Introduction
As an engineer, one must know about the materials used in the construction site. All structures are constructed of materials known as engineering materials or building materials. It is necessary for an engineer to be conversant with the properties of such materials.
The service conditions of buildings demand a wide range of materials with specific properties. Hence the properties of the materials are to be studied properly to select suitable building materials. In this section and in the subsequent sections, the properties and uses of some building materials, such as bricks, stones, cement, concrete and steel are discussed.
The common brick is one of the oldest building materials and it is extensively used at present because of its durability, strength, reliability, low cost, etc. Bricks are obtained by moulding clay in rectangular blocks of uniform size, then by drying and burning these blocks in brick kilns.Qualities of Good Bricks
1. Bricks should have perfect edges, well-burnt in kilns, copper coloured, free from cracks with proper rectangular shape and of standard size (23x 11x 9 cm).
2. Bricks should give a clear ringing sound when struck with each other.
3. Bricks must be homogeneous and free from voids.
4. The percentage absorption of water by weight should not be greater than 20 per cent for first-class bricks and 22 per cent for second-class bricks when soaked in cold water for 24 hours.
5. Bricks should be sufficiently hard, i.e., no nail impression must be present when scratched. The average weight of bricks should be 3-3.5 kg.
6. Bricks should not break when dropped from a height of 1 m.
7. Bricks should have low thermal conductivity and should be soundproof.
8. Bricks should not show deposits of salts when immersed in water and dried.
9. The minimum crushing strength of bricks must be 3.5 N/mm².Classification of Bricks
Bricks are classified based on the manufacturing process adopted. The classification is given as follows:
1. First-class bricks are table-moulded and of standard shape. These comply with all good qualities of bricks and are used for superior and permanent works.
2. Second-class bricks are ground-moulded and burnt in kilns. The surfaces of such bricks are rough and are slightly irregular in shape. Such bricks are used with a coat of plaster.
3. Third-class bricks are ground-moulded and are burnt in clamps. These bricks are not hard but rough with irregular and distorted edges. These give a dull sound when struck with each other. They are used for unimportant and temporarystructures and at places where there is less rainfall.
4. Overburnt bricks with irregular shape and dark colour are classified as the fourth class bricks. These are used as aggregates for concrete in foundations, floors, roads, etc.Uses of Bricks
1. Bricks are mainly used for the construction of walls.
2. Bricks when moulded in the shape of a gutter can be used as drains.
3. Bricks with cavities known as hollow bricks can be used for insulation purposes and because of their light weight they are more useful in speedy constructions.
4. Paving bricks prepared from clay containing higher percentage of iron can be used for pavements, since they resist abrasion in a better way.
5. Bricks with holes are used in multi-storied framed structures.
6. Fire bricks made of fire clay can be used as a refractory material.
7. Sand-lime bricks are used for ornamental work.
8. Bricks are used in the construction of compound walls, columns, etc. Broken pieces of bricks are used as aggregates in concrete.
9. Bricks of superior quality can be used in the facing of a wall.
10. Bricks are used in the construction of chimneys and other special works.Constituents of a Brick
1. Alumina It is the chief constituent of clay. A good brick should have 20-30 per cent of alumina. This imparts plasticity to the earth.
2. Silica It exists in clay in a free or combined form. A good brick earth should contain about 50-60 per cent of silica. The presence of silica prevents cracking, shrinking and warping of raw bricks. It imparts uniform shape to bricks. The durability depends on proper proportion of silica.
3. Lime Up to 5 per cent of lime is desirable in good brick earth. It prevents shrinkage in raw bricks. Sand alone is infusible, but it fuses at kiln temperature due to the presence of lime. Bricks may melt and lose their shape due to excess of lime content.
4. Oxide of iron This gives the red colour to bricks. A small quantity of iron oxide up to 5 or 6 per cent is desirable.
5. Magnesia This imparts yellow tints to bricks and it reduces shrinkage.
Advantages of using bricks
The following are the advantages of bricks over other construction materials, like stone, concrete, etc.
(a) Bricks are cheaper and easy to handle.(b) They are of standard size and hence easy to have proper bonding.(c) Consumes less mortar when compared to stone masonry.(d) Labour required for brick masonry is less.(e) Brick walls can be raised to a larger height, when compared to stone masonry.(f) Because of regular size the surface of wall will be plane and given a neat appearance.(g) Brick masonry consumes, less mortar for plastering.(h) Easy to drill holes for fixing service connection line.(i) Bricks have low thermal conductivity and high sound insulation properties.(j) They possess very high resistance to fire.(k) They are non-combustible and non-inflammable.
Disadvantages of using bricks
(a) The compressive strength of brick is less compared to stone and concrete.(b) Water absorption is more than that of stone or concrete.(c) Only a selected variety of clay can be used for manufacture of bricks.(d) Kilns are required to be constructed for manufacturing bricks.(e) It has got a very low tensile strength compared to other building materials.Tests on Bricks
The following are the field tests by judgment for assessing the quality of bricks.
Field tests
1. The bricks should be truly rectangular in shape with sharp edges and plane faces and of the same size.2. They should be hard and well burnt and should give a metallic ringing sound when struck with a steel rod.3. They should be of uniform red colour and of fine texture.4. When the bricks are dropped on the ground from one metre height, they should not crack or break.5. They should be free from cracks, fissures, pebbles or nodules of free lime.
