Town Planning Process

All stages of actions from defining the objectives till implementation and review of any planning project in the planning process. In plan preparation, the physical planning should associate with the socio-economical, geographical, political factors, for achieving the objective in desired direction. The various stages of planning process are as follows.

1.Identification and Definition of Problems

Various problems with reference to the results obtained by studies and surveys and with reference to the objectives are identified.

2. Defining the Objectives

Here the objectives of the planning are identified. The general objectives of any planning of urban area is

• To regulate growth
• To nullify the bad effects of past growth
• To improve the transportation facilities
• To optimize the resources utilization
• To balance population and economic activities
• To promote social integration among different categories
• To promote a convenient comfortable, beautiful and healthy environment

3. Studies and Survey

The following studies and surveys are taken up for plan preparation.

• Identification of growth (physical, economic, social, cultural, institutional, administrative and political)
• Identification of trend and direction of growth
• Traffic survey
• Study on demography
• Climate

4. Analysis of Data and Preparation of Study Maps

The data obtained is analyzed – observations and conclusions have to be derived out of the studies and surveys. The short-term objectives and long-term objectives are identified – various study maps, charts and graphs are prepared.

5. Forecasting

Period of demographic projection is prescribed. Forecasting of about migration, employment, industrialization and other rapid urbanization possibilities are to be made.

6. Design

This is an important aspect in the planning process. Need to relate existing pattern, interactions and trends is to be examined. Preparation of development plans, formulation of zones, alteration to the existing zoning regulations, widening of roads etc. are made in detail here. Alternatives are also made for effective and quick implementation of plan. The plan should also be able to cope with sudden and unexpected events. Possibility of changing from one strategy to another should be designed at the same time keeping in view its practicability and the total expenditure involved.

7. Fixation of Priorities

Since all the proposals cannot be taken up at one time due to financial and administrative difficulties, priorities should be fixed for taking up the implementation depending upon the importance and urgency.

8. Implementation

Implementation is the most important stage where all the earlier efforts to prepare plan is to be put into practice to achieve the objectives. The authority, which takes up the implementation, is to fulfill all the required legal obligations in time – zoning regulations, land acquisition for road widening and for other purposes is taken-up.

9. Review, Evaluation and Feedback

The work of implementation has to be monitored by taking – up periodical inspections and obtaining review reports. Feedback is essential periodically. The plan should be flexible for modifications depending upon the necessities.

Types of Surveys

Surveys can broadly be divided into two categories depending on the area upon which they are to be conducted. They are:

1. Regional Surveys

They are those surveys, which are done over a region dealing with physical factors like topography, physically difficult land, geology, landscape etc. Physical economic factors like agricultural value of the land, mineral resources and water gathering lands, areas with public services, transportation linkages etc. Social economic factors like areas of influence of towns and villages, employment, population changes etc.

2. Town Surveys

They are done at much small scale and apart from the above data collected from the regional surveys it also includes

• Land use surveys
• Density surveys
• Surveys for the age and condition of the buildings
• Traffic surveys

Other social surveys for conducting proper survey, primarily relevant enquiries should be framed in the form of questionnaires for presentation, when required.

Techniques of Surveys

1. Self-surveys (i.e. mailing questionnaires to the persons to be surveyed)

2. Interviews (i.e. by asking questions to the people to be surveyed)

3. Direct inspection (i.e. when the surveyor himself inspects the situations concerned)

4. Observers participation (i.e. when the observer himself participate in acquiring the data

required)

Approaches to Planning

1. Comprehensive planning

The traditional four-step planning process - the identification of problems and issues; the establishment of goals and objectives; data collection and analysis; and plan preparation and implementation.

2. Issue-oriented planning

Some communities do not get around to planning until a problem or a crisis has occurred. Issue-oriented planning focuses attention on the problems and issues of greatest concern to a community at a particular point in time. This is an “old-fashioned approach” to planning,

3. Functional planning

Functional planning is a slower version of comprehensive planning.

