Chapter 5
Bricks
Manufacture of bricks is mostly a village industry. Bricks have been produced since the dawn of civilization in the sun dried form. The Great Wall of China was made of both burnt and sun dried bricks. Bricks have been used all over the world in every class and kind of building. In places where plenty of clay is available, brickwork is cheaper. The cost of construction work is less with bricks. Bricks resist fire and, hence, they do not easily disintegrate. The atmospheric effects are resisted by bricks of good quality.
5.1 COMPOSITION OF GOOD BRICK EARTH
The constituents of good brick earth are:
- Alumina: A good brick earth should contain 20–30 per cent of alumina. It imparts the property of plasticity to the earth. An excess of alumina causes shrinkage and warping of bricks during drying and burning and it becomes too hard when burnt.
- Silica: Silica forms 50–60 per cent of good brick earth. It is seen either in the free or combined state. In the free state, it is mechanically mixed with clay and in the combined form it exists in a chemical composition with alumina. The cracking, shrinking and warping of raw bricks are being prevented by the presence of silica. The durability of bricks depends upon the proportion of silica. An excess of silica destroys the cohesion between particles and the bricks become brittle.
- Lime: A good brick earth should contain lime not exceeding 5 per cent. It should be present in a very finely powdered state in order to prevent the flaking of bricks. Lime prevents the shrinkage of bricks. An excess of lime causes the bricks to melt and, hence, to lose its shape.
- Oxides of iron: A small quantity of the oxide of iron to the extent of 5-6 per cent is desirable in good brick earth. It imparts red colour to the bricks. But excess of lime makes the colour dark blue or blackish. On the other hand, if the quantity of lime is less, the bricks will be yellowish in colour. It also helps to fuse sand and, thereby, increases the hardness of bricks.
- Magnesia: Presence of magnesia in small quantity imparts a yellowish tint to the bricks and decreases the shrinkage. But if in excess, it causes the decay of bricks.
The ingredients which are undesirable in the brick earth include excess of lime, the presence of iron pyrites, pebbles, alkalies, vegetation and organic matter.
5.2 MANUFACTURE OF BRICKS
The manufacture of bricks is carried out in a number of stages. It includes the following:
5.2.1 Selection and preparation of clay
As a practise, suitable deposits of clay are first located and thoroughly tested for the quality for brick making. Clay for bricks is prepared in the following order.
- Unsoiling: The top layer of the soil is taken out. This is because the clay in the top layer is full of impurities and, hence, it is to be rejected for the purpose of preparing bricks.
- Digging: The clay which is dug out is spread on a level ground, just little deeper than the general ground level. The height of the heap of clay is about 60–120 cm.
- Cleaning: The clay should be made free from stones, pebbles and vegetable matter. If these particles are in excess, the clay is to be washed and screened, which is considered to be uneconomical.
- Weathering: The softening of clay is done by exposing it to the atmosphere. The period of exposure varies from weeks to full seasons.
- Blending: The clay is made loose and any ingredient to be added is spread out at its top. Blending indicates intimate mixing. A small portion of clay is taken every time for mixing.
- Tempering: In this stage, the clay is brought to a proper degree of hardness and it is made fit for moulding. Water in the required quantity is added and the whole mass is mixed so as to form a mass of uniform character. A large-scale tempering is usually done in a pug mill. The process of grinding clay with water and making it plastic is known as pugging.
A pug mill consists of a conical iron tub with a cover at the top. It is fixed on a timber base which is made by fixing two wooden planks at right angles. The diameter of the pug mill at the bottom is about 80 cm and at the top is about 1 m. A vertical shaft with horizontal arms is provided at the centre of the iron tub. The small wedge-shaped knives of steel are fixed on the horizontal arms. Openings are provided at the top and bottom for charging clay and water and removing the mix respectively. The height of the pug mill is about 2 m.
5.2.2 Moulding
Moulding is the process of making rectangular-shaped brick units from properly tempered clay. The two types of moulding are
- Hand moulding
- Machine moulding.
