Basics of Bricks and Classification

UNIT 1

STONES, BRICKS AND CLAY CLAY PRODUCTS

Stones, Bricks and Clay Products

Structure 1.1

Introduction Objectives

1.2

1.3

1.4

Stone 1.2.1

Uses of Stones

1.2.2

Classification of Stones

1.2.3

Characteristics of Good Building Stones

1.2.4

Tests of Stones

1.2.5

Quarrying of Stones

1.2.6

Natural Bed

1.2.7

Deterioration of Stone

1.2.8

Preservation of Stone Work

1.2.9

Dressing of Stones

Bricks 1.3.1

Classification and Characteristics of Bricks

1.3.2

Uses of Bricks

1.3.3

Brick Sizes

1.3.4

Brick Earth

1.3.5

Manufacturing of Bricks

1.3.6

Qualities of Good Bricks

1.3.7

Tests for Burnt Clay Bricks

Tiles 1.4.1

Classification of Tiles

1.4.2

Manufacture of Tiles

1.4.3

Properties of Good Tiles

1.4.4

Types of Tiles

1.5

Summary

1.6

Answers to SAQs

1.1

INTRODUCTION

Stones, Bricks and clay products are inert materials used in building construction, roads, dockyards, airfields etc. Stone is a natural material of construction and it is in use since prehistoric days. Bricks are blocks of tempered t empered day moulded to suitable shape and sizes, used for most of the building works. Sometime bricks are used as substitute for stone, where stone is not cheaply available. Tile is one of the clay product used for roof covering and paving. The focus of this unit will be on stone, bricks and tile and we will be studying their classification, uses, characteristics, manufacturing and tests.

Objectives After studying this unit, you should be able to •

classify rocks, bricks and tiles,

•

describe uses of stones, bricks and tiles,

5

Engineering Materials

1.2

•

explain characteristics of good building stones, bricks and tiles,

•

appreciate the importance of tests of stones and bricks, and

•

distinguish between stone and brick as a material of construction.

STONE

Stone is a natural material of construction and is obtained from quarries. It has been used for constructing different components of buildings like foundations, walls, lintels, floors, roofs, bridges, weirs, dams etc. Stones that are used for construction of structures are known as building stones.

1.2.1 Uses of Stones Stones are extensively used for the following purposes : (i)

It is used as stones ballast for railway track, for road construction, for preparing cement concrete required for foundations, flooring, hollow and solid blocks, artificial stones and reinforced cement concrete.

(ii)

It is used as a crushed stone (stone dust) as a substitute for sand.

(iii) It is used as blocks in the t he construction of buildings, lintels, arches, walls, columns, abutments and piers of bridges, in weirs and in dams etc. (iv) It is used as blocks and slabs for face work of buildings requiring architectural treatments. (v)

It is used as thin slabs for f or roofing and for flooring buildings and pavements.

(vi) It is used as lime stone in the manufacture of lime, cement and in various other chemical processes. (vii) Thin slabs of impervious stones are used for laying Damp Proof Course in buildings. (viii) Base material for water and sewage filters, in case of water works and sewage treatment plants. (ix) Stone may be used to give massive and pleasing appearance. (x)

Lime stone is also used as a flux in the blast furnaces.

Limitations in Uses of Stones

Stones are gradually losing their Universal use as a building material because of following reasons : (i)

Important buildings these days are being constructed of steel or RCC, because of their being less bulky, stronger and more durable.

(ii)

Strength of structures made of stones cannot be so rationally analysed as of those made with steel and RCC.

(iii) Suitable, durable and more easily workable alternatives materials are now available. (iv) Cast stone, sand faced rusticated bricks, furnace ware etc. can be given architectural treatment more easily and have, therefore, overshadowed the use of stones. 6

(v)

Stones are not cheaply and conveniently available in the plains.


(vi) Bringing the stone to required shape and size is time-consuming and results in slow pace of construction.

1.2.2 Classification of Stones Stones are obtained from rocks. A rock forms portion of earth’s crust having no definite shape or chemical composition. It is usually a mixture of two or more minerals and is not homogeneous. Rocks from which stones can be had for building purposes are classified in the three different ways : (i)

Geologically

(ii)

Physically

(iii) Chemically Classification based on hardness of the stone are : Geological Classification

This classification depends upon how the rock was originally formed. Geologists classify rocks into three main categories: Igneous Rocks

Inner layers of earth are at such high temperatures that masses of silicates melt. This molten mass, known as magma, is forced up. Magma solidifies into rocks when it reaches the surface of earth and forms basalts and traps. If, however, the magma solidifies before reaching the surface of earth it forms solid crystalline rock known as Granite. Basalt, Trap and Granite are Igneous rocks. Sedimentary Rocks

Surface of earth is subjected to the destructive action of rain, frost, winds and chemical actions. These destructive agents break up the surface of earth which gets further broken up when carried down by rains and rivers. In the journey of river from mountains to sea the velocity goes on decreasing as the river moves further. Figure 1.1 shows the formation of sedimentary rocks.

Figure 1.1: Sedimentary Rock Formation

Characteristics of Sedimentary Rocks

(i)

Sedimentary rocks are often well stratified and show well-defined bedding planes. 7


(ii)

Sedimentary rocks can be split and cleared easily both in the bedding direction and normal to the bedding planes giving building blocks of fairly regular shape.

(iii) Properties of sedimentary rocks vary considerably depending upon the nature of sediment and the type of bonding that exists between the adjacent sediment grains. (iv) Sand stone formed by consolidation of sand deposits consist primarily of small rounded grains of silica. The sand stone will be relatively soft or hard depending upon the nature of binding material. Metamorphic Rocks

Due to structural changes in earth, igneous or sedimentary rocks find their way deep in earth where they are subject to high temperature and heavy pressure which cause changes in texture or in mineral composition or in both resulting in the formation of new types of rocks known as metamorphic rocks. Following are some of the changes to “metamorphic rocks”. Sl. No

Name of Original Rock

Name of Metamorphic Rock

1.

Granite (igneous rocks)

Gneiss

2.

Sand stone (sedimentary rock )

Quartzite

3.

Lime stone (sedimentary rock)

Marble

4.

