Few Examples of Engineering Materials!

Example # 1. Fuels:

These are the substances which are capable of creating heat energy. Such energy can then be put to some engineering or industrial use. The industrial fuels usually contain the elements of carbon and hydrogen. When oxygen of the air chemically combines with these elements, the heat is generated.

This generation of heat is seen as flame. The fuels also contain mineral matters which, on burning, are left behind as ash. It is not desirable to have ash in fuels because it does not in any way contribute to the generation of heat.

The calorific value of fuel is defined as the quantity of heat generated by completely burning unit weight of a given fuel. The unit of heat is gm calories and it indicates the quantity of heat required to raise 1 gm of water through 1°C.

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Following are the calorific values of the commonly used fuels:

Hydrogen – 34460 gm calories

Methane – 13060 gm calories

Carbon – 8080 gm calories

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Carbon monoxide – 2400 gm calories.

Classification of Fuels:

Depending upon the state in which they occur, the fuels are classified into the following three groups:

(1) Solid fuels

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(2) Liquid fuels

(3) Gaseous fuels.

(1) Solid Fuels:

The solid fuels are mostly used for the generation of heat required in kilns, furnaces and domestic chulhas. They are also used for heating and calcination of raw materials, burning of bricks and tiles, power generation, manufacture of engineering materials and goods for daily use, etc.

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The important solid fuels are as follows:

(i) Charcoal:

It is prepared by burning wood in a kiln with limited quantity of air. It is used to produce gas for domestic purposes.

(ii) Coal:

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It is the chief source for producing cheap power. It is formed by the bacterial decomposition of vegetable matter under great pressure and in absence of air. Its colour is black or dark brown. Tim is a natural solid fuel.

(iii) Coke:

It is prepared by heating the powdered coal in special ovens and then quenching the whole mass in cold water. It is better than coal and is the most important metallurgical fuel.

(iv) Peat and Lignite:

These are the intermediate products between wood and coal. They are available at a shallow depth as compared to the coal. They even sometimes occur on the surface of earth and can be cheaply obtained.

(v) Wood:

It is obtained from nature and can be used as fuel. It is not an important fuel because it contains too much moisture and is very costly.

(2) Liquid Fuels:

The source of liquid fuels is crude petroleum which is recovered from oil wells in Assam, Gujarat and Bombay High. It has a calorific value of about 11100 gm calories. The liquid fuels are mostly used for locomotion and power generation. They are required for driving automobiles, aircrafts, locomotives, generators and for lighting purposes.

The important liquid fuels are as follows:

(i) Alcohol:

These are of various types such as ethyl alcohol, methyl alcohol, etc. They are used to serve as fuels for motors, aeroplanes, etc.

(ii) Crude Oil:

This occurs in nature and it can be used as fuel after giving proper treatment to it.

(iii) Petroleum:

This is the product obtained by the distillation of organic matter in nature. It is refined and its distillation yields various artificial liquid fuels such as gasoline, gas oil, kerosene, petrol, aviation fuel, grease, lubricating oil, etc.

(3) Gaseous Fuels:

The gaseous fuels are used for heating, burning, lighting and running gas engines.

The important gaseous fuels are as follows:

(i) Coal Gas:

This gas is obtained by thermal decomposition of coal at high temperature in absence of air. It is a mixture of several gases like hydrogen, methane, carbon monoxide, nitrogen, ethylene, benzine, carbon dioxide and oxygen.

The usual percentage of various gases is as follows:

(ii) Natural Gas:

This is obtained from natural underground sources. It is associated with petroleum under pressure.

(iii) Producer Gas:

This gas is obtained by controlled combustion of coal, coke or charcoal in a blast of air. It is more powerful. It is used in the manufacture of glass, steel, etc.

Advantages of Gaseous Fuels:

The gaseous fuels are preferred wherever possible because they possess the following advantages over solid and liquid fuels:

(i) It is possible to economically convert low grade coals with high ash content into gas.

(ii) The amount of air and gas can be controlled as desired which will assist in controlling with great accuracy the flame and the temperature.

(iii) The furnace atmospheres can be made oxidizing, neutral or reducing as per requirements.

(iv) The furnaces can be lighted immediately without any previous preparations.

(v) They can be stored in large towers and tanks and their distribution to the various heating units can be economically and conveniently controlled.

