Classification of Bricks

I) Classification of Bricks Based on Building Process

a) Unburnt Bricks

These are half burnt bricks and the colour is yellow. The strength is low. They are used as surki in lime terracing. They are used as soiling under RCC footing or basement. Such bricks should not be exposed to rainwater.

b) Burnt Bricks

Burnt bricks are made by burning them in the kiln. First class, second class, third class and fourth class bricks are burnt bricks.

c) Over Burnt or Jhama Bricks

It is often known as the vitrified brick as it is fired at high temperature and for a longer period of time than conventional bricks. As a result, the shape is distorted. The absorption capacity is high. The strength is higher or equivalent to first class bricks. It is used as lime concrete for the foundation. It is also used as coarse aggregate in the concrete of slab and beam which will not come in contact with water.

II) Classification of Bricks Based on Quality

Burnt bricks are classified into the following four categories.

a) First Class Brick

These bricks are table moulded and of standard shape. The colour of these bricks is uniform yellow or red. It is well burnt, regular texture, uniform shape. The absorption capacity is less than 10%, crushing strength is 280 kg/cm² (mean) where it is 245 kg/cm² (minimum). It doesn’t have efflorescence. It emits a metallic sound when struck by another similar brick or struck by a hammer. It is hard enough to resist any fingernail expression on the brick surface if one tries to do with a thumbnail. It is free from pebbles, gravels or organic matters. The comply all the qualities of good bricks and used for superior work of permanent nature. The thickness of mortar joints doesn’t exceed 10mm. The uses are given below. In a building of long durability, say 100 years For building exposes to a corrosive environment For making coarse aggregates of concrete Recommended for exposed face work in masonry structures, flooring and reinforced brick work

b) Second Class Brick

These bricks are ground moulded and they are burnt in kilns. The size is standard, colour is uniform yellow or red. It is well burnt, slightly over burnt is acceptable. It has a regular shape; efflorescence is not appreciable. The absorption capacity is more than 10% but less than 15%. Crushing strength is 175kg/cm²(mean) where the minimum is 154 kg/cm². It emits a metallic sound when struck by another similar brick or struck by a hammer. It is hard enough to resist any fingernail expression on the brick surface if one tries to do with a thumbnail. It is used for the construction of one-storied buildings, temporary shed when intended durability is not more than 15 years. These bricks are commonly used at places where brick work is to be provided with a coat of plaster. The thickness of mortar joint is 12 mm.

c) Third Class Brick

These bricks are ground moulded and they 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. The shape and size are not regular. The colour is soft and light red coloured. It is under burnt, slightly over burnt is acceptable. It has extensive efflorescence. The texture is non-uniform. The absorption capacity is more than 15% but less than 20%. The crushing strength is 140kg/cm²(mean) where the minimum crushing strength is 105kg/cm². It emits a dull or blunt sound when struck by another similar brick or struck by a hammer. It leaves fingernail expression when one tries to do with the thumbnail. They are used for unimportant and temporary structures and at places where rainfall is not heavy.

d) Fourth Class Bricks

These are over burnt bricks with irregular shape and dark colour. These bricks are used as aggregate for concrete in foundation, floors, roads etc. because of the fact that the over burnt bricks have compacted structure and hence, they are sometimes found stronger than even first class bricks.

III) Classification of Bricks Based on Manufacturing Method

a) Extruded Brick

It is created by forcing clay and water into a steel die, with a very regular shape and size, then cutting the resulting column into shorter units with wires before firing. It is used in constructions with limited budgets. It has three or four holes constituting up to 25% volume of the brick.

b) Moulded Brick

It is shaped in moulds by hand rather being in the machine.

c) Dry pressed Brick

It is the traditional types of bricks which are made by compressing clay into moulds. It has a deep frog in one bedding surface and shallow frog in another.

IV) Classification Based on Strength

The Bureau of Indian Standards (BIS) has classified the bricks on the basis of compressive strength and is as given in Table 1.

