Dressing of Stones

The dressing of stone is defined as “The process of giving a proper size, shape and finish to the roughly broken stones as obtained from the quarry.” This process is done manually or mechanically. Stones obtained from the quarries are very rough and irregular in shape and quite bulky in size and weight. A dressed stone is fit for use in a particular situation in a building. A quarried stone has rough surfaces, which are dressed to obtain a definite and regular shape. Dressing of stones is done immediately after quarrying and before seasoning to achieve less weight for transportation. Dressing of stone provides pleasing appearance, proper bedding with good mortar joints, special shapes for arches, copings, pillars etc. The various types of dressed stones are shown in Fig.1.

Fig. 1 Dressed Stone Surfaces

With respect to the place of work, dressing can be divided into two types namely

1. Quarry dressing
2. Site dressing

Advantages of dressing, if it is carried out at quarry site.

1. At quarry site, it is possible to get chap labour for the process of dressing of stones.
2. It is possible to sort out stones for different works, if quarry dressing is practiced.
3. The irregular and rough portions of the stones are removed which decrease the weight of stones and it also facilitates easy transportation of the stones.
4. The stones when quarried freshly contain quarry sap and hence they are comparatively soft and can be easily dressed.

Various objectives of dressing are given below.

• To reduce the size of the big blocks of stones so that they are converted to easily liftable pieces. This reduction in size is generally carried out at the quarry itself because that saves a lot of transportation cost.
• To give a proper shape to the stone. It is known that stones can be used at different places in the building, e.g. in foundations, in walls, in arches or for flooring, each situation will require a proper shape. This can be given at the quarry and also at the site of construction.
• To obtain an appealing finish. In a residential building, stones are used not only because of their extra strength, hardness and durability but also because of their aesthetic value.

Methods / Types of Dressing of Stones

Dressing of stone can be done both manually as well as mechanically. Manually, skilled stone smiths can work wonders on the suitable type of stones with chisels and hammers and abrasives. Mechanically, machines can cut the stone to any desired size and shape. Their surfaces can be made extra smooth by polishing through machines. There are some traditional types of dressing of stones which are quite popular even at present. They are described below.

1) Pitched Dressing

In Pitched dressing, only the edges of a stone block are made level with the help of a hammer. The superfluous mass on the face is generally left intact.

2) Hammer Dressing

It is that type of dressing in which large raised portions of the stones are broken off, and the stone is shaped somewhat flat but rough due to hammer marks. These stone blocks are squared, and the bed and vertical sides are dressed to a distance of 8 to 10 cm from the face. This is done to enable the stone to have proper joints. This work is done by the use of Waller’s hammer. The obtained stones are termed as hammer faced, quarry-faced or rustic faced.

3) Chisel Drafting

In this method, drafts or grooves are made with the help of a chisel at all the four edges. Any superfluous stone from the center is then removed. Chisel drafted stones are specially used in plinths and corners of the buildings.

4) Rough Tooling

The edges are first squared by using a chisel and hammer. Then a series of grooves of variable width are developed over the surface of the stone.

5) Punched Dressing

In this method of dressing of stone, about 1 cm vertical or horizontal grooves are sunk with a chisel having it’s shaped as a hollow semicircle. The sides of the rock are kept chamfered or sunk. It is done on the stones that have already been rough tooled. With the help of Chisels, a series of parallel ridges are developed on the stone surface. It is also called furrowed finish.

6) Close Picked and Fine Tooling

This is an extreme type of dressing of stone in which almost every projection is removed from all the sides of the stone. Its surface is given a fine texture and appealing look.

7) Boasted or Droved Finish

It is a very common type of dressing of stone, in which the surface of the stone is covered with parallel marks that may run in any direction. A boaster which is actually a wide edged chisel is used for this purpose. These marks may be horizontal or at any angle. The chisel marks are not continuous across the whole width of the stone.

8) Scabbling

Irregular edges of the stones are broken off and the stone is shaped. This work is generally done in a quarry and the edges are broken with a scabbling hammer.

9) Reticulated Finish

In this type of dressing of stones, irregularly shaped sinking are made within the central portion of the stones having a 2 cm wide margin on its sides. These sinking are about 6 mm deep. The margin around the sinking is of constant width. The sunk surfaces may have punched marks to give a better appearance.

10) Vermiculated Finish

This type of dressing of stone is the same as the reticulated finish except that they are more curved and give a worm eaten type appearance. It is not very common as they need a lot of labour for construction.

11) Combed or Dragged Finish

This type of finish is done on soft stones. A comb is driven over the surface of this stone to remove it all elevating portions.

12) Picked Dressing

This type of Dressing of stones is obtained by finishing the stone with a point and the depression is smaller than the above type.

13) Moulded Finish

Moulding is done to improve the appearance of stones. These are either handmade or machine made.

14) Rubbed Finish

In this method of dressing of stone, the surfaces of stones are rubbed to get a smoother finish. One piece of stone is rubbed against the other. Water and sand are added to aid the operation. It can also be rubbed by hand or machines.

15) Polished Surfaces

Stones which can take polish, e.g., granites, marbles, limestone etc. are first rubbed and then polished by using rubber, pad, sand, water and putty powder. However, a machine can also be used for polishing.

16) Sand Blasting

This method of dressing of stone is done to imprint letterings and designs on the surface of granite. The polished surface is coated with a molten rubber like compound which solidifies on cooling. The desired design is cut on this coating with a sharp tool thereby exposing the stone surface which is to be cut. A blast of sand is then blown with compressed air, the part which is exposed is cut to the depth needed.

