Manufacturing of Bricks

The following operations are involved in the manufacturing of brick.

I) Preparation of Clay

The preparation of clay involves following operations.

a) Unsoiling

The soil used for making building bricks should be processed so as to be free of gravel, coarse sand (practical size more than 2 mm), lime and kankar particles, organic matter, etc. About 20 cm of the top layer of the earth, normally containing stones, pebbles, gravel, roots, etc. is removed after clearing the trees and vegetation.

b) Digging

Clay dug out from ground is spread on level ground about 60 to 120 cm heaps. After removing the top layer of the earth, proportions of additives such as fly ash, sandy loam, rice husk ash, stone dust etc. should be spread over the plane ground surface on volume basis. The soil mass is then manually excavated, puddled, watered and left over for weathering and subsequent processing. The digging operation should be done before rains.

c) Cleaning

Stones, pebbles, vegetable matter etc. should be removed from soil.

d) Weathering

Clay is exposed to atmosphere from few weeks to full season. Stones, gravels, pebbles, roots, etc. are removed from the dug earth and the soil is heaped on level ground in layers of 60 -120 cm. The soil is left in heaps and exposed to weather for at least one month in cases where such weathering is considered necessary for the soil. This is done to develop homogeneity in the mass of soil, particularly if they are from different sources and also to eliminate the impurities which get oxidized. Soluble salts in the clay would also be eroded by rain to some extent, which otherwise could have caused scumming at the time of burning of the bricks in the kiln. The soil should be turned over at least twice and it should be ensured that the entire soil is wet throughout the period of weathering. In order to keep it wet, water may be sprayed as often as necessary. The plasticity and strength of the clay are improved by exposing the clay to weather.

e) Blending

Clay is made loose and any ingredient to be added to it is spread out at top and turning it up and down in vertical direction. The earth is mixed with sandy earth and calcareous earth in suitable proportions to modify the composition of soil. Moderate amount of water is mixed so as to obtain the right consistency for moulding. The mass is then mixed uniformly with spades. Addition of water to the soil at the dumps is necessary for the easy mixing and workability, but the addition of water should be controlled in such a way that it may not create a problem in moulding and drying. Excessive moisture content may affect the size and shape of the finished brick.

f) Tempering

Clay is brought to a proper degree of hardness, then water is added to clay and whole mass is kneaded or pressed under the feet of men or cattle for large scale. Tempering is usually done in pug mill. Tempering consists of kneading the earth with feet so as to make the mass stiff and plastic (plasticity means the property which wet clay has of being permanently deformed without cracking). It should preferably be carried out by storing the soil in a cool place in layers of about 30 cm thickness for not less than 36 hours. This will ensure homogeneity in the mass of clay for subsequent processing.

Pug mill consists of a conical iron tube as shown in Fig. 1. The mill is sunk 60 cm into the earth. A vertical shaft, with a number of horizontal arms fitted with knives, is provided at the centre of the tube. This central shaft is rotated with the help of bullocks yoked at the end of long arms. Steam, diesel or electric power may be used for this purpose. Blended earth along with required water, is fed into the pug mill from the top. The knives cut through the clay and break all the clods or lump clays when the shaft rotates. The thoroughly pugged clay is then taken out from opening provided in the side near the bottom. The yield from a pug mill is about 1500 bricks.

Fig. 1 Pug Mill

II) Moulding

Clay, which is prepared form pug mill is sent for the next operation of moulding. It is a process of giving a required shape to the brick from the prepared brick earth. Moulding may be carried out by hand or by machines. The process 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 higher pressures). Fire-brick is made by the soft mud process. Roofing, floor and wall tiles are made by dry-press method. The stiff-mud process is used for making all the structural clay products.

Fig. 2 Details of Mould

1) Hand Moulding

Moulds are rectangular boxes of wood or steel, which are open at top and bottom. Steel moulds are more durable and used for manufacturing bricks on large scale. Bricks prepared by hand moulding are of two types.

a) Ground Moulding

In this process, the ground is levelled and sand is sprinkled on it. The moulded bricks are left on the ground for drying. Such bricks do not have frog and the lower brick surface becomes too rough. To overcome these defects, moulding blocks or boards are used at the base of the mould. The process consists of shaping in hands a lump of well pugged earth, slightly more than that of the brick volume. It is then rolled into the sand and with a jerk it is dashed into the mould. The moulder then gives blows with his fists and presses the earth properly in the corners of the mould with his thumb. The surplus clay on the top surface is removed with a sharp edge metal plate called strike or with a thin wire stretched over the mould. After this the mould is given a gentle slope and is lifted leaving the brick on the ground to dry.

This method is adopted when a large and level land is available. To prevent the moulded brick from sticking to the side of the mould, sand is sprinkled on the inner sides of the mould, or the mould may be dipped in water every time before moulding is done. The bricks so produced are respectively called sand moulded and slop moulded bricks, the former being better since they provide sufficient rough surface necessary for achieving a good bond between bricks and mortar.

Fig. 3 Type of Strikes

b) Table Moulding

The bricks are moulded on stock boards nailed on the moulding table. Process of moulding these bricks is just similar to ground bricks on a table of size about 2m x 1m. Stock boards have the projection for forming the frog. The process of filling clay in the mould is the same as explained above. After this, a thin board called pallet is placed over the mould. The mould containing the brick is then smartly lifted off the stock board and inverted so that the moulded clay along with the mould rests on the pallet. The mould is then removed as explained before and the brick is carried to the drying site.