(vi) They do not leave any residue.

(vii) They require the minimum amount of excess air and ensure high thermal efficiency.

Example # 2. Gypsum:

The gypsum is the hydrated sulphate of calcium and its chemical composition is CaSO4, 2H2O. It contains 79.1 per cent calcium sulphate and 20.9 per cent water. It is a white crystalline substance. Its density is 0.023 N/cm3. It is soft, hardness being equal to 2. As a binding material, the gypsum quickly sets and hardens.

Its initial setting time after addition of water is about 4 to 6 minutes and its final setting time is about 30 minutes. Its solubility in water is very poor, about 1 per cent of gypsum in 495 parts of water. It is soluble in hydrochloric acid. But it is insoluble in sulphuric acid.

The gypsum very seldom occurs in nature in pure state. It contains impurities such as alumina, calcium carbonate, magnesium carbonate and silica. The gypsum containing upto 70 per cent of CaSO4, 2H2O can be used as a building material.

The physical forms of gypsum may be crystals as selenite, fibrous as stain spar and massive as alabaster. It is mainly used in the manufacture of cement to increase its setting time. It is also used to prepare plaster of Paris and gypsum boards. These boards are popularly known as the gypboards and they are formed by mixing gypsum with asphalt. The gypsum is also used as filler in paint, paper and rubber industries.

The Egyptians were the first to use the gypsum as a building material in their pyramids. The gypboard is formed by enclosing and bonding together a core of set gypsum plaster by two sheets of heavy paper. The gypboard is an ideal substitute for plywood and other wood-based products for partitioning and panelling.

The gypboard offers the following advantages over other conventional materials:

(1) Cost:

It is cheaper than plywood partitioning systems.

(2) Easy Workability:

It is flexible in nature and is as easy to work with as any wood-based boards. It is easy to nail, screw and cut with ordinary carpenter’s tools.

(3) Final Finish:

Its unique tapered edges make it possible to joint boards and provide a finish to the partitions like plastered brick walls and to ceilings like smooth plaster of Paris. The dry construction completely eliminates the messy plastering.

(4) Fire Resistance:

It has the ability to retard the spread of fire. It can give fire protection upto 2 hours.

(5) Light Weight:

It is a very light weight product with its wall weighing less than 500 N/m2. Its light weight offers economy in structural costs and transportation.

(6) Smooth Surface:

Its surface is smooth and hence any type of interior finish such as painting, veneering, wall papering, etc. can be easily carried out.

(7) Sound Insulation:

It possesses good sound insulating properties because it is made from a non-resonant material.

(8) Strength:

It is strong, durable and dimensionally stable.

(9) Time of Installation:

It offers scope of better planning and greater control over construction schedule.

The gypsum-based products can be obtained from gypsum, water and aggregate. They may be solid or hollow and they may be reinforced or non-reinforced. The artificial strengthening of gypsum items by reinforcing or fibrous materials becomes necessary because gypsum is very brittle in nature. If steel reinforcement is used, a protective coat should be provided as steel is susceptible to the corrosion.

Table 17-1 shows the areas from which gypsum is available in India. Rajasthan contributes about 90 per cent of the annual production of gypsum.

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The gypsum items possess the following important properties:

(i) They are incombustible.

(ii) They possess relatively small bulk density.

(iii) They serve as good sound absorbers.

However these items have poor strength in wet state and they develop high creep under load, especially in the moist surroundings. Hence the gypsum items give excellent performance in dry state and at places having not more than 60% relative air humidity. Sometimes the water-proof paints or pastes are applied on the surface of gypsum items to improve their moisture resistance and water-resisting properties.

Example # 3. Gypsum Plaster:

When finely ground gypsum is heated at the temperature of 160°C to 170°C, it loses about 14.7 per cent of its water content in the form of steam. The resulting product is the hemihydrate of calcium sulphate and it is known as first settle plaster or the plaster of Paris.

When water is added to plaster of Paris, it hardens in three to four minutes. Hence to extend the setting time, suitable retarders are added to it. The usual retarders are clay, citric acid, glue, gum, starch and sugar. The term hemihydrate gypsum plaster is used to indicate the plaster of Paris with retarder.