Stone Dressing Tools

The dressing tools are shown in Fig. 2. They are wedge, pitching tool, boaster, scabbing hammer, mash hammer, separated pick, punch, scabbing pick, crow bar, axe punch, dressing knife and splitting chisel. Some other tools like mason’s hammer, spacing hammer, club hammer, soft stone chisel, claw chisel, punching chisel, point chisel, drafting chisel etc. are also used.

Fig. 2 Tools for Cutting and Dressing Stones

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Properties and Requirements of Good Building Stones

Properties of Good Building Stones

The following properties of the stones should be looked into before selecting them for engineering works.

1) Structure

The structure of the stone may be stratified (layered) or unstratified. Stratified stones should be easily dressed and suitable for super structure. Unstratified stones are hard and difficult to dress. They are preferred for the foundation works.

2) Texture

Fine grained stones with homogeneous distribution look attractive and hence they are used for carving. Such stones are usually strong and durable.

3) Density

Denser stones are stronger. Light weight stones are weak. Hence stones with specific gravity less than 2.4 are considered unsuitable for buildings.

4) Appearance

A stone with uniform and attractive colour is durable, if grains are compact. Marble and granite get very good appearance, when polished.

5) Strength

Strength is an important property to be looked into before selecting stone as building block. Indian standard code recommends, a minimum crushing strength of 3.5 N/mm² for any building block. Due to non-uniformity of the material, usually a factor of safety of 10 is used to find the permissible stress in a stone. Hence even laterite can be used safely for a single storey building, because in such structures expected load can hardly give a stress of 0.15 N/mm². However, in stone masonry buildings, care should be taken to check the stresses when the beams are placed on laterite wall. A good building stone should have better crushing strength i.e. greater than 100 N/mm².

6) Hardness

It is an important property to be considered when stone is used for flooring and pavement. Coefficient of hardness is to be found by conducting test on standard specimen in Dory’s testing machine. For road works coefficient of hardness should be at least 17. For building works stones with coefficient of hardness less than 14 should not be used.

7) Percentage Wear

It is measured by attrition test. It is an important property to be considered in selecting aggregate for road works and railway ballast. A good stone should not show wear of more than 2%.

8) Porosity and Absorption

All stones have pores and hence absorb water. The reaction of water with material of stone causes disintegration. Absorption test is specified as percentage of water absorbed by the stone when it is immersed under water for 24 hours. For a good stone it should be as small as possible and in no case more than 5.

9) Weathering

Rain and wind cause loss of good appearance of stones. Hence stones with good weather resistance should be used for face works.

10) Toughness

The resistance to impact is called toughness. It is determined by impact test. Stones with toughness index more than 19 are preferred for road works. Toughness indexes 13 to 19 are considered as medium tough and stones with toughness index less than 13 are poor stones.

11) Resistance to Fire

Sand stones resist fire better. Argillaceous materials, though poor in strength, are good in resisting fire.

12) Ease in Dressing

Cost of dressing contributes to cost of stone masonry to a great extent. Dressing is easy in stones with lesser strength. Hence an engineer should look into sufficient strength rather than high strength while selecting stones for building works.

13) Seasoning

The stones obtained from quarry contain moisture in the pores. The strength of the stone improves if this moisture is removed before using the stone. The process of removing moisture from pores is called seasoning. The best way of seasoning is to allow it to the action of nature for 6 to 12 months. This is very much required in the case of laterite stones.

14) Durability

A good building stones must be durable long lasting nature.

15) Water Resistance

They should have less water absorption properties.

16) Economy

They should be economical and easily available.

Requirements of Good Building Stone

The following are the quality requirements of good building stones.

1) Strength

Strength is an important property to be looked into before selecting stone as a building block. Generally, most of the building stones have high strength to resist the load coming on it. Therefore, it is not of prime concern when it comes to check the quality of stones. But when the stones are to be used in large structures, it becomes necessary to check the compressive strength of stones. Compressive strength of building stones generally falls within the range of 60 to 200 N/mm². Indian standard code recommends, a minimum crushing strength of 3.5 N/mm² for any building block.

2) Durability

Building stones should be capable to resist the adverse effects of natural forces like wind, rain and heat. It must be durable and should not deteriorate due to the adverse effects of the above natural forces.

3) Hardness

It is an important property to be considered when a stone is used for flooring, pavement or aprons of bridges, they become subjected to wearing and abrasive forces caused by movement of men or machine over them. So it is required to test hardness of stone. The coefficient of hardness is to be found by conducting a test on a standard specimen in Dory’s testing machine. For road works coefficient of hardness should be at least 17. For building works stones with a coefficient of hardness less than 14 should not be used.

4) Toughness

Toughness of stones means it ability to resist impact forces. It is determined by the impact test. Building stones should be tough enough to sustain stresses developed due to vibrations. The vibrations may be due to the machinery mounted over them or due to the loads moving over them. The stone aggregates used in the road constructions should be tough. Stones with toughness index more than 19 are preferred for road works.

5) Specific Gravity

The more the specific gravity of stone, the heavier and stronger the stone is. Therefore, stones having higher specific gravity values should be used for the construction of dams, retaining walls, docks and harbours. The specific gravity of good building stone is between 2.4 and 2.8. Stones with a specific gravity less than 2.4 are considered unsuitable for buildings.