Shale (sedimentary rock)

Slate

Physical Classification of Rocks

This classification depends upon the structure of rock. Physically rocks are classified in three types : (i)

Stratified rocks,

(ii)

Unstratified rocks, and

(iii) Foliated or laminated rocks. Stratified Rocks

These rocks show distinct layers along which it can be easily split into thin slabs. Slate, sand stone and lime stone belong to this category. Unstratified Rocks

These rocks do not show any sign of stratification and cannot be easily split into thin layers. Granite, Basalt and Trap belong to this category. Foliated or Laminated Rocks

These rocks comprise of thin laminations. They can be split in definite direction and size. Metamorphic rocks come under this category. Chemical Classification of Rocks

8

This classification depends upon the chief constituent mineral of the rock. Chemically, rocks are classified into three categories : (i)


Agrillaceous Rocks

The chief constituent of agrillaceous rocks is clay (Alumina, Al 2O3). Slate and laterite belong to this category. (ii)

Silicious Rocks

The chief constituent of silicious rocks is sand (Silica, SiO2). Quartzite and Granite come under this category. (iii) Calcareous Rock The chief constituent of calcareous rocks is lime. Lime stone and marble stone belong to this category. Classification Based on Hardness of the Stone

According to this classification stone may be classified as (i)

very hard ,

(ii)

hard,

(iii) medium, and (iv) soft. Very Hard Rocks

Granite, trap, taconite are the very hard varieties of rocks. Hard Rocks

Granite, basalt, trap, gravel, quartzite are the hard varieties of rocks. Medium Rocks

Dolomite and lime stone are the medium varieties. Soft Rocks

Talc, gypsum, sand stone, slate etc. are the soft varieties of stones.

1.2.3 Characteristics of Good Building Stones A good building stone should essentially have the following qualities: (i)

Appearance

This property is of extreme importance for face work of buildings. Also, from architectural point of view, colour of the stone should be such as to go well with the surroundings. Stones should be of uniform colour and free from clay holes, bands or spots of colour whatsoever. (ii)

General Structure

It should show uniformity of texture. It must be either crystalline in structure or homogeneous and close-grained. It should be free from cavities, cracks or patches of soft or loose material. (iii) Heaviness Heavier varieties of stones are more compact, less porous and have greater specific gravities. Heavier varieties of stones are preferred for constructions in water like weirs, barrages, dams, docks, harbours and

9


retaining walls. Lighter varieties of stones are used for construction of domes and roof coverings and similar other usages. (iv) Strength The stones used should be quite strong to withstand the forces likely to be encountered yet in case of construction where unusually bigger forces are likely to come the stone to be used should be tested for its strength. (v)

Hardness

It is the resistance of stone to abrasive forces caused by much wear and friction as in floors, pavements and aprons of bridges and weirs in rivers. Stones to be used at such places should be hard. (vi) Toughness It is a measure of the impact that a stone can withstand. Stones used at places subjected to vibrations of machinery and to moving loads should be tough. (vii) Ease of Working The ease with which the stone can be worked upon, i.e. cut, dressed, carved and mounded etc. is an important consideration from economy point of view. The ease of working is opposed to strength, durability and hardness. (viii) Porosity More porous building stones are unsuitable for use in construction especially for exposed surfaces of structures. These stones should be used at places where they are not likely to encounter frost, rain or moisture in any other form. (ix) Seasoning All freshly quarried stones contain a certain amount of moisture known as quarry sap, which makes them soft and easier to work upon. Dressing, carving and moulding etc., should be done as early after quarrying as possible. The stones should be left for seasoning after quarrying when all the work has been done. Stones become considerably harder on seasoning. (x)

Weathering

It is the extent to which the face of a stone resists the action of weather. Stones with good weathering properties only should be used in the construction of important buildings. (xi) Resistance of Fire To be fire resistant stones should be free from calcium carbonate and oxide of iron and be not composed of minerals with differing co-efficients of thermal expansion. (xii) Fineness of Grains Fine grained stones are considered most suitable for carving and moulding works. Stones having non crystalline or amorphous texture are liable to be easily disintegrated. A good building stone should 10

have crystalline structure, because such stones are generally durable and strong.


(xiii) Compactness Compact stones are generally more durable. The stones found at larger depths under earth are considered the best stones, as they have been subjected to a larger pressure of earth above them. (xiv) Durability A good building stone should be durable. The durability of stone depends on its chemical composition, physical structure, homogeneity and closeness of grains and type of cementing material. Location of the stone in the structure also affects durability. (xv) Presence of Wear Resistance to wear is measured by attrition test. Good stone should not show wear of more than 3%. This property is useful for the stone to be used as road aggregate and railway ballast.

1.2.4 Tests of Stones Building stones are required to be tested for their different properties before they may be recommended for any specific use. Following are the tests which can be carried out on stones : (i)

Smith’s test

(ii)

Freezing and thawing test

(iii) Water absorption test (iv) Frost action test (v)

Impact test

(vi) Attrition test (vii) Crushing strength test (viii) Hardness test (ix) Acid test (x)

Microscopic examination

(xi) Crystallization test You may go through any book on building material listed in the section “Further Readings” for details of these tests.

1.2.5 Quarrying of Stones The site from where stones are excavated is known as quarry or quarry site. The process of taking out stones of various sizes from quarry is known as quarrying of stones. Purpose of quarrying is to obtain stones for masonry or ballast in concrete or road construction or railway track or for any other purpose. Selection of Quarry Site

Selection of site for quarrying of stones should be done very carefully. The following points should be considered while making selection of site : (i)

Distance of quarry from road, railways, etc. should not be very large.

11


(ii)

Sufficient stone should be assured from the site.

(iii) Availability of equipment, labour, power, water etc. also affect the selection of site. (iv) Quality of stone available from quarry should be good. (v)

Drainage from quarry should be easy.