On heating further upto a temperature of about 200°C, the entire water of crystallization is driven off and the resulting product is known as the gypsum anhydrite or hard burnt plaster. The setting time of gypsum anhydrite is more and to shorten it, the accelerators are added to it. The common accelerators are alum, potassium sulphate and raw gypsum. The term anhydrous gypsum plaster is used to indicate the gypsum anhydrite with accelerator.

Following are the properties and uses of the gypsum plaster:

(i) It is a fire resisting material and it does not allow heat to pass easily. Hence it is used as an insulating material to protect wood or metal columns and beams from high temperature.

(ii) It is light in weight. To decrease the weight further, the fillers such as granulated cork, saw dust, wood shavings, etc. may be added to it. Such fillers also improve heat and sound insulating quality of the gypsum plaster.

(iii) It is practically not affected by bacteria.

(iv) It is slightly soluble in water, about 2 gm per litre. Hence it cannot be adopted for damp conditions and external work. The bags containing gypsum plaster should therefore be stored in dry place.

(v) It is used for ornamental plaster work and for preparation of boards and blocks. The gypsum plaster boards are used for ceiling, for internal lining of wall and for partition walls. They are cheap, easy to work, light in weight and fire-proof.

(vi) It requires small proportion of sand and other aggregates. When used as base coat, the sand or light weight aggregate or wood fibre is added. When used as final coat, the lime putty is added.

(vii) It sets by natural process of crystallization. Hence it can be applied with ease and without wastage.

(viii) It sets with little change in volume and with negligible shrinkage or drying.

(ix) It shows good adhesion to the fibrous materials.

(x) The plaster of Paris is used in artwork, pottery, dentistry and in surgery for the shaping of fractured bones.

Example # 4. Plaster Boards:

They are made from a large sheet of gypsum plaster faced on both sides with stout paper as a reinforcement. They are available in thickness varying from 9.5 mm to 12.5 mm. There are two types of plaster boards depending upon the nature of facing plaster.

(i) Gypsum lath board

(ii) Plaster wall board.

In gypsum lath board, facing paper is rough to provide an adhesive grip for plaster, while in plaster wall board, the facing paper is of self-finish type to provide decorative finish such as a wood veneer. Because of good insulating properties, plaster boards are normally used for partition walls, internal lining of walls and for partition walls.

Example # 5. Pyrocell:

When water is added to finally ground gypsum powder, gas is liberated and the mixture is expanded 3 to 4 times its volume. It hardens after sometime. This mixture is known as pyrocell. It is light, cellular and fire-resistant. It is normally used for acoustical and insulating purposes in the buildings.

Example # 6. Heat Insulating Materials:

The heat insulating materials are required to grant protection against heat and cold. These materials are generally porous and their properties are governed not only by their porosity but also by the nature of pores, their distribution, size and whether they are open or closed.

The materials with a greater number of fine, closed and air-filled pores are the best heat insulating materials. The bulk density of heat insulating materials is usually below 7000 N/m3 and their coefficient of heat conductivity does not exceed 0.18 k cal per m. hr.°C.

If heat insulating materials are properly used in building construction, they greatly reduce the heat losses to the environmental medium through wall structures and as a result of this, the fuel consumption is reduced. Thus, the economic efficiency of thermal insulation is very high and the investments made in heat insulating materials can be recovered in a short duration of time.

In general, it can be stated that the low heat conductivity of heat insulating materials is due to their air-filled pores. Hence, if their efficiency is to be maintained, it should be seen that these pores are not covered with a film of water or are filled with water because the coefficient of heat conductivity of water is about 25 times higher than that of air. Hence, the heat insulating materials should be protected against the moisture.

The choice of an insulating material depends on its cost, area to be covered, standard of insulation required and the cost of heating or cooling. The thermal insulating material should be reasonably fire-proof, non-absorbent of moisture, able to resist attack of small insects and not liable to undergo deformation.

The usual insulating materials are rock wool, slag wool, fibre board, flexible blankets, saw dust, wood shavings, cork board slabs, mineral wool slabs, aluminium foils, products of cement concrete with light weight aggregates, gypsum boards, asbestos cement boards, chip boards, foam glass, gasket cork sheet, foam plastic, etc.

Table 17-2 shows the density, thermal conductivity and thermal resistivity of some of the common building and insulating materials. In general, it may be stated that the materials of low density provide better thermal insulation than those of higher density.