4. Strategic planning

Major emphasis should be placed on the selection and application of strategies to resolve identified issues and attain desired goals. The principal steps in strategic planning are

• Scan the environment and select key issues.
• Formulate goals or missions for the key issues.
• Analyze each issue, developing a list of community strengths and weaknesses.
• Develop strategies that are realistic and take into account those strengths and weaknesses.
• Implement strategies using public and private resources.
• Monitor and update the plan to ensure implementation.

5. Vision-based Planning

Imagines what a community should look like in the future and then develops a plan to achieve that vision. This visioning process is unique, but it is often included as an element of a variety of master plans. It is a positive approach to planning and can be very inspiring.

6. Urban Planning (urban, city, and town planning)

It is a technical and political process concerned with the control of the use of land and design of the urban environment, including transportation networks, to guide and ensure the orderly development of settlements and communities.

Elements of City/Town Planning

1.  Communication & Transportation
2. Built up Area
3. Open space (Parks, Playgrounds, Recreational unit)
4. Public utility services (Electricity, Gas, Telephone, Fire safety)
5. Public amenities (Post office, Bank, Police, Solid waste disposal, Petrol pump, Court)

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Principles of Town Planning

Town planning cannot be studied in isolation. It involves the study of various subjects such as engineering, architecture, surveying, transportation planning etc. The intention of the town planning is to satisfy the needs of our future generations and prevent the haphazard growth of the town. Some of the guiding principles of town planning are as follows. 

1. Zoning 

The town should be divided into suitable zones such as commercial zone, industrial zone, residential zone, etc. and suitable rules and regulations should be formed for the development of each zone. 

2. Green Belt 

Green belt is non-development zone on the periphery of the town. It prevents the haphazard sprawl of the town restricting its size. In essence, a green belt is an invisible line designating a border around a certain area, preventing development of the area and allowing wildlife to return and be established. Greenways and green wedges have a linear character and may run across the town and not around the town. 

3. Housing 

Housing has to be carefully studied and designed to suit the local population. Care should be taken to see that there is no development of slums since it would be responsible for degrading the life of the citizens. There are various types of housing styles. When a landuse plan is made, zones for independent housing, midrise buildings, high rise buildings are allocated. 

4. Public Buildings 

Public buildings should be well grouped and distributed throughout the town. Unnecessary concentration of public buildings should be avoided. Factors such as parking facilities, road widths have to be taken into consideration while allocating the space for public buildings. 

5. Recreation Centres 

Recreation centres have to be given importance while designing a town. They are necessary for the recreational activities of the general public. They include parks for walking and cycling, amusement parks etc. 

6. Road Systems 

Road network hierarchy is very important. The efficiency of any town is measured by the layout of its roads. A nicely designed road system puts a great impression in the minds of people, especially the visitors to the town. The provision of a faulty road system in the initial stages of town formation proves to be too difficult and costly to repair or to re-arrange in future. 

7. Transport Facilities 

The town should be provided with suitable transport facilities so that there is minimum loss of time from place of work to the place of residence. Efficiency in transport facilities includes both public and private networks. Public transportation network includes access to buses, trains, trams and trolleybuses. Efficiency in using the public transport will determine the success of that town in terms of design.

Objectives of Town Planning

The main objectives of the town planning may be summarized in three words viz. Health, Convenience, Beauty and Environment.

1. Health

To make right use of the land for the right purpose by proper division of land called zoning such as residential, commercial industrial, institutional and recreational etc. inorder to avoid the encroachment of one zone upon other for smooth and orderly development of the town or city without causing future conflicts. To create and promote healthy conditions and environments for all the people – rich and poor, to live, to work, to play or relax.

2. Convenience

The object of convenience is meant in the form of various needs of the community such as social, economic, cultural and recreational amenities etc. Public amenities requiredfor the proper upkeep of the citizens include water supply, sanitation, electricity, post,telegraph, gas etc., proper sites for industrial, commercial, business enterprises toencourage them in trade with cheap power, transport services, drainage etc. Recreational amenities include open spaces, parks, gardens and playgrounds, for children and town halls stadiums, community centers, cinema houses, and theatres for adults.