5.2.2.1 Hand moulding
This is presently the most common method for brick manufacture. This is adopted where manpower is cheap and readily available. The moulds used for hand moulding are rectangular boxes made from well-seasoned wood or steel open at the top and bottom. Hand moulding is of two types:
- Ground moulding
- Table moulding
Ground moulding
In this method, the ground is first levelled and fine sand is sprinkled over it. The mould is dipped in water and placed over the ground. The clay is pressed in the mould in such a way that it fills all the corners of the mould. Any surplus earth from the top of the mould is removed using a cutting wire or a metal with a sharp edge, which has to be dipped in water every time it is used. The mould is then lifted up and the brick is left on the ground. The mould is dipped in water and placed close by and another brick is moulded in the same way. If the mould is dipped in water every time, such preparation of bricks is known as slope moulded bricks. If fine sand or ash is sprinkled on the inside surface of the mould instead of dipping the mould in water, such bricks are called sand moulded bricks.
In pallet moulding, bricks of higher quality and with frogs are produced. The frogs are made using a pair of pallet boards and a wooden block. A frog is a mark of depth about 10–20 mm made on raw bricks during moulding. The frog is provided for mainly two purposes:
- It serves as a key of mortar when other bricks are placed over it.
- It indicates the trade name of the manufacturer.
Table moulding
The process of moulding operations are carried out on a specially designed moulding table. The clay, the mould, water pots, stock board, etc. are placed on this table. The bricks are moulded similar to the ground moulding on the table. The cost of brick moulding increases when table moulding is adopted.
5.2.2.2 Machine moulding
The moulding can also be achieved by using machines. It is quite economical when bricks are produced in huge amounts. The machine moulding is broadly classified into two categories:
- Plastic clay machines
- Dry clay machines
Plastic clay machines
The machines contain rectangular openings of size equal to the length and width of a brick. The pugged clay is placed in the machine and as it comes out through the openings it is cut into strips by wires fixed in frames. Hence, it is known as wire cut bricks.
Dry clay machines
In this machine, the strong clay is first converted to powder form. A small quantity of water is added to the stiff plastic paste. Such paste is placed in the mould and pressed by machines to form well-shaped hard bricks. These bricks are known as pressed bricks.
The machine moulded bricks have regular shape, sharp edges and corners; they are heavier and stronger than hand moulded bricks.
5.2.3 Drying
The drying of bricks is necessary, firstly to make them strong enough for rough handling during subsequent stages and secondly to save fuel during burning. For drying the bricks are laid longitudinally in stocks of bricks with width equal to two bricks. Drying of bricks is achieved by either natural or artificial methods.
- The bricks are generally dried by natural process. But when bricks are to be rapidly dried, artificial drying may be adopted. In artificial drying, bricks are made to pass through driers in the form of tunnels or hot channels or floors. The tunnel driers are more economical than hot floor driers.
- The brick in stocks should be arranged in such a way that sufficient air space is left between them for circulation of air.
- Special drying yards should be prepared and accumulation of rainwater should be prevented.
- The period of drying depends upon the prevailing weather conditions.
5.2.4 Burning
Burning of dried bricks is essential to develop the desired engineering properties, like hardness, durability and resistance to decay. Three chemical changes are known to take place in the brick earth during burning, namely dehydration, oxidation and vitrification.
Dehydration is completed within 425–750°C temperature range and it results in expulsion of most of the water from the bricks.
During oxidation, carbon and sulphur are eliminated as oxides, whereas the fluxes are also oxidized. Oxidation starts at the range of dehydration temperatures and is completed at about 900°C.
Vitrification is the extreme reaction and occurs when heating is carried out beyond 900°C. This is commonly not required in building bricks although in other clay products like sewer pipes it is necessary.
Burning of bricks is either done in clamps or kilns. Clamps are temporary structures while kilns are permanent structures. Clamps are adopted to manufacture bricks on a small scale while kilns are adopted to manufacture bricks on a large scale.
5.2.4.1 Clamps
The shape of the clamp is generally trapezoidal. The brick wall is constructed on the short end and a layer of fuel is placed on the prepared floor. The fuels generally used are cow dung, litter, husk of rice, wood, coal, etc. The thickness of fuel layer varies from 70 to 80 cm. The layer consisting of 4 or 5 courses of raw brick is then put up. Sufficient space for circulation of air is provided. Alternate layers of fuel and bricks are placed over this. The total height of the clamp is around 3-4 m. When nearly one-third of the height is reached, the lower portion of the clamp is ignited so as to burn the bricks in the lower part when the construction of bricks in the upper part is in progress. After construction is complete, it is completely plastered with mud in order to prevent the escape of heat. The clamp is allowed to burn for 1 or 2 months and cooling is also done for 2 months and later the burnt bricks are taken out.