Example # 7. Lubricants:

In order to prevent the contact between two surfaces or parts of a moving machine, the lubricants or lubricating materials are inserted.

The functions of a lubricant are:

(i) To facilitate easy motion of the moving parts;

(ii) To prevent the loss of energy and power due to friction by separating rubbing surfaces in relative motion and thereby to increase the efficiency of machine;

(iii) To reduce wear and tear of parts; and

(iv) To remove heat and to protect metal surfaces against corrosion.

Thus the lubrication is the art of minimizing friction between different components of a machine and it is indispensable to the mechanization. It is also of vital importance for mechanical efficiency as well as the functioning of a machine.

The history of lubrication reveals that it was used in the old bullock-cart leading over the years to the automobile and the modern machine. The use of castor oil is mentioned in the epic of Mahabharat as a lubricant for axles of chariots.

Until petroleum (Petra-rock-oil) made advent in the nineteenth century, the natural oils and fats were the chief ingredients used for lubrication and even at present, they are partial components of some of the important lubricants.

The lubricant is basically a petroleum product. The nineteenth century was a period of mechanical advancement and a new market opened for the lubricants with the availability of crude petroleum which was refined and distilled to suit the requirements of various industries. These mineral oils are by far the most frequently used form of lubricants because they are chemically unreactive and can be produced with a wide range of viscosities.

The industrialised society of the present day requires many designed lubricants to satisfy the needs of the modern machinery. The viscosity i.e. the frictional resistance within the substance which opposes any internal movement is an essential property of lubricating oil.

Depending upon the source of crude, refining process, viscosity and various other factors affecting the qualities of a lubricant, more than 300 varieties of lubricants are put in the market and many of them are actually tailor-made to suit the requirements of different industries or of various machines. It may be noted that the automobile industry is the largest consumer of lubricants.

The demand for lubricants is so acute that there will always be a shortage of lubricants and it is high time for finding out some of the alternatives for lubricants. The concept of conservation has been floated and in the future, it is hoped that properly recycled used oils will be as good as virgin oil.

Similarly the new lubricant sources such as synthetic lubricants will also have to be encouraged. The world today is in the grip of an oil revolution affecting the economy and destiny of many nations. The lubricant industry as such is facing new challenges every day because of its close association with the petroleum industry.

Example # 8. Magnetic Materials:

General:

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet, i.e. a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.

A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door.

Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.

Example # 9. Nano Materials:

Nano-technology is a technology that enables to develop materials with improved or totally new properties. It is an extension of the sciences and technologies already developed for many years to examine the nature of our world at an ever smaller scale. A nanometer is one billionth of a meter. Nano particles are defined as a particle that has at least one dimension less than 100 nm.

Nano Science and Nanotechnology is going to drastically change the construction industry in the years to come. Nanotechnology has a range of possibilities in making the construction industry come up to the expectations of the current times. Nano materials in construction industry will bring a big revolution.

Main features and advantages of nano materials are as follows:

(i) Nano materials offer longer life to the building materials.

(ii) With the use of nano materials maintenance cost and efforts will be almost nullified for a very long time.

(iii) Nano materials can improve fluidity, strength and durability of the concrete.

(iv) Nano materials can improve the reinforcement qualities like and corrosion.

(v) Nano enabled coating of construction materials is going to constitute the largest application of nanotechnology in construction.

(vi) Nano products like architectural paints, water sealers and deck treatments; treatments applied during fabrication, such as scratch-resistant coatings on vinyl or wood flooring, insulation coatings etc. offer immense market opportunities for nano-materials.

(vii) Nanotech products and applications, among other benefits, may enhance the performance with regard to blocking of the ultra violet rays, transparency of the structures, photo reactivity and resistance to stain and odour.

(viii) Nanotechnology based coatings can enable creating self-cleaning surfaces.

(ix) Long lasting scratch resistant floors can be made.

(x) Super strong structural components can be made to solve the problem of corrosion in structure.

(xi) Paints prepared using nano technology will be long lasting and prevents fading of paints.

(xii) Anti-fogging and self-cleaning glass manufactured using nano technology are good for maintaining glass structure.

Thus, nano materials and nanotechnology based applications will thus take the construction industry much beyond bricks and mortar.