3. Beauty

To preserve the individuality of the town by developing it on its most suited natural conditions. To preserve the aesthetics in the design of all elements of town or city plan, which includes preservation of trees, natural greenery, improved types of domestic buildings and buildings of civic dignity and beauty, architectural control on public as well as semi-public buildings, ancient architectural buildings, temples, churches, mosques and buildings of cultural and historical importance.

4. Environment

It is an important factor in town planning. The environment of the town should be in such a way that people can lead their normal activities with least difficulty. The complex problems of modern society such as tiresome travel to work, long hours of work, limited time spent within the family and community, etc. has led to serious problems in the lifestyle of the people living in the towns, therefore, town planning is mainly concerned with brining about a better relation between man and his environment.

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Quarrying of Stones

The method of removal of stones from their natural bed by using different operations is called quarrying. The only operation involved in the production of natural stone is the quarrying process. The open part of the natural rock from which useful stone is obtained is known as quarry. While selecting a quarry site, the points to be borne in mind are availability of sufficient quantity of the stone of desired quality, proper transportation facilities, cheap local labour, problems associated with drainage of rain water, location of important and permanent structures in the vicinity and site for dumping refuse.

Stone Quarrying Tools

Some of the quarrying tools shown in Fig. 1 are wedge, pin, hammer, dipper or scraping spoon, tamping bar, priming needle, jumper, borer, claying iron, crow bar.

Fig. 1 Tools for Quarrying Stones

Methods of Quarrying

Rocks suitable for the manufacture of stone materials are called useful minerals and the operations involved in obtaining minerals are called mining. In the process of mining, voids formed are called excavations and the mined deposits are the quarries. The purpose of quarrying is to obtain stones for various engineering purposes. A knowledge of various quarrying methods is essential but does not make one very much more competent to choose or specify a stone for building work. Depending upon the nature and surface of rocks and the purpose for which stones are needed, quarrying is done by excavating, wedging, heating or blasting.

1) Excavation/Digging

Stones buried in earth or under loose overburden are excavated with pick axes, crow bars, chisels, hammers, etc. This method is used when the quarry consists of small & soft pieces of stones.

Fig.2 Digging of Stones

2) Wedging

This method of quarrying is suitable for costly, soft and stratified rocks such as sandstone, limestone, laterite, marble and slate. This method is used when the hard rock consists of natural fissure. When natural fissures are absent then artificial fissures are prepared by drilling holes. The tools used for wedging are

• Steel wedge
• Conical steel pin or plug
• Flat steel wedge
• Wooden plug
• Pneumatic drill

About 10–15 cm deep holes, at around 10 cm spacing, are made vertically in the rock. Steel pins and wedges or plugs (conical wedges) and feathers (flat wedges) as shown in Fig. 3 are inserted in them. The latter arrangement of plugs and feather is better. These plugs are then struck simultaneously with sledge hammer. The rock slab splits along the lines of least resistance through holes. In case of soft rocks, dry wooden pegs are hammered in the holes and water is poured over them. The pegs being wet swell and exert pressure causing the rocks to crack along the line of holes. Then, the wedges are placed on the plane of cleavage (the joint of two layers) on the exposed face of rock and are hammered. The slab is completely detached and taken out with the help of crow bars and rollers. In this method, the wastage is minimum and the slabs of required size and shape can be quarried.

Fig. 3 Quarrying by Wedging

3) Heating

Heating is most suitable for quarrying small, thin and regular blocks of stones from rocks, such as granite and gneiss. This method is used when the natural rock bed is horizontal and small in thickness. A heap of fuel is piled and fired on the surface of rock in small area. The two consecutive layers of the rock separate because of uneven expansion of the two layers. The loosened rock portions are broken into pieces of desired size and are removed with the help of pick-axes and crow-bars. Stone blocks so obtained are very suitable for coarse rubble masonry. Sometimes, intermediate layers are to be separated from the top and bottom layers. In such a case, the intermediate layer is heated electrically and the expansion separates it from the other two.

