Clay Bricks, Bricks Selection, Properties, manufacturing Process – 2022: Bricks are the structural units of rectangular shape and convenient size that are made of suitable clays by different processes involving moulding, drying and burning. It is the oldest and extensively used building material because of its durability, strength, low cost, reliability, and easy availability, etc. It is believed that bricks were firstly used in Egypt some 6000 years ago.

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Also Read: Introduction of Civil Engineering | Definition, Branches, Roles & Scopes in Civil Engineering | 2022


Properties of Bricks:

  • They are manufactured from naturally available Clay.
  • Possess good strength.
  • Easily Available.
  • Low cost material.
  • They are durable.
  • Light weight compared to stones.

Ingredients of Good Brick Earth

  • Alumina (20- 30 %)
  • Silica (50- 60%)
  • Iron Oxide (4- 6%)
  • Lime (4- 6%)
  • Magnesia (<1%)
  • Ferric Oxide (<7%)
  • Alkalis (<10%)

1. Alumina (20- 30%):-

  • All clays are chemically hydrous aluminum silicates.
  • The alumina content is responsible for giving the plastic character to the clay in wet conditions.
  • When alumina content is higher than 30 % the bricks becomes more plastic and also shrink on drying.
  • But if Alumina Content is less than 20 %, clay may be difficult to mould to proper shapes. Therefore required content of Alumina is about 20 – 30 %.

2. Silica ( 50- 60%):-

  • Silica present in ideal proportion i.e. 50- 60% imparts the qualities of hardness and strength to the bricks.
  • It is also responsible for resistance against shrinkage and durability of the brick to weather.
  • But if the percentage of silica are pretty excessive within the clay such bricks when burnt could be quite brittle and porous.

3. Iron Oxide (4-6%):-

  • This Oxide act as a flux, i.e. it lowers down softening temperature of silica and other clay components during firing. Further, the iron oxide in the clays may make their burning difficult and give them yellowish appearance.
  • In normal condition its gives red color of brick.
  • Excess of this makes the brick dark blue.

4. Lime (4-6 %):-

  • This Components make the burning of bricks quicker. Provided,
  • It should not be more than 4 %, because in this case may result in excessive softening of the clay on heating.
  • It should be present simplest in fine powder shape otherwise it could supply rise to slaking, that is dangerous and may reason sluggish disintegration of bricks.

5. Magnesia (<1%):-

  • The amount of magnesia should not exceed 1% in Brick.
  • It makes the brick Yellow.
  • During burning, it causes the clay to melt at slower fee than does lime and decreases warping.

Manufacturing Process(Bricks Selection) of Bricks

The Stages are listed below:-

  1. Selection of suitable type of clay
  2. Preparation and tempering of mud
  3. Moulding of brick units
  4. Drying of moulded bricks
  5. Firing or burning of dried bricks

1. Selection of Suitable type of Clay:-

Bricks Selection: A very good form of bricks cannot be made from every form of clay. The soil used for making building bricks should be processed in a good way to be free of gravel, coarse sand (particle size more than 2mm), lime and kankar particles, natural matters, etc. The soil to be used have to include the specified amount of components as defined above. If in case of soil not meeting the required quantity additional ingredients must be added.

When the manufacture of brick is on big scale, it’s really helpful to go through survey of deposits of clays which consist of mapping of place for vast analysis of chemical composition checking out the engineering properties of the specimen crafted from such earth. Such survey assure the entire quantity and quality of the clay(Bricks Selection).

2. Preparation and tempering of mud:-

After removing the top layer of the earth, the soil is excavated, puddled, watered and left for weathering and subsequent processing. Stones, gravels, pebbles, roots, etc. are removed from the dug earth and the soil is heaped on level ground in layers of 60- 120cm. The soil is left in heaps and exposed to weather for at least one month to develop homogeneity, plasticity and strength in the mass of soil. The earth is then with sandy-earth and calcareous-earth in suitable proportions to modify the composition of soil. Moderate amount of water is blended in order to achieve the proper consistency for moulding.

Tempering includes kneading the earth with feet in order to make the mass stiff and plastic. It have to be carried out by way of storing the soil in a cool location in layers of approximately 30cm thickness for no longer less than 36 hours. This can make sure homogeneity in the mass of clay for next processing. For manufacturing quality bricks, tempering is done in pug mills and the operation is called pugging.


3. Moulding of brick units:-

Moulding is a technique of giving a required shape to the brick from the prepared brick earth. Moulding may be executed through hand or through machines. The system of moulding of bricks may be the soft -mud (hand moulding), the stiff -mud (machine moulding) or the dry press process (moulding using maximum 10 per cent water and forming bricks at better pressures). Fire-brick is made through the smooth mud method. Roofing, floor and wall tiles are made by using dry-press technique. However, the stiff-mud technique is used for making all of the structural clay products.

4. Drying of moulded bricks:-

Green bricks incorporate moisture in them relying upon the technique of manufacture The object of drying is to eliminate the moisture to control the shrinkage and save fuel and time during burning. The drying shrinkage depends upon pore spaces inside the clay and the quantity of water used for blending. The moisture content is reduced down to  approximately  3  percent  below  uncovered conditions within 3 to 4 days. So that, the strength of the green bricks is accelerated and the bricks can be handled properly(Bricks Selection).