(vi) Adequate facilities for transportation of stone should be available. (vii) Geological formation of the site should be properly studied. (viii) The site should be away from built up areas, in case blasting has to be resorted to. (ix) Site for the dumping of refuse should be easily available nearby. Methods of Quarrying Stones

Depending upon the nature and structure of rocks and the purpose for which stones are needed, quarrying is done by one of the following methods in common use : (i)

Excavating or digging

(ii)

Wedging

(iii) Heating (iv) Use of channelling machine (v)

Blasting

Excavating or Digging

The method is employed when stones to be quarried are lying buried in earth or under loose overburden. Tools like crow bars, pick-axes, kassi, etc. are mainly required. Wedging

This method is suitable for quarrying soft stratified rocks. In this method, first of all naturally occurring cracks or fissures are located in the rocks, to be excavated. The steel wedges or points are then driven with the help of hammer, in such fissures or cracks and stones are detached. The split out blocks of stone can be converted into marketable forms and supplied to users. This method is employed for the rock of sedimentary type, which are comparatively soft, such as sand stone, lime stone, marble, slate and laterite. Heating

12

In this method of quarrying, fire wood is piled on the surface of the stone to be excavated. The fire is burnt steadily for some time. Heating of the stone causes upper layers of stone expand, in relation to the lower layers. Due to unequal expansion, upper layers of rock separate from the lower layers. Splitting of the rock is indicated by dull bursting sound. The detached stones are then taken out of quarry with the help of wedges and crow bars etc. The stones are suitably, shaped with the help of hammers and marketed.

This method is suitable where only smaller blocks of more or less regular shape are required and suitable rocks bedded in horizontal layers of not much thickness are to be quarried.


Blasting

Blasting is a method in which quarrying of stone is done with the help of explosives first. This method is used when stone to be excavated is of very hard variety and it has no cracks or fissures. Moreover, if stone is to be excavated on very large scale, blasting method will have to be adopted. No definite size blocks can be excavated by this method. After blasting, the excavated stone is sorted out in different sizes and categories. Channelling Machine Method

This method of excavation or rocks is adopted when stone in definite large sizes is required, and stone involved is of softer variety. Marble, lime stone, and soft sand stones can be quarried by this method. In this method a special machine channelling machine is used. It is such a machine which can make vertical as well as oblique grooves in the stone. This machine can cut out 5 to 8 cm wide and 2 to 4 m deep grooves. A groove is cut around the stone block to be excavated with the help of this machine. The stone block is then detached from the bottom by drilling horizontal holes and then by wedging.

1.2.6 Natural Bed The stone used in buildings is obtained from rocks. The rocks, particularly sedimentary type have distinct planes of division along which stones can be easily split. The plane alongwhich stones can be easily split is known as natural bed of stone. In stone masonry work, stones should always be placed in such a manner that the natural bed is at right angles to the pressure that the stone is to carry. In other words, the position of the stone blocks in the walls should be same in which they were originally deposited. This position gives maximum strength to the stone work in the walls. Stone in the walls should further be placed with layers at right angle to the face of wall. This will offer greatest resistance to disintegration of the stone by frost and other weather actions (Figure 1.2).

Figure 1.2 : Natural Bed of Stone

In the case of arches, the bedding plane should be radial and at right angles to the face of the arch. Such an arrangement causes the thrust of arch, act normal to the direction of the natural bed. In the case of cornices and strong courses, stones remain unsupported. In their case, natural bed of the stone should be kept vertical.

13


1.2.7 Deterioration of Stones Deterioration of stones is caused by the chemical and physical changes brought about by various agencies of weather. Following are the main climatic agencies bringing about deterioration of stones. Rain

Frequent wetting and drying of stones by rains and sun causes internal stresses thereby resulting in the disintegration of stones causing its deterioration. Frost

At very cold places, frost on entering the pores of stones freezes and consequent increase in volume resulted in its disintegration. Wind

Dusty winds cause deterioration of stones because of the abrasive effect of dust particles. Also wind force, rain water and frost enter the pores of stones deeper resulting in deterioration of stone as explained above. Atmospheric Impurities

Atmosphere, especially in big industrial towns, is generally polluted with smoke and acidic gases. These react readily with stones containing carbonate of lime resulting in their deterioration. Nature of Binding Material Used

Binding materials used for laying stones sometimes are such that they react chemically with any one of the constituents of stones, thereby causing the stones to disintegrate. Such a combination should be avoided. Vegetable Growth

Roots of trees that penetrate the joints of stone masonry keep stones damp and secrete organic and acidic matters which cause deterioration of stones. Temperature Variations

Stones should not be used at places where temperature variations are too much or quite frequent. Stones, if used at these places, would crumble because of the setting up of internal stresses. Mutual Decay

If materials having different physical characteristics are used together then they may cause mutual decay. For example, if sand stone were used under a lime stone then the chemicals brought down from the lime stone by rain water will cause decay of the sand stone. Living Organism

There are some living organism which slowly act upon stones and cause their disintegration. Lichens destroy lime stone, and worms or molluses destroy all the stones except granite. Water

Sea water, river water, or ground water may also cause deterioration of stone. The water coming in contact with stone may dissolve some of the mineral constituents of stone and cause their decay. 14

1.2.8 Preservation of Stone Work

To increase the life of a stone structure, it should be cleaned with water or with steam so that dirt and soluble salts are removed and threat of decay is reduced. The following points, if given due consideration, will help to increase the life of a stone structure particularly in the polluted atmosphere of big industrial towns.


Selection

It is suggested to initially use a durable compact and crystalline stone rather than to depend upon preservatives later. Seasoning

Use of seasoned stones after they had been cut and dressed immediately after quarrying will increase the life of structure. Also, seasoned stones are less liable to deterioration because of frost and acids. Size

Bigger sized stones are more durable than the smaller ones. Natural Bed

Care should be taken to place the stones in such a way that loads act at right angles to the natural bed of stones used as otherwise t he stone will flake off. Also with the natural bed vertical, effect of rain and frost shall be more detrimental. Surface Finish

Well dressed, smoothly finished and polished surfaces are more durable than the rough and rugged ones. Workmanship

Good workmanship helps in the preservation of stone work. All joints should be properly filled in leaving no hollows or cavities inside the masonry. External Rendering

Either the entire external surface should be plastered wit h cement sand plaster or at least it should be properly pointed so as to stop rain water from entering the joints. Proper Maintenance

Maintenance of structure in neat condition is quite effective in preserving it. To ensure this, it should be washed with water or with steam so that the dirt and soluble salts are removed and the rate of decay is reduced. Application of Preservative

The best way of preserving a stone is to eliminate the causes of its deterioration as far as possible. A good preservative should possess following properties : (i)

It should be economical, non-corrosive and harmless for health.

(ii)

It should maintain its effectiveness for long time.

(iii) It should easily penetrate into the surface of stone. (iv)

It should be easy in applying on the surface of stone. 15


(v)

It should not develop objectionable colour and it should not allow deep penetration of moisture into the stone.