Fig. 4 Quarrying by Heating

4) Blasting

It is the process of removal of stones with the help of controlled explosives is filled in the holes of the stones. Line of least resistance plays very important role in the blasting process. Explosives such as blasting powder, blasting cotton, dynamite and cordite are used. The operations involved are boring, charging, tamping and firing.

a) Boring

Holes are drilled or bored in the rock to be dislodged. For vertical holes, jumper is used whereas for inclined or horizontal holes, boring bars are used. One person holds the jumper exactly in the place where hole is to be made. The other person strikes it up and down and rotates it simultaneously. Water is poured in the hole regularly during the operation to soften the rock and facilitate drilling. The muddy paste generated in the process is removed from holes by scrapping. For hard rocks, machine drilling is employed instead of hand drilling.

b) Charging

The holes are dried completely and the required amount of charge is placed in the holes. For drying the holes, rag is tied in the scrapper and is moved in the hole from where it absorbs the moisture, if any. In case it is found that water is oozing into the hole, water tightness is ensured inside the hole.

c) Tamping

After placing the charge in the hole, a greased priming needle, projecting a little outside the hole, is placed in the hole which is then filled up with damp clay or stone dust in layers tamped sufficiently with a braced tamping rod. The priming needle should be kept on rotating while tamping is going on. This is done so that the needle remains loose in the hole. The priming needle is then taken out and 60 to 75 per cent of space created by withdrawal of needle is filled with gun powder. A Bickford fuse, a small rope of cotton coated with tar, is placed just touching the needle. The other end of the fuse is kept of sufficient length so that the person igniting it can move away to a safe place. Blasting powder and cordite are ignited by means of a fuse, whereas gun cotton and dynamite are exploded by detonation.

Detonation is achieved with detonators. These are copper tubes about 5 mm in diameter and 25 mm long containing 5 to 20 grains of fulminate of mercury. These can be exploded by an ordinary fuse or by an electric current. An electric detonator is shown in Fig. 5. Tamping should be done very carefully otherwise the explosive fires back in the hole, since the line of least resistance (LLR) is the shortest distance (Fig. 6) from the explosive in the hole to the nearest rock face, fixture, crack, fault or plane of cleavage.

Fig. 5 Electrical Detonator

Fig. 6 Tamped and Charged Blast Hole

d) Firing

The fuses of blasting holes are fired by using electrical power supply or match sticks.

Precautions in Blasting

Accidents may take place during blasting. The following points to be considered while blasting process.

1. Blasting should not be carried out in late evening or early morning hours. The blasting hours should be made public and a siren should warn the workmen and nearby public timely to retire to a safe distance.
2. The danger zone, an area of about 200 m radius, should be marked with red flags.
3. First aid should be available.
4. The number of charges fired, the number of charges exploded and the misfires should be recorded.
5. Explosives should be stored and handled carefully.
6. Detonators and explosives should not be kept together.
7. Cartridges should be handled with rubber or polythene gloves.
8. A maximum of 10 bore holes are exploded at a time and that also successively and not simultaneously.

Storage of Explosives

The explosives should be stored in a magazine (a special type of building) which should be away from residential areas, petrol depots etc. The magazine should have ventilators at high levels and should have concealed wiring. It should be protected from lightning. Smoke or fire should not be allowed in the nearby area. Explosives should be protected from extreme heat or cold and also from moisture. They should be handled carefully and gently. The magazine should be surrounded by barbed wire and the entry should be restricted.

Quantity of Explosive Required

The quantity of explosive required depends upon several factors such as strength of explosive, method of blasting, number of bore holes and their size, position etc. and the type and mass of rock to be dislodged. It is very difficult to incorporate all the factors in an expression and obtain the exact amount of explosive required. A rough estimate can be made by the equation given below.

where  A = quantity of gunpowder or dynamite (g)

              L = length of line of least resistance (m)

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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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