6) Porosity and Water Absorption

All stones have pores and hence absorb water. The reaction of water with a material of stone cause disintegration. Porosity of building stones depend upon the mineral constituent and structural formation of the parent rock. If stones used in building construction are porous then rain water can easily enter into the pore spaces and cause damage to the stones. Therefore, building stone should not be porous. Water absorption of stone is directly proportional to the porosity of rock. If a stone is more porous then it will absorb more water and cause more damage to stone. In higher altitudes, the freezing of water in pores takes place and it results into the disintegration of the stone. The absorption test is specified as the percentage of water absorbed by the stone when it is immersed underwater for 24 hours. For a good stone it should be as small as possible and in no case more than 5.

Table 1 - 24 Hours Water Absorption of Stones by Volume

Sl. No	Types of Stone	Water absorption (% not greater than)
1	Sandstone	10
2	Limestone	10
3	Granite	1
4	Trap	6
5	Shale	10
6	Gneiss	1
7	Slate	1
8	Quartzite	3

7) Dressing

Giving required shape to the stone is called dressing. It should be easy to dress so that the cost of dressing is reduced. However, the care should be taken so that, this is not being at the cost of the required strength and the durability.

8) Appearance

A stone with uniform and attractive colour is durable if grains are compact. Marble and granite get a very good appearance, when polished. Hence, they are used for face works in buildings. In case of the stones to be used for face works, where appearance is a primary requirement, its colour and ability to receive polish is an important factor. Light coloured stones are more preferred than dark coloured stones as the colour are likely to fade out with time.

9) Seasoning

The stones obtained from the quarry contain moisture in the pores. The strength of the stone improves if this moisture is removed before using the stone. The process of removing moisture from pores is called seasoning. The best way of seasoning is to allow it to the action of nature for 6 to 12 months. This is very much required in the case of laterite stones. Good stones should be free from the quarry sap. Lateritic stones should not be used for 6 to 12 months after quarrying. They are allowed to get rid of quarry sap by the action of nature.

10) Workability

Stone should be workable. Stone is said to be workable when the work involved in stone working (such as cutting, dressing & shaping) is economical and easy to conduct.

11) Cost

Cost is an important consideration in selecting a building material. Proximity of the quarry to building site brings down the cost of transportation and hence the cost of stones comes down.

12) Fire Resistance

Stones should be free from calcium carbonate, oxides of iron and minerals having different coefficients of thermal expansion. Igneous rock show marked disintegration principally because of quartz which disintegrates into small particles at a temperature of about 575°C. Limestone can withstand a little higher temperature; i.e. up to 800°C after which they disintegrate. Sandstones resist fire better. Argillaceous materials, though poor in strength, are good in resisting fire.

13) Percentage Wear

It is measured by the attrition test. It is an important property to be considered in selecting aggregate for road works and railway ballast. A good stone should not show the wear of more than 2%.

14) Weathering

Rain and wind cause loss of the good appearance of stones. Hence stones with good weather resistance should be used for face works.

15) Structure

The structure of the stone may be stratified (layered) or unstratified. Stratified stones should be easily dressed and suitable for super structure. Unstratified stones are hard and difficult to dress. They are preferred for the foundation works.

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Nominal Stress, True Stress and Factor of Safety

Direct stress is the value obtained by dividing the load by original cross-sectional area. That is the reason why the value of stress started dropping after neck is formed in mild steel (or any ductile material). But actually as material is stressed its cross-sectional area changes. We should divide load by the actual cross-sectional area to get true stress in the material. To distinguish between the two values, the terms nominal stress and true stress is introduced.

Because we consider nominal stress, after neck formation started (after ultimate stress), stress-strain curve started sloping down and the breaking took place at lower stress (nominal). If we consider true stress, it is increasing continuously as strain increases as shown in Fig. 1.

Fig. 1 Nominal Stress - Strain Curve and True Stress - Strain Curve for Mild Steel

Factor of Safety

In practice, it is not possible to design a mechanical component or structural component permitting stressing up to ultimate stress for the following reasons.

1. Reliability of material may not be 100 per cent. There may be small spots of flaws.
2. The resulting deformation may obstruct the functional performance of the component.
3. The loads taken by designer are only estimated loads. Occasionally there can be overloading. Unexpected impact and temperature loadings may act in the lifetime of the member.
4. There are certain ideal conditions assumed in the analysis (like boundary conditions).

Actually ideal conditions will not be available and, therefore, the calculated stresses will not be 100 per cent real stresses. Hence, the maximum stress to which any member is designed is much less than the ultimate stress and this stress is called Working Stress. The ratio of ultimate stress to working stress is called factor of safety. Thus

In case of elastic materials, since excessive deformation create problems in the performance of the member, working stress is taken as a factor of yield stress or that of a 0.2 proof stress (if yield point does not exist). Factor of safety for various materials depends up on their reliability. The following values are commonly taken in practice.

1. For steel – 1.85
2. For concrete – 3
3. For timber – 4 to 6

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Stress-Strain Relationship of Mild Steel

The stress-strain relation of any material is obtained by conducting tension test in the laboratories on standard specimen. Different materials behave differently and their behaviour in tension and in compression differ slightly.

Behaviour in Tension

Fig.1 shows a typical tensile test specimen of mild steel. Its ends are gripped into universal testing machine. Extensometer is fitted to test specimen which measures extension over the length L1, shown in Fig. 2. The length over which extension is measured is called Gauge Length. The load is applied gradually and at regular interval of loads extension is measured. After certain load, extension increases at faster rate and the capacity of extensometer to measure extension comes to an end and hence, it is removed before this stage is reached and extension is measured from scale on the universal testing machine. Load is increased gradually till the specimen breaks.

Fig. 1 Tension Test Specimen

Fig. 2 Tension Test Specimen after Breaking

Load divided by original cross sectional area is called as nominal stress or simply as stress. Strain is obtained by dividing extensometer readings by gauge length of extensometer (L1) and by dividing scale readings by grip to grip length of the specimen (L2). The uniaxial tension test is carried out on tensile testing machine and the following steps are performed to conduct this test.

