3

BRICKS

3.1 INTRODUCTION

Bricks have been in use since the dawn of civilization. In the initial stages they were used as sun-dried bricks. Burnt bricks have succeeded sun-dried bricks. It has been reported that bricks have been used to build monuments in different parts of the world. Refined brick making and burning techniques have remarkably improved the quality of buildings and other works.

Now, brick plays a prime role in construction at all levels starting from thatched roofs to multi-storeyed buildings. But, the process of brick making has not changed over many years in India except for minor refinements. It is the oldest construction material which has been extensively used at present because of its durability, easy availability and low cost.

3.2 CONSTITUENTS OF BRICK EARTH

The main constituents of good brick earth are alumina, silica, lime, oxide of iron and magnesia.

3.2.1 Functions of Constituents of Brick Earth

1. Alumina

This is the main constituent of every kind of brick earth. This imparts plasticity to the earth so that it can be moulded. If alumina is present in excess, it leads to shrinking and warping during drying and burning. This can be prevented by the addition of sand.

2. Silica

It exists in brick earth as free sand or in a combined form as silicate of alumina. The presence of silica prevents cracking, shrinking and warping of raw bricks. Thus, it imparts uniform shape to bricks. The addition of silica also increases hardness, durability and resistance to heat. Excess of silica removes the cohesion between particles and makes the brick brittle.

3. Lime

It enables the silica to melt during burning and bind the particles together. Lime should be in the form of powder, otherwise lumps of lime are converted into quick lime after burning. Quicklime slakes and expands in the presence of moisture and results in the splitting of bricks into pieces. Excess of lime causes the brick to melt too readily, and the shape will be lost.

4. Iron Oxide

It acts as a flux and helps the grains of sand to melt and bind the particles of clay together. It is responsible for imparting a red colour to the brick. It improves the durability of the brick. Excess of iron oxide makes the bricks dark blue or blackish, and less quantity of iron oxide makes the brick yellowish in colour.

5. Magnesia

A small quantity of magnesia in brick earth imparts a yellow tint and decreases shrinkage. Excess of magnesia causes decay of bricks.

3.2.2 Desirable Chemical Composition of Brick Earth

The desirable chemical composition of brick earth for good quality bricks are given below (IS: 1077, 1976):

1. Alumina or clay – 20 to 30% by weight
2. Silica or sand – 35 to 50% by weight
3. Silt – 20 to 25% by weight
4. Other required ingredients are
   1. Iron oxide
   2. Magnesia
   3. Lime
   4. Sodium potash, etc.
5. In the case of alluvial soils, the total lime and magnesia should not be more than 1%; in other soils it should be less than 15%.
6. Water-soluble materials in total should be less than 1%.

3.2.3 Harmful Ingredients in Brick Earth

Naturally available soil may contain ingredients other than those required for making quality bricks, which are discussed below.

1. Alkalis

Alkalis in the form of soda and potash lower the fusion point of clay, and cause bricks to fuse, twist and warp during burning. Alkalis present in bricks absorb moisture from the atmosphere and create dampness. Such dampness dries and leaves behind grey or white deposits on the surface of the wall.

2. Pebbles

Although the presence of pebbles causes harm chemically, they are not desirable as they do not permit the clay to be thoroughly mixed uniformly resulting in weak and porous bricks. Regular breaking of bricks during masonry work is not possible because of the presence of pebbles.

3. Iron Pyrites

The presence of iron pyrites in brick earth causes the earth to decompose and oxidize resulting in the splitting of bricks.

4. Vegetation and Organic Matter

Vegetation and organic matter when present in brick earth assists in burning. Incomplete burning of these materials causes the bricks to become porous.

5. Limestone

Limestone and kankar nodules present in brick earth are harmful as that of lumps of lime. On heating limestone is converted into lime, which comes into contact with water, swells and finally results in splitting and crumbling.

3.2.4 Field Testing of Brick Earth

Brick earth in the field has to be tested for consistency, moulding properties and shrinkage.

In order to test for the first property, a handful of soil sample is taken and formed into a ball. The ball is dried in the sun and then examined. Crumbling of the dried ball shows the presence of excess sand. If the ball is hard and shows cracks on the surface, it indicates the presence of less sand. Thus, the soil has to be modified by mixing different proportions of sand or clay such that the above-mentioned defects do not occur.