(vi) Preservatives should be hard enough to withstand the effects of atmospheric agencies. Following are preservatives which may be used : •

Paint

•

Linseed oil

•

Coaltar

•

Paraffin

•

Alum and Soap

•

Use of baryta or barium hydroxide

•

Pointing and plastering

1.2.9 Dressing of Stones Stone blocks, obtained from quarry, are in irregular size and shapes and cannot be used as such in masonry work, without dressing. Dressing of stones is a process, carried out with the help of hand tools, to give proper shape and surface to the stone, before its use in masonry or in any other work. The dressing of the stones is done for the following purposes : (i)

To get the good appearance of stone masonry.

(ii)

To suit to the requirements of stone masonry.

(iii) To make transportation of stone easy and economical from the quarry. (iv) To take the advantage of softness in dressing due to presence of quarry sap in freshly quarried stones. The degree of dressing of the stone, depends upon the type of masonry and the type of stone available. Following are the varieties of finishes that can be obtained by dressing of stones : Tooled Finish

In this type of dressing, the surface of the stone is finished by means of a chisel. This finishing has continuous parallel chisel marks throughout the width and length of stone blocks. By using different chisels, different pattern of finished surfaces can be obtained. Punched Finish

In this type of finishing, the surface of the exposed face of the stone is prepared by using punches. The full surface consists of uniformly distributed punch mark on its surface. Cut Stone Finishing

In this type of dressing, the surface is finished using very sharp chisel in such a way that chisel marks can hardly be recognized. It is a sort of very high class of finishing. 16

Furrowed Finish

The surface of the exposed face is finished by making a number of vertical or horizontal grooves about 10mm wide. 2 cm wide margin is also made around the edges of the exposed face.


Reticulative Finish o

In this finishing, 2 cm wide margin chamfered at 45 is marked around the edges and irregular depressions are developed on the enclosed space of the exposed face. Axed Finish

This finish is mostly given to the surfaces of hard stones such as granite. This finish is given with the help of an axe. Scabbling Finish

In this type of finish, the projections from the quarried stone blocks are removed using scabbling hammer. It is also a sort of rough dressing of stones. Hammer Dressed Finish

This finish is slightly more finer than quarry and scabbling finishes. The stones in this dressing are made roughly square or rectangular and 2 cm to 5 cm wide margin is made about the edges of the exposed face. Vermiculated Finish

This finished surface appears just like worm eaten surface. It is more or less similar to reticulated finish except that depressions in this surfacing are more curved and good looking in appearance. Quarry Faced Finish

Some of the stone blocks, as obtained from the quarry, can be used in masonry work directly without any dressing at the site of work. The large projections are knocked off from the stone blocks with the help of quarry hammer, as soon as the stone is quarried. The faces of such stone blocks are rough. The surface of such a stone is termed as self-faced, rock faced or hammer faced or quarry faced. Rubbed Finish

In this finishing, instead of finishing surface with the help of suitable chisel, it is finished by rubbing the surface either with the help of hand or with machine. Polished Finish

The rubbed surface may be furthur improved in its appearance by polishing. Polishing may be done using pumice stone, sand and water, or by rubbing machine. Chisel-draughted Margin

In this case, a uniform margin about 2 cm wide is draughted around the stone with the help of chisel. Rest of the face may have any finishing.

SAQ 1 (a)

What are the essential qualities of good building stone?

(b)

Describe the methods of preservations of building stones.

17


(c)

Differentiate between stratified and unstratified rocks.

(d)

Fill in the blanks : (i)

Main ingredient of silicious rocks is ___________

(ii)

Clay is the main ingredient of ______________ rocks.

(iii) Geologically marble is called ________________. (iv)

Rocks having Alumina as their main constituent are called ______________.

(v)

______________ rocks are formed by the cooling of magma at a great depth.

(vi) _______________rocks can be split along definite planes. (vii) _______________ rocks are formed by gradual deposition. (viii) _______________ rocks are formed by alteration of original structure under heat and pressure. (e)

Choose the correct answers from amongst those given below : (i)

(ii)

Natural bed of stone is always kept: (a)

Parallel to applied load

(b)

At 90 to applied load

(c)

At 45 to applied load

(d)

None of above

o o

For construction of heavy engineering works like piers, bridges, dams, docks and harbours the best suited stone is: (a)

Granite

(b)

Marble

(c)

Slate

(d)

Lime stone

(iii) For ornamental carved work the best suited stone is : (a)

Sand Stone

(b)

Slate

(c)

Lime Stone

(d)

Marble

(iv) Stones should be dressed :

(v) 18

(a)

Immediately after quarrying

(b)

After seasoning

(c)

Immediately before use in work

(d)

None of these

Hardest rock is

(a)

Slate

(b)

Marble

(c)

Granite

(d)

Sand stone


1.3 BRICKS Brick is not only one of the oldest but also the most extensively used material of construction. Popularity of bricks as a material of construction is because of their local and cheap availability, strength, durability, reliability and insulating property against heat and sound. Brick is a regular sized rectangular unit, used for most of the building works. Bricks are obtained by moulding plastic mass of suitably proportioned earth in timber or steel moulds. Moulded bricks are first allowed to dry and then burnt in kilns designed for the purpose. Bricks are always rectangular in shape and of such proportion that the length is generally twice the width plus the thickness of mortar joint. Thickness of the brick is less than or at the most equal to the width of the brick. The size of the brick is such that it can be easily lifted and handled with one hand. They do not require any lifting appliances. It is used as substitute for stone, where stone is not available. Comparison of stone with brick as a material of construction is given in Table 1.1. Table 1.1 : Comparison of Stone with Bricks as a Material of Construction Sl. No.

Stone

Brick

1.

It is a natural material.

It is manufactured from clay.

2.

It is heavier.

It is lighter.

3.

It costs much to dress it to required shape and size.

It can easily be moulded to any shape or size.

4.

It is more costly except in hilly areas.

Except in hilly areas, it can be locally manufactured and is cheaper.

5.

It is less porous and therefore better suited for construction of water retaining structures.

It is more porous and requires costly water proofing treatment when used for constructing water retaining structures.

6.

Because of more strength, it is better suited for constructing structures carrying extra heavy loads or subjected to heavy pressures, e.g. harbour, dock and forts etc.

Reasonably good for normal loads.

7.

It is a better conductor of heat.

It is a poorer conductor of heat.

8.

It withstands attacks of weather better.