• The ends of the specimen are secured in the grips of the testing machine.
• There is a unit for applying a load to the specimen with a hydraulic or mechanical drive.
• There must be some recording device by which you should be able to measure the final output in the form of Load or stress. So the testing machines are often equipped with the pendulum type lever, pressure gauge and hydraulic capsule and the stress vs strain diagram is plotted. Fig.3 shows stress vs strain diagram for the typical mild steel specimen.

Fig. 3 Stress-Strain Curve of Mild Steel

The following salient points are observed on stress-strain curve.

a) Limit of Proportionality (A)

It is the limiting value of the stress up to which stress is proportional to strain.

b) Elastic Limit

This is the limiting value of stress up to which if the material is stressed and then released (unloaded) strain disappears completely and the original length is regained. This point is slightly beyond the limit of proportionality.

c) Upper Yield Point (B)

This is the stress at which, the load starts reducing and the extension increases. This phenomenon is called yielding of material. At this stage strain is about 0.125 per cent and stress is about 250 N/mm².

d) Lower Yield Point (C)

At this stage the stress remains same but strain increases for some time.

e) Ultimate Stress (D)

This is the maximum stress the material can resist. This stress is about 370 - 400 N/mm². At this stage cross sectional area at a particular section starts reducing very fast. This is called neck formation. After this stage load resisted and hence the stress developed starts reducing.

f) Breaking Point (E)

The stress at which finally the specimen fails is called breaking point. At this strain is 20 to 25 per cent. If unloading is made within elastic limit the original length is regained i.e., the stress-strain curve follows down the loading curve shown in Fig. 3. If unloading is made after loading the specimen beyond elastic limit, it follows a straight line parallel to the original straight portion as shown by line FF′ in Fig. 3. Thus if it is loaded beyond elastic limit and then unloaded a permanent strain (OF) is left in the specimen. This is called permanent set.

Stress-Strain Relation in Aluminium and High Strength Steel

In these elastic materials there is no clear cut yield point. The necking takes place at ultimate stress and eventually the breaking point is lower than the ultimate point. The typical stress-strain diagram is shown in Fig. 4. The stress at which if unloading is made there will be 0.2 per cent permanent set is known as 0.2 per cent proof stress and this point is treated as yield point for all practical purposes.

Fig. 4 Stress-Strain Relation in Aluminium and High Strength Steel

Stress-Strain Relation in Brittle Material

The typical stress-strain relation in a brittle material like cast iron, is shown in Fig. 5. In these material, there is no appreciable change in rate of strain. There is no yield point and no necking takes place. Ultimate point and breaking point are one and the same. The strain at failure is very small.

Fig. 5 Stress-Strain Relation for Brittle Material

Percentage Elongation and Percentage Reduction in Area

Percentage elongation and percentage reduction in area are the two terms used to measure the ductility of material.

a) Percentage Elongation

It is defined as the ratio of the final extension at rupture to original length expressed, as percentage. Thus,

where

     L – Original length

     L′– Length at rupture

The code specify that original length is to be five times the diameter and the portion considered must include neck (whenever it occurs). Usually marking are made on tension rod at every ‘2.5d’ distance (d - diameter of rod) and after failure the portion in which necking takes place is considered. In case of ductile material percentage elongation is 20 to 25.

(b) Percentage Reduction in Area

It is defined as the ratio of maximum changes in the cross sectional area to original cross-sectional area, expressed as percentage. Thus,

where

     A – Original cross-sectional area

     A′ – Minimum cross-sectional area

In case of ductile material, A′ is calculated after measuring the diameter at the neck. For this, the two broken pieces of the specimen are to be kept joining each other properly. For steel, the percentage reduction in area is 60 to 70.

Behaviour of Materials under Compression

As there is chance to bucking (laterally bending) of long specimen, for compression tests short specimens are used. Hence, this test involves measurement of smaller changes in length. It results into lesser accuracy. However precise measurements have shown the following results.

a) In case of ductile materials stress-strain curve follows exactly same path as in tensile test up to and even slightly beyond yield point. For larger values the curves diverge. There will not be necking in case of compression tests.

b) For most brittle materials ultimate compressive stress in compression is much larger than in tension. It is because of flows and cracks present in brittle materials which weaken the material in tension but will not affect the strength in compression.

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Relationship between Elastic Constants

 Modulus of elasticity, modulus of rigidity and bulk modulus are the three elastic constants.

1) Modulus of Elasticity (Young’s Modulus) ‘E’

It is defined as the ratio of linear stress to linear strain within elastic limit.

2) Modulus of Rigidity (G or N)

It is defined as the ratio of shearing stress to shearing strain within elastic limit and is usually denoted by letter 'G' or 'N'. Thus

where

     G = Modulus of rigidity

     q = Shearing stress

     ϕ = Shearing strain

3) Bulk Modulus (K)

When a body is subjected to identical stresses p in three mutually perpendicular directions, as shown in Fig. 1, the body undergoes uniform changes in three directions without undergoing distortion of shape.

Fig. 1 Stresses p acting in three mutually perpendicular directions

The ratio of change in volume to original volume has been defined as volumetric strain (e_v). Then the bulk modulus, 'K' is defined as

where

      p = identical pressure in three mutually perpendicular directions

  , Volumetric strain

      Δ v = Change in volume

      v = Original volume

Thus bulk modulus may be defined as the ratio of identical pressure ‘p’ acting in three mutually perpendicular directions to corresponding volumetric strain.

