In this article we will discuss about:- 1. Meaning of Bricks 2. Composition of Good Brick Earth 3. Harmful Ingredients in Brick Earth 4. Classification of Brick Earth 5. Qualities of Good Bricks 6. Factors Affecting Quality of Bricks 7. Tests for Bricks 8. Classification of Bricks 9. Uses of Bricks 10. Colours of Bricks 11. Size and Weight of Bricks 12. Shape of Bricks 13. Fire-Clays 14. Fire-Bricks.
Meaning of Bricks:
The bricks are obtained by moulding clay in rectangular blocks of uniform size and then by drying and burning these blocks. As bricks are of uniform size, they can be properly arranged and further, as they are light in weight, no lifting appliance is required for them.
The bricks do not require dressing and the art of laying bricks is so simple that the brickwork can be carried out with the help of unskilled labourers. Thus, at places where stones are not easily available, but if there is plenty of clay suitable for the manufacture of bricks, the bricks replace stones.
The common brick is one of the oldest building material and it is extensively used at present as a leading material of construction because of its durability, strength, reliability, low cost, easy availability, etc.
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The bricks seem to have been produced since the dawn of the civilization in the sun-dried form. The Great Wall of China (210 B.C.) was built with both, burnt and sun-dried bricks. The other examples of the use of bricks in early stage of civilization could be cited in Rome and other places.
The medieval cities were of wood and because of the disastrous fire potential of wood, the bricks replaced the wood over the years. For instance, the great fire of London in 1666 changed London from being a city of wood to one of brick. A number of country farm houses still exists in Great Britain and profess to be the monuments of the excellent hand-made bricks.
The bricks have been used all over the world in every class and kind of building. If the total bricks produced till today are to be counted, the figure would indeed be astronomical. At present, India has the production capacity to manufacture over 100000 million bricks through about 45000 local kilns in the unorganized sector.
It is understood that about 65 per cent of the bricks in the world goes into dwellings and the balance into commercial, industrial and institutional buildings.
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The bricks have established as an age old material right from the thatched house to the multi-storied buildings. They were initially handmade and used as load bearing material for various structures.
With the passage of time and advent of cement and steel, the frames only are filled up with the burnt clay bricks. The production of burnt clay bricks on a scientific and modern basis including proper mining of clays can lead to the availability of quality bricks.
In India, the process of brick making has not changed since many centuries except some minor refinements. There have been hardly any efforts in our country to improve the brick-making process for enhancing the quality of the bricks. The main reason for this attitude is that the production of bricks has been largely remained confined to the unorganized small sector. Some of the large mechanised brick plants came up in the past.
But they seem to have gone sick for some reason or the other. The result is that the construction industry in our country is largely dependent on the small sector which is unable to deliver high quality bricks in view of rising fuel cost, outdated technology and lower efficiency of production.
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Composition of Good Brick Earth:
Following are the constituents of good brick earth:
(1) Alumina:
It is the chief constituent of every kind of clay. A good brick earth should contain about 20% to 30% of alumina. This constituent imparts plasticity to the earth so that it can be moulded. If alumina is present in excess, with inadequate quantity of sand, the raw bricks shrink and warp during drying and burning and become too hard when burnt.
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(2) Silica:
It exists in clay either as free or combined. As free sand, it is mechanically mixed with clay and in combined form, it exists in chemical composition with alumina. A good brick earth should contain about 50 per cent to 60 per cent of silica. The presence of this constituent prevents cracking, shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks. The durability of bricks depends on the proper proportion of silica in brick earth. The excess of silica destroys the cohesion between particles and the bricks become brittle.
(3) Lime:
A small quantity of lime not exceeding 5 per cent is desirable in good brick earth. It should be present in a very finely powdered state because even small particles of the size of a pin-head cause flaking of the bricks. The lime prevents shrinkage of raw bricks. The sand alone is infusible. But it slightly fuses at kiln temperature in presence of lime.
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Such fused sand works as a hard cementing material for brick particles. The excess of lime causes the brick to melt and hence its shape is lost. The lumps of lime are converted into quick lime after burning and this quick lime slakes and expands in presence of moisture. Such an action results in splitting of bricks into pieces.
(4) Oxide of Iron:
A small quantity of oxide of iron to the extent of about 5 to 6 per cent is desirable in good brick earth. It helps as lime to fuse sand. It also imparts red colour to the bricks. The excess of oxide of iron makes the bricks dark blue or blackish. If, on the other hand, the quantity of iron oxide is comparatively less, the bricks will be yellowish in colour.