It is good for normal conditions but needs protection by way of plastering and pointing.

9.

Superior qualities of stones are used for constructing monuments and for decoration.

Good quality bricks are sometimes left unplastered to achieve some Architectural effect.

1.3.1 Classification and Characteristic of Bricks I.S.I. Classification of Bricks

19


The I.S.I. has classified the bricks into HI, HII, FI, FII, I, II, LI and LII categories, primarily according to the compressive strength. Table 1.2 gives the I.S.I. classification of bricks. Table 1.2 : I.S.I. Classification of Bricks Class of Bricks

Minimum Compressive Strength in 2 (kg/cm )

Minimum Absorption in 24 hours (%) of Dry Weight

Efflorescence

Tolerance in Dimension (%)

HI

440

5

No

±3

Metallic sound, smooth, rectangular.

HII

440

5

No

±8

Slight deformation in shape permitted.

FI

175

12

Very little

±3

Smooth, rectangular, metallic sound when two bricks strike.

FII

175

12

Very little

±8


I

70

20

Very little

±3

Smooth, rectangular, metallic sound when two bricks strike.

II

70

20

Very little

±8


LI

35

25

Very little

±3

Rectangular, sharp edge, metallic sound on striking need not be present.

LII

35

25

Little

±8

Slight deformation in shape allowed.

Shape and other Properties

In general, bricks may be classified into following five categories : (i)

First class bricks

(ii)

Second class bricks

(iii) Third class bricks (iv) Over burnt or Jhama bricks (v)

Under burnt or pilla bricks

First Class Bricks

The earth from which bricks have been made should be suitably proportioned. It should be free from small pebbles, lime lumps, organic, matter, and sodium salts. A first class bricks should possess the following properties : (i) 20

Size of the burnt brick should be exactly 19 × 9 × 9cm.

(ii)


It should be well burnt

(iii) Its colour should be uniform yellow or red. (iv)

It should not absorb water more than 20% of its own dry weight after 24 hours of immersion.

(v)

It should have a firm, compact, and uniform texture.

(vi)

Its surface should be regular and sides parallel. Edges should be sharp and at right angles to each other.

(vii) Its fractured surface should not show fissures and bubbles or lumps of lime. 2

(viii) Its crushing strength should not be less than 105 kg/cm . (ix)

Two first class bricks when struck with each other give a sharp metallic ringing sound.

(x)

It should not show any signs of efflorescence when bricks are dried after soaking in water.

(xi)

The brick should not break when it is dropped on a hard ground from a height.

(xii) Its surface should be so hard that finger nails should not be able to make any impression on its surface. Second Class Bricks

Brick of this category should possess the following properties : (i)

The brick of this class should also be well burnt. They can even slightly be over burnt. 2

(ii)

Minimum crushing strength should be 70 kg/cm .

(iii)

They should not absorb water more than 22% of its own dry weight after 24 hours of immersion in cold water.

(iv)

Two second class bricks when struck with each other give a metallic ringing sound.

(v)

These bricks should not show appreciable signs of efflorescence when soaked bricks are dried.

(vi)

The shape, size and colour of the brick should be generally regular and uniform. Slight distortion and surface cracks may however be allowed.

(vii)

Texture of these bricks should be uniform. Presence of small pebbles may however be allowed. Presence of limestone and kanker should not be allowed to occur.

Third Class Bricks

A third class brick should possess the following properties : (i)

These bricks are generally under burnt.

(ii)

They are soft and light coloured.

(iii) They may suffer from intensive efflorescence. (iv)

They emit a dull sound when struck against each other.

21


(v)

It should not absorb water more than 25% of its own dry weight when kept immersed in water for 24 hours.

(vi) Texture of above brick is not uniform and it may show lot of pebbles on freshly fractured surface of the brick. (vii) Size and shape of these bricks is not regular. But the irregularities should not be so much that it may become difficult t o have the course of uniform thickness, in masonry work. Over Burnt or Jhama Bricks

These are over burnt vitrified bricks. They are so distorted that they cannot be used in construction works. They are used for making aggregate for lime concrete for foundations and as a road metal in the construction of roads. Under Burnt or Pilla Bricks

Under burnt bricks are known as pilla bricks. They are just half burnt and have yellow colour. These bricks do not have any strength. These bricks may be burnt by using them again in the kilns, or they can be ground to powder form and used as surkhi.

1.3.2 Uses of Bricks (i)

(ii)

First class bricks are used for following purposes : (a)

It is used for all sound work of a permanent character.

(b)

It is used in the face work of structures not to be plastered but only pointed.

(c)

It is used in flooring and reinforced brick work.

Second class bricks are used for the following purposes : (a)

It is used in unimportant situations or at places where the masonry is to be plastered.

(b)

Used as brick ballast in R.C.C. work and in Lime concrete.

(iii) Third class bricks are used for the following purposes : (a)

These bricks are not used for important and permanent works.

(b)

These bricks are used in temporary constructions not subjected to heavy loads or heavy rains.

1.3.3 Brick Sizes According to I.S.I., the size of the brick without the thickness of mortar joint is 19 cm × 9 cm × 9 cm. The size with mortar joint becomes 20 cm × 10 cm × 10 cm. Hence, the effective size of brick is taken as 20 cm × 10 cm × 10 cm. If thinner bricks are required, their standard size is 19 cm × 9 cm × 4 cm and effective size is 20 cm × 10 cm × 5 cm. The minimum compressive strength of the standard bricks should be 35 kg/cm 2.

1.3.4 Brick Earth Composition

22

A good brick earth mainly consists of silica (sand) and alumina (clay), mixed in such a proportion that the resulting mass with water is a plastic mass which could be easily moulded and dried without undergoing shrinkage, cracking or warping. It should contain a littl e finely divided lime which enables silica to melt at furnace heat and bind the particles of brick together. It should also contain a small quantity of oxide of iron. It acts as a flux like lime, and more over gives red colour to the bricks on burning.