Fig. 4 Table Moulding

Fig. 6 Stock Board

2) Machine Moulding

This method proves to be economical when bricks in huge quantity are to be manufactured at the same spot. It is also helpful for moulding hard and string clay. These machines are broadly classified in two categories.

a) Plastic Clay Machines

This machine containing rectangular opening of size equal to length and width of a brick. Pugged clay is placed in the machine and as it comes out through the opening, it is cut into strips by wires fixed in frames, so these bricks are called wire cut bricks. This is a quick and economical process.

b) Dry Clay Machines

In these machines, strong clay is first converted into powder form and then water is added to form a stiff plastic paste. Such paste is placed in mould and pressed by machine to form hard and well-shaped bricks. These bricks have well behaviour than ordinary hand moulded bricks. They carry distinct frogs and exhibit uniform texture. These are burnt carefully as they are likely to crack.

III) Drying

Green bricks contain about 7–30% moisture depending upon the method of manufacture. The object of drying is to remove the moisture to control the shrinkage and save fuel and time during burning. The drying shrinkage is dependent upon pore spaces within the clay and the mixing water. The addition of sand or ground burnt clay reduces shrinkage, increases porosity and facilities drying. The moisture content is brought down to about 3 percent under exposed conditions within three to four days. Thus, the strength of the green bricks is increased and the bricks can be handled safely.

Clay products can be dried in open air driers or in artificial driers. The artificial driers are of two types, the hot floor drier and the tunnel drier. In the former, heat is applied by a furnance placed at one end of the drier or by exhaust steam from the engine used to furnish power and is used for fire bricks, clay pipes and terracotta. Tunnel driers are heated by fuels underneath, by steam pipes or by hot air from cooling kilns. They are more economical than floor driers. In artificial driers, temperature rarely exceeds 120°C. The time varies from one to three days. In developing countries, bricks are normally dried in natural open air driers. They are stacked on raised ground and are protected from bad weather and direct sunlight. A gap of about 1.0 m is left in the adjacent layers of the stacks so as to allow free movement for the workers. The drying of brick is by the following means.

i) Natural Drying – usually about 3 to 10 days to bricks to become dry under sunlight.

ii) Artificial Drying – drying by tunnels usually 120⁰C about 1 to 3 days.

Fig. 7 Method of Drying Bricks

IV) Burning

This is very important operation in the manufacturing of bricks to impart hardness, strength and makes them dense and durable. The burning of clay may be divided into three main stages.

a) Dehydration (400 – 650ºC)

This is also known as water smoking stage. During dehydration,

• The water which has been retained in the pores of the clay after drying is driven off and the clay loses its plasticity
• Some of the carbonaceous matter is burnt
• A portion of sulphur is distilled from pyrites
• Hydrous minerals like ferric hydroxide are dehydrated
• The carbonate minerals are more or less decarbonated

Too rapid heating causes cracking or bursting of the bricks. On the other hand, if alkali is contained in the clay or sulphur is present in large amount in the coal, too slow heating of clay produces a scum on the surface of the bricks.

b) Oxidation Period (650 – 900ºC)

During the oxidation period, remainder of carbon is eliminated and the ferrous iron is oxidized to the ferric form. The removal of sulphur is completed only after the carbon has been eliminated. Sulphur on account of its affinity for oxygen, also holds back the oxidation of iron. Consequently, in order to avoid black or spongy cores, oxidation must proceed at such a rate which will allow these changes to occur before the heat becomes sufficient to soften the clay and close its pore. Sand is often added to the raw clay to produce a more open structure and thus provide escape of gases generated in burning.

c) Vitrification

To convert the mass into glass like substance, the temperature ranges from 900 – 1100°C for low melting clay and 1000 – 1250°C for high melting clay. Great care is required in cooling the bricks below the cherry red heat in order to avoid checking and cracking. Vitrification period may further be divided into

i) Incipient Vitrification

It is the stage at which 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.

ii) Complete Vitrification

The stage at which more or less well-marked by maximum shrinkage.

iii) Viscous Vitrification

This stage produced by a further increase in temperature which results in a soft molten mass, a gradual loss in shape, and a glassy structure after cooling. Generally, clay products are vitrified to the point of viscosity. However, paving bricks are burnt to the stage of complete vitrification to achieve maximum hardness as well as toughness.

Burning of bricks is done either in clamps or in kilns. Clamps are temporary structures and they are adopted to manufacture bricks on small scale. Kilns are permanent structures and they are adopted to manufacture bricks on a large scale.

a) Burning in Clamp ox Pazawah

A typical clamp is shown in Fig. 8. The bricks and fuel are placed in alternate layers. The amount of fuel is reduced successively in the top layers. Each brick tier consists of 4–5 layers of bricks. Some space is left between bricks for free circulation of hot gasses. The total height of clamp in alternate layers of brick is about 3 to 4 m. After 30 per cent loading of the clamp, the fuel in the lowest layer is fired and the remaining loading of bricks and fuel is carried out hurriedly. The top and sides of the clamp are plastered with mud. Then a coat of cow dung is given, which prevents the escape of heat. The production of bricks is 2–3 lacs and the process is completed in six months. This process yields about 60 per cent first class bricks.

