Constituents of good brick earth: Bricks are the most commonly used construction material. Bricks are prepared by moulding clay in rectangular blocks of uniform size and then drying and burning these blocks. In order to get a good quality brick, the brick earth should contain the following constituents: 1. Silica 2. Alumina 3. Lime 4. Iron oxide 5. Magnesia Silica 1. Brick earth should contain about 50 to % of silica. 2. It is responsible for preventing cracking, shrinking and warping of raw bricks. 3. It also affects the durability of bricks. 4. If present in excess, then it destroys the cohesion between particles and the brick becomes brittle. Alumina 1. Good brick earth should contain about 20% to 30% of alumina. 2. It is responsible for plasticity characteristic of earth, which is important in moulding operation. 3. If present in excess, then the raw brick shrink and warp during drying. Lime 1. The percentage of lime should be in the range of 5% to 10% in a good brick earth. 2. It prevents shrinkage of bricks on drying. o It causes silica in clay to melt on burning and thus helps to bind it. 3. Excess of lime causes the brick to melt and brick looses its shape. Iron oxide 1. A good brick earth should contain about 5% to 7% of iron oxide. 2. It gives red color to the bricks. 3. It improves impermeability and durability. 4. It gives strength and hardness. 5. If present in excess, then the color of brick becomes dark blue or blackish. 6. If the quantity of iron oxide is comparatively less, the brick becomes yellowish in color. Magnesia 1. Good brick earth should contain less a small quantity of magnesia about 1% 2. Magnesium in brick earth imparts yellow tint to the brick. 3. It is responsible for reducing shrinkage 4. Excess of magnesia leads to the decay of bricks Harmful Ingredients in Brick: Below mentioned are some of the ingredients which are undesired in brick earth. Lime o A small quantity of lime is required in brick earth. But if present in excess, it causes the brick to melt and hence brick looses its shape. o If lime is present in the form of lumps, then it is converted into quick lime after burning. This quick lime slakes and expands in presence of moisture, causing splitting of bricks into pieces. Iron pyrites o The presence of iron pyrites in brick earth causes the brick to get crystallized and disintegrated during burning, because of the oxidation of the iron pyrites. o Pyrites decolorise the bricks. Alkalis o These are exist in the brick earth in the form of soda and potash. It acts as a flux in the kiln during burning and it causes bricks to fuse, twist and warp. Because of this, bricks are melted and they loose their shape. o The alkalis remaining in bricks will absorb moisture from the atmosphere, when bricks are used in masonry. With the passage of time, the moisture gets evaporated leaving grey or white deposits on the wall surface (known as-efflorescence). This white patch affects the appearance of the building structure. Pebbles o Pebbles in brick earth create problem during mixing operation of earth. It prevents uniform and through mixing of clay, which results in weak and porous bricks o Bricks containing pebbles will not break into shapes as per requirements. Vegetation and Organic Matter o The presence of vegetation and organic matter in brick earth assists in burning. But if such matter is not completely burnt, the bricks become porous. This is due to the fact that the gasses will be evolved during the burning of the carbonaceous matter and it will result in the formation of small pores. Manufacturing of bricks In the process of manufacturing bricks, the following distinct operations are involved: • Preparation of clay • Moulding • Drying • Burning Each of the above operation of the manufacturing bricks will now be studied at length. Preparation of clay The clay for brick is prepared in the following order: • Unsoiling • Digging • Cleaning • Weathering • Blending • Tempering Unsoiling: The top layer of the soil, about 200mm in depth, is taken out and thrown away. The clay in top soil is full of impurities and hence it is to be rejected for the purpose of preparing bricks. Digging: The clay is then dug out from the ground. It is spread on the leveled ground, just a little deeper than the general level. The height of heaps of clay is about 600mm to 1200mm. Cleaning: The clay as obtained in the process of digging should be cleaned of stones, pebbles, vegetable matters. If these particles are in excess, the clay is to be washed and screened. Such a process naturally will prove to be troublesome and expensive. Weathering: The clay is then exposed to atmosphere for softening and mellowing. The period varies from few weeks to full season. Blending: The clay is made loose and any ingredient to be added to it , is spread out at its top. The blending indicates intimate or harmonious mixing. It is carried out by taking a small amount of clay every time and turning it up and down in vertical direction. The blending makes clay fit for the next stage of tempering. Tempering: In the process of tempering, the clay is brought to a proper degree of hardness and it is made fit for the next operation of moulding .Kneaded or pressed under the feet of man or cattle .The tempering should be done exhaustively to obtain homogeneous mass of clay of uniform character.For manufacturing good bricks on a large scale, tempering is done in pug mill.A typical pug mill capable of tempering sufficient earth for a daily output of about 15000 to20000 bricks. A pug mill consists of a conical iron tub with cover at its top .It is fixed on a timber base which is made by fixing two wooden planks at right angle to each other. The bottom of tub is covered except for the hole to take out pugged earth. The diameter of pug mill at bottom is about 800mm and that at top is about 1 m.The provision is made in top cover to place clay inside pug mill .A vertical shaft with horizontal arms is provided at center of iron tub.The small wedge-shaped knives of steel are fixed at arms.The long arms are fixed at vertical shaft to attach a pair of bullocks .The ramp is provided to collect the pugged clay .The height of pug mill is about 2m. Its depth below ground is 600m to800mm lessen the rise of the barrow run and to throw out the tempered clay conveniently.In the beginning, the hole for pugged clay is closed and clay with water is placed in pug mill from the top. When vertical shaft is rotated by a pair of bullock, the clay is thoroughly mixed up by the action of horizontal arms and knives and homogeneous mass is formed. The rotation of vertical shaft can also be achieved by using steam, diesel or electrical power.When clay has been sufficiently pugged, the hole at the bottom of the tub, is opened out and pugged earth is taken out from the ramp by barrow i.e. a small cart with wheels for next operation of moulding.The pug mill is then kept moving and feeding of clay from top and taking out of pugged clay from bottom are done simultaneously.If tempering is properly carried out, the good brick earth can then be rolled without breaking in small threads of 3mm diameter. Classification of Bricks as per common practice: Bricks, which are used in construction works, are burnt bricks. They are classified into four categories on the basis of its manufacturing and preparation, as given below. 