BRICKS
Bricks are structural units of rectangular shape and convenient size, and are made of suitable
clay by the process of moulding, drying and burning.
Since long, bricks are believed to have been used by the people of ancient civilization. This
has been established beyond doubt from the excavation of prehistoric sites like Indus Valley
civilization at Mohen-jo-daro and Harrappa. Bricks are the most favoured structural units
used for construction in the modern day world around the globe.
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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.
- Silica
- Alumina
- Lime
- Iron oxide
- Magnesia
Silica
o Brick earth should contain about 50 to % of silica.
o It is responsible for preventing cracking, shrinking and warping of raw bricks.
o It also affects the durability of bricks.
o If present in excess, then it destroys the cohesion between particles and the brick becomes brittle.
Alumina
o Good brick earth should contain about 20% to 30% of alumina.
o It is responsible for plasticity characteristic of earth, which is important in moulding operation.
o If present in excess, then the raw brick shrink and warp during drying.
Lime
o The percentage of lime should be in the range of 5% to 10% in a good brick earth.
o It prevents shrinkage of bricks on drying.
o It causes silica in clay to melt on burning and thus helps to bind it.
o Excess of lime causes the brick to melt and brick looses its shape.
Iron oxide
o A good brick earth should contain about 5% to 7% of iron oxide.
o It gives red colour to the bricks.
o It improves impermeability and durability.
o It gives strength and hardness.
o If present in excess, then the colour of brick becomes dark blue or blakish.
o If the quantity of iron oxide is comparatively less, the brick becomes yellowish in colour.
Magnesia
o Good brick earth should contain less a small quantity of magnesia about1%)
o Magnesium in brick earth imparts yellow tint to the brick.
o It is responsible for reducing shrinkage
o Excess of magnesia leads to the decay of bricks.
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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 pyrits.
o Pyrites discolourise 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 efflorescences ). 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.
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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 levelled 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.
Pug mill
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.
Moulding:
The clay which is prepared as above is then sent for the text operation of moulding.
Following are two types of moulding:
i. Hand Moulding
ii. Machine Moulding
Qualities of good brick
1. The bricks should be table-moulded, well-burnt in kilns, copper-colored, free from cracks and with sharp and square edges. The color should be uniform and bright.
2. The bricks should be uniform in shape and should be of standard size.
3. The bricks should give a clear metallic ringing sound when stuck with each other.
4. The bricks when broken or fractured should show a bright homogenous and uniform compact
structure free from voids.
5. The bricks should not absorb water more than 20 percent by weight for I class brick and 22
percent by weight for II class brick when soaked in water for 24hrs.
6. The bricks should be sufficiently hard. No impression should be left on brick surface, when it
is scratched with finger nail.
7. The bricks should not break into pieces when it is dropped on hard ground from a height of 1
meter.
8. The bricks should have low thermal conductivity and they should be sound-proof.
9. The bricks when soaked in water for 24hrs should not show deposits of white salts when allowed to dry in shade.
10. No brick should have the crushing strength less than 3.5 N/mm2.
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 colour appearance is grey instead of the
regular reddish colour.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 colours 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
(1) Absorption: A brick is taken and it is weighed dry. It is then immersed in water for a period of 16hrs. It is weighed again and the difference in weight indicates the amount of water absorbed by brick. It should not in any case exceed 20% of weight of drybrick.
(2) Crushing strength: The crushing strength of a brick is found out by placing it in a compression testing machine. It is pressed till it breaks. As per the code IS: 1077-1970, the minimum crushing strength should not be less than 3.5N/mm2. The brick with crushing strength of about 7 to 14 N/mm2 are graded as A and those having greater than 14N/mm2 are graded as AA.
(3) Hardness: In this test, a scratch is made on brick surface with the help of a finger nail. If no impression is left on the surface, the brick is treated to be sufficiently hard.
(4) Presence of soluble salts: the soluble salts, if present in bricks, will cause efflorescence on the surface of bricks. For finding out the presence of soluble salts in a brick, it is immerse in water for 24hrs. It is then taken out and allowed to dry in shade. The absence of grey or white deposits on its surface indicates the absence of soluble salts.
If the white deposits cover about 10% surface, the efflorescence is said to be slight and it is considered as moderate, when the white deposits cover about 50% o surface. If grey or white deposits are found on more than 50% of surface, the efflorescence becomes heavy and it is treated as serious.
(5) Shape and size: In this test, a brick is closely inspected. It should be of standard size and its shape should be truly rectangular with sharp edges. For this purpose, 20 bricks (19cm X 9cm X 9cm) are selected at random and they are stacked lengthwise, along the width and along the height.
For good quality bricks, the results should be within the following permissiblelimits. Length: 3680mm to 3920mm
Breadth: 1740mm to 1860mm
Height: 1740mm to 1860mm
(6) Soundness: In this test, the two bricks are taken and they are strucked with each other. The bricks should not break and a clear ringing sound should be produced.
(7) Structure: A brick is broken and its structure is examined. It should be homogeneous, compact and free from any defects such as holes, lumps etc.
Classification of bricks
He bricks can be broadly classified into two categories as follows:
1. The un-burnt or sun-dried bricks are dried with the help of heat received from sun after the process of moulding. These bricks can only be used in the construction of temporary and cheap structures. Such bricks should not be used at places where they are exposed to heavy rains.
2. The bricks used in construction field are burnt bricks and they are classified into following four categories:
I 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 satisfy all the qualities of good bricks which are mentioned earlier.
- These bricks are used for superior work of permanent nature.
II 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 straightly 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 plaster.
III 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 strucked with each other.
- They are used for unimportant and temporary structures and at places where rainfall is not heavy.
IV Class Bricks:
- These are over-burnt bricks with irregular shape and dark colored.
- 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 they are seems to be stronger than I Class bricks.
Uses of Bricks
- Construction of walls of any size
- Construction of floors
- Construction of arches and cornices
- Construction of brick retaining wall
- Making Khoa (Broken bricks of required size) to use as an aggregate in concrete
- Manufacture of surki (powdered bricks) to be used in lime plaster and lime concrete
- Brick is a durable material, and can last hundreds or thousands of years
- Brick is fireproof and can withstand exposure to high temperatures
- Brick offers good noise reduction and sound isolation capabilities
- Brick does not require the application of paints or other finishes for protection from the environment
- As a modular building component, problems with individual bricks can be addressed without the need to tear down and rebuild the entire structure.
- Since clay is available almost everywhere, brick can be fabricated locally, eliminating the costs associated with their shipment. This can mean that construction using brick as a material may be less expensive than with using stone, concrete, or steel.
- Brick is simpler to work with because of its uniformity in size, unlike stone which needs to be sized and dressed.
- Brick is easy to handle and skilled tradespeople that can construct with brick are plentiful.
In construction applications, bricks offer several advantages over alternative materials that serve the same purpose.