Fig. 8 Clamp

Advantages

• The bricks produced are tough and strong because burning and cooling are gradual
• Burning in clamps proves to be cheap and economical
• No skilled labour and supervision are required for the construction of clamps
• There is considerable saving of clamps fuel

Disadvantages

• Bricks are not of required shape
• It is very slow process
• It is not possible to regulate fire in a clamp
• Quality of brick is not uniform

b) Kiln Burning

The kiln used for burning bricks may be underground, e.g. Bull’s trench kiln or overground, e.g. Hoffman’s kiln. These may be rectangular, circular or oval in shape. When the process of burning bricks is continuous, the kiln is known as continuous kiln, e.g. Bull’s trench and Hoffman’s kilns. On the other hand, if the process of burning bricks is discontinuous, the kiln is known as intermittent kiln. The different types of kiln are explained below.

i) Intermittent kiln

The example of this type of an over ground and rectangular kiln. After loading the kiln, it is fired, cooled and unloaded and then the next loading is done. Since the walls and sides get cooled during reloading and are to be heated again during next firing, there is wastage of fuel. Bricks manufactured by intermittent up drought kilns are better than those prepared by clamps. But, bricks burnt by this process is not uniform. Intermittent kiln is of two types.

Fig. 9 Intermittent Kiln

a) Intermittent Up-Draught Kiln

This is in the form of rectangular with thick outside walls. Wide doors are provided at each end for loading and unloading of kilns. A temporary roof may be installed to protect from rain and it is removed after kiln is fired. Flues are provided to carry flames or hot gases through the body of kiln. The stages are given below.

1. Raw bricks are laid in row of thickness equal to 2 to 3 bricks and height 6 to 8 bricks with 2 bricks spacing between rows
2. Fuels are filled with brush wood which takes up a fire easily
3. Loading of kiln with raw bricks with top course is finished with flat bricks and other courses are formed by placing bricks on edges
4. Each door is built up with dry bricks and are covered with mud or clay
5. The kiln is then fired for a period of 48 to 60 hours draught rises in the upward direction from bottom of kiln and brings about the burning of bricks.
6. Kiln is allowed to cool down and bricks are then taken out
7. Same procedure is repeated for the next burning

b) Intermittent Down-Draught Kiln

These kilns are rectangular or circular in shape. They are provided with permanent walls and closed tight roof. Floor of the kiln has opening which are connected to a common chimney stack through flues. Working is same as up-draught kiln. But it is so arranged in this kiln that hot gases are carried through vertical flues upto the level of roof and they are then released. These hot gases move downward by the chimney draught and in doing so, they burn the bricks.

ii) Continuous Kiln

These kilns are continuous in operations. This means that loading, firing, cooling and unloading are carried out simultaneously in these kilns. The examples of continuous kiln are Hoffman’s kiln and Bull’s trench kiln. 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 is fired, the bricks in the next set of chambers are dried and preheated while bricks in the other set of chambers are loaded and in the last are cooled. There are three types of continuous kilns.

a) Bull’s Trench Kiln

This kiln may be of rectangular, circular or oval shape in the plan as shown in Fig 10. It is constructed in a trench excavated in ground either fully underground or partially projecting above ground openings is provided in the outer walls to act as flue holes. Dampers are in the form of iron plates and they are used to divide the kilns in suitable sections and it is the most widely used kiln in India.

The bricks are arranged in such a way that flues are formed. Fuel is placed in flues and it is ignited through flue holes after covering top surface with earth and ashes to prevent the escape of heat. Usually, two movable iron chimneys are employed to form draught. These chimneys are placed in advance of section being fired. Hence, hot gases leaving the chimney warm up the bricks in next section. Each section requires about one day to burn. The tentative arrangement for different sections as shown in Fig. 10 may be as follows

   Section 1 – Loading

   Section 2 – Emptying

   Section 3 – Unloading

   Section 4 – Cooling

   Section 5 – Burning

   Section 6 – Heating

Fig 10 Bull’s Trench Kiln

b) Hoffman’s Kiln

This kiln is constructed over ground and hence, it is sometimes known as flame kiln. Its shape is circular to plan and it is divided into a number of compartments or chambers. A permanent roof is provided; the kiln can even function during rainy season. Fig.11 shows plan and section of Hoffman’s kiln with 12 chambers

   Chamber 1 - Loading

   Chamber 2 to 5 – Drying and Pre-Heating

   Chambers 6 and 7 - Burning

   Chambers 8 to 11 - Cooling

   Chamber 12 – Unloading

The initial cost in stalling this kiln is high. The advantages of Hoffman’s kiln are given below.

• Good quality of bricks are produced
• It is possible to regulate heat inside the chambers through fuel holes
• Supply of bricks is continuous and regular
• There is considerable saving in fuel due to pre heating of raw bricks by flue gases

Fig.11 Hoffman’s Kiln

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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

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Brick

A brick is a type of block used to build walls, pavements and other elements in masonry construction. Bricks are obtained by moulding clay in rectangular blocks of uniform size and then by drying and burning these blocks. The term brick denotes a block composed of dried clay, but is now also used informally to denote other chemically cured construction blocks. Bricks can be joined using mortar, adhesives or by interlocking them. In India, standard brick size is 190 x 90 x 90 mm as per the recommendation of BIS. With mortar thickness, the dimension of the brick becomes 200 x 100 x 100 mm which is also known as the nominal size of the modular brick. As bricks are of uniform size, they can be properly arranged, light in weight and hence bricks replace stones. Block is a similar term referring to a rectangular building unit composed of similar materials, but is usually larger than a brick. Lightweight bricks (also called lightweight blocks) are made from expanded clay aggregate.