5.2.4.2 Kilns
The kilns used for the manufacture of bricks are of two types:
- Intermittent kilns
- Continuous kilns
Intermittent kilns
These kilns may be underground or overground in model. They are classified in two ways:
Intermittent up-draught kilns These kilns are in the form of rectangular structures with thick outside walls. Doors are provided at each end for loading and unloading of kilns. The flues are channels or passages which are provided to carry flames or hot gases through the body of the kiln. A roof is provided to protect the raw bricks from rain.
The quality of the bricks is not uniform; the bricks at the bottom are overburnt and at the top are under-burnt. The supply of bricks is not continuous and there is a considerable wastage of fuel in the kiln.
Intermittent down-draught kilns These kilns are rectangular or circular in shape. They are provided with permanent walls and a closed tight roof. The floor of the kiln has openings which are connected to a common chimney stacked through flues. They are so arranged that in this kiln the hot gases are carried through the vertical flues upto the level of roof and then released. As a result the bricks are evenly burnt and the performance is much better than intermittent up-draught kilns. Here, there is close control of heat and the bricks obtained are evenly burnt.
Continuous kilns
These kilns are continuous in operation where loading, firing, cooling and unloading are simultaneously carried out.
Bull's trench kiln This is one of the continuous type kilns. These kilns are rectangular, circular or oval shaped in plan. These kilns are constructed in a trench excavated on the ground. It may be fully underground or partially projecting above the ground. The outer and inner walls are to be constructed in bricks. Openings are provided on the outer walls to act as flue holes. Iron plates are used to divide the kiln into suitable sections. The fuel is placed in flues and is ignited after covering the top surface with earth to prevent the escape of heat. Usually, two movable iron chimneys are employed to form draught. The chimneys are placed in advance of the firing sections so that the warm gases leaving the chimney warm up the bricks in the next section. As the section has burnt, the flue holes are closed and allowed to cool down. Later the fire is advanced to the next section.
Hoffmann kiln This kiln is constructed under ground, is circular in plan and consists of a number of chambers. A permanent roof is provided so that the kiln can function even in the rainy season. The chamber in Hoffmann's kiln is provided with a main door for loading and unloading bricks. Communicating doors should act as flues in the open condition. A radial flue connected with a central chimney and fuel holes are also provided. The advantages are that the bricks are uniformly, equally and evenly burnt and that there is no air pollution in the locality. Also, there is saving in fuel and a high percentage of good bricks are produced.
Tunnel kiln This type of kiln is in the form of a tunnel, which is oval, circular or straight in plan. It contains a stationary source of fire. The raw bricks are placed in trolleys which are then moved from one end to another end of the tunnel. The raw bricks get dried and preheated as they approach the zone of fire and in the zone of fire the bricks are burnt and pushed forward for cooling. Later, after cooling, they are unloaded.
Table 5.1 shows the comparison between clamp and kiln burning.
5.3 SIZE AND WEIGHT OF BRICKS
Bricks are prepared in various sizes. If the bricks are too large, it is difficult to burn them and handle them. But, on the other hand, if the bricks are small, more quantity of mortar is required while placing.
In India, the standard size recommended by the Bureau of Indian Standards (BIS) is 19 cm × 9 cm × 9 cm and the size of brick including the mortar thickness is 20 cm × 10 cm × 10 cm, which is known as the nominal size of the brick.
The test carried out for inspecting the size is that 20 bricks of standard size (19 cm × 9 cm × 9 cm) are stacked length wise, along the width and along the height. For good quality bricks the results should be within the following permissible limits:
Length: 368–392 cm
Width: 174–186 cm
Height: 174–186 cm
The weight of 1 m3 of brick earth is 18 kN. Hence, the average weight of a brick will be around 30–35 N.
5.4 QUALITIES OF A GOOD BRICK
A good brick should possess the following properties:
- The brick should be uniform in shape and should be of standard size.
- The brick when broken should show a uniform compact and homogeneous structure free from voids.