Following nano materials are used in construction:

(i) Carbon nano tubes

(ii) Silicon dioxide nano particles (SiO2)

(iii) Titanium dioxide nano particles (TiO2)

(iv) Aluminium oxide nano particles (AI2O3)

(v) Copper nano particles

(vi) Silver nano particles

(vii) Quantum dots

Table 17-5 shows characteristics and uses of each above mentioned nano materials.

Carbon nanotubes and nanofibres present an important classification of nano-materials. They are made from Grapheme. Grapheme is defined a monolayer of carbon atoms packed into a honeycomb lattice. It can also be defined as an atomic-scale chicken wire made of carbon atoms and their bonds.

If grapheme layers are arranged as stacked cones, cups or plates, it is known as Carbon nanofibres (CNF) and if the grapheme layers are wrapped into perfect cylinders, they are termed as Carbon Nano Tubes (CNT).

Nano composites are produced by adding nano-particles to a bulk material in order to improve the bulk material properties. Materials reduced to nano-scale can suddenly show very different properties compared to what they exhibit on a macro scale, enabling unique applications. For instance, opaque copper substances become transparent and inert platinum materials attain catalytic properties.

Nano-technology is a dynamic research field that covers a large number of disciplines including construction industry.

The materials like nano-Titania (TiO2), Carbon nanotubes, nano-silica (SiO2) and nano-alumina (AI2O3) are being combined with portland cement to manufacture nano cement.

The use of finer particles has following advantages:

(i) The finer particles have higher surface area.

(ii) They can fill pores more effectively to enhance the overall strength and durability.

(iii) They impart higher strength and faster chemical reactions.

(iv) They can accelerate cement hydration due to their high activity.

Thus, nano-particles can lead to the production of a new generation of cement composites with enhanced strength, and durability.

Use of nano technology is advantageous in the following list of areas:

(i) Replacement of steel cables by much stronger carbon nano tubes in suspension bridges and cable-stayed bridges.

(ii) Using nano-silica, dense cement composite materials can be produced.

(iii) Using of resistive carbon nano fibres in concrete roads in snowy areas.

(iv) Using nano-Titania, to produce photo catalytic concrete.

(v) Use of nano-clays in concrete to enhance its plasticity and flowability.

(vi) Urban air quality could be improved by if the civil structures are treated with nano-Titania.

Example # 10. Rubber:

The rubber is a very important engineering material for any nation as it is widely used for military accessories, the most important one being tyre industry.

India is the fifth largest natural rubber producing country in the world after Malaysia, Indonesia, Thailand and Sri Lanka. Kerala State accounts for about 91 per cent of the total area under rubber industry and for Kanyakumari district in Tamil Nadu, the percentage is about 5 per cent.

The remaining 4 per cent area is located in Karnataka, Andaman and Nicobar Islands, Goa, Maharashtra and north eastern States. The total area under rubber cultivation in the country is about 522000 hectares.

The rubber consumption during 1995-96 was about 53 lakh kN and the production was about 51.10 lakh kN. It is estimated that by the year 2000 A.D., the demand for rubber would go upto 70 lakh kN and the production would be about 65 lakh kN. In 2010 the consumption was about 98 lakh kN and the production was about 90 lakh kN.

Columbus is understood to have discovered rubber during his second voyage to America towards the end of the fifteenth century. He reported the existence of a gum which was used by the natives of America for making balls used in games.

An Englishman used the term rubber when he observed that the substance could be used for removing the pencil marks. The commercial use of rubber came into existence with the discovery of process known as the vulcanization in year 1836 by Good Year.

Example # 11. Sealants for Joints:

The development of construction technique has posed the problem of providing durable sealants in the joints between different engineering materials such as aluminium, concrete, glass, marble, masonry wall, steel and stone. As a matter of fact, the concept of joints and joint designs has been evolved and the structures are to be provided with joints which would accommodate movement of the building materials.

The spacing of joints should be kept in such a way that the stresses developed due to the movement of building materials are properly regulated and maintained within permissible limits.

It was observed that a sealant material in a joint has to face the following conditions:

(i) It changes shape with the change in the width of the joint.

(ii) It is always in shear in lap joints.

(iii) It is always in tension.

(iv) It undergoes stress and stress relief cycles.