Functions of Constituent Silica or Sand

It is present in the earth either in free or combined form. As free sand, it remains mechanically mixed with clay. In combined form, it exists in chemical composition with alumina forming silicate of alumina. The percentage of silica in a good brick earth should lie between 50 to 60%. Presence of silica prevents cracking, shrinking and warping of raw bricks. Its excess amount destroys the cohesion between particles and makes the bricks brittle and weak. Hence, durability of the bricks depends largely on the proper proportion of silica in brick earth. Alumina (Clay)

Alumina is the chief constituent of every kind of clay. A good brick earth should contain between 20 to 30% of alumina. Alumina provides plasticity to the earth, so that it can be moulded. If bricks contain excess amount of alumina, and insufficient sand they shrink, crack and warp on drying and burning. Lime

When present in small quantities in finely divided state, it reduces shrinkage of bricks and acts as a flux causing silica to melt. It results in binding the particles of brick together resulting in greater strength of brick. Excess of lime causes the brick to melt and lose its shape. Oxide of Iron

Small quantity of oxide of iron to the extent of about 5 to 6 % is considered desirable in good brick earth. In the presence of silica and alumina, it helps the fusion of brick particles. Also it influences the colour of bricks. It produces a tint varying from light yellow to red depending upon the percentage of iron present in clay. Excess of it makes the colour dark blue. It should not be present in the form of iron pyrites. Magnesia

A small proportion of it decreases the shrinkage and gives yellow tint to the bricks. But larger amounts of magnesia cause bricks to decay. Harmful Ingredients of Brick Earth

Some of the ingredients, if present in the brick earth, render the resulting bricks unsuitable for any safe construction. These ingredients are :

Lime

If present in excess, it melts the brick particles as a result of which the brick loses shape. Lime should also be not present in brick-earth in

23


the form of limestone or kankar modules. On the burning of bricks, these get converted to quicklime which expands on absorption of moisture and causes the cracking and disintegration of bricks. Iron Pyrites

Presence of iron pyrites in the brick earth causes crystallization and disintegration during burning of the bricks. It is a common impurity found in most of the clays. It causes discolouration of bricks in form of black slag. Pebbles of Stone and Grave

Their presence makes it difficult to mix the brick-earth thoroughly as a result of which the bricks are not homogeneous. It gives weak and porous bricks. Also such bricks cannot be readily cut or worked. Alkalies

Their excessive presence in earth renders it unsuitable for bricks. These act as flux causing the bricks to melt, twist and warp. Presence of common salt in earth taken from seashores or from near salt formations has similar effects to those narrated above and also make the bricks hygroscopic, thereby causing efflorescence. Presence of Reh or Kallar

These consists of sodium chloride, sodium carbonate and sodium sulphate. The bricks cannot be properly burnt in the presence of sodium sulphate. These elements re-crystallize after burning of the bricks and deposit on the surface of masonry in form of whitish spots. This causes pealing off the plaster and bricks and ultimately leads to the failure of the structure. Presence of reh or kallar in the earth can be detected inspecting freshly dug pits or by noticing the signs of efflorescence. Organic Matter

Presence of organic matter and vegetation in the brick earth render the bricks porous. The porosity in bricks is the result of burning carbonaceous matter present in the earth. In order to avoid the adverse effect of the organic matter, the bricks should be well burnt.

1.3.5 Manufacturing of Bricks The complete process of manufacturing the bricks can be broadly divided into following heads : (i)

Preparation of brick earth or clay

(ii)

Pugging or tempering of the clay

(iii) Moulding of bricks (iv) Drying of bricks (v)

Burning of bricks

Each of these operations will now be discussed in detail. Preparation of Brick Earth or Clay

Following are the various operations involved in preparing brick-earth. 24

Digging


The top layer of soil about 20 cm thick which contains lot of impurities is first of all removed as it is unsuitable for brick making. After removing the top unsuitable soil, the clay is dug out either with manual labour or with the help of power excavators. The excavated lumps of soil are broken. The dug out soil is spread on the levelled ground. Height of the heap of clay may vary from 60 cm to 120 cm. It is ensured that the soil is free from gravel, coarse sand, lime and kanker particles, vegetable matter etc. Weathering

Excavated soil, after the clods have been broken, is mixed with a little water and is left in heaps to weather for a period varying from a few weeks to as long as it can be left. This improves its plasticity and strength. Water may be sprayed on the heap from time to time to keep the soil wet and heap turned over. Blending

After weathering, the earth is chemically analysed and if t here is any deficiency of any ingredient it is spread on the heap of the weathered earth and mixed with it with the help of phorah. The whole mass is thoroughly mixed up and reasonable amount of water is added if needed. Pugging or Tempering of the Clay

Tempering or pugging the clay means, breaking up of prepared clay, watering, and kneading till the earth becomes a homogeneous mass. Water is added to clay in required quantity and the whole mass is kneaded under the feet of men or cattle. But this method of tempering is outdated and not used much these days. Where good bricks are required to be manufactured on a large scale, tempering of clay is usually done by pugmill. Moulding of Bricks

It is done to give the required shape to the prepared brick earth. After tempering of the clay, bricks should be moulded as soon as possible, as otherwise pugged clay may become stiff and moulding of bricks may become difficult. The bricks can be moulded by following two methods : (i)

Hand moulding, and

(ii)

Machine moulding.

Hand Moulding

Hand moulding of bricks is extensively used in India. This could be done on ground or on table known, respectively, as ground moulding and as table moulding. A team of a moulder and a helper can mould about 1000 bricks in a day. Machine Moulding of Bricks

Machine moulding of the bricks may be adopted when very large number of bricks are required to be manufactured at one place. This

25


method proves economical only when bricks in huge quantity are to be moulded. The machine moulded bricks are hard and strong than table or ground moulded bricks. They are heavier in weight and have external surfaces smooth. Machine moulding of the bricks can be done by following two methods : •

Plastic clay method

•

Dry pressed clay method

Drying of Bricks

Wet bricks have to be dried before they are fed for burning in the kilns. Following are the objects of drying the bricks : (i)

To remove as much of moisture from the bricks as possible, so as to save time and fuel during the burning.

(ii)

To avoid the chances of cracking and distortion of bricks during the burning.

(iii) To increase the mechanical strength of the bricks without any damage to the bricks. Bricks have to staked after two or three days of drying. Although after two or three days of drying, bricks do not become dry and fit for burning, but they become sufficiently strong to get stacked. The bricks are stacked in such a way, that even in staked position free circulation of air maintained around them. Bricks are stacked in two brick (length wise) wide and 8 to 10 courses high stacks with sufficient space between individual brick. A specimen stack is shown in Figure 1.3.