Fig.1 Bricks

One of the oldest building material brick continues to be a most popular and leading construction material because of being cheap, durable and easy to handle. Clay bricks are used for building-up exterior and interior walls, partitions, piers, footings and other load bearing structures. A brick is rectangular in shape and of size that can be conveniently handled with one hand. Brick may be made of burnt clay or mixture of sand and lime or of Portland cement concrete. Clay bricks are commonly used since these are economical and easily available. The length, width and height of a brick are interrelated as below.

     Length of brick = 2 × Width of brick + Thickness of mortar

     Height of brick = Width of brick

Characteristics of Good Brick

The essential requirements for building bricks are sufficient strength in crushing, regularity in size, a proper suction rate and a pleasing appearance when exposed to view. The various characteristics of good brick are listed below.

1) Size and Shape

The bricks should have uniform size and plane, rectangular surfaces with parallel sides and sharp straight edges. Bricks should be table moulded, well burnt in kilns, free from cracks and with sharp and square edges.

2) Colour

The brick should have a uniform deep red or cherry colour as indicative of uniformity in chemical composition and thoroughness in the burning of the brick.

3) Texture and Compactness

The surfaces should not be too smooth to cause slipping of mortar. The brick should have pre compact and uniform texture. A fractured surface should not show fissures, holes grits or lumps of lime.

4) Hardness and Soundness

The brick should be so hard that when scratched by a finger nail no impression is made. When two bricks are struck together, a ringing sound should be produced.

5) Bricks should not absorb water more than 20 percent by weight for first class bricks and 22

percent by weight for second class bricks, when soaked in cold water for a period of 24 hours.

6) Crushing Strength should not be less than 10 N/mm².

7) Brick Earth should be free from stones, kankars, organic matter, saltpetre, etc.

8) Bricks, when soaked in water for 24 hours, should not show deposits of white salts when allowed to dry in shade.

9) Bricks when broken should show a bright homogeneous and compact structure free from voids.

10) Bricks should be low thermal conductivity and they should be sound proof.

11) Bricks should not break when dropped flat on hard ground from a height of about one meter.

Ingredients of Good Brick Earth

For the preparation of bricks, clay or other suitable earth is moulded to the desired shape after subjecting it to several processes. After drying, it should not shrink and no crack should develop. The clay used for brick making consists mainly of silica and alumina mixed in such a proportion that the clay becomes plastic when water is added to it. It also consists of small proportions of lime, iron, manganese, sulphur etc. The proportions of various ingredients are as follows.

1) Alumina

It is the chief constituent of every kind of clay. A good brick earth should contain 20 to 30 percent of alumina. This constituent imparts plasticity to earth so that it can be moulded. If alumina is present in excess, raw bricks shrink and warp during drying and burning.

2) Silica

A good brick earth should contain about 50 to 60 percent of silica. Silica exists in clay either as free or combined form. As free sand, it is mechanically mixed with clay and in combined form; it exists in chemical composition with alumina. Presence of silica prevents crackers shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks. Durability of bricks depends on the proper proportion of silica in brick earth. Excess of silica destroys the cohesion between particles and bricks become brittle.

3) Lime

A small quantity of lime is desirable in finely powdered state to prevents shrinkage of raw bricks. Excess of lime causes the brick to melt and hence, its shape is last due to the splitting of bricks.

4) Oxide of Iron

A small quantity of oxide of Iron to the extent of 5 to 6 percent is desirable in good brick to imparts red colour to bricks. Excess of oxide of iron makes the bricks dark blue or blackish.

5) Magnesia

A small quantity of magnesia in brick earth imparts yellow tint to bricks and decreases shrinkage. But excess of magnesia decreases shrink leads to the decay of bricks.

The ingredients like, lime, iron pyrites, alkalies, pebbles, organic matter should not present in good brick earth.

Harmful Ingredients in Brick

1) Lime

When a desirable amount of lime is present in the clay, it results in good bricks, but if in excess, it changes the colour of the brick from red to yellow. When lime is present in lumps, it absorbs moisture, swells and causes disintegration of the bricks. Therefore, lime should be present in finely divided state and lumps, if any, should be removed in the beginning itself. Experience has shown that when lime particles smaller than 3 mm diameter hydrate and they produce only small pock mark. Particles larger than this might, if present in any quantity, cause unsightly blemishes or even severe cracking.

2) Pebbles, Gravels, Grits

It does not allow the clay to be mixed thoroughly and spoil the appearance of the brick. Bricks with pebbles and gravels may crack while working.

3) Iron Pyrites

It tends to oxidise and decompose the brick during burning. The brick may split into pieces. Pyrites decolourise the bricks.

