is scratched by fluorite and not by calcite its hardness is between 3 and 4.

Classification of rocks:

a. Geological properties: i) Igneous ii) Sedimentary & iii) Metamorphic Rocks.

• Igneous Rocks: The slower cooling of lava that takes place underground, allows larger crystal formation. Granite is an example of this type of rock formation. Extrusive Igneous rocks form above the ground and Intrusive rocks form below the ground. Examples: Granite, Basalt, Trap, Diorite Syenite etc. Granite: It is hard, durable and available in different colours. This is highly resistant to natural forces. Basalt: This is very hard and tough. Trap: This is very hard and tough. It is used for road metal, rubble stone masonry and other foundation works.
• Sedimentary Rocks: The characteristic of sedimentary rocks is layering. Gravel, Sandstone, Lime stone,Laterite, Marble, Shale are the examples. Impure Lime Stone (Kankar): Used as road aggregate and to manufacture hydraulic lime etc., Laterite is porous, spongy structure, easily quarried in blocks. This contains high percentage of iron oxide and available in different colours. Lime stone: It contains carbonate of lime. It is easy to work. Marble: It takes nice finish. It is easy to carve.
• Metamorphic Rocks: Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.

Wood:

Wood has become indispensible material in human life today. A typical cross sectional view of wood as obtained naturally is shown below.

Teak, Rose, Honni, Matti , Nandi , Deodar , Neem , Rubber wood , Acacia , Jackfruit-wood, Bamboo, Arcanut, are generally used for building construction.

Teak wood: There are three types of teak wood which are generally in use. They are mainly from India, Burma and Malaysia. Teak is hard and used for making doors, windows, ventilators frame and shutters. It is also used for furniture, gymnasium floorings, platform, etc., Burma and Malaysian teak are softer than Indian teak.

Rose wood: Seasoned Rose wood is Purple in colour. Rose wood is very hard and costly. This is used for decorative items like sofa, cot, chairs, tables, ward robes, furniture of home and offices, etc. It takes nice carving and polish.

Honne wood: This is costlier compared to Matti and Nandi woods. This wood is widely used for making doors and windows. The texture and colour resembles that of teak wood. When this wood comes in contact with water it leaves red colour. So this property can be used as confirmatory test for Honne wood.

Matti wood: Matti is widely used for making doors and windows of buildings. This is available in white and black colour. Black matti resembles rose wood but very hard compared to rose wood.

Nandi woods: Nandi wood is light and pink in colour. Nandi is widely used for making doors and window frames of shutters of buildings. This wood decays early, when it comes to contact with water. Yearly maintenance is required for this wood. This can be protected by painting.Other Jungle woods like tamarind, Neelgiri (Eucalyptus) etc., are used for centering and form-work purpose. The Circumference of a tree/log is called the Girth. The average girth is measured at the centre of the log (as the long timber log has many times an uneven surface all along). Knowing the length and girth of the log quantity of wood can be calculated using the formula given below.Qty = (L x G2) / 2304 =____cft. (Where L – length in ft. G=Girth (Circumference) in inches).

Defects in Timber:

Knots: These weaken the timber under compression and seen with hard dark pieces on the surface of wood. Less than 6 mm dia is called -nail knot, 6-20 mm as small knot, 20-40mm as medium knot, >40mm as large knots. If securely joined with wood then they are “tight knots, otherwise loose knots. Wood with large and loose knots should be avoided.

Shakes: These are cracks and splits in felled log. They may be of cup shape, heart shape or circumferential shrinkage. A few cracks in woods are shown below.

Twisted grain or fiber: Twisting of tree occurs in wood, due to high wind. Due to this grains will be twisted. A few views of twisted grains are as shown below.

For cold-worked bars (Fig. 2), the stress is proportional to the strain up to a stress of 0.8 fy. Thereafter, the inelastic curve is defined as given below:

Upset or rupture: Discontinuity of fibre due to strong wind or felling of tree occours during its growth itself.

Wane: It is original outside rounded part and important when timber is used for important uses.

Presence of Sap wood: It is outer layer of wood and has colour lighter than heart wood. This does not take good polish, compared to heart wood. Sloping grains: Simple sloping grains are not large defects of wood. But if sloping spiral pattern grains around the axis are called defects. Cracks (Fissures, resin pockets): These are fiber disruptions and affect the strength of wood. Resin pockets with resin are defects. A long narrow crack is called streak. Gutters or channels: Actually these are not the defects as far as quality of wood is concerned. Log with larger and longer channel will produce less quantity of wood after it is cut.

Seasoned wood should always be used for constructions.