The soil found or made suitable in the first test is taken and ground well. Then a required quantity of water is added and mixed. The mixed soil is kneaded well to be rolled to form threads of about 3 mm diameter without crumbling. Such a mixture of soil and water is used to make a brick using a mould. The moulded brick should be with clear edges and corners which ensures perfect requirement of water. Otherwise water is added or removed to get a satisfactory brick.

Sample of bricks moulded as above are burnt in a clamp until bricks become red hot. These bricks are examined for shrinkage and deformation after cooling. Bricks which have shrunk evenly and do not show any defects are separated out and others are rejected. The percentage of sorted out bricks should be high, otherwise the soil is modified to get a better percentage of good bricks.

3.3 SELECTION OF SITE FOR BRICK EARTH

The following factors are to be considered in the selection of a site for brick earth:

1. The site to be selected should be capable of providing an adequate quantity of soil during the entire planned production period.
2. Additional materials, such as sand or silt, required to make good bricks should also be available near the site.
3. Water and fuel should also be available near the site; otherwise extra cost may be incurred for transport.
4. It should be connected by road and rail for easy conveyance of the produced bricks.
5. The ground of the site must be situated as a plain ground.
6. The location of the water table should be at a deeper depth.

3.4 MANUFACTURING OF BRICKS

The manufacturing of bricks involves four stages, viz., preparation of soil, moulding, drying and burning.

3.4.1 Preparation of Soil

1. Removal of Top Soil

The preparation of soil involves first renewal of loose materials at the top of the ground for a depth of about 200 mm. As it contains a lot of impurities, this material is not used for the preparation of bricks.

2. Digging and Spreading

The earth is dug out 200 mm from the ground. The soil is spread on the level ground, and heaps of clay are about 600 to 1200 mm.

3. Cleaning

The soil after being spread on the ground should be cleaned of stones, pebbles, vegetable matter, etc. If non-clay materials are in excess, the clay should be washed and screened. Such a process will be expensive and cumbersome. All the lumps of soil should be broken into a powder form.

4. Weathering

The soil is then exposed to the atmosphere for softening or mellowing. The period of exposure may last for a few weeks depending on the nature of soil. This imparts plasticity and strength to the soil.

5. Blending

To increase the quality of soil, additional soil such as sandy or calcareous clays may be added in suitable proportions with a small quantity of coal, ash, etc. The whole mass is mixed uniformly and water is added.

6. Tempering

It is the process of kneading the soil under the feet of men or cattle after adding the necessary quantity of water in order to make the soil stiff and homogeneous. In general, a soft plastic clay could be prepared by using about 25 to 30% of water. This procedure is adopted for the majority of common handmade bricks. For making superior bricks on a large scale, the earth is tempered in a pug mill.

A pug mill comprises of a truncated conical tub of 120 cm diameter at the top and 75 cm diameter at the bottom with a height of about 180 cm. A long vertical shaft is placed at the centre (Fig. 3.1). The central shaft is provided with a number of horizontal arms attached with cutting knives for breaking clay lumps, if any. A long horizontal arm is fitted at the top of the vertical shaft. The shaft is rotated with the help of bullocks or sometimes by electric power. Clay and water in the ratio of 1:3/4 are fed into the vessel from the top. The tempered clay is collected from the hole provided at the bottom of the vessel. This type of pug mill, as shown in Fig. 3.1, can be used to get sufficient soil for a daily output of about 20,000 bricks.

3.4.2 Moulding of Bricks

Bricks are made in traditional size (in inch) and in metric size (cm) as prescribed by the Bureau of Indian Standards. Metric size bricks are called modular bricks. Nominal size of bricks is the size including thickness of the mortar. Hence, the actual size of modular bricks is 19 cm × 9 cm × 9 cm and the nominal size of modular bricks is 20 cm × 10 cm × 10 cm.

A brick mould is a rectangular box of steel or wood. Both ends, the top and bottom, of the box are open. The inside dimensions of the mould are 20 cm × 10 cm × 10 cm (Fig. 3.2).