4) Alkalis (Alkaline Salts)

It forms less than 10 per cent of the raw clay, are of great value as fluxes, especially when combined with silicates of alumina. These are mainly in the form of soda or potash. When present in excess, alkali makes the clay unsuitable for bricks. They melt the clay on burning and make the bricks unsymmetrical. When bricks come in contact with moisture, water is absorbed and crystallise. On drying, the moisture evaporates, leaving behind grey or white powder deposits on the brick which spoil the appearance. This phenomenon is called efflorescence. Efflorescence should always be dry brushed away before rendering or plastering a wall; wetting it will carry the salts back into the wall to reappear later. If bricks become saturated before the work is completed, the probability of subsequent efflorescence is increased and be protected from rain at all times. During laying, the bricks should be moistened only to the extent that is found absolutely essential to obtain adequate bond between bricks and mortar; newly built brickwork should be protected from rain.

5) Organic Matter

On burning green bricks, the organic matter gets charred and leave pores making the bricks porous; the water absorption is increased and the strength is reduced.

6) Carbonaceous Material

It is in the form of bituminous matter or carbon greatly affects the colour of raw clay. Unless proper precaution is taken to effect complete removal of such matter by oxidation, the brick is likely to have a black core.

7) Sulphur

It is usually found in clay as the sulphate of calcium, magnesium, sodium, potassium or iron, or as iron sulphide. Generally, the proportion is small. If, there is carbon in the clay and insufficient time is given during burning for proper oxidation of carbon and sulphur, the latter will cause the formation of a spongy, swollen structure in the brick and the brick will be decoloured by white blotches.

8)Water

A large proportion of free water generally causes clay to shrink considerably during drying, whereas combined water causes shrinkage during burning. The use of water containing small quantities of magnesium or calcium carbonates, together with a sulphurous fuel often causes similar effects as those by sulphur.

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Artificial Stones

Where durable natural stone is not available at reasonable cost, artificial stone, also known as cast stone becomes the choice. Artificial stone is made with cement and natural aggregates of the crushed stone and sand with desired surface finish. Suitable colouring pigments may be added. However, colouring should not exceed 15% by volume. Cement and aggregates are mixed in proportion of 1:3. Artificial stone can be moulded into the most intricate forms, cast into any size, reinforced to have higher strength, are most suitable for face work, grooves, rebates, etc., can be cast easily and are economical. Some of the artificial stones available are as follows.

1) Concrete Blocks

These are cast at site in the construction of piers or cast in moulds for steps, window sills, etc.

2) Ransom Stones

These are prepared by mixing soda silicate with cement to provide decorative flooring. These are also known as chemical stones. These have compressive strength of about 32 N/mm².

3) Victoria Stones

These are granite pieces with the surfaces hardened by keeping immersed in soda silicate for about two months.

4) Bituminous Stones

Granite and diorite are impregnated with prepared or refined tar to form bituminous stone. These are used for providing noise, wear and dust resistant stone surfaces.

5) Imperial Stones

Finely crushed granite is washed carefully and mixed with Portland cement. The mix is moulded in desired shape and then steam cured for 24 hours. The cured blocks are immersed in silicate tanks for three days. These stones are similar to Victoria stones.

6) Artificial Marbles

It can be either pre-cast or cast-in-situ. These are made from portland gypsum cement and sand. In the precast variety, the cast-stone is removed after three days. On the fifth day of casting these are treated with a solution, liquid fluorite of magnesia. It is then washed and wrapped in paper for 24 hours and then once again treated with the liquid. After one month the stone is polished by rubbing emery over the surface with a linen rag ball dipped in mixture of lime water and silicate of potash and then the process is repeated without emery. It is used for external works. Cast-in-situ variety is made by laying the mix on canvas, in thickness about 1.5 mm more than the required thickness of the stone. The surface is rubbed over and the air holes are filled with mix. Grinding is done by hand or machine. The surface is then rubbed with a polishing stone. Final rubbing is done with a ball of wool moistened with alum water dipped into a 1:3 mix of hartshorn powder and diatomite.

7) Garlic Stone

It is produced by moulding a mixture of iron slag and portland cement. These are used as flag stones, surface drains, etc.

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Tests on Building Stones

Following are different tests on building stones.

1) Acid Test

This test is carried out to understand the presence of calcium carbonate in building stone. A sample of stone weighing about 50 to 100 gm is taken. It is placed in a solution of hydrophobic acid having strength of one percent and is kept there for seven days. Solution is agitated at intervals. A good building stone maintains its sharp edges and keeps its surface free from powder at the end of this period. If the edges are broken and powder is formed on the surface, it indicates the presence of calcium carbonate and such a stone will have poor weathering quality.

2) Attrition Test

This test is done to find out the rate of wear of stones, which are used in road construction. The results of the test indicate the resisting power of stones against the grinding action under traffic. The following procedure is adopted.

• Samples of stones is broken into pieces about 60mm size.
• Such pieces, weighing 5 kg are put in both the cylinders of Deval’s attrition test machine.
• Diameter and length of cylinder are respectively 20 cm and 34 cm.
• Cylinders are closed. Their axes make an angle of 30 degree with the horizontal.
• Cylinders are rotated about the horizontal axis for 5 hours at the rate of 30 rpm.
• After this period, the contents are taken out from the cylinders and they are passed through a sieve of 1.5mm mesh.
• Quality of material which is retained on the sieve is weighed.

Percentage wear worked out as follows

3) Crushing Strength Test

Samples of stone is cut into cubes of size 40 mm and are finely dressed and finished. Maximum number of specimen to be tested is three. Such specimen should be placed in water for about 72 hours prior to test and therefore tested in saturated condition. Load bearing surface is then covered with plaster of paris of about 5mm thick plywood. Load is applied axially on the cube in a crushing test machine. Rate of loading is 140 kg/sq.cm per minute. Crushing strength of the stone per unit area is the maximum load at which the sample crushes or fails divided by the area of the bearing face of the specimen.

