Engineering key Point

ASBESTOS Asbestos is a general name for several varieties of fibrous minerals which are available in nature. But presently, most of the commercial asbestos produced is ‘chriotile’ [Mg6SiO11(OH)6.H2O]. Properties of Asbestos 1. It is flexible, soft and non-porous. 2. It is fireproof and acid proof material. 3. It is a good insulator of heat and electricity. 4. When it is mixed with cement and water, it retains shape firmly. 5. Its colour is brown or grey. 6. It can be cut into pieces or can be drilled. 7. It possesses high tensile strength in the direction of its fibres. 8. Its specific gravity is 3.10. Uses of Asbestos 1. Asbestos cement sheets are the cheapest roofing materials. 2. Asbestos cement pipes are used as down take pipes of rain water from the roof. 3. With bitumen it forms good damp proof layer. 4. It is used for preparing fire proof ropes and clothes. 5. It is used as covering material for fuse and electric switch boxes. 6. It is

BITUMEN Ashalt, bitumen and tar are referred as bituminous materials, which are essentially hydrocarbon materials.The asphalt is a mixture of inert mineral matter lime alumina, lime, silica etc. and a hydrocarbon known as asphaltic bitumen. In some places like Trinidad and Bermudez, asphalt is available in nature at a depth of 3 to 60 metres. It is known as natural asphalt.  Common variety used all over the world is residual asphalt, which is obtained by fractional distillation of crude petroleum oil. Bitumen is the binding material which is present in asphalt. It is a hydrocarbon.  It is obtained by partial distillation of crude oil. It contains 87 per cent carbon, 11 per cent hydrogen and 2 per cent oxygen.Tar is obtained in the distructive distillation of coal, wood or other organic materials. When coal or wood is heated to redness in an closed chamber, it yields volatile product and residue coke. After separating and cooling volatile product gives tar.

DISTEMPERS Distempers are the cheaper variety of paints in which chalk is used as base and water is used as a carrier.The emulsifying agent which is commonly used is glue or casein. Distempers are available in powder form or in the form of paste.  They are to be mixed with hot water before use.The surface to be distempered should be thoroughly rubbed and cleaned. The cracks, if anyshould be filled by lime putty. The surface should be kept dry for about two months before applying distemper. Thus a primary coat is applied and is allowed to dry. Distemper is usually applied in two coats. Properties of Distemper 1. They are generally light in colour. 2. The coatings are generally thick. 3. They give reflective coating. 4. They are less durable than oil paints but are cheaper.

Requirements of Good Building Stone (i) Strength: The stone should be able to resist the load coming on it. Ordinarily this is not of primary concern since all stones are having good strength. However in case of large structure, it may be necessary to check the strength. (ii) Durability:  Stones selected should be capable of resisting adverse effects of natural forces like wind, rain and heat. (iii) Hardness: The stone used in floors and pavements should be able to resist abrasive forces caused by movement of men and materials over them. (iv) Toughness :   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. (v) Specific Gravity:  Heavier variety of stones should be used for the construction of dams, retaining walls, docks and harbours. The specific gravity of

(a) Soundness Test: It is conducted by sieve analysis. 100 gms of cement is taken and sieved through IS sieve No. 9 for fifteen minutes. Residue on the sieve is weighed. This should not exceed 10 per cent by weight of sample taken. (b) Setting Time: Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity. Vicat apparatus is used for finding the setting timesVicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder. Before finding initial and final setting time it is necessary to determine water to be added to get standard consistency. Fo