Unit 1 Bricks 1 Bricks One of the

Unit -1 Bricks 1

Bricks � One of the oldest building material brick continues to bea most popular and leading construction material because of being cheap, durable and easy to handle and work with. � 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 b e conveniently handled with one hand. � Brick may be made of burnt clay or mixture of sand andlime or of Portland cement concrete. � Clay bricks are commonly used since these are economical and easily available 2

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� 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 � Size of a standard brick (also known as modular brick)should be 19 × 9 cm and 19 × 4 cm. When placed in masonry the 19 × 9 cm brick with mortar becomes 20 × 10 cm. � The bricks available in most part of the country still are 9" × 4½" × 3" and are known as field bricks. Weight of such a brick is 3. 0 kg. � An indent called frog, 1– 2 cm deep, is provided for 9 m chigh bricks. � The size of frog should be 10 × 4 × 1 cm. 4

� The purpose of providing frog is to form a key o fr holding the mortar and therefore, the bricks are laid with frogs on top. � Frog is not provided in 4 cm high bricks andextruded bricks. 5

Chemical Composition of Brick Earth � Alumina or clay = 20 -30 % by weight � Silica or sand = 35 – 50 % by weight � Silt = 20 -25% by weight (Total content of clay and silt may preferably be not less than 50% by weight) � Remaining ingredients, include i. Iron oxide ii. Magnesia (Mg. O) 1 -2% by weight iii. Lime (Ca. O) iv. Sodium potash, etc. , Total lime and magnesia in case of alluvial soil should not more than 1 % and in other cases should not be preferably more than 15%. The total soluble material should not be more than 1 % by weight 6

Functions of constituents of brick earth � Alumina: � It is the chief constituent of every clay. A good brick earth should contain about 20% to 30% of alumina. � This � If impart plasticity to the earth so that it can be moulded. 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. 7

Functions of constituents of brick earth (contd. ) � 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 earth brick should contain 35% to 50% of silica. � It prevents cracking, shrinkage and warping of raw bricks. It imparts uniform shape to the bricks. � The durability of bricks depends on the proportion ofsilica in brick earth. � The excess of silica destroys the cohesion between particles and the bricks become brittle. 8

Functions of constituents of brick earth (contd. ) � Lime � It should be present in a very finely powdered statebecause even small particles of the size of a pin-head cause flaking of the bricks. � It prevents shrinkage of raw bricks. � The sand alone is infusible. But it slightly fuses at kiln temperature in presence of lime. Such fused sand works as a hard cementing material for brick particles. � The excess of lime causes the brick to melt and hence tis 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. 9

Functions of constituents of brick earth (Contd. ) � Oxide of iron: � It helps as lime to fuse sand. � It also impart red colour to the bricks. � The excess of oxide of iron makes brick dark blue or blackish. � If the quantity of iron oxide is comparatively less, the bricks will be yellowish in colour. � Magnesia �A small quantity of magnesia in brick earth imparts yellow colour to the bricks and decreases shrinkage. � But excess of magnesia leads to the decay of bricks 10

Harmful ingredients in brick earth � Lime: � The excess of lime causes the brick to melt and hence tis 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. � Iron � If pyrites iron pyrites are present in brick earth, the bricks are crystallized and disintegrated during burning because of the oxidation of the iron pyrites. 11

Harmful ingredients in brick earth (Contd. ) � Pebbles: The presence of pebbles or grits of any kind is undesirable becauseit 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 a s desired. � � Vegetation and organic matter: The presence of vegetation and organic matter in brick earthassists in burning. If such matter is not completely burnt, the bricks become porous. � This is due to the fact that the gases will be evolved during 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. � 12

Field tests ( to determine the suitability of soil for Brick manufacturing) � First Test - Consistency � The test soil is to be tested is ground to a fine powder and sufficient quantity of water is then mixed. It is then kneaded and converted into plastic mass of required consistency. � Then the balls of about 80 mm diameter are moulded with hands and these balls are allowed to dry on the sun. � If the dry balls deform in shape and crumble down easily on pressing, it indicated 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. 16