Fig. 1 shows a body subjected to identical compressive pressure ‘p’ in three mutually perpendicular directions. Since hydrostatic pressure i.e. the pressure exerted by a liquid on a body within it, has this nature of stress, such a pressure ‘p’ is called as hydrostatic pressure.

Relationship between Modulus of Elasticity and Modulus of Rigidity

Consider a square element ABCD of sides ‘a’ subjected to pure shear ‘q’ as shown in Fig. 2. AEC’D shown is the deformed shape due to shear ‘q’. Drop perpendicular BF to diagonal DE. Let ′ϕ′ be the shear strain and G is the modulus of rigidity.

Fig. 2

              

Since angle of deformation is very small we can assume ∠BEF = 45°, hence EF = BE cos 45°

                    

                      

Now, we know that the above pure shear gives rise to axial tensile stress ‘q’ in the diagonal direction of DB and axial compression q at right angles to it. These two stresses cause tensile strain along the diagonal DB.

Relationship between Modulus of Elasticity and Bulk Modulus

Consider a cubic element subjected to stresses p in the three mutually perpendicular direction x, y, z as shown in Fig. 3.

Fig. 3

Now the stress p in x direction causes tensile strain p/E in x direction while the stress p in y and z direction cause compressive strains μp/E in x direction.

Relationship between E,G and K

                                 We know that       E = 2G(1 + μ)                  (1)

                                                                     E = 3K(1 – 2 μ) (2)

By eliminating ′μ′ between the above two equations we can get the relationship between E, G and K, free from the term μ.

     

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Water Quality - Physical Parameters

The quality of water is determined by the impurities present in it. The impurities may be physical, chemical or bacteriological in nature. In order to ascertain the quality of water, it is subjected to various tests viz., physical, chemical and bacteriological tests. It is essential for devising water quality management programme to properly use water in any project. It gives information for following decisions to be taken.

• Helps in identifying the present and future problems of water pollution.
• Identifying the present resources of water as per various usages.
• It helps in developing plans and setting priorities for water quality management programme so as to meet future water requirements.
• It helps in evaluating the effectiveness of present management actions being taken and devising future course of actions.

Impurities in Water

It is not possible to find pure water in nature. The rain water as it drops down to the surface of earth absorbs dust and gases from the atmosphere. It is further exposed to organic matter on the surface of earth and by the time, it reaches the source of water supply, it is found to contain various other impurities also. For the purpose of classification, the impurities present in water may be divided into the following three categories.

1. Physical impurities
2. Chemical impurities
3. Bacteriological impurities

Analysis of water

In order to ascertain the quality of water, it is subjected to various tests. These tests can be divided into the following three categories.

1.  Physical tests
2.  Chemical tests
3.  Bacteriological tests

Collection of Water Samples

The sampling is the most important part of any analysis because the final results obtained, even from the most accurate analysis, will be misleading, if the samples on which such analysis is carried out, are not representative ones of the liquids to be tested. As a matter of fact, it will be ideal to carry out all the analysis immediately after the collection of samples and quicker the analysis, the more representative will be the results of analysis of the liquid at the time the samples are taken. These precautions while sampling of water are as follows.

• The water should be collected in bottles, especially of white glass, having well fitted stoppers. Bottles having holding capacity of about 2 liters of water are necessary for chemical analysis. For bacteriological examination, bottles with smaller capacities will be sufficient.
• Bottles should be thoroughly cleansed, filled thrice with water and thrice emptied before collecting the sample. However, it will not be necessary to carry out such process, if the sealed bottles are directly obtained from the laboratories.
• When the sample of water is to be collected from a pipe, the water tap should be turned on and the water should be allowed to go waste for at least two minutes so as to prevent the entry of impurities of the pipe in the sample of water. If the sample is to be collected for conducting a bacteriological analysis, the nozzle of the tap should be flamed and made unbearably hot and then cooled by the running water before the bottle is filled.
• For the collection of water for bacteriological tests, the person who collects the water must be free from any disease. The containers and bottles must be cleaned with sulphuric acid, potassium dichromate or alkaline permanganate and then, they should be thoroughly rinsed with distilled water and finally sterilization should be done. Immediately after collection of the samples, bottles should be closed and covered with clot to prevent accumulation of dirt, etc.
• For collecting the sample of water from lake, streams, spring or well the whole bottled with stopper closed should be immersed deep into the surface of water and then only the stopper of the bottle should be removed by means of a clean piece of string and the bottle is filled. Thus the entry of floating materials will be prevented in the bottle.
• The bottle should be held as far away from its neck as possible. In no case, the water entering the bottle should come into contact with the hand.
• In case the water is being collected from the ground sources i.e. through well or tube well, sufficient quantity of water should be pumped out before collecting the samples.
• After collecting the sample, the stopper of bottle should be well secured and the bottles containing samples of water should be labelled stating the source, date and time of collection.

Physical Tests

Under this category, tests are carried out to examine water for the following properties.

1) Colour

The colour of water is usually due to presence of organic matter in colloid condition and due to the presence of mineral and dissolved organic and inorganic impurities. An undesirable appearance is produced by colour in water. It spoils the clothes and affects various industrial processes. The measurement of colour in water is carried out by means of a tintometer. The instrument has an eye piece with two holes. A slide of standard coloured water is seen through one hole and in the other hole, the slide of water to be tested is inserted. The intensity of colour in water is measured on an arbitrary scale.

The unit of colour on cobalt scale is the colour produced by one milligram of platinum cobalt in one litre of distilled water. The slide of standard numbers is kept ready in the laboratory. For public water supply, the number on cobalt scale should not exceed 20 and should preferably less than 10.