(5) Magnesia:
A small quantity of magnesia in brick earth imparts yellow tint to the bricks and decreases shrinkage. But excess of magnesia leads to the decay of bricks.
Harmful Ingredients in Brick Earth:
Following are the ingredients which are undesirable in the brick earth:
(1) Lime
(2) Iron Pyrites:
If iron pyrites are present in brick earth, the bricks are crystallized and disintegrated during burning because of the oxidation of the iron pyrites.
(3) Alkalies:
These are mainly in the form of soda and potash. The alkalies act as a flux in the kiln during burning and they cause bricks to fuse, twist and warp. As a result, the bricks are melted and they loose their shape. Further, the alkalies remaining in bricks will absorb moisture from the atmosphere, when bricks are used in masonry.
Such moisture, when evaporated, leaves behind grey or white deposits on the wall surface. The appearance of the building as a whole is then seriously spoiled.
(4) Pebbles:
The presence of pebbles or grits of any kind is undesirable in brick earth because it will not allow the clay to be mixed uniformly and thoroughly which will result in weak and porous bricks. Also, the brick containing pebbles will not break regularly as desired.
(5) Vegetation and Organic Matter:
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 gases will be evolved during the burning of the carbonaceous matter and it will result in the formation of small pores. Hence it is necessary to see that all these gases are removed during the process of burning for getting bricks of good quality.
Classification of Brick Earth:
The brick earth is classified in the following three categories:
(1) Loamy, mild or sandy clay
(2) Marls, chalky or calcareous clay
(3) Plastic, strong or pure clay.
(1) Loamy, Mild or Sandy Clay:
This type of earth consists of considerable amount of free silica in addition to alumina. The presence of sand helps in preventing cracking, shrinking and warping of bricks. The addition of lime in such clay helps to fuse sand and thereby to increase hardness of bricks.
A typical analysis of such clay is as follows:
(2) Marls, Chalky or Calcareous Clay:
This clay consists of considerable amount of chalk in addition to alumina and silica. Such clay generally makes good bricks. But to avoid undesirable effects of excess lime, the sand is sometimes added to such clay.
A typical analysis of such clay is as follows:
(3) Plastic, Strong or Pure Clay:
This clay consists of alumina and silica and it is sometimes referred to as strong clay or fat clay. The raw bricks will crack, shrink and warp during drying, if pure clay alone is used in making of bricks. Hence such clay is corrected by the addition of sand and ash. The sand prevents shrinkage and the ash provides lime to act as flux.
A typical analysis of such clay is as follows:
It should be noted that the best guide in the selection of brick clay would be the preparation of sample bricks from such clay. The sample bricks should be burnt in a simple kiln and their behaviour in drying and burning should be carefully noted. The sample bricks should be exposed to the sun and wind and their various properties should be tested to determine their utility.
If result is satisfactory, such clay should be adopted to manufacture bricks on a large scale. Otherwise necessary ingredients may be added to such clay to make it fit for brick making.
However the following two field tests may be carried out to determine the suitability of soil for the purpose of brick manufacture:
First Test:
The soil to be tested is ground to a fine powder and sufficient quantity of water is then mixed. It is then kneaded and converted into a plastic mass of required consistency. Then the balls of about 80 mm diameter are moulded with hands and these balls are allowed to dry in the sun.
If the dry balls deform in shape and crumble down easily on pressing, it indicates the excessive sand content in the soil. On the other hand, if the sand content is deficient, the balls will develop surface cracks on drying.
Second Test:
The soil is prepared as in case of first test with plastic consistency of such nature that it can be rolled in threads of 3 mm diameter. The bricks of standard size are then moulded with sharp edges and corners. These bricks are then allowed to dry in the sun for four days and they are examined for cracks developed due to the shrinkage.
Qualities of Good Bricks:
The good bricks which are to be used for the construction of important structures should possess the following qualities:
(i) The bricks should be table-moulded, well-burnt in kilns, copper-coloured, free from cracks and with sharp and square edges. The colour should be uniform and bright.
(ii) The bricks should be uniform in shape and should be of standard size.
(iii) The bricks should give a clear metallic ringing sound when struck with each other.
(iv) The bricks when broken or fractured should show a bright homogeneous and uniform compact structure free from voids.
(v) The brick should not absorb water more than 20 per cent by weight for first class bricks and 22 per cent by weight for second class bricks, when soaked in cold water for a period of 24 hours.
(vi) The bricks should be sufficiently hard. No impression should be left on brick surface, when it is scratched with finger nail.
(vii) The bricks should not break into pieces when dropped flat on hard ground from a height of about one metre.