4) Freezing and Thawing Test

Stone specimen is kept immersed in water for 24 hours. It is then placed in a freezing machine at -12 ⁰C for 24 hours. Then it is thawed or warmed at atmospheric temperature. This should be done in shade to prevent any effect due to wind, sun rays, rain etc. This procedure is repeated several times and the behaviour of stone is carefully observed.

5) Hardness Test

For determining the hardness of a stone, the test is carried out as follows.

• A cylinder of diameter 25mm and height 25mm is taken out from the sample of stone.
• It is weighed.
• The sample is placed in Dorry’s testing machine and it is subjected to a constant pressure.
• Annular steel disc machine is then rotated at a speed of 28 rpm.
• During the rotation of the disc, coarse sand of standard specification is sprinkled on the top of disc.
• After 1000 revolutions, specimen is taken out and weighed.
• The coefficient of hardness is found out from the following equation

6) Impact Test

For determining the toughness of stone, it is subjected to impact test in an Impact Test Machine as followed.

• A cylinder of diameter 25mm and height 25mm is taken out from the sample of stones.
• It is then placed on cast iron anvil of machine.
• A steel hammer of weight 2 kg is allowed to fall axially in a vertical direction over the specimen.
• Height of first blow is 1 cm, that of second blow is 2 cm, that of third blow is 3 cm and so on.
• Blow at which specimen breaks is noted. If it is n^th blow, ‘n’ represents the toughness index of stone.

7) Microscopic Test

The sample of the test is subjected to microscopic examination. The sections of stones are taken and placed under the microscope to study the various properties such as

1. Average grain size
2. Existence of pores, fissures, veins and shakes
3. Mineral constituents
4. Nature of cementing material
5. Presence of any harmful substance
6. Texture of stones etc.

8) Smith’s Test

This test is performed to find out the presence of soluble matter in a sample of stone. Few chips or pieces of stone are taken and they are placed in a glass tube. The tube is then filled with clear water. After about an hour, the tube is vigorously stirred or shaken. Presence of earthy matter will convert the clear water into dirty water. If water remains clear, stone will be durable and free from any soluble matter.

9) Water Absorption Test

The test is carried out as follows.

• From the sample of stone, a cube weighing about 50gm is prepared. Its actual weight is recorded as W1 gm.
• Cube is then immersed in distilled water for a period of 24 hrs.
• Cube is taken out of water and surface water is wiped off with a damp cloth.
• It is weighed again. Let the weight be W2 gm.
• Cube is suspended freely in water and its weight is recorded. Let this be W3 gm.
• Water is boiled and cube is kept in boiling water for 5 hours.
• Cube is removed and surface water is wiped off with a damp cloth. Its weight is recorded. Let it be W4 gm.

From the above observations, values of the following properties of stones are obtained.

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Deterioration and Preservation of Stone Work

Deterioration of Stone Work

The various natural agents such as rain, heat, etc. and chemicals deteriorate the stones with time.

1) Rain

Rain water acts both physically and chemically on stones. The physical action is due to the erosive and transportation powers and the latter due to the decomposition, oxidation and hydration of the minerals present in the stones.

2) Physical Action

Alternate wetting by rain and drying by sun causes internal stresses in the stones and consequent disintegration.

3) Chemical Action

In industrial areas the acidic rain water reacts with the constituents of stones leading to its deterioration.

4) Decomposition

The disintegration of alkaline silicate of alumina in stones is mainly because of the action of chemically active water. The hydrated silicate and the carbonate forms of the alkaline materials are very soluble in water and are removed in solution leaving behind a hydrated silicate of alumina (Kaolinite). The decomposition of feldspar is represented as

5) Oxidation and Hydration

Rock containing iron compounds in the forms of peroxide, sulphide and carbonate are oxidised and hydrated when acted upon by acidic rain water. As an example the peroxide - FeO is converted into ferric oxide - Fe₂O₃ which combines with water to form FeO.nH₂O. This chemical change is accompanied by an increase in volume and results in a physical change manifested by the liberation of the neighbouring minerals composing the rocks. As another example iron sulphide and siderite readily oxidize to limonite and liberates sulphur, which combines with water and oxygen to form sulphuric acid and finally to sulphates.

6) Frost

In cold places frost pierces the pores of the stones where it freezes, expands and creates cracks.

7) Wind

Since wind carries dust particles, the abrasion caused by these deteriorates the stones.

8) Temperature Changes

Expansion and contraction due to frequent temperature changes cause stone to deteriorate especially if a rock is composed of several minerals with different coefficients of linear expansion.

9) Vegetable Growth

Roots of trees and weeds that grow in the masonry joints keep the stones damp and also secrete organic and acidic matters which cause the stones to deteriorate. Dust particles of organic or nonorganic origin may also settle on the surface and penetrate into the pores of stones. When these come in contact with moisture or rain water, bacteriological process starts and the resultant microorganism producing acids attack stones which cause decay.

10) Mutual Decay

When different types of stones are used together mutual decay takes place. For example, when sandstone is used under limestone, the chemicals brought down from limestone by rain water to the sandstone will deteriorate it.