Moulding of bricks is carried out either by hand or by machine.

1. Hand Moulding

Two types of hand moulding, viz., ground moulding and table moulding, are adopted in India. In hand moulding, bricks are moulded manually. This is preferred in areas where the manpower is cheap and available readily and where only a small quantity of bricks is needed.

(i) Ground Moulding

The process of moulding bricks on the ground by manual labour is called ground moulding. At first a level ground is prepared, and a thin layer of fine sand is spread over the ground.

The mould is wetted and placed on the ground firmly. The tempered brick earth is dashed into the mould. The earth is pressed in the mould such that the earth fills all the corners of the mould without leaving any air gap within the brick. The excess earth is removed using a wooden or metal strike dipped in water.

The mould is then lifted, and the raw brick is left on the ground. The mould is cleaned, dipped in water and placed near the previous brick; the process is repeated till the ground is completely covered with the prepared raw bricks. On an average, a moulder can mould about 750 bricks per day. When the bricks have sufficiently dried they are taken to the drying shed and placed in an ordered manner.

Brick prepared by dipping moulds in water every time called slop-moulded brick, and if sand is sprinkled on the sides of the mould, the brick is called a sand-moulded brick.

(ii) Table Moulding

Table moulding is done on a table of size 2 m × 1 m × 0.7 m instead of on the ground. The process of moulding is almost similar to ground moulding except for a few changes.

Invariably, table-moulded bricks are provided with a frog. A frog is a mark of depth of about 10 to 20 mm provided in a mould. This serves two purposes, viz., it provides a key for the mortar when the next brick is placed with its flat surface over this and to place the trademark of the manufacturer.

A stock board of the same site as the inside dimensions of the mould with a projection for the frog with the trademark of the manufacturer is kept on the moulding table, and the moulder stands behind the table.

The mould is placed to fit the stock board and the tampered earth is dashed against the mould, carefully filled, and excess earth is removed. Then a thin board called the pallet board is placed on the mould; the mould and pallet board are lifted together followed by lifting the mould leaving the brick on the pallet board. Another pallet board is kept on the brick and carried to the drying yard where it is placed on its edge and the pallet boards are removed. This procedure is repeated. Figure 3.3 shows a moulded brick lying over the pallet board.

Figure 3.3 Moulded brick lying over the pallet board

2. Machine Moulding

Moulding machines are used when a large number of bricks are to be manufactured within a short time. Machine-moulded bricks are heavier and stronger than the hand-moulded ones. These bricks have a sharp regular shape and size, a smoother surface and sharp edges.

There are two types of machine moulding, viz., plastic method and dry method.

(i) Plastic Method

In the plastic method, pugged earth is used. The machine adopted for the plastic method contains a rectangular opening of size equal to the length and width of the brick. The pugged earth is placed in the machine and a beam of the moulded earth comes out. This is cut into strips of width equal to the depth of the brick by wires fixed in the frames. These bricks are also known as wire-cut bricks.

(ii) Dry Method

In the Dry Method, the machine first converts the hard earth into a powder form. A small quantity of water is added to the powder to form a stiff plastic paste. The plastic paste is placed in the mould and pressed by the machine to form hard and correct-shaped bricks. These bricks are called pressed bricks. Such bricks do not require drying and can be sent directly to the burning section.

3.4.3 Drying of Bricks

Moulded bricks can not be burnt directly as they may get cracked or distorted. Hence, before burning they are dried. Natural drying or artificial drying may be resorted to. The bricks are left to dry for about two weeks.

1. Natural Drying

It is also called hack drying. It comprises placing moulded bricks in rows on their edges on a slightly raised ground called a hack. A small space is given between bricks for the circulation of air. Direct exposure to sun is avoided by providing a cover and it is also protected from the rain. The air- and sun-dried bricks are strong enough and can be used for the construction of small structures.

2. Artificial Drying

When bricks are needed continuously and to a large scale, artificial drying is resorted to. The bricks are dried in special dryers which receive heat from special furnaces that are made especially for this purpose. Hot flue gases from the chambers of a kiln and waste steam from engines may also be used for the artificial drying of bricks.