1. First class bricks 2. Second class bricks 3. Third class bricks 4. Fourth class bricks First Class Bricks: These bricks are table moulded and of standard shape and they are burnt in kilns. The surface and edges of the bricks are sharp, square, smooth and straight. They comply with all the qualities of good bricks. These bricks are used for superior work of permanent nature. Second Class Bricks: These bricks are ground moulded and they are burnt in kilns. The surface of these bricks is somewhat rough and shape is also slightly irregular. These bricks may have hair cracks and their edges may not be sharp and uniform. These bricks are commonly used at places where brick work is to be provided with a coat of plaster. Third Class Bricks: These bricks are ground moulded and they are burnt in clamps. These bricks are not hard and they have rough surfaces with irregular and distorted edges. These bricks give dull sound when struck together. They are used for unimportant and temporary structures and at places where rainfall is not heavy. Fourth Class Bricks: These are over burnt bricks with irregular shape and dark color. These bricks are used as aggregate for concrete in foundations, floors, roads etc, because of the fact that the over burnt bricks have a compact structure and hence they are sometimes found to be stronger than even the first class bricks. Classification of Bricks as per constituent materials There are various types of bricks used in masonry. • Common Burnt Clay Bricks • Sand Lime Bricks (Calcium Silicate Bricks) • Engineering Bricks • Concrete Bricks • Fly ash Clay Bricks Common Burnt Clay Bricks Common burnt clay bricks are formed by pressing in moulds. Then these bricks are dried and fired in a kiln. Common burnt clay bricks are used in general work with no special attractive appearances. When these bricks are used in walls, they require plastering or rendering. Sand Lime Bricks Sand lime bricks are made by mixing sand, fly ash and lime followed by a chemical process during wet mixing. The mix is then moulded under pressure forming the brick. These bricks can offer advantages over clay bricks such as: their color appearance is grey instead of the regular reddish color. Their shape is uniform and presents a smoother finish that doesn’t require plastering.These bricks offer excellent strength as a load-bearing member. Engineering Bricks Engineering bricks are bricks manufactured at extremely high temperatures, forming a dense and strong brick, allowing the brick to limit strength and water absorption.Engineering bricks offer excellent load bearing capacity damp-proof characteristics and chemical resisting properties. Concrete Bricks Concrete bricks are made from solid concrete. Concrete bricks are usually placed in facades, fences, and provide an excellent aesthetic presence. These bricks can be manufactured to provide different colors as pigmented during its production. Fly Ash Clay Bricks Fly ash clay bricks are manufactured with clay and fly ash, at about 1,000 degrees C. Some studies have shown that these bricks tend to fail poor produce pop-outs, when bricks come into contact with moisture and water, causing the bricks to expand Tests on Bricks To know the quality of bricks following 7 tests can be performed. In these tests some are performed in laboratory and the rest are on field. • Compressive strength test • Water Absorption test • Efflorescence test • Hardness test • Size, Shape and Color test • Soundness test • Structure test Compressive strength test: This test is done to know the compressive strength of brick. It is also called crushing strength of brick. Generally 5 specimens of bricks are taken to laboratory for testing and tested one by one. In this test a brick specimen is put on crushing machine and applied pressure till it breaks. The ultimate pressure at which brick is crushed is taken into account. All five brick specimens are tested one by one and average result is taken as brick’s compressive/crushing strength. Water Absorption test: In this test bricks are weighed in dry condition and let them immersed in fresh water for 24 hours. After 24 hours of immersion those are taken out from water and wipe out with cloth. Then brick is weighed in wet condition. The difference between weights is the water absorbed by brick. The percentage of water absorption is then calculated.The less water absorbed by brick the greater its quality. Good quality brick doesn’t absorb more than 20% water of its own weight. Efflorescence test: The presence of alkalies in bricks is harmful and they form a grey or white layer on brick surface by absorbing moisture. To find out the presence of alkalis in bricks this test is performed. In this test a brick is immersed in fresh water for 24 hours and then it’s taken out from water and allowed to dry in shade.If the whitish layer is not visible on surface it proofs that absence of alkalis in brick. If the whitish layer visible about 10% of brick surface then the presence of alkalis is in acceptable range. If that is about 50% of surface then it is moderate. If the alkalies’ presence is over 50% then the brick is severely affected by alkalies. Hardness test: In this test a scratch is made on brick surface with a hard thing. If that doesn’t left any impression on brick then that is good quality brick. Size, shape and color test: In this test randomly collected 20 bricks are staked along lengthwise, width wise and height wise and then those are measured to know the variation of sizes as per standard. Bricks are closely viewed to check if its edges are sharp and straight and uniform in shape. A good quality brick should have bright and uniform color throughout. Soundness test: In this test two bricks are held by both hands and struck with one another. If the bricks give clear metallic ringing sound and don’t break then those are good quality bricks. Structure test: In this test a brick is broken or a broken brick is collected and closely observed. If there are any flows, cracks or holes present on that broken face then that isn’t good quality brick.