Transparent water with a low accumulation of dissolved materials appears blue. Dissolved organic matter such as humus, peat or decaying plant matter, etc. produce a yellow or brown colour. Some algae or dinoflagellates produce reddish or deep yellow waters. Water rich in phytoplankton and other algae usually appears green. Soil runoff water has a variety of yellow, red, brown and grey colours.

The colour in water is not harmful but it is objectionable. The colour of a water sample can be reported as Apparent or True colour. Apparent colour is the colour of the whole water sample and consists of colour from both dissolved and suspended components. True colour is measured after filtering the water sample to remove all suspended material. Before testing the colour of the water, first of all total suspended matter should be removed from the water by centrifugal force in a special apparatus. When multicolour industrial wastes are involved, such colour measurement is meaningless. It should be remembered that the examinations of colour by matching with slides of standard colours will be sufficient for most of the purposes and it is obvious that the results will be influenced by the personal factor, the conditions of lighting under which the tests are carried out etc.

2) Temperature

The test for temperature of water has no meaning in the sense that it is not possible to give any treatment to control the temperature in any water supply project. The temperature of water to be supplied from storage reservoir depends on the depth from which it is drawn. The most desirable temperature for public supply between 4.4°C to 10°C. The temperature above 35°C is unfit for public supply, because it is not palatable.

The multiplication of bacteria in the waters is more rapid at higher temperatures than in the waters at lower temperature. Hence, when waters with a temperature of about 15°C are collected for bacteriological analysis, they should be cooled down as quickly as possible. It should further be remembered that the air temperature at the time of taking the water sample should always be recorded.

The measurement of temperature of water is done with the help of ordinary thermometers. From the study of temperature, the characteristics of water such as density, viscosity, vapour pressure and surface tension can be determined. It also helps in determining the saturation values of solids and gases which can be dissolved in water and also the rates of chemical, biochemical and biological activity.

Density, viscosity, vapour pressure and surface tension of water are all dependent upon the temperature. The saturation values of solids and gases that can be dissolved in water and the rates of chemical, biochemical and biological activity are also determined on the basis of temperature. The temperature of surface water is generally same as the atmospheric temperature while that of ground water may be more or less than atmospheric temperature.

3) Taste and Odour

The water possesses taste and odour due to various causes and they make the water unpleasant for drinking. Tastes and odours in water are due to the presence of any of the following.

1.  Dead or living microorganisms
2.  Dissolved gases such as hydrogen sulphide, methane, carbon dioxide or oxygen combined with organic matter
3.  Mineral substances such as sodium chloride, iron compounds
4.  Carbonates and sulphates

The test is conducted for odour by inhaling through two tubes of osmoscope. One tube is kept in a flask containing diluted water and other one in a flask containing water to be tested. The odour of water also changes with temperature. The odour may be classified as sweetish, vegetable, greasy etc. The odour of both cold and hot water should be determined.

The intensities of the odours are measured in terms of threshold odour number (TON). In this method, water to be tested is diluted with odour free water and mixture at which odour becomes detectable is determined. It indicates threshold number and other intensities of odour are then worked out. The results of test are greatly affected by the sensitiveness of the observer. For public water supply, the threshold number should not be more than 3.

4) Turbidity

Mainly, the colloidal matter present in water imparts turbidity to water. It is also caused due to presence of suspended matter in the water. Turbidity is a measure of the resistance of water to the passage of light through it. Turbidity is expressed as NTU (Nephelometric Turbidity Units) or ppm (parts per million) or Milligrams per litre (mg/l). The turbidity in water may also be due to clay and silt particles, discharges of sewage or industrial wastes, presence of large numbers of micro-organisms etc. and the cloudy appearance developed in water due to turbidity is aesthetically unattractive and it may also be harmful to the consumers.

The standard unit of turbidity is the form of finely divided silica in a million parts of distilled water. The permissible turbidity for drinking water is 5 to 10 ppm. The measurement of turbidity in the field is done by means of a turbidity rod. For laboratory, various turbidimeters are found out to measure the turbidity of water, the most common being Jackson turbidimeter and Baylis turbidimeter. Jacksons Turbidimeter is used to measure turbidity when it is more than 100 ppm. Bali's Turbidimeter is used to measure the turbidity of the sample when it is less than 5 ppm.

Fig. 1 Jacksons Turbidimeter

Fig. 2 Bali's Turbidimeter

Ground waters are generally less turbid than the surface water. The character and amount of turbidity depends on the type of soil over which the water has moved. Earlier, the turbidity produced by one milligram of silica in one litre of distilled water was considered as the unit of turbidity. Turbidity was previously determined by Jackson candle Turbidity units (JTU). This unit is now replaced by more appropriate unit called Nephelometric Turbidity unit (NTU) which is the turbidity produced by one milligram of formazin polymer in one litre of distilled water. Nephelometry method has better sensitivity, precision and applicability over a wide range of particle size and concentrations as compared to older methods.

5) Specific Conductivity of Water

The total amount of dissolved salts present in water can be estimated by measuring the specific conductivity of water. The specific conductivity of water is determined by means of a portable ionic water tester and is expressed as micro-mho per cm at 25°C. ‘mho’ is the unit of conductivity and it equals to 1 Ampere per volt. The specific conductivity of water in micro mho per cm at 25°C is multiplied by a coefficient generally 0.65 so as to directly obtain the dissolved salt content in mg/L or ppm. The actual value of this coefficient depends upon the type of salt present in water.