Following are the properties of a good sealant so as to fulfill its provision in a joint:

(i) It should have good bond.

(ii) It should not deteriorate either due to weather effects or due to stress and stress relief cycles.

(iii) It should remain flexible and soft.

It is found that the sealants possessing above properties are classified as elastomeric sealants and these sealants are silicone based, urethane based, acrylic based and polysulphide based sealants. Out of all these, the polysulphide based sealants have become more popular because of their superior performance.

Polysulphide Based Sealants:

These sealants are cold applied compounds which are available in thick pasty form. They are chemically cured to form a firm synthetic rubber.

These sealants are available in the following two systems:

(1) One part system

(2) Two part system.

(1) One Part System:

In this system, the sealants are supplied in premixed ready to use condition. Such sealants cure chemically by absorbing atmospheric moisture. The time required for curing will naturally depend on the existing weather conditions of the locality. It usually varies from 3 to 4 weeks.

(2) Two Part System:

In this system, the sealants are supplied in two parts, namely, base and accelerator. This system achieves curing chemically after the base and accelerator are thoroughly mixed. They become touch dry within a short period of about 48 hours, but they require about 8 days for a full cure.

The two part polysulphide based sealants are available in two forms, namely, gun grade and pour grade. The former is used for inclined, overhead and vertical applications while the latter is used for the horizontal applications.

These sealants can be specifically formulated for different application areas such as:

(i) Joints in building structures such as basements, retaining walls, glazing frames, ceiling, floors, roofs of sheet metal and concrete, external walls, cladding, etc.

(ii) Joints in traffic surfaces such as bridges, car parks, roads, aerodromes, etc.

(iii) Joints in water retaining structures such as dams, canals, culverts, reservoirs, water treatment works, etc.

Sealant Application Equipment:

The equipment required for the polysulphide based sealants are as follows:

(1) Filling Device:

Some sort of filling device becomes necessary when the packing is of 30 N or more and it helps in filling cartridges for gun application without wasting the sealant.

(2) Gun:

It is a caulking gun and includes PVC cartridges and nozzles. It can be used on all inclined, overhead and vertical applications.

(3) Mixer:

For effective mixing of base and accelerator in two part system, a slow speed (150 to 250 r.p.m.) hand drill with a paddle stirrer is an ideal mixer.

(4) Spatula or Putty Knife:

In absence of a caulking gun, the spatula or putty knife can be used for application of gun grade material.

All the above application equipment should be cleaned with a recommended solvent after use. They should be made dry and clean before application. In a similar way, the recommended primer should be used to develop effective bonding on the porous surfaces.

Sealant Application Accessories:

The application accessories required for a proper application of polysulphide based sealant are as follows:

(1) Back up Material:

This is in the form of a compressible and non-absorbent filler bond and it controls the depth of the sealant in the joint.

(2) Bond Breakers:

This is an adhesive tape and it is used to prevent three face adhesions. It may be in the form of PVC tape, paper tape or metal foil.

(3) Masking Tape:

This is used on the sides of the joint to ensure neat application and it helps in avoiding the spreading of primer and sealant on adjacent surface during application. It need not be required when the job is being executed by a skilled and highly experienced worker.

Working Criteria for Sealants:

Following are the working criteria for polysulphide based sealants:

(1) Application Temperature:

They can be applied in the temperature range of 5°C to 50°C.

(2) Chemical Resistance:

These sealants offer good chemical resistance to dilute acids and alkalies, aviation fuel, diesel oil, lubricating oils, paraffin, petrol and white spirit.

(3) Joint Size:

The joint width should be within a range of 5 mm to 50 mm. The minimum depth of sealant should be 5 mm for joints in metal, glass and other smooth surfaces and it should be 10 mm for brick and concrete surfaces. As a rule-of-thumb, the ideal depth of sealant should be one-half the width of the joint.

(4) Movement Capability:

It is observed that two part polysulphide based sealant is capable of accepting 25% movement for butt joints and 50% for lap joints.

(5) Service Temperature:

These sealants will work effectively within the service range of temperature of – 40°C to + 80°C.

(6) Setting Time and Cure Time:

Table 17-7 shows the setting time and cure time of these sealants at different temperatures.