Table 1 Classification of Bricks based on Compressive Strength (IS: 1077) 

Class	Average compressive strength not less than (N/mm2)
35	35.0
30	30.0
25	25.0
20	20.0
17.5	17.5
15	15.0
12.5	12.5
10	10.0
7.5	7.5
5	5.0
3.5	3.5

The burnt clay bricks having compressive strength more than 40.0 N/mm² are known as heavy duty bricks and are used for heavy duty structure such as bridge, foundation for industrial building, multi-storey building, etc. The water absorption of these bricks is limited to 5 per cent. Each class of bricks as specified above is further divided into subclasses A and B based on tolerance and shape. Subclass-A bricks should have smooth rectangular face with sharp corner and uniform colour. Subclass - B bricks may have slightly distorted and round edges.

V) Classification of Bricks Based on Raw Materials

a) Burnt Clay Brick

It is obtained by pressing the clay in moulds and fried and dried in kilns. It is the most used bricks. It requires plastering when used in construction works.

b) Fly Ash Clay Brick

It is manufactured when fly ash and clay are moulded in 1000 degree Celsius. It contains a high volume of calcium oxide in fly ash. That is why usually described as self-cementing. It usually expands when coming into contact with moisture. It is less porous than clay bricks. It proved a smooth surface so it doesn’t need plastering.

c) Concrete Brick

It is made of concrete and is the least used bricks. It has low compression strength and is of low quality. These bricks are used above and below the damp proof course. These bricks can be used for facades, fences and internal brickworks because of their sound reductions and heat resistance qualities. It is also called mortar brick. It can be of different colours if the pigment is added during manufacturing. It should not be used below ground.

d) Sand-Lime Brick

Sand, fly ash and lime are mixed and moulded under pressure. During wet mixing, a chemical reaction takes place to bond the mixtures. Then they are placed in the moulds. The colour is greyish as it offers something of an aesthetic view. It offers a smoother finish and uniform appearance than the clay bricks. As a result, it also doesn’t require plastering. It is used as a load bearing member as it is immensely strong.

e) Fire Brick

It is also known as refractory bricks. It is manufactured from a specially designed earth. After burning, it can withstand very high temperature without affecting its shape, size and strength. It is used for the lining of chimney and furnaces where the usual temperature is expected to be very high.

VI) Classification of Bricks Based on Using Location

a) Facing Brick

Facing Bricks are made primarily with a view to have good appearance, either of colour or texture or both. The face material of any building is known as facing brick. Facings bricks are standard in size, are stronger than other bricks and also have better durability. The colour is red or brown shades to provide a more aesthetic look to the building. There are many types of facing bricks which use different techniques and technology. Facing bricks should be weather resistant as they are most generally used on the exterior wall of buildings.

b) Backing Brick

These types of brick don’t have any special features. They are just used behind the facing bricks to provide support.

VII) Classification of Bricks Based on Weather-resisting Capability

a) Severe Weather Grade

These types of bricks are used in the countries which are covered in snow most of the time of year. These bricks are resistant to any kind of freeze-thaw actions.

b) Moderate Weather Grade

These types of bricks are used in tropical countries. They can withstand any high temperature.

c) No Weather Grade

These bricks do not have any weather resisting capabilities and used on the inside walls.

VIII) Classification of Bricks Based on Their Use

a) Common Bricks

These bricks are the most common bricks used. They don’t have any special features or requirements. They have low resistance, low quality, low compressive strength. They are usually used on the interior walls.

b) Engineering Bricks

These bricks are known for many reasons. They have high compressive strength and low absorption capacity. They are very strong and dense. They have good load bearing capacity, damp proof, and chemical resistance properties. They have a uniform red colour. They are classified as Class A, class B, class C. Class A is the strongest but Class B is most used. They are used for mainly civil engineering works like sewers, manholes, ground works, retaining walls, damp proof courses etc.