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Population Forecasting

Population forecasting is a method to predict or forecast the future population of an area. Design of water supply and sanitation scheme is based on the projected population of a particular city, estimated for the design period. Any underestimated value will make system inadequate for the purpose intended; similarly overestimated value will make it costly. Change in the population of the city over the years occurs and the system should be designed taking into account of the population at the end of the design period. Factors affecting changes in population are given below.

• Increase due to births
• Decrease due to deaths
• Increase/ decrease due to migration
• Increase due to annexation

The present and past population record for the city can be obtained from the census population records. After collecting these population figures, the population at the end of design period is predicted using various methods as suitable for that city considering the growth pattern followed by the city. The various population forecasting methods are mentioned below. 

1) Arithmetical Increase Method 

The arithmetical Increase Method is mainly adopted for old and developed towns, where the rate of population growth is nearly constant. Therefore, it is assumed that the rate of growth of the population is constant. It is similar to simple interest calculations. The population predicted by this method is the lowest of all. If it is used for small, average or comparatively new cities, it will give low result than actual value. In this method the average increase in population per decade is calculated from the past census reports. This increase is added to the present population to find out the population of the next decade. 

Hence, 

i.e. rate of change of population with respect to time is constant. 

Therefore, Population after n^th decade will be

Pn = Po + n x̄ 

where, 

    Po - last known population 

    Pn - population (predicted) after 'n' number of decades 

    n - number of decades between Po and Pn 

    x̄ - the rate of population growth 

Example Question

Predict the population for the year 2021, 2031 and 2041 from the following population data. 

Year	1961	1971	1981	1991	2001	2011
Population	8,58,545	10,15,672	12,01,553	16,91,538,	20,77,820,	25,85,862

Solution

Year	Population	Increment
1961	858545	-
1971	1015672	157127
1981	1201553	185881
1991	1691538	489985
2001	2077820	386282
2011	2585862	508042
	Average increment, x̄	345463

     Population after n^th decade is Pn = Po + n x̄ 

     Population in year 2021 is, P₂₀₂₁ = 2585862 + 345463 x 1 

                                                                       = 2931325 

Similarly,                                 P₂₀₃₁ = 2585862 + 345463 x 2 

                                                                = 3276788 

                                                   P₂₀₄₁ = 2585862 + 345463 x 3 

                                                               = 3622251 

2) Geometrical Increase Method or Geometrical Progression Method 

This method is adopted for young and developing towns, where the rate of growth of population is proportional to the population at present (i.e., dP/dt ∝ P). Therefore, it is assumed that the percentage increase in population is constant. It is similar to compound interest calculations. The population predicted by this method is the highest of all. Geometric mean increase is used to find out the future increment in population. Since this method gives higher values and hence should be applied for a new industrial town at the beginning of development for only few decades. 

The population at the end of n^th decade ‘Pn’ can be estimated as 

where, 

     Po - last known population 

     Pn - population (predicted) after 'n' number of decades 

     n - number of decades between Po and Pn 

     r - growth rate in percentage 

r could be found as 

a) Arithmetic Mean Method 

b) Geometric Mean Method 

Note: According to Indian standards 'r' should be calculates using geometric mean method.

Example Question

Considering data given in above example predict the population for the year 2021, 2031 and 2041 using geometrical progression method. Solution

Solution

Year	Population	Increment	Geometrical increase Rate of growth
1961	858545	-
1971	1015672	157127	(157127/858545) = 0.18
1981	1201553	185881	(185881/1015672) = 0.18
1991	1691538	489985	(489985/1201553) = 0.40
2001	2077820	386282	(386282/1691538) = 0.23
2011	2585862	508042	(508042/2077820) = 0.24

By Geometric Mean Method 

 

                                                                 = 0.235 i.e., 23.5% 

Population in year 2021 is, P₂₀₂₁ = 2585862 x (1+ 0.235)¹

                                                                 = 3193540 

Similarly, for year 2031 and 2041 can be calculated  by,

                                            P₂₀₃₁ = 2585862 x (1+ 0.235)²  

                                                      = 3944021 

                                           P₂₀₄₁ = 2585862 x (1+ 0.235)³  

                                                      = 4870866 

3) Incremental Increase Method 

This method is adopted for average sized towns under normal conditions, where the rate of population growth is not constant i.e., either increasing or decreasing. It is a combination of the arithmetic increase method and geometrical increase method. Population predicted by this method lies between the arithmetical increase method and the geometrical increase method. While adopting this method the increase in increment is considered for calculating future population. The incremental increase is determined for each decade from the past population and the average value is added to the present population along with the average rate of increase.

Hence, population after n^th decade is 

where, 

      Po - last known population 

     Pn - population (predicted) after 'n' number of decades 

     n - number of decades between Po and Pn 

     x̄ - mean or average of increase in population 

     ȳ - algebraic mean of incremental increase (an increase of increase) of population

Example Question

Considering data given in the above example, predict the population for the year 2021, 2031 2041 using incremental increase method.

Year	Population	Increase (X)	Incremental Increase (Y)
1961	858545	-	-
1971	1015672	157127	-
1981	1201553	185881	+28754
1991	1691538	489985	+304104
2001	2077820	386282	-103703
2011	2585862	508042	+121760
	Total	1727317	350915
	Average	345463	87729

Population in year 2021 is P₂₀₂₁ = 2585862 + (345463 x 1) + {(1 (1+1))/2} x 87729

                                                                = 3019054

            For year 2031      P2031 = 2585862 + (345463 x 2) + {((2 (2+1)/2)}x 87729

                                                           = 3539975

                                         P2041 = 2585862 + (345463 x 3) + {((3 (3+1)/2)}x 87729

                                                      = 4148625 

4) Decreasing Rate of Growth Method 

Since the rate of increase in population goes on reducing as the cities reach towards saturation, this method is suitable. In this method, the average decrease in the percentage increase is worked out and is then subtracted from the latest percentage increase for each successive decade. This method is applicable only when the rate of growth shows a downward trend. 