11) Chemical Agent

Smokes, fumes, acids and acid fumes present in the atmosphere deteriorate the stones. Stones containing CaCO₃, MgCO₃ are affected badly.

12) Lichens

These destroy limestone but act as protective coats for other stones. Molluses gradually weaken and ultimately destroy the stone by making a series of parallel vertical holes in limestones and sand stones.

Durability of Stones

Quarrying and cutting have a great bearing on the weathering properties of stones. Stone from top ledges of limestone, granite and slate and from the exposed faces of the rock bed is likely to be less hard and durable. Highly absorbent stone should not be quarried in freezing weather since the rock is likely to split. The method of blasting and cutting also influences the strength of the stone and its resistance to freezing and temperature changes. Small, uniformly distributed charge of blasting powder has a lesser weakening effect than large concentrations of explosives.

A porous stone is less durable than a dense stone, since the former is less resistant to freezing. Also, rocks with tortuous pores and tubes are more apt to be injured by freezing than those of equal porosity having straight pores and tubes. Repeated hammering in cutting is likely to injure the stone. Polished stone is more enduring than rough surfaced work, since the rain slides off the former more easily. Stones from stratified rocks should be placed along the natural bed in order to secure maximum weathering resistance. Pyrite, magnetite and iron carbonate oxidize in weathering and cause discolouration of the stone in which they are present. Since oxidation is accompanied by a change in volume, the surrounding structure is weakened.

Preservation of Stones

Preservation of stone is essential to prevent its decay. Different types of stones require different treatments. But in general stones should be made dry with the help of blow lamp and then a coating of paraffin, linseed oil, light paint, etc. is applied over the surface. This makes a protective coating over the stone. However, this treatment is periodic and not permanent. When treatment is done with the linseed oil, it is boiled and applied in three coats over the stone. Thereafter, a coat of dilute ammonia in warm water is applied.

The structure to be preserved should be maintained by washing stones frequently with water and steam so that dirt and salts deposited are removed from time to time. However, the best way is to apply preservatives. Stones are washed with thin solution of silicate of soda or potash. Then, on drying a solution of CaCl₂ is applied over it. These two solutions called Szerelmy’s liquid, combine to form silicate of lime which fills the pores in stones. The common salt formed in this process is washed afterwards. The silicate of lime forms an insoluble film which helps to protect the stones.

Sometimes lead paint is also used to preserve the stones, but the natural colour of the stone is spoiled. Painting stone with coal tar also helps in the preservation but it spoils the beauty of the stone. Use of chemicals should be avoided as far as possible, especially the caustic alkalis. Although cleaning is easy with chemicals, there is the risk of introducing salts which may subsequently cause damage to the stone.

In industrial towns, stones are preserved by application of solution of baryta, Ba(OH)₂ — Barium hydrate. The sulphur dioxide present in acid reacts on the calcium contents of stones to form calcium sulphate. Soot and dust present in the atmosphere adhere to the calcium sulphate and form a hard skin. In due course of time, the calcium sulphate so formed flakes off and exposes fresh stone surface for further attack. This is known as sulphate attack. Baryta reacts with calcium sulphate deposited on the stones and forms insoluble barium sulphate and calcium hydroxide. The calcium hydroxide absorbs carbon dioxide from the air to form calcium carbonate.

The treatments, if carefully applied under favourable circumstances, may result in an apparent slowing down of the rate of decay. However, the rate of decay of stone is so slow that a short period experience is of very little value in establishing the effectiveness of the treatment. Also, there is some evidence that treatments which appear to be successful for few years, fail to maintain the improvement. In fact, the value of preservatives is not yet proved and they may actually be detrimental if judged over a long period.

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Couple

A pair of two equal and unlike parallel forces (i.e. forces equal in magnitude, with lines of action parallel to each other and acting in opposite directions) is known as a couple.

A couple is unable to produce any translatory motion (i.e., motion in a straight line). But it produces a motion of rotation in the body on which it acts. The simplest example of a couple is the forces applied to the key of a lock, while locking or unlocking it.

Arm of a Couple

The perpendicular distance (a), between the lines of action of the two equal and opposite parallel forces is known as arm of the couple as shown in Fig.1.

Fig. 1

Moment of a Couple

The moment of a couple is the product of the force (i.e., one of the forces of the two equal and opposite parallel forces) and the arm of the couple.

Mathematically,

                  Moment of a couple = P × a

Where,

     P = Magnitude of the force

     a = Arm of the couple

Classification of Couple

The couple may be classified into the following two categories, depending upon their direction, in which the couple tends to rotate the body, on which it acts.

1) Clockwise Couple

A couple, whose tendency is to rotate the body, on which it acts, in a clockwise direction, is known as a clockwise couple as shown in Fig. 2 (a). Such a couple is also called positive couple.

Fig. 2

2) Anticlockwise Couple

A couple, whose tendency is to rotate the body, on which it acts, in an anticlockwise direction, is known as an anticlockwise couple as shown in Fig. 2 (b). Such a couple is also called a negative couple.

Characteristics of a Couple

A couple (whether clockwise or anticlockwise) has the following characteristics.

1. The algebraic sum of the forces constituting the couple is zero.
2. The algebraic sum of the moments of the forces constituting the couple about any point is the same and equal to the moment of the couple itself.
3. A couple cannot be balanced by a single force. But it can be balanced only by a couple of opposite sense.
4. Any no. of coplanar couples can be reduced to a single couple, whose magnitude will be equal to the algebraic sum of the moments of all the couples.