IX) Classification of Bricks Based on Shape

a) Bullnose Brick

These bricks are moulded into round angles. They are used for rounded quoin.

b) Airbricks

These bricks contain holes to circulate air. They are used on suspended floors and cavity walls.

c) Channel Bricks

They are moulded into the shape of a gutter or channel. They are used in drains.

d) Coping Bricks

They can be half round, chamfered, saddleback, angled varied according to the thickness of the wall.

e) Cow Nose Bricks

Bricks having double bullnose known as cow nose bricks.

f) Coping Bricks

These bricks are used to cap the tops of parapets or freestanding walls.

g) Brick Veneers

These bricks are thin and used for cladding.

h) Curved Sector Bricks

These are curved in shape. They are used in arcs, pavements, etc.

i) Hollow Bricks

These bricks are around one-third of the weight of the normal bricks. They are also called cellular or cavity bricks. Their thickness is from 20-25mm. These bricks pave the way to quicker construction as they can be laid quickly compared to the normal bricks. They are used in partitioning.

j) Paving Bricks

These bricks contain a good amount of iron. Iron vitrifies bricks at low temperature. They are used in garden park floors, pavements. These bricks withstand the abrasive action of traffic thus making the floor less slippery.

k) Perforated Bricks

These bricks contain cylindrical holes. They are very light in weight. Their preparation method is also easy. They consume less clay than the other bricks. They can be of different shapes like round, square, rectangular. They are used in the construction of the panels for lightweight, structures and multi-storeyed frame structures.

l) Purpose Made Bricks

These bricks are made for specific purposes. Engineering bricks are made for civil engineering constructions such as sewers, manholes, retaining walls. Fire bricks are made for chimneys and fireworks. Ornamental bricks are made to use for cornices and corbels. Arch bricks are used in arches.

X) Classification of Bricks Based on the Basis of Manufacture

a) Hand-made

These bricks are hand moulded.

b) Machine-made

Depending upon mechanical arrangement, bricks are known as

Wire Cut Bricks - bricks cut from clay extruded in a column and cut off into brick sizes by wires

Pressed Bricks - when bricks are manufactured from stiff plastic or semi-dry clay and pressed into moulds

Moulded Bricks - when bricks are moulded by machines imitating hand mixing.

XI) Classification of Bricks Based on the Basis of Burning

a) Pale Bricks

These are under burnt bricks obtained from outer portion of the kiln.

b) Body Bricks

These are well burnt bricks occupying central portion of the kiln.

c) Arch Bricks

These are over burnt also known as clinker bricks obtained from inner portion of the kiln.

XII) Classification of Bricks Based on the Basis of Type

a) Solid

Small holes not exceeding 25 per cent of the volume of the brick are permitted; alternatively, frogs not exceeding 20 per cent of the total volume are permitted.

b) Perforated

Small holes may exceed 25 per cent of the total volume of the brick.

c) Hollow

The total of holes, which need not be small, may exceed 25 per cent of the volume of the brick.

d) Cellular

Holes closed at one end exceed 20 per cent of the volume.

XIII) Classification of Bricks Based on the Basis of Finish

a) Sand-faced Brick

It has textured surface manufactured by sprinkling sand on the inner surfaces of the mould.

b) Rustic Brick

It has mechanically textured finish, varying in pattern.

Different Forms of Bricks

Some of the common type of bricks, depending upon the places of use, are shown in Fig.1. Round ended and bull nosed bricks are used to construct open drains. For door and window jambs, cant brick, also called splay brick are most suitable. The double cant brick is used for octagonal pillars. Cornice brick is used from architectural point of view. A compass brick which is tapering in both directions along its length is used to construct furnaces. Perforated brick is a well burned brick, but is not sound proof. The hollow bricks are about l/3rd of the weight of normal bricks and are sound and heat proof, but are not suitable where concentrated loads are expected. Top most bricks course of parapets is made with coping bricks and it drain off the water from the parapets. When the brick is cut along the length, it is called queen closer and when cut at one end by half header and half stretcher, it is known as king closer.

Fig. 1 Forms of Bricks