Field tests ( to determine the suitability of soil for Brick manufacturing) –(contd) � Second �A test – Moulding properties small quantity of the original or modified soil used inthe first test is taken and ground well. � Then required quantity of water is added in the soil o sthat soil mixture can be kneaded well to be rolled to form threads of about 3 mm diameter. � The above kneaded soil is then put into the mould and a brick is moulded out of it. � If the edges and corners of the moulded brick are not wel formed or sharp, this test is repeated with varying water contents till a satisfactory result is obtained. � Then an approx. soil and water ratio may be found when the correct moisture content is obtained. 17

Field tests ( to determine the suitability of soil for Brick manufacturing) –(contd) � Third � The test – Deformation and shrinkage on burning. sample of bricks moulded as above are burnt in the ordinary potter’s clamp for 3 days and 4 nights until the bricks become red hot. � Then the bricks are allowed to cool and removed from the clamp. These burnt bricks should be examined for shrinkage and deformation. � The bricks which have shrunk evenly and do not showany bad effect due to burning should be sorted out and those which have lost their shape or deformed, should be rejected. 18

Classification of bricks Classification of Bricks Based on field practice Based on use Based on finish First class bricks Common bricks Sand-faced Brick Second class bricks Engineering bricks Rustic brick Third class bricks Facing bricks Fourth class bricks 19 Based on strength

Classification Of Bricks � On Field Practice Clay bricks are classified based on their physical and mechanical properties � First Class, � Second Class, � Third Class and � Fourth Class 20

First Class Bricks These are thoroughly burnt and are of deep red, cherry or copper colour. 2. The surface should be smooth and rectangular, with parallel, sharp and straight edges and square corners. 3. These should be free from flaws, cracks and stones. 4. These should have uniform texture. 5. No impression should be left on the brick when a scratch is made by a finger nail. 6. The fractured surface of the brick should not show lumps of lime. 7. A metallic or ringing sound should come when two bricks are struck against each other. 8. Water absorption should be 12– 15% of its dry weight when immersed in cold water for 24 hours. 9. The crushing strength of the brick should not be less than 10 N/mm 2. This limit varies with different Government organizations around the country. Uses: First class bricks are recommended for pointing, exposed face work in masonry structures, flooring and reinforced brick work 1. 21

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Second Class Bricks are supposed to have the same requirements as the first class ones except that Small cracks and distortions are permitted. 1. A little higher water absorption of about 16– 20% of its dry weight is allowed. 2. The crushing strength should not be less than 7. 0 N/mm 2. Uses: Second class bricks are recommended for all important or unimportant hidden masonry works and centering of reinforced brick and reinforced cement concrete (RCC) structures. 23

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Third Class Bricks � Third Class Bricks are underburnt. � They are soft and light-coloured producing a dull sound when struck against each other. � Water absorption is about 25 per cent of dry weight. Uses : � It is used for building temporary structures. 25

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Fourth Class Bricks � Fourth Class Bricks are over-burnt and badly distorted in shape and size and are brittle in nature. Uses: � The ballast of such bricks is used for foundation and floors in lime concrete and road metal. 27

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Classification of bricks based on Strength Classification of Bricks based on Compressive Strength (IS: 1077) 29

Notes: 1. The burnt clay bricks having compressive strength more than 40. 0 N/mm 2 are known as heavy duty bricks and are used for heavy duty structures such as bridges, foundations for industrial buildings, multistory buildings, etc. The water absorption of these bricks is limited to 5 per cent. 2. Each class of bricks as specified above is further divided into subclasses A and B based on tolerances and shape. Subclass-A bricks should have smooth rectangular faces with sharp corners and uniform colour. Subclass-B bricks may have slightly distorted and round edges 30