Figure 1.3 : Stacking of Bricks

Burning of Bricks

Burning of the bricks imparts strength and hardness to the bricks and makes them dense and durable. The burning or heating of bricks is continued to o o higher temperature of about 1100 C. At about 1100 C, the two main constituents of brick, silica and alumina, combine with each other and bricks become dense and strong. At this temperature, fusible glass which is a flowing matter at high temperature is formed in very small amount which keeps clay particles binded together. Temperature of the bricks should not o be allowed to exceed 1100 C as otherwise fusible glass will be formed in large amount. This will cause bricks to flow and make them destorted and vitrified. Bricks should neither be over burnt nor under burnt, as in both the cases quality of bricks suffers. 26

Bricks are burnt either in clamps or in kilns. Clamps are temporary structures and are adopted when bricks are to be burnt on very small scale. Kilns are permanent structures and they are adopted when manufacture of bricks is to be carried out on a large scale.


1.3.6 Qualities of Good Bricks The general qualities that a good brick should possess are given below : (i)

Colour

The colour of good brick should be uniform. It may be deep red, cherry or copper coloured. Uniformity of colour indicates uniformity of chemical composition. (ii)

Water Absorption

No brick should absorb water more than 20% of its dry weight when kept immersed in water for 24 hours. (iii) Crushing Strength 2

It should not be less than 105 kg/cm . (iv) Shape Bricks should be uniform in shape with all its edges sharp, straight and at right angles to each other. All the faces should be true in shape. (v)

Size

Size of the bricks should be standard as prescribed by Indian standards. (vi) Texture and Compactness The bricks should have fine, dense, compact and uniform texture. Fractured surface of the brick should not show lumps of lime, loose grit, fissures and cavities. (vii) Soundness Two bricks should give clear ringing sound when struck against each other. (viii) Hardness The bricks should be so hard that finger nail should not be able to make any impression on its surface when scratched. (ix) The earth used for moulding the bricks should be free from gravel, pebbles, kankar, salt petre and other harmful ingredients. (x)

Bricks should be sound proof and also of low thermal conductivity.

(xi) Bricks should not break when dropped flat on hard ground from a height of about 1m.

1.3.7 Tests for Burnt Clay Bricks Certain tests are necessary to be conducted for judging the quality of a brick lot. These tests are : (i)

Water absorption test

(ii)

Crushing strength test

(iii) Hardness test (iv) Shape and size test

27


(v)

Soundness test

(vi) Test for presence of soluble salts. Water Absorption Test

There are two test to determine the water absorption (i)

24-hour immersion cold water test

(ii)

5-hour boiling water test

24-hour Immersion Cold Water Test

Dry specimen is put in an oven maintained at a temperature of 105 to o 115 C, till it attains substantially constant mass. Weight of specimen (W 1) is recorded after cooling it to room temperature. The dry specimen is then o immersed completely in water at a temperature of 27 ± 2 C for 24 hours. Take the specimen out of water and wipe out all traces of water with damp cloth. Complete weighing of the specimen, 3 minutes after the specimen has been removed from water. Let this weight be W 2. Water absorption percent by mass, after 24 hours immersion in cold water is given by W 2

−

W 1

W 1

×

100

5-hour Boiling Water Test o

The specimen is dried in an oven at 105 to 115 C till it attains constant mass. Cool the specimen at room temperature and record its weight ( W 1). The brick is immersed in boiling water for 5 hours. The water is allowed to o cool at 27 ± 2 C with brick immersed. The brick is taken out and wiped with damp cloth. Complete the weighing of the specimen in three minutes. Let it be W 3. Water absorption, percent by mass, is given by W 3

−

W 1

W 1

×

100

Test for Presence of Soluble Salts

Soluble salts, if present in the brick, cause efflorescence. Presence of such salts can be determined as follows. Place one end of the bricks in 25 mm depth of water in a dish of minimum diameter 150 mm and depth 30 mm. The dish is made of glass, porcelain or of glazed stone work. The experiment is performed in a well ventilated o room between 20 to 30 C till all the water in the dish is either absorbed by the specimen or is evaporated. After the specimen have dried, add similar quantity of water to the dish and let it too be absorbed by the specimen for efflorescence after the second evaporation. Presence of efflorescence shall be classified as nil, slight, moderate, heavy or serious as defined below : Nil

When the deposit of efflorescence is imperceptible. Slight

28

When the deposit of efflorescence does not cover more than 10% of the exposed area of the brick.


Moderate

When the deposit of efflorescence is heavier than slight and does not cover more than 50 percent of the exposed area of the brick surface. The deposit should not, however, powder or flake of the surface. Heavy

When the deposit of efflorescence salts is heavy and covers 50 percent or more of the exposed area of brick surface. The deposit, however, does not powder or flake of the surface. Serious

When the deposit of efflorescence salts is heavy and is accompanied by powdering and/or flaking of the exposed surfaces. Crushing Strength Test

In this test, well burnt brick’s areas are elected. Grind the two bed faces to provide smooth, even and parallel faces. Immerse the specimen in water at room temperature. Fill up flush the frog and all voids with cement mortar (1 part cement and 1 part clean coarse sand of grade 3 mm and down), store under damp jute bags for 24 hours and then immerse in clean water for 3 days. Remove and wipe out any traces of moisture. Place the specimen between two plywood sheets, each 3 mm thick, with flat faces horizontal and mortar filled face facing upwards. The specimen sandwiched between the ply sheets are carefully centred between plates of compression testing machine. Apply axial load at a uniform rate of 2 140 kg/cm per minute till failure. The maximum load at failure divided by the average area of the bed faces gives the compressive strength. Hardness Test

Hardness of the bricks can be estimated with the help of the scratch of the finger nail. If no nail scratch is left on the brick, it is considered to be having sufficient hardness. Shape and Size Test

All the faces of the brick should be truly rectangular and size truely standard as specified by Indian Standards. All the edges should be sharp and right angled. Soundness Test

Soundness of the bricks is estimated by striking two bri cks against each other. They should emit ringing sound. Soundness of the brick is also tested by the fall of the brick. A good sound brick should not break, when made to fall flat on hard ground, from a height of about 1 m.

SAQ 2 (a)

Compare stone work with brick work.

(b)

On what considerations the size of the brick is fixed? What relation should the breadth of a brick bear with length and why? 29


(c)

What are the various operations involved in the manufacture of bricks?

(d)

What is the importance of chemical analysis of the brick earth?