3.4.4 Burning of Bricks

Bricks are burnt in kilns after moulding and drying so as to impart hardness and strength and to increase the density of the brick so that it absorbs less quantity of water.

Physical and chemical changes take place in burning of bricks. Heating brick earth up to about 640°C produces only physical changes. At this temperature moisture and water of crystallization are driven out, and the organic matter is burnt. Such a brick that can absorb moisture from the air can get back hydrated. Such bricks are said to be poorly burnt and disintegrate when subjected to moisture contact.

If brick earth is heated up to 700–1,000°C, it undergoes chemical changes. During chemical action alumina and silica in brick earth fuse together resulting in a compound which is strong and stable. After this chemical transformation, it does not turn back to break earth on cooling. Further, it does not crumble when immersed in water. Thus, the break earth burnt above 700°C is different from the original brick earth. Hence, to get a good quality brick it has to be heated to the required temperature.

On heating the brick earth beyond 1,300°C, the above materials get vitrified. The bricks begin to lose their shape.

1. Clamp or Open Kiln

Clamp or open kiln is a temporary structure where dried bricks are stacked in alternate layers of fuel, i.e., both bricks and fuels are placed in alternate layers. Locally available materials such as grass, rice, husk, wooden chippings, cheap quality woods and dried cow dung are used as fuel. About 20,000 to 1,00,000 bricks are available after burning and cooling. It takes around 3–6 months to complete the burning and cooling of the bricks (Fig. 3.4) in the clamp.

Figure 3.4 Typical arrangement of fuel and bricks in clamp or open kilns

The main advantages of this method are that its initial cost is low, fuel cost is low and there is no need of a permanent structure and skilled labourers. The regulation and circulation of heat are not possible, and hence only 60% of good-quality bricks can be expected. Further, only a small quantity of bricks can be manufactured at a time.

2. Intermittent Kiln

When a large quantity of good-quality bricks are needed, intermittent or continuous kilns have to be used. In intermittent kilns, the process of burning is discontinuous.

Figure 3.5 shows the plan of an intermittent kiln. The operations are in the order of loading the bricks, firing, cooling and unloading, which are performed one after the other. It is rectangular in shape with thick outside walls and is constructed over the ground.

Trenches are dug across the floor of the kiln. Small openings are made in between the trenches. Sun-dried bricks are laid above the trenches with the bricks being laid on edges with gaps in between. This gap enables the hot flue gases to flow to each brick.

Flue gases are let in through the longitudinal walls through the small openings. Dampers are provided in the flue openings to regulate the air supply. Circulation of flues is kept up for 3–4 days. The bricks are cooled for a week’s time.

3. Continuous Kilns

In continuous kilns, the process of burning is continuous. There are three types of continuous kilns, viz.,

1. Bull’s Trench Kiln
2. Tunnel Kiln
3. Hoffman’s Kiln

(i) Bull’s Trench Kiln

This type of kiln is usually oval in plan and is constructed in a trench excavated in the ground. It may be fully underground or partly projecting above ground. The depth of trench is about 2 metres. The outer and inner walls are constructed of bricks, and the flue holes are provided in the outer walls (Fig. 3.6). Dampers are provided to conveniently divide the kiln into sections.

Bricks are arranged in sections in such a way that flues are formed. Fuel is placed in the flues, and the top surface is covered with earth and ashes to prevent the escape of heat. The fuel is burnt through the flue holes. Additional flue holes are provided at the top to insert fuel when burning is in progress. Two movable chimneys are used to form a draught. These chimneys are placed before the section is fired. This arrangement makes the hot gases leaving the chimney warm up the bricks in the next section. When the burning is over in a section the flue holes are closed and the bricks are cool down gradually. The fire is advanced to the next section and the chimneys are moved forward.

As loading, burning, cooling and unloading are carried out simultaneously, a continuous supply of bricks is available. This is the mostly used kiln in India.

(ii) Tunnel Kiln

This kiln is in the form of a tunnel which may be of any shape in plan, viz., straight, circular or oval. The zone of fire is at one place. The moulded bricks are loaded on a trolleys, which are moved from one end of the tunnel to the other end. During this process when they approach the zone of fire that are completely dried and pre-heated.