Classification of bricks based on the Basis of Use Brick is a general multi-purpose unit manufactured economically without special reference to appearance. These may vary greatly in strength and durability and are used for filling, backing and in walls where appearance is of no consequence. � Facing Bricks are made primarily with a view to havegood appearance, either of colour or texture or both. These are durable under severe exposure and are used in fronts of building walls for which a pleasing appearance is desired. � Engineering Bricks are strong, impermeable, smooth, table moulded, hard and conform to defined limits of absorption and strength. These are used for all load bearing structures. � Common 32

Classification of bricks on the Basis of Finish Brick has textured surface manufactured by sprinkling sand on the inner surfaces of the mould. � Rustic Brick has mechanically textured finish, varying in pattern. � Sand-faced 33

Additives in the Manufacture of Bricks � Certain additives such as fly ash, sandy loam, rice husk ash, basalt stone dust, etc. are often required not only to modify the shaping, drying and firing behaviour of clay mass, but also to help conserve agricultural land utilise waste materials available in large quantities. � These additives should, however, have a desirable level of physical and chemical characteristics so as to modify the behaviour of clay mass within the optimum range without any adverse effect on the performance and durability. 35

Characteristics of Good Bricks �Size and shape – uniform size, rectangular surfaces, parallel sides, sharp straight edges �Color – uniform deep red or cherry �Texture and compactness – uniform texture, fractured surface should not show fissures, holes, grits or lumps of lime �Hardness and soundness – not scratch able b y finger nail. Produce metallic ringing sound �Water absorption – should not exceed 20% wt �Crushing strength – not less than 10. 5 N/mm 2 �Brick earth – free from stones, organic matter 40

Manufacturing of Bricks � Preparation of Brick Earth � Un-soiling � Digging � Weathering � Blending � Tempering � Molding of Bricks � Drying of Bricks � Burning of Bricks 41

Excavation of soil 42

43 Carting of Water from Source for mixing in the soil

44 Mixing of sand in the soil

45 Mixing of clay with Feet to form clay paste

46 Preparation of Bricks by inserting lump of clay paste in the mould

47 Placing the newly moulded bricks on the ground for drying

49 Transportation of fully dried bricks for placing in the kiln

50 Arranging the bricks in the Kiln for firing

51 Fully prepared Kiln ready for firing

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Preparation of Brick Earth � Unsoiling: The soil used for making building bricks should be processed o sas 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. � � Digging: After removing the top layer of the earth, proportions ofadditives 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. � 54

� Weathering: Stones, gravels, pebbles, roots, etc. are removed from the dugearth 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. � The soil should be turned over at least twice and it should b e 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 b yexposing the clay to weather. � 55

� Blending: � The earth is then mixed with sandy-earth and calcareousearth 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 o fr 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 effect the size and shape of the finished brick. 56

Blending brick clay 57

Tempering: � Tempering consists of kneading the earth with feetso as to make the mass stiff and plastics (by plasticity, we mean 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 o fr subsequent processing. � For manufacturing good brick, tempering is done n i pug mills and the operation is called pugging 58

Pug Mill 59

Manufacturing of Burnt Bricks Molding – giving right shape � ◦ Hand molding Ground molding. Table ◦ Molded on sand. No frog in bricks molding. Molded on stock boards with frog Machine molding Plastic method or Stiff-Mud process. Molded stiff clay bar cut by wire into brick size pieces. Dry Press method. Moist powdered clay fed into machine to be molded into bricks. � Drying Natural drying: also known as hack drying, bricks are dried in open air and sunlight. Artificial drying: brick are dried in Kiln chamber or tunnel. 60

Brick Molds 61

Hand molding of bricks 62

Table Molding 63

Strikes 64

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Method of Drying Bricks 68

Drying of bricks 69

Drying of bricks 70

Protecting dry bricks 71

Manufacturing of Burnt Bricks � Burning Stages � Dehydration (400 -650 °C). Water smoking stage in which water from pores is driven off � Oxidation (650 -900 °C). Carbon eliminated and ferrous iron oxidized to ferric form. Sulphur is removed � Vitrification substance 72 (900 -1250 °C). Mass converted into glass like