5. Firing or burning of dried bricks:-

Burning of bricks is the technique of eliminating the moisture content from the bricks and make it ready to use in construction. There are three stages in burning of clay:-

(a) Dehydration (400-650 ˚C):-

It is also called water smoking level. During dehydration,

  • The clay loses its plasticity.
  • The carbonaceous matter is burnt.
  • The portion of water which has been retained in the pores of the clay after drying is driven off and Sulphur is distilled from pyrites.
  • Hydrous minerals like ferric hydroxide are dehydrated.
  • The carbonate minerals are more or less decarbonated.
(b) Oxidation Period (550-900 ˚C):-

During the oxidation period,

  • Remainder of carbon is eliminated.
  • The ferrous iron is oxidized to the ferric form.

The elimination of sulphur is finished only after the carbon has been removed. Sulphur because of its affinity for oxygen, also holds back the oxidation of iron. Therefore, with a view to keep away from black or spongy, oxidation have to continue at such a rate which will permit these modifications to arise before the heat will become enough to soften the clay and close its pore. Sand is often introduced to the raw clay to produce a greater open shape and as a result provide escape of gases generated in burning.

(c) Vitrification:-

Extreme care is needed in cooling the bricks under the cherry red heat so as to keep away from checking and cracking. Vitrification period may further be divided into :-

  • Incipient vitrification :- At this the clay has softened sufficiently to cause adherence but not enough to close the pores or cause loss of space on cooling the material cannot be scratched by the knife.
  • Complete vitrification :- In it the clay is more or less well-marked by maximum shrinkage.
  • Viscous vitrification :- It is produced through a additional increase in temperature which leads to a soft molten mass, a gradual loss in shape, and a glassy shape after cooling. Usually, clay products are vitrified to the factor of viscosity, but paving bricks are burnt to the level of complete vitrification to gain maximum hardness as well as toughness.

Methods of Burning Bricks

There are two methods of burning i.e. Burning in Clamp or Pazawah and Burning in Kiln.

Burning in Clamp or Pazawah:
  • In Clamp burning the bricks and fuel are placed in alternate layers.
  • The quantity of fuel is decreased successively within the top layers.
  • Every brick tier includes four-five layers of bricks.
  • A few space is left among bricks for free movement of hot gasses.
  • After 30 percent loading of the clamp, the gas in the lowest layer is fired and the final loading of bricks and fuel is executed hurriedly.
  • The top and sides of the clamp are plastered with mud.
  • Then a coat of cow dung is given, which prevents the break out of heat.
  • The manufacturing of bricks is two-three lacs and the procedure is finished in six months.
  • This technique yields about 60 percent first class bricks.
Kiln Burning :-
  • The kiln used for burning bricks can be underground, e.g. Bull’s trench kiln or overground, e.g. Hoffman’s kiln.
  • These can be rectangular, circular or oval in shape.
  • When the technique of burning bricks is non-stop, the kiln is called continuous kiln, e.g. Bull’s trench and Hoffman’s kilns.
  • However if the procedure of burning bricks is discontinuous, the kiln is called intermittent kiln.
Intermittent Kiln :-

In this type of kiln after loading the kiln, it is fired, cooled and unloaded and then the next loading is done. Because the walls and sides get cooled in the course of reloading and are to be heated once more at some point of subsequent firing, there is wastage of fuel.

Continuous :-
  • In a continuous kiln, bricks are stacked in various chambers wherein the bricks undergo different treatments at the same time.
  • When the bricks in one of the chambers are fired, the bricks in the subsequent set of chambers are dried and preheated at the same time as bricks within the different set of chambers are loaded and within the remaining one they are cooled.

Requirements of Bricks

  • The color of the brick should be red or copper.
  • It should be well burnt in the furnace.
  • The surface should be free from cracks.
  • The edges should be sharp.
  • They should be hard enough when scratched with fingers, no scratch marks should be formed.
  • When two bricks are struck they should form an metallic ringing sound.
  • They should not break when dropped form a height of 1 mt.
  • Bricks shall not absorb water more than 20% of its dry weight.
  • Bricks should have an crushing strength between 3N/mm² to 15N/mm².

Classification of Bricks on basis of Quality(Bricks Selection):-

  • 1st class
  • 2nd Class
  • 3rd Class
  • 4th Class

(a) First Class Brick:-

  • These are thoroughly burnt and are of red, cherry or copper colour.
  • The surface should be smooth and rectangular, with parallel, sharp and straight edges and square corners.
  • These should be free from flaws, cracks and stones.
  • These should have uniform texture.
  • No impression should be left on the brick when a scratch is formed by a finger nail.
  • The fractured surface of the brick mustn’t show lumps of lime.
  • A metallic or ringing sound should come when two bricks are struck against one another.
  • Water absorption:- 12 to 15% when immersed in cold water for 24 hours.
  • The crushing strength of the brick shouldn’t be <10 N/mm².

Uses :- First-class bricks are used for exterior as well as interior walls, decoration works in masonry structures, flooring and reinforced brick work.

(b) Second Class Brick:-

  • Small cracks and distortions are permitted.
  • Water absorption:- 16 to 20% of its dry weight.
  • The crushing strength shouldn’t be <7.0 N/mm².

Uses :- Second class bricks are recommended for all important or unimportant hidden masonry works and centering of reinforced brick and reinforced cement concrete (RCC ) structures.

(c)Third Class Bricks:-

  • They are under burnt.
  • They are soft and light coloured producing a dull sound when struck against one another.
  • Water absorption is about 25 percent of dry weight.

Uses :- It is used for building temporary structures.


(d) Fourth Class Bricks:-

  • They may be overburnt and badly distorted in shape and size and are brittle in nature.

Uses :- These bricks are used for foundation and flooring in lime concrete and street base.

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