These bricks are burnt in the zone of fire and are then moved for cooling. After adequate cooling, the bricks are unloaded. As the temperature can be controlled, better quality bricks are produced. The bricks from this kiln are said to be economical.

(iii) Hoffman’s Kiln

This is constructed over the ground to produce a continuous supply of bricks on a large scale.

D₁ to D₁₂ – Main doors; D₁ and D₂ – Opened doors, D₃ to D₁₂ – Closed doors

1 to 12 – Chambers; C – Chimney, F₁ to F₁₂ – Radial flues

A₁ to A₁₂ and B₁ to B₁₂ – Communicating doors

Figure 3.7 Hoffman’s continuous kiln

This is circular in plan and is provided with a chimney at the centre. Around the chimney are 12 chambers that are in an annular shape. Each chamber comprises of the following parts (Fig. 3.7):

1. A main door for the loading and unloading of bricks (e.g. D₁).
2. Communicating doors for the flow of flue gases between the chambers (e.g. A₁ and B₁).
3. A radical flue from each chamber to the chimney (e.g. F₁).
4. Fuel holes for providing fuel, and powdered coal is used as fuel.

   Functions that occur in the chambers are listed below:

   Chamber 1 – Loading

   Chambers 9–12 – Drying and pre-heating

   Chambers 7 and 8 – Burning

   Chambers 3– 6 – Cooling

   Chamber 2 – Unloading

The working of a kiln is as follows:

1. Cool air enters through Chambers 1 and 2 as they are open doors.
2. It crosses the cooling Chambers 3–6 and enters the burning Sections 7 and 8 in a heated condition.
3. It moves to Chambers 9–12 to dry and pre-heat the raw bricks.
4. It escapes into the atmosphere through the damper of Chamber 12 and the chimney.

The flow of air and fuel gas are shown by arrows in each chamber. Although the initial cost is more, Hoffman’s Kiln claims several advantages:

1. High-quality bricks with uniform burning are obtained with regulation of heat.
2. Supply of bricks are continuous in all seasons because the top of the kiln is closed, and the working is not stopped.
3. Considerable saving in fuel due to pre-heating of raw bricks by fire gases.
4. No air pollution in the locality, as the exhaust gases do not contain black smoke or dust particles.

3.5 QUALITIES OF GOOD BRICKS

The qualities of good bricks are as follows:

1. Bricks should have perfect edges, must be adequately burnt, should be uniform red or copper in colour and should be free from cracks.
2. Bricks should have rectangular plane surfaces with parallel sides and sharp right-angled edges. The size of a standard brick is 190 mm × 90 mm × 90 mm.
3. It should be hard enough such that no impression is left when scratched with one’s finger nails.
4. Bricks when struck with each other should produce a ringing sound.
5. Bricks should not break when dropped flat from a height of 1 m.
6. Bricks should be homogeneous and compact throughout, and the brick should not have any voids or grit.
7. Bricks should have a percentage of absorption of water by weight less than 20%.
8. Bricks should not show deposits of salts when immersed in water and dried.
9. Bricks should have less thermal conductivity and must be sound proof.
10. Brick should have a minimum crushing strength of 3.5 N/mm².

3.6 CLASSIFICATION OF BUILDING BRICKS AND USES

Bricks are broadly classified into two broad categories as follows:

1. Sun-dried bricks and
2. Burnt bricks

Sun-dried bricks also called un-burnt or katcha bricks, and these are dried directly under the sun after the process of moulding. These bricks are of inferior quality and are used for the construction of temporary and cheap structures. Such bricks should not be used in areas exposed to heavy rains.

Burnt bricks are of superior quality, which are generally used for civil engineering constructions. These are discussed in depth below.

3.6.1 Quality Classification of Burnt Bricks

The classification of bricks under the following four categories on the basis of constituents, preparation and burning is called quality classification:

1. First-class Bricks

First-class bricks are table-moulded bricks and are burnt in kilns. These bricks should not have any defects like cracks, stones or lumps of clay. They should be of standard size and have uniform colour, sharp edges, even surfaces, correctly burnt and hard. These bricks are used for superior quality works and works of permanent nature.