Dehydration (400– 650°C): This is also known as water smoking stage. During dehydration, (1) The water which has been retained in the pores of the clay after drying is driven off and the clay loses its plasticity, (2) Some of the carbonaceous matter is burnt ( 3 ) A portion of sulphur is distilled from pyrites. (4) Hydrous minerals like ferric hydroxide are dehydrated, and (5) The carbonate minerals are more or less decarbonated. Too rapid heating causes cracking or bursting of the bricks. ( 6 ) 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. � 73

Oxidation Period (650– 900°C): � During the oxidation period, 1. Remainder of carbon is eliminated and, 2. The ferrous iron is oxidized to the ferric form. � The removal of sulphur is completed only after thecarbon 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. 74

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 redheat in order to avoid checking and cracking. � Vitrification period may further be divided into � (a) incipient vitrification, 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; � (b) complete vitrification, more or less well-marked by maximum shrinkage; � (c) viscous vitrification, produced by a further increase intemperature 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. 75

Manufacturing of Burnt Bricks �Clamp ◦ ◦ Alternate layers of bricks and fuel encased in mud plaster. Fuel consists of grass, cow dung, wood, coal dust Brick layer consists of four to five courses of brick 25, 000 to 100, 000 bricks in three months cycle �Kiln ◦ ◦ ◦ 76 or Pazawah Burning Intermittent kiln. Loaded, fired, cooled and unloaded before next loading Continuous kiln. Bricks are loaded, fired, dried and cooled simultaneously in different chambers. Example: Bull’s trench kiln and Hoffman’s kiln

Clamp Burning 77

Burning of bricks in kilns 78

Burning fuel 79

Preheating bricks Loading raw bricks Unloading cooled bricks Bull’s Trench Kiln 80 Cooling bricks

Hoffman’s Continuous Kiln Preheating Fuel supply Hot air leaving Loading raw bricks Cool air entering Removing cooled bricks 81 Cooling bricks

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Testing of Bricks � Compressive strength Test. �Specimen brick is immersed in water for 24 hours �The frog of brick is filled flush with 1: 3 mortar and brick is stored under damp jute bags for 24 hours followed by immersion in clean water for three days. �The specimen is then placed between plates of compression testing machine. �Load is applied axially at uniform rate till failure. Maximum load at failure divided by average area of bed face gives compressive strength. � 83 �������� ��������������������� ℎ =

Testing of Bricks �Absorption ◦ Test. 24 hours immersion cold water test. Dry bricks are oven dried at 105° ± 5° C Room temperature cooled bricks weighed W 1 Bricks immersed in water at 27° ± 2° C for 24 hrs Soaked bricks weighed W 2 Water absorption in % = (W 2 – W 1 )/W 1 x 100 ◦ Five hours boiling water test Oven dried bricks weight : W 1 Bricks immersed in water and boiled for 5 hours and then cooled down at room temperature in 16 -19 hours Cooled down weight as W 2 Water absorption in % = (W 2 – W 1 )/W 1 x 100 84

Testing of Bricks �Efflorescence Test. The soluble salts if present in bricks cause efflorescence on the surface of brick. �Brick is immersed in water for 24 h. 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. ◦ Nil imperceptible efflorescence ◦ Slight deposit covers area < 10% of exposed area ◦ Moderatedeposit covers exposed area 10% to 50% ◦ Heavy Serious away ◦ 85 deposit covers exposed area > 50% deposits are heavy and powder or flake the surface

Testing of Bricks �Structure Test. A specimen structure is examined. � is broken and tsi It should be homogeneous, compact, and free from defects e. g. , lumps and holes, etc. �Soundness test. This test is performed bystriking two specimen bricks with each other. The bricks should not break and a clear ringing sound should be produced. �Hardness test. This test is performed by making a scratch on brick surface with the help of finger nail. If no impression is left on surface, the brick is considered to be sufficiently hard. 86