Decreasing Rate of Growth Method Formula 

where, 

     Pn - population at required decade 

     P(n-1) - population at previous decade (predicted or available) 

     r (n-1) - growth rate at previous decade 

     S - average decrease in growth rate 

Due to the very nature of the formula, which requires population data at the previous decade i.e., P(n-1), this method requires the calculation of population at each successive decade (from the last known decade) instead of directly calculating population at the required decade.

Example Question

Considering data given in the above example, predict the population for the year 2021, 2031 2041 using incremental increase method. 

Year	Population	Increase in population	Growth rate (r) (%)	Decrease in Growth rate (%)
1961	858545	-	-	-
1971	1015672	157127	18	-
1981	1201553	185881	18	0
1991	1691538	489985	40	-22
2001	2077820	386282	23	17
2011	2585862	508042	24	-1

Average of decrease in growth rate 

                       S = (0-22+17-1)/4 

                          = -1.5 S 

                          = 0.015% 

By using the equation, 

                                                                      = 3206081 

(Here r₍₂₀₃₁₎ is directly found as (24 - 0.015) i.e., r₍₂₀₂₁₎ - S, which equals to 23.985.

                                                                    = 3974579 

                         r₍₂₀₄₁₎ = 23.985 - 0.015

                                    = 23.97

                                                                      = 4926689

5) Graphical Method 

In this method, the population of last few decades are correctly plotted to a suitable scale on a graph. The graph is plotted from the available data between time and population, the curve is then smoothly extended up to the desired year. It is to be done by an experienced person and is almost always prone to error. As per the graph shown in Fig.1, the population up to the year 2001 is known and the population of the year 2021 can be found by smoothly extending the graph.

Fig.1 Graphical Method of Population Forecasting

6) Comparative Graphical Method 

Cities of similar conditions and characteristics are selected which have grown in similar fashion in the past and their graph is plotted and then mean graph is also plotted. This method gives quite satisfactory results. In this method, the population of a town is predicted by comparing it with a similar town. The advantage of this method is that the future population can be predicted from the present population even in the absence of some of the past census report.

Example Question

Let the population of a new city ‘X’ be given for decades 1970, 1980, 1990 and 2000 were 32000, 38000, 43000 and 50000 respectively. The cities A, B, C and D were developed in similar conditions as that of city X. It is required to estimate the population of the city X in the years 2010 and 2020. The population of cities A, B, C and D of different decades were given below.

1. City A was 50000, 62000, 72000 and 87000 in 1960, 1972, 1980 and 1990 respectively.
2. City B was 50000, 58000, 69000 and 76000 in 1962, 1970, 1981 and 1988 respectively.
3. City C was 50000, 56500, 64000 and 70000 in 1964, 1970, 1980 and 1988 respectively.
4. City D was 50000, 40000, 58000 and 62000 in 1961, 1973, 1982 and 1989 respectively. 

Population curves for the cities A, B, C, D and X were plotted. Then an average mean curve is also plotted by dotted line as shown in the Fig.2. The population curve X is extended beyond 50000 matching with the dotted mean curve. From the curve the populations obtained for city X are 58000 and 68000 in year 2010 and 2020.

Fig.2 Comparative Graphical Method

7) Master Plan Method 

The big and metropolitan cities are generally not developed in haphazard manner, but are planned and regulated by local bodies according to master plan. The master plan is prepared for next 25 to 30 years for the city. According to the master plan, the city is divided into various zones such as residence, commerce and industry. The population densities are fixed for various zones in the master plan. From this population density total water demand and wastewater generation for that zone can be worked out. So by this method it is very easy to access precisely the design population.

8) The Ratio Method or Apportionment Method

In this method, the city’s census population record is expressed as the percentage of the population of the whole country, in order to do so, the local population and the country’s population for last 4 - 5 decades is obtained from the census records. The ratios of local population to national population is worked out a graph is then plotted between those ratios and time and extended up to the design period and then ratio is multiplied by expected national population at the end of design period. This method does not take into consideration abnormalities in local areas.

9) The Logistic Curve Method

This method is given by P.F. Verhulst. This method is mathematical solution for logistic curve. This method is used when the growth rate of population due to births, deaths and migrations takes place under normal situation and it is not subjected to any extraordinary changes like epidemic, war, earth quake or any natural disaster etc. the population follow the growth curve characteristics of living things within limited space and economic opportunity. If the population of a city is plotted with respect to time, the curve so obtained under normal conditions is look liked ‘S’ shape curve and is known as logistic curve.

Fig. 3 Logistic Curve

In Fig. 3, the curve shows an early growth AB at an increasing rate i.e. geometric growth or log growth, dP/dt ∝𝑃, the transitional middle curve BD follows arithmetic increase i.e. dP/dt = constant and later growth DE the rate of change of population is proportional to difference between saturation population and existing population, i.e. dP/dt ∝ (Ps-P). Verhaulst has put forward a mathematical solution for this logistic curve AE.

The population at any time t from the start is given by

where,

      P_S = Saturation population

      P = Population at any time ‘t’ from start point

     P_O =Population at the start point of the curve

P_O, P₁, P₂ are population at times t₀, t₁, t₂ and t₂ = 2 t₁.

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