(viii) The bricks should have low thermal conductivity and they should be sound-proof.
(ix) The bricks, when soaked in water for 24 hours, should not show deposits of white salts when allowed to dry in shade.
(x) No brick should have the crushing strength below 5.50 N/mm2.
Factors Affecting Quality of Bricks:
Following factors affect the quality of bricks:
(i) Composition of brick earth;
(ii) Preparation of clay and blending of ingredients;
(iii) Nature of moulding adopted;
(iv) Care taken in drying and stacking of raw or green bricks;
(v) Type of kiln used including type of fuel and its feeding;
(vi) Burning and cooling processes; and
(vii) Care taken in unloading.
It is thus obvious that not only the bricks of different brick fields will have different strengths, but in the same brick field, the bricks of the same batch may have different strengths.
The average crushing strength and tensile strength of hand moulded bricks are 60000 kN/m2 and 2000 kN/m2 respectively. The shearing strength of bricks is about one-tenth of the crushing strength. In practice however the bricks are not subjected to the tensile stresses.
It may be noted that the strength of brickwork mainly depends on the type of mortar used and not so much on the individual strength of the bricks.
Tests for Bricks:
A brick is generally subjected to the following tests to find out its suitability for the construction work:
(1) Absorption
(2) Crushing strength
(3) Hardness
(4) Presence of soluble salts
(5) Shape and size
(6) Soundness
(7) Structure.
(1) Absorption:
A brick is taken and .it is weighed dry. It is then immersed in water for a period of 16 hours. It is weighed again and the difference in weight indicates the amount of water absorbed by the brick. It should not, in any case, exceed 20 per cent of weight of dry brick.
(2) Crashing 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 IS: 1077-1970, the minimum crushing or compressive strength of bricks is 3.50 N/mm2. The bricks with crushing strength of 7 to 14 N/mm2 are graded as A and those having above 14 N/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 immersed in water for 24 hours. It is then taken out and allowed to dry in shade. The absence of grey or white deposits on its surface indicates absence of soluble salts.
If the white deposits cover about 10 per cent surface, the efflorescence is said to be slight and it is considered as moderate, when the white deposits cover about 50 per cent of surface. If grey or white deposits are found on more than 50 per cent of surface, the efflorescence becomes heavy and it is treated as serious, when such deposits are converted into powdery mass.
(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 of standard size (190 mm X 90 mm X 90 mm) 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 permissible limits:
(6) Soundness:
In this test, the two bricks are taken and they are struck 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:
The bricks can broadly be divided into two categories as follows:
(i) Un-burnt or sun-dried bricks; and
(ii) Burnt bricks.
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 exposed to heavy rains.
The bricks used in construction works are burnt bricks and they are classified into the following four categories:
(1) First class bricks
(2) Second class bricks
(3) Third class bricks
(4) Fourth class bricks.
(1) First Class Bricks:
These bricks are table-moulded and of standard shape and they are burnt in kilns. The surfaces and edges of the bricks are sharp, square, smooth and straight. They comply with all the qualities of good bricks. These bricks are used for superior work of permanent nature.
(2) Second 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 slightly irregular. These bricks may have hair cracks and their edges may not be sharp and uniform. These bricks are commonly used at places where brickwork is to be provided with a coat of plaster.
(3) Third 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 struck together. They are used for unimportant and temporary structures and at places where rainfall is not heavy.
(4) Fourth Class Bricks:
These are over-burnt bricks with irregular shape and dark colour. 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 hence they are sometimes found to be stronger than even the first class bricks.
It is thus seen that the above classification of bricks is based on the method of manufacturing or preparing bricks.
Uses of Bricks:
Following are the uses of bricks:
(i) The first class and second class bricks are widely used for all sorts of sound work especially of permanent nature. These structures include buildings, dams, roads, sewers, bridge piers, tunnels, pitching works, etc.
(ii) The use of first class bricks is specified for obtaining the architectural effects on faces of structures where they are to be kept exposed for beauty.
(iii) The masonry with second class bricks is generally plastered to make the smooth surface obtained due to the irregular shape and size of bricks. The mortar required in brick masonry using second class bricks will also be more.
(iv) The third class and sun-dried bricks are used for construction work of temporary nature. These bricks are not used in damp situations or at places subjected to heavy rains.
(v) The fourth class bricks are used as road metal and as aggregates in the foundation concrete.