2. Second-class Bricks

Second-class bricks are ground moulded and are burnt in kilns. In general, they have to satisfy the requirement – quality of first-class bricks. However, they may have a slightly irregular shape, rough uneven surfaces or may have slight cracks. These bricks are hard and are correctly burnt and used where the brick masonry is to be plastered.

3. Third-class Bricks

Third-class bricks are ground moulded and burnt in clamps. These bricks have irregular edges with less sharpness, uneven surfaces and are not hard enough. They give a dull sound when struck against each other. They are used for unimportant and temporary constructions.

4. Fourth-class Bricks

Fourth-class bricks are either over burnt or under burnt with irregular shapes, edges and surfaces. These are used as aggregates for concrete in road, floor and foundation construction.

3.6.2 Indian Standard Classification of Burnt Bricks

As per the Indian Standard (IS: 3102 – 1971), bricks are classified according to their strength as given in Table 3.1.

Table 3.1 Classification of bricks

Source: IS: 3102–1971.

3.7 TESTS ON BRICKS

Before recommending bricks for construction work, their suitability is to be assessed by conducting the following tests:

1. Compressive Strength Test
2. Water Absorption Test
3. Efflorescence Test
4. Dimensional Tolerance Test
5. Hardness Test
6. Soundness Test
7. Structure Test

The sampling and testing of bricks are carried out as per IS: 3495 – 1992 and are as shown in Table 3.2.

Table 3.2 Sampling and testing of bricks

Source: IS: 3495–1992.

3.7.1 Compressive Strength Test

As per norms, five bricks are taken at random and their dimensions are measured accurately to 1 mm. They are immersed in water at 25–29°C for a period of 24 hours. After that they are taken out, and excess moisture is allowed to drain. If the bricks have frogs they are filled with C.M 1:3. They are again kept under a jute bag for another 24 hours. They are again immersed in clean water for three days.

Just at the time of testing they are taken out, one at a time, from the water and wiped dry. The horizontal and mortar-filled surface is placed facing upwards with three thin plywood sheets on a brick-testing machine.

Load is applied on the brick at a rate of 140 kg/cm² per minute till the failure of the brick. An average of five test values of bricks is reported. While computing the average value, any single value obtained as compressive strength which is higher than the upper value of the class of the bricks tested should be taken only as the upper limit of the class. Test values less than 20% of the average should be rejected. Also the average value should not be less than the specified value of the class of the brick.

3.7.2 Water Absorption Test

For the test, five bricks are taken at random from the lot. They are dried in an oven at 110–115°C till they attain constant weight. Generally it takes 48 hours. Bricks are then cooled at room temperature, which generally takes 4–6 hours and are then weighed. Let the weight of a dry brick be W₁.

Bricks are then kept in clean water at 27 ± 2°C for 24 hours. They are taken out, wiped dry with a damp cloth, and the wet weight W₂ is noted.

The average percentage of water absorbed as a percentage of dry weight is reported. This value should not be more than the standard value of a particular class of brick.

3.7.3 Efflorescence Test

The presence of soluble salts causes efflorescence on the surface of the brick. Here also five test samples of bricks are taken at random. The brick is placed in a dish with 2.5 cm immersed in distilled water. The brick is allowed to absorb the water fully, and then the water evaporates through it. When the brick has dried, an additional and equal quantity of water is placed in the dish. As before the water is allowed to evaporate. After the second evaporation, the brick is examined and evaporated as under:

For general construction, bricks should not have more than slight to moderate efflorescence.

3.7.4 Dimensional Tolerance Test

Twenty bricks are taken at random and their dimensions, length, width and depth are measured. Variations in dimensions generally allowed up to ± 3% for class one and ± 8% for other classes.

3.7.5 Hardness Test

The hardness of a brick surface is determined by making a scratch on the surface of a brick with the help of a finger nail. If no impression is left on the surface of the brick, the brick is considered to be hard.

3.7.6 Soundness Test

Two bricks are struck against each other slightly. A good brick has a clear ringing sound, and the bricks should not get break.

3.7.7 Structure Test

In this test a brick is broken, and the broken surfaces are examined for the structure. It should be uniformly burnt, homogeneous in structure, compact and free from any defects such as holes, lumps, etc.