(7) Storage Life:

If stored in dry and cool place in unopened original containers, the two part polysulphide sealant can be stored for a period of about 12 months from the date of manufacture.

(8) Useful Life:

These sealants can last for about 10 years in joints in traffic- surfaces and for about 25 years in other conditions.

(9) Water Resistance:

These sealants possess excellent water resistance after they are fully cured and hence they can safely be permanently immersed in water after full cure is achieved.

Example # 12. Sheets for Pitched Roof Coverings:

The two varieties of commonly used sheets for pitched roof coverings are as follows:

(1) Asbestos cement sheets

(2) Galvanized iron sheets.

(1) Asbestos Cement Sheets:

The cement is mixed with about 15 per cent of asbestos fibres and the paste so formed is pressed under rollers with grooves or teeth. Thus, sheets, commonly known as the A.C. sheets, with a series of waves or corrugations are formed and they are used for factories, workshops, garages, big halls, etc.

The corrugations help to increase strength and rigidity and they permit the easy flow of rain water. They are available under different trade names and in standard sizes. They are cheap, fire-resisting, light in weight and sound-proof.

(2) Galvanized Iron Sheets:

The galvanized iron sheets are prepared by pressing flat wrought-iron plates between rollers with grooves or teeth and then they are galvanized with a coat of zinc. These sheets are commonly known as the G.I. sheets.

The process of galvanizing is named after the 18th century Italian scientist Galvanic and it has been perfected to highly sophisticated degree achieving improvement in quality, economy, productivity and labour savings. The use of computer controlled sophisticated modern equipment ensures a quality product with a uniform tightly adherent layer of zinc.

The corrugations help to increase strength and rigidity and they permit easy flow of rain water. They are available in lengths varying from 1.20 m to 3.60 m and in widths varying from 600 mm to 900 mm. The thickness varies from 0.18 mm to 1.60 mm. They are costly and do not offer resistance to fire and sound.

Example # 13. Solder:

The solder is an alloy which is used to join two or more pieces of metal. The melting point of solder is lower than the materials to be joined. The molten solder joins the pieces of metal and the process is known as the soldering.

The solders are of the following two types:

(1) Hard solders

(2) Soft solders.

(1) Hard Solders:

A hard solder is an alloy of copper and zinc. It melts at a very high temperature. It is used for joining brass, copper, iron and steel. There are two varieties of hard solder, namely, brazing solder and silver solder. The term brazing is used to indicate soldering at high temperatures. The flux commonly used for hard solders is borax.

(2) Soft Solders:

A soft solder is an alloy of tin and lead. The most popular composition is 50% tin and 50% lead. It melts at a low temperature varying from 110°C to 180°C. It is used for joining copper, lead, tinned iron, zinc, etc. There are three varieties of soft solder, namely, plumber solder, wireman’s solder and tin liner’s solder. The flux commonly used for soft solders is sal ammoniac.

Example # 14. Sound Absorbent Materials:

The sound absorbent materials can be incorporated in building structures either in compressed state or in suspended state or in Free State. In compressed state, they are provided between the load bearing panels of ceiling and floor. In suspended state, they are provided in the form of slabs fastened to ceiling so as to provide an air space. In Free State, they are provided in non-compressed or loose manner.

According to the nature of absorbing sound, the sound absorbent materials can be classified as follows:

(1) Porous Materials:

This type includes light weight concrete with porous aggregate, foam glass, etc.

(2) Porous-cum-Elastic Materials:

This type includes porous materials with an elastic backing.

(3) Baffle Materials:

This type includes thin panels from veneer, rigid wood fibre slabs, solid cardboard, etc.

(4) Perforated Materials:

This type includes perforated panels and slabs. The holes may be of equal diameter or different diameters and they may be symmetrically arranged or located at random on the surface of panels or slabs.

Most of the common building materials absorb sound to a small extent and hence, for better acoustical requirement, some other materials are to be incorporated on the surfaces of the room. Such materials are known as the sound absorbent materials and they help a great deal in making the room acoustically good.

These materials are used for:

(i) Damping sound in ventilation installations;

(ii) Developing special acoustic effects in TV, radio and film shooting studios, etc.

(iii) Facing interiors of premises which require a low noise level such as offices, restaurants, commercial centres, banks, etc.;

(iv) Providing adequate acoustic in theatre halls, auditoriums, etc.