Colours of Bricks:
The colours of bricks, as obtained in its natural course of manufacture, depend on the following factors:
(i) Degree of dryness achieved before burning,
(ii) Natural colour of clay and its chemical composition,
(iii) Nature of sand used in moulding operation,
(iv) Quality of fuel used in burning operation,
(v) Quantity of air admitted to the kiln during burning, and
(vi) Temperature at which bricks are burnt.
Table 4-4 shows the colours produced by clays with various constituents.
The artificial colouring of brick is achieved by adopting one of the following two methods:
(1) Addition of colouring material
(2) Dipping in colouring liquid.
(1) Addition of Colouring Material:
In this method, the required colouring material is added in brick earth. The bricks prepared from such earth will present the desired colour. The usual colouring materials are iron oxide, manganese, French ultramarine, Indian red, etc. This method is adopted when the colouring material is cheap and when it is available in plenty.
(2) Dipping in Colouring Liquid:
In this method, an earthenware box which is slightly larger each way than a common brick is taken. It is filled nearly to ½ depth with liquid which is in the form of thick paste. The bricks to be coloured are placed on an iron plate and with a fire underneath, they are heated to such an extent that they can be easily handled.
One brick is taken at a time and it is allowed to stay for few seconds in the box. It is then placed on a table to dry. Just after a few minutes, they are cleaned with cold water and placed aside to dry.
The colouring liquid is formed by the addition of colouring material to a mixture of linseed oil, litharge and turpentine.
Table 4-5 shows the proportions of various components of colouring liquid for different colours.
Following are the advantages of this method:
(i) The bricks which are coloured by this method do not lose their colour, when exposed to the atmosphere.
(ii) It can be adopted for expensive colours.
(iii) It is possible to develop a variety of colours cheaply and easily.
(iv) The penetration of colouring liquid in ordinary bricks is about 3 mm or so.
(v) This method can also be used for brick walls which are already constructed. The wall surface is carefully cleaned. The colouring liquid is slightly heated and it is applied on the wall surface with a brush.
Size and Weight of Bricks:
The bricks are prepared in various sizes. The custom in the locality is the governing factor for deciding the size of a brick. Such bricks which are not standardized are known as the traditional bricks.
If bricks are large, it is difficult to burn them properly and they become too heavy to be placed with a single hand. On the other hand, if bricks are small, more quantity of mortar is required.
For India, a brick of standard size 190 mm x 90 mm x 90 mm is recommended by the BIS. With mortar thickness, the size of such a brick becomes 200 mm x 100 mm x 100 mm and it is known as the nominal size of the modular brick. Thus the nominal size of brick includes the mortar thickness.
It is found that the weight of 1 m3 of brick earth is about 18 kN. Hence the average weight of a brick will be about 30 to 35 N.
Shape of Bricks:
The ordinary bricks are rectangular solids. But sometimes the bricks are given different shapes to make them suitable for particular type of construction.
Following are such few shapes of bricks:
(1) Bullnose Brick:
A brick moulded with a rounded angle is termed as a bullnose. It is used for a rounded quoin. A connection which is formed when a wall takes a turn is known as a quoin. The centre of the curved portion is situated on the long centre-line of brick. Fig. 4-8 shows a bullnose brick.
(2) Channel Bricks:
These bricks are moulded to the shape of a gutter or a channel and they are very often glazed. These bricks are used to function as drains.
(3) Coping Bricks:
These bricks are made to suit the thickness of walls on which coping is to be provided. Such bricks take various forms such as chamfered, half-round or saddle-back as shown in fig. 4-9.
(4) Cow-Nose Bricks:
A brick moulded with a double bullnose on end is known as a cownose.
(5) Curved Sector Bricks:
These bricks are in the form of curved sector and they are used in the construction of circular brick masonry pillars, brick chimneys, etc.
(6) Hollow Bricks:
These are also known as the cellular or cavity bricks. Such bricks have wall thickness of about 20 mm to 25 mm. They are prepared from special homogeneous clay. They are light in weight about one-third the weight of the ordinary brick of the same size.
These bricks can be laid almost about four times as fast as the ordinary bricks and thus the use of such bricks leads to speedy construction. They also reduce the transmission of heat, sound and damp. They are used in the construction of brick partitioning. Fig. 4-10 shows a typical hollow brick.
(7) Paving Bricks:
These bricks are prepared from clay containing a higher percentage of iron. The excess iron vitrifies the bricks at a low temperature. Such bricks resist better the abrasive action of traffic. The paving bricks may be plain or chequered. Fig. 4-11 shows a chequered brick. These bricks are extensively used for garden walks, street pavements, stable floors, etc. These bricks also render the floor less slippery.