Testing of Bricks � Shape and size test � In this test, a specimen brick should be closely inspected. � It should be of standard size and its shape should be truly rectangular with sharp edges. � 20 bricks of standard size (190 mm x 90 mm) are randomly selected and stacked length-wise, along the width and along the height. � For good quality bricks, the results should be within the following permissible limits: � Length : 3680 mm to 3920 mm � Width : 1740 mm to 1860 mm � Height: 1740 mm to 1860 mm 87

Special Bricks � Specially shaped bricks � Burnt clay facing bricks � Heavy duty bricks � Perforated bricks � Burnt clay hollow bricks � Sandlime bricks � Sewer bricks � Acid resistant bricks 88

Special shaped bricks � Bull-nosed bricks � Cant or plinth bricks � Circular bricks � Squint bricks � Cornice bricks � Coping bricks � Paving bricks 89

Specially Shaped Bricks 90

Burnt clay facing bricks � These bricks are used in the exposed face of masonry without any further surface protection � The use of these is economical where external plastering or rendering have to be frequently renewed due to corrosive atmosphere and also for high rise buildings. 91

Heavy duty bricks � These bricks are characterised by high compressive strength, low water absorption, high durability and high bulk density. � They should be free from cracks and other flaws and lime nodules. Uses: � Masonry in heavy engineering works such as bridge structures, multi-storeyed buildings and industrial foundations. 92

Perforated bricks � Standard size bricks and contain cylindrical holes throughout their thickness. � The aperture of the perforations is such that it gives maximum amount of ventilation but does not permit the entry of rats or mice. � Light in weight � Require less quantity of clay for their preparation � Easy to burn Use: � For constructing panel walls for multi-storeyed buildings and for providing walls. 93

Burnt clay hollow bricks � Prepared from special homogeneous clay � Clay – uniform colour, fine, compact and uniform texture. � Light in weight � Impart insulation against heat, sound and dampness to the building. Uses: � Construction of load bearing walls, partition walls or panel walls in multi-storeyed buildings. 94

Sand lime bricks � Calcium silicate bricks � An intimate and uniform mixture of siliceous sand or crushed siliceous rock and lime combined by the action of saturated steam under pressure. Uses � Masonry construction 95

Sewer bricks � Manufactured from surface clay, fire clay or shale ora combination of these materials. � Uses: � Lining of walls, roofs and floors of sewers (for ordinary sanitary sewage. 96

Acid resistant bricks � Made of raw materials such as clay or shale of suitable composition with low lime and iron content, feldspar, flint or sand vitrified at high temperature at ceramic kiln. Uses: � Masonry constructions � Flooring subjected to acid attacks, � Lining of chambers and towers in chemical plans, � Lining of sewers carrying industrial effluents. 97

Refractory bricks �made from fire-clay, using same process as an ordinary clay. �The color of the Brick is yellow or light brown. �The water absorption varies from 4 -10 %. �The compressive strength b/w 15 -2 0 N/mm 2 �Used for the lining blast furnaces, O vens, Boilers and Chimneys. 98 Kiln,

Varieties of fire bricks � Acidic bricks � Ordinary fire bricks � Prepared from natural fire-clay. � Used for acidic refractory lining. � Silica bricks � Silica 95 -97% � Lime 1% - added as binding material. � These bricks moulded under pressure and burnt @high temperature. � Withstand up to 2000 o C � Compressive strength – 15 N/mm 2 99

� Basic � � Bricks Used for basic lining. Prepared from lime ad magnesia rocks � Neutral Bricks � Used for neutral lining. � Offers resistance to corrosive action of slags and fumes. � More inert to slags � Chromite bricks � Prepared from chrome, iron ore, ferrous oxide, bauxite and silica. � Unaffected by acidic or basic actions � High alumina bricks � Contain high % of alumina � More inert to slags. 100