The various types of absorbent materials are available in the market under different trade names. The value of coefficient of absorption is supplied by the manufacturer.

Following are some of the common types of absorbent materials:

(1) Hairfelt:

This material was used by Prof. Sabin in his experimental works. The average value of coefficient of absorption of 25 mm thick hairfelt is 0.60.

(2) Acoustic Plaster:

This is also known as the fibrous plaster and it includes granulated insulation material mixed with cement. If quantity of cement is more than required, the plaster will not have sufficient pores to become effective for acoustics. If quantity of cement is less, the plaster will not have enough strength.

Thus the quantity of cement should be carefully decided. For thickness of 20 mm and density of 1 kN/m3, the acoustic plaster possesses an absorbent coefficient of 0.30 at 500 cycles per second. The acoustic plaster boards are also available. They can be fixed on the wall and their coefficient of absorption varies from 0.15 to 0.30.

(3) Acoustical Tiles:

These are made in factory and sold under different trade names. The absorption of sound is uniform from tile to tile and they can be fixed easily. However the acoustical tiles are relatively costly than other absorbent materials. They are most suitable for rooms in which small area is available for the acoustical treatment.

(4) Strawboard:

This material can also be used as absorbent material. With a thickness of 13 mm and density of 2.4 kN/m3, it possesses a coefficient of absorption of 0.30 at 500 cycles per second.

(5) Pulp Boards:

These are soft boards which are prepared from the compressed pulp. They are cheaper and can be fixed by ordinary panelling. The average value of coefficient of absorption is 0.17.

(6) Compressed Fibre-Board:

This material may be perforated or un-perforated. The average coefficient of absorption for the former is 0.30 and for the later is 0.52. It has a density of 3 kN/m3.

(7) Compressed Wood Particle Board:

This material is provided with perforations and it can be painted also. With a thickness of about 13 mm, the average coefficient of absorption is 0.40.

(8) Perforated Plywood:

This material can be used by forming composite panels with mineral wool and cement asbestos or with mineral wool and hardboard. It is generally suspended from trusses. The average value of coefficient of absorption for the former composite panel is as high as 0.95 and for the latter composite panel, it is about 0.20.

(9) Wood Wool Board:

This material is generally used with a thickness of 25 mm and it has a density of 4 kN/m3. The average value of coefficient of absorption is 0.20.

(10) Quilts and Mats:

These are prepared from mineral wool or glass wool and are fixed in the form of acoustic blankets. The absorption coefficients of such quilts and mats depend on the thickness, density, perforations, mode of fixing, nature of backing and frequency of sound.

Example # 15. Tar:

The tar is a dark black liquid with high viscosity.

According to its source, the tar is classified into the following three categories:

(1) Coal tar

(2) Mineral tar

(3) Wood tar.

(1) Coal Tar:

It is heavy, black and strong smelling liquid. The variety of tar is prepared by heating coal in closed iron vessels. The escaping gases are allowed to pass through tubes which are kept cool by circulation of water. The coal tar gets deposited in these tubes. It is usually derived as a by-product during the manufacture of coal gas. The coal tar is used for making macadam roads, preserving timber, etc.

(2) Mineral Tar:

This variety of tar is obtained by distilling the bituminous shales. It contains less volatile matter.

(3) Wood Tar:

This variety of tar is obtained by the distillation of pines and similar other resinous wood. It contains creosote oil which can be obtained by further distillation of wood and hence it possesses strong preservative property. However the wood creosote is not so good as coal tar creosote for the preservation of timber.

Example # 16. Turpentine:

The turpentine is a transparent volatile liquid and it is thin like water. It is obtained by taping juice or gum from certain varieties of pine trees. The juice is collected in spring time and it is heated in furnaces. The vapours formed during heating are collected and distilled to obtain turpentine. It is also called the oil of turpentine or spirits of turpentine or turps.

It possesses the following properties:

(i) It is colourless.

(ii) It is highly fluid and inflammable.

(iii) It oxidizes on exposure to the air and is converted into a resinous substance.

(iv) Its good variety has a pleasant pungent odour or sharp smell.

The turpentine is mainly used as solvent for paints and varnishes. It also acts as a solvent for rubber. It is used for making printing ink, synthetic camphor, etc.