3.8 TYPES OF BRICKS

There are different kinds of bricks which differ from the conventional building bricks with respect to their shape, specifications and special purpose for which they are made. Those bricks are discussed in the subsequent sections.

3.8.1 Specially Shaped Bricks

1. Bull-nosed Bricks

A brick moulded with a rounded angle is termed as the bull nose. It is used for rounded quoin. A quoin is a connection which is formed when a wall takes a smooth circular turn. The centre of the curved position is located on the long centre line of the brick (Fig. 3.8(a)).

Figure 3.8 Typical shapes of bull-nosed and cast bricks

2. Cant or Plinth Bricks

This has a slant-cum-straight edge at one end which is used in a plinth or in a door and window joints (Fig. 3.8(b)).

3. Circular Bricks

These bricks are provided with internal and external faces to be curved to meet the requirement of the particular curve and radius of the wall. These bricks are used for structures like towers, wells, etc. These bricks have to be specially made with a particular curvature (Fig. 3.9(a)).

4. Squint Bricks

These bricks have a special edge. These are used in the construction of active and obtuse squint quoins. Actual requirements should be provided for the manufacture of such bricks (Fig. 3.9(b)).

Figure 3.9 Typical shapes of circular and squint bricks

5. Coping Bricks

These bricks are made to suit the thickness of walls on which coping is to be provided. Different forms such as chamfered, half-round or saddle-back can be made (Fig. 3.10(a)).

Figure 3.10 Typical shapes of brick copings and a cornice brick

6. Cornice Bricks

In the construction of a cornice, different shapes are made to give a beautiful appearance. Such bricks are to be made especially for a particular purpose (Fig. 3.10(b)).

7. Perforated Bricks

These bricks are provided with cylindrical holes throughout their thickness (Fig. 3.11(a)). Because of holes the bricks are light in weight and easy to dry and burn. These are used for panel walls in small and multi-storeyed buildings. It provides maximum amount of ventilation, and the perforations are placed such that they do not permit the entry of rats or mices.

Figure 3.11 Typical shapes of perforated and clay hollow bricks

8. Hollow Clay Bricks

These bricks are made out of specially made homogeneous clay. They are uniform in colour and have a fine, compact and uniform texture. These bricks are of light weight. As bricks are hollow they provide insulation against heat, sound and dampness to the building (Fig. 3.11(b)).

9. Paving Bricks

Paving bricks are made from rock clay. The clay is burnt at a very high temperature than that of ordinary bricks. Paving bricks are used in roads and to resist the abrasive action of traffic. The paving bricks may be plain or chequered. These bricks are non-slipping and are hence preferred for street pavements, garden walks, etc. (Fig. 3.12).

Figure 3.12 Typical chequered paving brick

3.8.2 Refractory Bricks

Refractory bricks, also called Refractory Fire Bricks, are prepared from fire clay in the same manner as ordinary bricks. Refractory bricks contain about 30% alumina and 70% silica. After drying, they are burnt in kilns at high temperature ranging from 1400–1900°C. These bricks are yellowish white in colour.

As the fire bricks can resist high temperatures without softening or melting, they are used for the linings of interior surfaces of furnaces, chimneys, kilns, ovens, fireplaces, etc. The compressive strength of these bricks varies from 200–220 N/mm², and the percentage absorption varies from 5–10.

3.8.3 Pressed Bricks

These bricks are made by pressing the clay to a high pressure of about 40 kg/sq.cm. They are directly burnt without drying. A special type of oil is used in the process which gives a glazed surface. The glazed surface may get peeled off when exposed to weather.

These bricks are of a regular shape, are compact and have high strength. These are used for decorative purposes of very high quality.

3.8.4 Sand–Lime Bricks

Autoclaved calcium silicate bricks are popularly known as sand–lime bricks. These bricks are made from a mixture of 95% of sand and 5% of lime (CaO) by weight. These bricks are hard, strong and are uniform in colour and texture. It presents a smooth and soft surface and hence may not be suitable for plastering. They are used for ornamental purposes.

3.8.5 Heavy-duty Bricks

These bricks are of very high quality, viz., high compressive strength, low water absorption, high durability and high bulk density. These bricks are free from any defects. They are used in heavy engineering works such as bridge structures, multi-storeyed buildings and industrial foundations.