Lab tests
1. Test for water absorptions
(a) 3 samples of clean well dried bricks are taken and their dry weight is found out individually.(b) The bricks are then immersed in water for 24 hours.(c) After 24 hours, the bricks are taken out, surface dried and weighed in a balance and wet weight found out.(d) If the wet weight of each bricks is W₂, the percentage water absorption of each brick= W₂ - W₁ / W₁ x 100(e) The average percentage of water absorption of three samples is the water absorption of the bricks.
Required standard The average absorption should not be greater than 20%. Too much of water absorption indicates under-burnt condition and poor strength.
2. Test for efflorescence (For the presence of salt)
Salts like sulphates of calcium, magnesium, sodium and potassium present in the brick will cause efflorescence on the brick surface, when they get dissolved in water. Bricks containing too much of salt are less resistant to weathering and will have poor strength.
1. Three samples of bricks are immersed in good water for 24 hours.2. After 24 hours, the bricks are taken out and examined for white patches of salt on the surfaces.3. If the white patches of salt present are heavy, the bricks are poor and are to be rejected.4. If the white patches present are small to medium, the bricks can be accepted.
3. Test for compressive strength
The load carrying capacity of bricks is increased, as the compressive strength increases.
(a) Three samples of bricks are taken and immersed in good water for 24 hours.(b) After 24 hours of immersion, the bricks are taken out and surface dried.(c) Each brick is placed on the compression testing machine and the load on the brick is gradually increased until the brick fails. The failure load of each brick is found out.(d) Average failure load of the 3 bricks is the compressive strength of the bricks.
Requirement standards1. Country Bricks → 35 to 50 kg/cm²2. II Class bricks → 50 to 75 kg/cm²3. I Class bricks → 75 to 125 kg/cm²Manufacture of Bricks
The following are the four processes involved in the manufacture of bricks.
1. Preparation of brick earth2. Moulding of bricks3. Drying of bricks4. Burning of bricks
1. Preparation of Brick Earth
Preparation of brick earth involves the following operations.
(i) Removal of loose soil(ii) Digging, Spreading and Cleaning(iii) Weathering(iv) Blending(v) Tempering
(i) Removal of loose soil The top layer of the loose soil about 20 cm depth contains lot of impurities and hence it should be taken out and thrown away.
(ii) Digging, spreading and cleaning The earth is then dug out from the ground. This earth is spread into heaps about 60 cm to 120 cm height. All the undesirable matters like stones, vegetable matter, etc. are removed. Lumps of clay should be converted into powder form.
(iii) Weathering The earth is then exposed to atmosphere for softening. The period of exposure varies from weeks to full season.
(iv) Blending The clay is then mixed with suitable ingredients. It is carried out by taking a small portion of clay every time and by turning it up and down in vertical direction.
(v) Tempering This is done to make the whole mass of clay homogeneous and plastic. Required water is added to clay and the whole mass is kneaded under the feet of men or cattle.
When bricks are manufactured on a large scale, tempering is usually done in a pug mill. A pug mill consists of a conical iron tub with cover at its top. A vertical shaft with horizontal arms is provided at the centre of iron tub. Several cutting blades are attached to this horizontal arm. The clay with water is put inside the mill and the vertical shaft is rotated by bullocks or stream, diesel or electric power. Due to the action of the horizontal arms the clay is thoroughly mixed and tempered.
2. Moulding of Bricks
The tempered clay is then sent for the next operation of moulding. There are two methods of moulding.
(i) Hand moulding(ii) Machine moulding
1. Hand Moulding This is done by a mould which is a rectangular box with open at top and bottom. It may be of wood or steel.
Following are the ways of hand moulding:
(a) Ground moulding(b) Table moulding
(a) Ground moulding First a small portion of ground is cleaned and levelled. Fine sand is sprinkled over it. Moulding is started from one end of the ground. Mould is dipped in water and kept on the ground and clay is pressed by hand nicely so that all is again dipped in water and it is placed just near the previous brick to prepare another brick. Process is repeated till the ground is covered with bricks. A mark of depth about 10 mm to 20 mm is placed on raw brick by a pallet during moulding. This mark is called as frog. After the bricks become sufficiently dry, they are sent for the next process of drying.
(b) Table moulding This should be done by an experienced supervisor. The moulder stands near a table of size about 2 m x 1 m. Clay, mould water pots, stock board, strikes and pallet boards are placed on this table. Bricks are moulded on the table and sent for the next process of drying.
2. Machine moulding When bricks are manufactured in huge quantity at the same spot then moulding is done by machines. These machines contain a rectangular opening of size equal to the length and width of the brick. The Tampered clay is placed in the machine and as it comes out through the opening under pressure it is cut into strips by wire fixed in frames. Arrangement is made in such a way that strips of thickness equal to that of the brick are obtained.The machine moulded bricks have sharp edges and corners, smooth external surface and uniform texture.
3. Drying of Bricks
After the bricks are moulded they are dried. This is done on specially prepared drying yards. Bricks are stacked in the yard with 8 to 10 bricks in each row. Bricks are dried for a period of 5 to 12 days.
During drying it must be protected from wind, rain and direct sun. Sometimes, bricks, are dried artificially by hot gases from kiln. But there is change of warping of bricks in case of artificial drying. After drying, the bricks are sent for the next operation of burning.
4. Burning of Bricks
Burning imparts hardness and strength to bricks and makes them dense and durable. It must be done carefully and properly because underburnt bricks remain soft and hence cannot carry loads and overburnt bricks become brittle and hence, break easily. Burning of bricks is done either in clamp or in kilns.
Er Ashish Shah
Making A Difference
concept.shah@gmail.com