Defects of Bricks � Over-burning. Burnt beyond complete vitrification � Under-burning. vitrification Burnt less not to cause complete Spongy swollen mass over the surface due to excess carbonaceous matter and sulphur � Bloating. � Black 102 Core. Due to bituminous matter or carbon

Grey of white crystallization of alkalis on the surface, due to water absorption �Efflorescence. �Spots. Dark sulphur spots due to iron sulphides Broken blisters due to air entrapped during molding �Blisters. Thin lamina produced due to ari entrapped in voids of clay �Laminations. 103

Substitutes for Bricks � Reason for Alternatives � High wastage during transportation � High water absorption of bricks � Inconsistency in size of bricks � Increasing cost of kiln fuel � Unavailability of skilled labour. � Substitutes � Concrete blocks � Flyash bricks � Sand-lime or calcium bricks. 104

Cement and concrete blocks � Cement and concrete blocks are also used for masonry construction. They are available in 3 types � Solid � Hollow � Cellular. � Concrete blocks are usually made larger in size that o s the block work is faster and consume less cement in joints than brick work. � These blocks are extensively used for compound walls and non-loadbearing walls. � Specially made hollow blocks are also used for load bearing walls. 105

Solid Concrete Block Hollow concrete block Light weight concrete block 106

main disadvantage of concrete blocks is shrinkage due to movement of moisture which is absent in bricks. � These blocks are much larger in size than bricks, any foundation movement will cause block work to crack more than the brick work. � The mortar strength should not be higher than thestrength of the block. With high mortar strength, the cracks will be few and very large, but with low mortar strength, the cracks will be small and distributed. � The blocks should be cured properly for 14 days anddried for 4 weeks. � The ordinary unreinforced block works in walls is very weak for resisting lateral loads that are caused due to expansion of roof or earthquake and cyclones. � The 107

Manufacturing of Concrete Blocks � The concrete mix for concrete blocks shall not richer be than one part of cement to six part of combined aggregates. � Lean mixes upto 1: 8 are also commonly used. � BIS recommends a fineness modulus of combined aggregates between 3. 6 to 4. � Coarse aggregates of size 6 to 12 mm is generally used. � 60% Fine and 40% Coarse aggregates is the mix recommended. 108

Manufacturing of Cement concrete blocks � Cement concrete blocks can be hand-made and also preferably machine made. � The cast block is then cured in a water tank or yard for at least 14 days. � When immersed in the tank, the water shall be changed at least every 4 days. � After curing, the blocks are dried for a period of 4 weeks before being used on the work. � They should be stacked with voids in the horizontal direction to facilitate easy drying. Otherwise, they should be steam cured and dried. � This process allows the complete shrinkage of the block to take place before to take place they are laid on the wall. 109

Process � Selection and proportioning of ingredients � The main criteria for the selection of the ingredients is the desired strength of the block. � The greater the proportion of C A , the greater will be the strength of the quantity of cement used. � Mixing � The of ingredients blending of aggregates, cement and water should be done very carefully. � The mixing should preferably take place in mechanical mixer. � For hand mixing, extreme care should be taken to see that cement and aggregates are first mixed thoroughly in dry state and the water is then added gradually. 110

� Placing and vibration � The mixed concrete material is fed into the mould box upto the top level and it is ensured that the box is evenly filled. � The vibration of concrete is done till it has uniformly settled in the mould box. � Curing � The block is watered after about one day of casting and continued for a minimum of 7 days and preferably 28 days. � The longer the curing period, the better will be the block. 111

Dimensions and tolerances � Nominal dimensions � Length – 400, 450, 500 or 600 mm � Height – 200 or 100 mm � Width – 50, 75, 100, 150, 200, 250 or 300 mm (Actual sizes will be less by 10 mm the mortar thickness) 112

Classification of concrete blocks � Hollow concrete blocks (Open and closed cavity types) � Grade A � Grade B � Grade C 113