3.8.6 Sewer Bricks

These bricks are manufactured using surface clay, fire clay or shale or a combination of these materials. They are used for the lining of walls, roofs and floors of sewers for ordinary domestic sewage. They are not suitable for industrial use as they are not acid resistant.

1. The main constituents of good brick earth are alumina, silica, lime, oxide of iron and magnesia.
2. Alumina imparts plasticity to the earth so that it can be moulded. Excess alumina leads to shrinking and warping.
3. The presence of silica prevents cracking, shrinking and warping of raw bricks. It imparts a uniform shape to the bricks.
4. Lime enables the silica to melt during burning and binds the particles together. Excess lime causes the brick to melt too readily, and the shape is thus lost.
5. Iron oxide acts as a flux and helps the grains of sand to melt and bind the particles of clay together.
6. A small quantity of magnesia in brick earth imparts a yellow tint and decreases shrinkage. Excess of magnesia causes decay of bricks.
7. The required chemical combination of earth is:

   Alumina or clay – 20–30% by weight

   Silica or sand – 35–50% by weight

   Silt – 20–25% by weight

   Others – 1–2% by weight

8. Harmful ingredients in brick earth are alkalis, pebbles, iron pyrites, vegetation and organic matter and limestone.
9. Brick earth in the field has to be tested for consistency, moulding properties and shrinkage.
10. The preparation of soil comprises of the removal of top soil, digging and spreading, cleaning, weathering, blending and tempering.
11. Moulding is the process by which wet bricks are made using a mould of size 200 mm × 10 mm.
12. Moulding of bricks is carried out either by hand or by a machine. Hand moulding consists of ground moulding and table moulding.
13. There are two types of machine moulding, viz., the plastic method and the dry method.
14. Bricks are dried by natural drying or artificial drying.
15. Bricks are burnt in kilns after moulding and drying so as to impart hardness and strength and to increase the density of the brick so that it absorbs less quality of water.
16. Kilns are of three types, viz., open kilns or clamps, intermittent kilns and continuous bricks. There are three types of continuous kilns, viz., Bull Trench Kiln, Tunnel Kiln and Hoffman’s Kiln.
17. Bricks are broadly classified into two categories, viz., sun-dried bricks and burnt bricks.
18. The following tests are conducted on bricks to assess their suitability:

    a. Compressive Strength Test

    b. Water Absorption Test

    c. Efflorescence Test

    d. Dimensional Tolerance Test

    e. Soundness Test

    f. Hardness Test

    g. Structure Test

19. Specially shaped bricks are:

    (i) Bull-nosed bricks

    (ii) Cast or plinth bricks

    (iii) Circular bricks

    (iv) Squint bricks

    (v) Coping bricks

    (vi) Cornice bricks

    (vii) Perforated bricks

    (viii) Hollow clay blocks

    (ix) Paving bricks

20. Refractory bricks are prepared from fire clay in the same manner as ordinary bricks. Refra ctory bricks contain about 30% alumina and 70% silica and are burnt at a high temperature of 1400–1900ºC.

1. What are the constituents of good brick earth?
2. Explain the chemical composition of brick earth.
3. Explain briefly the harmful ingredients in brick earth.
4. How do you conduct field tests to assess the qualities of brick earth?
5. What factors are to be considered in the selection of a site for brick earth?
6. Discuss the operation of preparation of soil for the manufacture of bricks.
7. What is moulding of bricks? Explain.
8. Explain the advantages of machine moulding over hand moulding.
9. Why is drying of bricks needed?
10. Compare the merits and demerits of burning bricks in clamp and kilns.
11. Describe the process of burning bricks in intermittent kilns.
12. Explain the principles of continuous kilns.
13. Briefly explain the qualities of good bricks.
14. How do you classify bricks? Explain.
15. Distinguish between sun-dried bricks and burnt bricks.
16. What is the basis on which bricks are classified under the Indian Standard?
17. What are the field tests to determine the suitability of bricks for construction?
18. Enumerate specially shaped bricks and their uses.
19. What are refractory bricks? Where are they used?
20. Explain the advantages of sand–lime bricks.