(e)

Describe the characteristics and essential features of good quality bricks.

(f)

What is efflorescence in bricks? What are its causes and remedies?

1.4 TILES Tiles are also a clay product, used mainly for roof covering and pavings. They are manufactured from superior type of clay. Tiles, being thinner than bricks, should be carefully handled to avoid any damage to tiles.

1.4.1 Classification of Tiles Tiles are classified as (i)

common tiles, and

(ii)

encaustic tiles.

Common Tiles

These tiles are of different shapes and sizes and are mainly used for flooring, paving and roofing. Encaustic Tiles

These tiles are used for decorative purposes in floors, walls, r oofs ceilings.

1.4.2 Manufacture of Tiles The following operations are involved in the manufacture of tiles : (i)

Preparation of clay.

(ii)

Moulding of tiles.

(iii) Drying of tiles. (iv) Burning of tiles. Preparation of Clay

30

The clay to be used in the manufacture of tiles is taken and all the pebbles, grit, and other impurities, picked out from it. The clay is then pulverized in grinding machine. Water is added to the ground clay, and put in the pugmill to temper. For manufacturing tiles of superior quality, the ground clay is mixed with plenty of water in a large pucca tank. The mixture is allowed to

stand quietly, during which coarse heavy particles settle at the bottom of the tank. Fine particles are held in suspension and mixture of f ine particles and water is transferred to another tank. This mixture is allowed to dry in open during which water is evaporated and fine clay is left at the bottom of the tank, which is used for the manufacture of the tiles. In order to make tiles impervious and hard, a finely ground mixture of glass and pottery ware may be added to the clay.


Moulding of Tiles

Moulding of tiles is done in the similar manner as has been already explained in the moulding of bricks. Tiles can be moulded by following three methods : (i)

Moulding by wooden moulds.

(ii)

Machine moulding.

(iii) Potter’s wheel moulding. Moulding by Wooden Moulds

In this method of moulding, wooden moulds are prepared in shape and size of which tiles are required to be moulded. Mould is placed in position and pressed clay is pressed in the mould. Care should be taken to preserve the shape of the tiles during the removal from mould. This method of moulding tiles is very common. Only plain tiles are generally moulded by this method. In this method, tiles are first moulded plain and when they become a bit hard and dry, they can be given any shape with the help of wooden patterns. Machine Moulding

In machine moulding of tiles, the clay is forced to come out of machine, under pressure and tiles are cut in specific size fr om the clay spout with the help of wire frame. Potters Wheel Moulding

In this method, round tiles are first shaped on the potters when, and after taking out of wheel, when moulded cylinder is quite hard, it is cut longitudinally to develop two half round tiles. These tiles are not exactly cylindrical, but have varying diameter as tiles moulded on potter’s wheel are not of same diameter throughout. Drying of Tiles

The tiles are stacked as they come out of mould one above the other and heaps of tiles, about 15 tiles high, are formed. After about 2 days time when tiles slightly get hard, the irregularities developed due to working are corrected with the help of flat wooden mallets. The tiles by now become hand hard and they can be lifted. Each tile is lifted up and its edges and under surfaces cleared. They are stacked on edge under a shade, to dry for about two-three days. The tiles are dried under the shade as this will prevent cracking and warping of tiles, due to rain and direct sun. Burning of Tiles

The tiles are subjected to burning in kilns after drying. A typical kiln is capable of burning 30,000 to 40,000 tiles. 31


The kiln is circular in plan and is protected by a shed. Long narrow flues are formed at the bottom of the kiln and they are covered by a layer of bricks. The bricks are arranged in such a way that open spaces are left between them. Dried tiles are now placed on edge, layer after layer, on the layer of bricks. After loading the kiln to its full capacity, all the doors in the kiln are closed by brick work in mud. Top of the kiln is covered with a layer of old tiles, placed in loose conditions. Heating or burning of the tiles is affected by firing wood in the flues. In the beginning, fire in the kiln should be gentle so that moisture is completely driven out of the tiles. The temperature is then raised to about o 800 C. This temperature is known as red hot. The fire is kept slackened for o about 5 hours and then it is raised to about 1300 C. This temperature is maintained for 3 hours. The process of slackening the fire for 6 hours and o then raising to about 1300 C temperature is also called white heat o temperature. Finally, the temperature of 1300 C is maintained for 3 hours and firing is stopped. The kiln is now allowed to cool gradually. In order to achieve gradual cooling of tiles in the kiln, the flues are filled with fire and their mouths closed with bricks in mud and fire is allowed to burn gradually. The complete process of burning the tiles may take about 3 days time. Now kiln is started unloading. Good well burnt tiles are stacked separately and dispatched for use. Under burnt tiles are stacked separately, and they are used on the top of the kiln in next load of burning of tiles.

1.4.3 Properties of Good Tiles Good tiles should possess the following properties : (i)

It should be of a regular shape and size.

(ii)

It should be well burnt.

(iii) It should have a uniform colour. (iv) It should be free from twists, cracks or flaws. (v)

It should give a clear ringing sound when one tile is struck against another.

(vi) It should be strong, hard, sound and durable. (vii) Its fractured surfaces should show the compact and even structure. (viii) Thickness (minimum) of machine made tiles should be 10 mm and hand mould tiles between 12 to 15 mm.

1.4.4 Types of Tiles Tiles are mainly of three types which are described (depicted) in Figure 1.4.

32


Figure 1.4: Types of Tiles

SAQ 3 (a)

What are the characteristics of good tiles?

(b)

Describe the complete process of manufacturing the good tiles.

1.5 SUMMARY You have seen that in construction engineering, the execution of different types of works always involves use of some inert materials. These inert materials have significant impact on strength, rigidity, durability and face appearance. Stones are extensively used for construction of buildings, lintels, arches, walls, columns, abutments etc. Stones are placed in such a way that their position offers maximum strength. Bricks are now-a-days used popularly for different types of construction. Its use is increasing day by day due to its local manufacturing and low cost. Bricks can be prepared in different sizes and shapes. Suitability of bricks i s judged by performing various tests on it. Tiles are mainly used for flooring, paving, roofing and for decorative purposes in floors, walls, roofs ceilings.

1.6 ANSWERS TO SAQs Refer the relevant preceding text in the unit or other useful books on the topic listed in Section “Further Reading” to get the answers of the SAQs.

33

Basics of Bricks and Classification

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