(8) Perforated Bricks:
These bricks contain cylindrical holes throughout their thickness as shown in fig. 4-12. These bricks are light in weight and they require less quantity of clay for their preparation. The drying and burning of these bricks are also easy.
If perforated bricks of large size are used, it will result in the increase of output of mason. It has been observed that for tropical countries like India, the bricks with perforations of about 30 to 45 per cent of the total area of the corresponding face of the brick would offer adequate thermal insulation property.
The perforated bricks are used in the construction of brick panels for light weight structures and multi-storeyed framed structures. The perforations may be circular, square, rectangular or any other regular shape in cross-section. The distance between the side of brick and edge of perforation should not be less than 15 mm. The distance between the edges of successive perforations should preferably be not less than 10 mm.
The water absorption after immersion for 24 hours in water should not exceed 15 per cent by weight. The compressive strength of perforated bricks should not be less than 7 N/mm2 on gross area.
(9) Purpose-Made Bricks:
In order to achieve certain purpose, these bricks are made. The splay or cant bricks are made for jambs of doors and windows. The arch bricks are made of wedge shape to keep mortar joint of uniform thickness.
The ornamental bricks are prepared for corbels, cornices, etc. The engineering bricks having considerable strength, 50 to 80 N/mm2 and water absorption about 4 to 6 per cent, can be prepared from specially selected earth for use in constructions where high durability, compression strength and adequate resistance to sudden shocks are required.
These bricks are usually more costly than the ordinary bricks. But they grant safe, clean and quick construction. Hence, their cost is justified by their excellent performance in situations for which they are purposely prepared.
Fire-Clays:
The fire-clay is a refractory clay which is capable of resisting a high temperature without being melted or softened. It is used for making refractory materials. A refractory material is able to stand a high temperature without losing its shape. Thus the fire-clay is used in the manufacture of fire-bricks, crucibles, lining materials for furnaces, hollow tiles, etc.
The earth that is available from under the coal seams is generally found to be good fire-clay. The constituents of a good fire-clay are two — alumina and silica. The percentages of alumina varies from 25 to 35 and that of silica from 75 to 65. In any case, the impurities such as lime, magnesia, iron oxide and alkalies should not exceed 5 per cent.
Depending upon the fire resisting capacity, the fire-clays are classified into the following three categories:
(1) High duty fire-clays
(2) Medium duty fire-clays
(3) Low duty fire-clays.
The high duty fire-clays can resist temperature range of 1482°C to 1648°C; the medium duty fire-clays can resist temperature range of 1315°C to 1482°C; and the low duty fire-clays can resist temperature upto 870°C only.
Fire-Bricks:
These bricks are made from fire-clay. The process of manufacture is the same as that of ordinary clay bricks. The burning and cooling of fire-bricks are done gradually.
These bricks are usually white or yellowish white in colour. The weight of a fire-brick is about 30 to 35 N. The fire-bricks can resist high temperature without softening or melting. Hence they are used for linings of interior surfaces of furnaces, chimneys, kilns, ovens, fireplaces, etc. The compressive strength of these bricks varies from 200 to 220 N/mm2. The percentage of absorption for these bricks varies from 5 to 10.
Following are the three varieties of fire-bricks:
(1) Acidic bricks
(2) Basic bricks
(3) Neutral bricks.
(1) Acidic Bricks:
These bricks are used for acidic lining.
Following are the types of acidic bricks:
(i) Ordinary Fire-Bricks:
These bricks are prepared from natural fire-clay and they provide a good material for acidic refractory lining.
(ii) Silica Bricks:
These bricks contain a very high percentage of silica to the extent of about 95 to 97 per cent. A small quantity of lime, about 1 to 2 per cent, is added to work as binding material. These bricks are moulded under pressure and burnt at high temperature. The silica bricks can stand a high temperature upto about 2000°C. The compressive strength of such bricks is about 15 N/mm2.
(2) Basic Bricks:
These bricks are used for basic lining and basic refractory materials are used in the manufacture of such bricks. The magnesia bricks are prepared from lime and magnesia rocks. The dolomite may also be adopted for the manufacture of these bricks.
(3) Neutral Bricks:
These bricks are used for neutral lining. They offer resistance to the corrosive action of slags and acid fumes. As compared to the basic bricks, the neutral bricks are more inert to the slags.
Following are the types of neutral bricks:
(i) Chromite Bricks:
These bricks are prepared from a mixture of chrome, iron ore, ferrous oxide, bauxite and silica. Such bricks are unaffected by acidic or basic actions.
(ii) High Alumina Bricks:
These bricks contain a high percentage of alumina and they are found to be more inert to the slags.