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Varignon’s Principle of Moments (Law of Moments)

Varignon’s theorem states that “If a number of coplanar forces are acting simultaneously on a particle, the algebraic sum of the moments of all the forces about any point is equal to the moment of their resultant force about the same point.”

Proof

Case (i) When the forces are concurrent

Let ‘P’ and ‘Q’ be any two forces acting at a point O along lines OX and OY respectively and let D be any point in their plane as shown in Fig. 1. Line DC is drawn parallel to OX to meet OY at B. The line OB represent the force Q in magnitude and direction and OA represent the force P in magnitude and direction.

Fig. 1

With OA and OB as the adjacent sides, parallelogram OACB is completed and OC is joined. Let ‘R’ be the resultant of forces P and Q. 

Then, according to the “Theorem of parallelogram of forces”, R is represented in magnitude and direction by the diagonal OC of the parallelogram OACB.

The point D is joined with points O and A. The moments of P, Q and R about D are given by 2 x area of ΔAOD, 2 x area of ΔOBD and 2 x area of ΔOCD respectively.

With reference to Fig1. (a), the point D is outside the <AOB and the moments of P, Q and R about D are all anti-clockwise and hence these moments are treated as positive.

Now, the algebraic sum of the moments of P and Q about

      D = 2ΔAOD + 2ΔOBD

          = 2 (ΔAOD + ΔOBD)

          = 2 (ΔAOC + ΔOBD)

          = 2 (ΔOBC + ΔOBD)

          = 2ΔOCD

          = Moment of R about D

[As AOC and AOD are on the same base and have the same altitude. ΔAOD = ΔOBC. As AOC and OBC have equal bases and equal altitudes. ΔAOC = ΔOBC]

With reference to Fig 1. (b), the point D is within the <AOB and the moments of P, Q and R about D are respectively anti-clockwise, clockwise and anti-clockwise.

Now, the algebraic sum of the forces P and Q about

      D = 2ΔAOD - 2 ΔOBD

          = 2 (ΔAOD-ΔOBD)

          = 2 (ΔAOC- ΔOBD)

          = 2(ΔOBC - ΔOBD)

          = 2ΔOCD

          = Moment of R about D

Case (ii) When the forces are parallel

Let P and Q be any two like parallel forces (i.e. the parallel forces whose lines of action are parallel and which act in the same sense) and O be any point in their plane.

Let R be the resultant of P and Q.

Then,

       R = P + Q

From O, line OACB is drawn perpendicular to the lines of action of forces P, Q and R intersecting them at A, B and C respectively as shown in Fig 2.

Fig 2

Now, algebraic sum of the moments of P and Q about O

     = P x OA + Q x OB

     = P x (OC - AC) + Q x (OC + BC)

     = P x OC – P x AC + Q x OC + Q x BC

  But P x AC = Q x BC

Algebraic sum of the moments of P and Q about O

     = P x OC + Q x OC

     = (P + Q) x OC

     = R x OC

     = Moment of R about O

In case of unlike parallel forces also it can be proved that the algebraic sum of the moments of two unlike parallel forces (i.e. the forces whose lines of action are parallel but which act in reverse senses) about any point in their plane is equal to the moment of their resultant about the same point.

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Moment of a Force

Moment is the turning effect produced by a force, on the body, on which it acts. The moment of a force is equal to the product of the force and the perpendicular distance of the point, about which the moment is required and the line of action of the force.

Fig. 1

Mathematically, moment,

                        M = P × d

  Where,

           P = Force acting on the body

           d = Perpendicular distance between the point, about which the moment is required and the

                  line of action of the force.

Fig. 2

Let a force ‘P’ act on a body which is hinged at O. Then, moment of P about the point O in the body is

                                       Moment = F x ON

Where 

           ON = perpendicular distance of O from the line of action of the force F.

Graphical Representation of Moment

Consider a force P represented in magnitude and direction, by the line AB. Let O be a point, about which the moment of this force is required to be found out, as shown in Fig. 3. From O, draw OC perpendicular to AB. Join OA and OB.

Fig. 3 Representation of Moment

Now moment of the force P about O

                       = P × OC

                       = AB × OC

But AB × OC is equal to twice the area of triangle ABO.

Thus the moment of a force about any point is equal to twice the area of the triangle, whose base is the line to some scale representing the force and whose vertex is the point about which the moment is taken.

Units of Moment

Since the moment of a force is the product of force and distance, therefore the units of the moment will depend upon the units of force and distance. Thus, if the force is in Newton and the distance is in meters, then the units of moment will be Newton-meter (N-m). Similarly, the units of moment may be kN-m (i.e. kN × m), N-mm (i.e. N × mm) etc.

Types of Moment

The moments are of two types.

1) Clockwise Moment

It is the moment of a force, whose effect is to turn or rotate the body, about the point in the same direction in which hands of a clock move as shown in Fig. 4 (a).

2) Anticlockwise Moment

It is the moment of a force, whose effect is to turn or rotate the body, about the point in the opposite direction in which the hands of a clock move as shown in Fig. 4 (b).

The general convention is to take clockwise moment as positive and anticlockwise moment as negative. But there is no hard and fast rule regarding sign convention of moments.

Fig. 4 Clockwise and Anticlockwise Moments

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