- The brick should not absorb water more than 20 per cent for first-class bricks and 22 per cent for second-class bricks when soaked in cold water for a period of 24 hours.
- The brick should be hard enough. No impression should be left when scratched.
- The brick should not break into pieces when dropped from a height of 1 m.
- The brick when soaked in water for 24 hours should not show deposits of white salts when allowed to dry in shade.
- The brick should have low thermal conductivity and should be sound proof.
- The crushing strength of brick should not be below 5.5 N/mm2.
- The brick should be table moulded, well burnt and free from cracks with sharp and square edges.
- The colour should be uniform and bright.
- The bricks should give a good metallic sound when struck with each other.
Concrete blocks, hollow blocks and bricks made of various materials like fly ash are being used successfully as substitutes.
5.5 FALG BRICKS
The rapid increase in the capacity of thermal power generation in India has resulted in the production of a huge quantity of fly ash, which is approximately 50 million tons per year. The prevailing disposal methods are not free from environmental pollution and ecological imbalance. Large stretches of scarce land, which can be used for shelter, agriculture or some other productive purposes, are being wasted for disposal of fly ash. Fly ash, lime and gypsum (FALG) can be used to make bricks and hollow blocks of adequate strength, an economical alternative to conventional burnt clay bricks will be available. Lime and gypsum are either available from mineral sources or can be procured from industrial wastes.
Fly ash bricks are made of fly ash, lime, gypsum and sand. Fly ash, lime sand and gypsum are manually fed into a pan mixer where water is added in the required proportion for intimate mixing. The proportion of the raw material is generally in the ratio of 60–80 per cent of fly ash, 10–20 per cent lime, 10 per cent Gypsum and 10 per cent sand, depending upon the quality of raw materials. The mixture is slow-setting pozzalona cement mix. After mixing, the mixture is shifted to the hydraulic/mechanical presses. A specially designed machine which gives a very high pressure load at slow rate (in the order of 280–350 kg/inch) is used to mould the bricks. Holding the pressure at specific times gives more strength to the finished product. The moulded bricks are then transferred to hydraulic-operated wooden pellets manually and stored in covered space for 3 days (minimum) for setting.
Then the bricks are taken to the yard for water curing for 15–20 days. Then it is sorted and tested before despatch. These can be extensively used in all building constructional activities similar to that of common burnt clay bricks. The fly ash bricks are comparatively lighter in weight and stronger than common clay bricks. Fly ash bricks are used in multi-storeyed apartment houses for non-load bearing purposes and in making curtain and partition walls of these houses. Use of fly ash bricks in this type of construction is meant mainly to achieve economy and make profits. The domestic buildings of low- or middle-income groups mostly have single or two-storied dwelling units. Therefore the cost effectiveness along with the strength and durability of fly ash bricks are very important for them.
5.6 FLY ASH BRICKS
Fly ash is one of the residues generated in the combustion of coal. It is generally captured from the chimneys of coal-fired power plants and is one of two types of ash that jointly are known as coal ash; the other, bottom ash, is removed from the bottom of coal furnaces. Depending upon the source and makeup of the coal being burnt, the components of fly ash vary considerably, but all fly ash includes substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline) and calcium oxide (CaO).
Fly ash has been used for over 50 years to make concrete building blocks. They are widely used for the inner skin of cavity walls. They are naturally more thermally insulating than blocks made with other aggregates.
Fly ash bricks have been used in house construction since the 1970s. There is, however, a problem with the bricks in that they tend to fail. This happens when the bricks come into contact with moisture and a chemical reaction occurs causing the bricks to expand.
REVIEW QUESTIONS
- What are the constituents of good bricks?
- What are the different stages involved in the manufacture of bricks?
- Write short notes on
- Moulding of bricks
- Drying of bricks
- Different types of kilns used for the manufacture of bricks
- Size of bricks
- Fly ash bricks
- What are the qualities of a good brick?
- Briefly discuss FALG bricks.
Great post! In comparison to clay bricks, fly ash bricks offer the advantages of uniform quality, faster speed of construction, lower labor involvement and longer durability.
ReplyDeleteHi, really appreciate you for this awesome article you have covered all the pin points of the topic in this article keep up the good work. Cheers! Terracotta Bricks
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