Asbestos :Properties,Uses, Asbestos cement sheets,Types of AC sheets,Asbestos cement flat sheets,unreinforced corrugated asbestos sheet,Asbestos cement boards,Bakelite
Asbestos is available in nature as a mineral in Marwar, Garhwal (in U.P.); Bhandara (in MP) and Mysore. It is a silicate of calcium and magnesium found in the form of very thin fibers in venin’s of metamorphic rocks.
It has low water permeability, is vermin proof and resists rot.
It is brown, grey and white in colour.
It is good for heat sound and electric insulation.
It is resistant to acids and alkalies.
Its fibres are capable of being woven into fabrics.
It is highly durable and is strong.
Its specific gravity is 3.10
Its melting point is 1200-11 50ºC
It acts as reinforcing material when mixed with ordinary Portland Cement.
Asbestos is used extensively for heat insulation. It is used for making fire proof padding, packing sheets and clothes for firemen and electricians. In the form of paper and mill boards asbestos is used for low voltage insulation.
Various products for use in buildings are made with a mixture of 8 to 20 per cent of asbestos with Portland cement. These products are called asbestos cement products.
Asbestos cement products are light in weight, strong, durable, water proof and resist corrosion. These can be sawn, drilled and drilled, nailed and screwed. It is rolled into various forms such as sheets, tiles and pipes for drainage etc. Everest Asbestos Cement Ltd. manufacture a variety of quality products made with asbestos.
Asbestos cement sheets.
Asbestos cement paste obtained by mixing 15% asbestos fibres with cement and mixed with water is pressed under grooved or toothed rollers so as to give a series of waves or corrugations. Corrugations are given to increase strength and rigidity of sheets and ensure quick flow of rain water.
These are light, durable, impervious and fire resisting commonly used for roofing big halls, factories, workshops. These provide a reasonably priced roof covering which is a non-conductor of heat and is durable. A.C. sheets require no maintenance.
Types of A.C. sheets.
There are three types of A.C. sheets.
- Everite ‘Big Six’.
- Everite standard : Sheets 0.76 m wide, 6.5 m thick and 1.1 m to 3 m in length. Overall depth of corrugations is 2.56 to 2.85 m. There are 10½ corrugations per sheet at a pitch of 5.4 cm.
- Turnal-trafford sheets : 1.12 m wide, 1.3 to 3 m long and 6.5 mm thick. Every sheet has 4-5 cm deep corrugations alternating with flat portions. Pitch of corrugations is 34 cm. Excellent roof covering specially for large spans.
Asbestos cement flat sheets.
These may be compressed or uncompressed and are intended for interior and exterior use. The compressed sheets are stronger than the uncompressed one’s. Nominal thicknesses of these sheets are : 5, 6, 8, 10 and 15 mm with + 5 per cent tolerance. Their nominal lengths are 2400, 1800 and 1200 mm; and the width is 1200 mm with tolerance of + 5 mm in each case.
For details refer to IS : 2096-1966.
Unreinforced corrugated asbestos cement sheets.
These are designed for roof covering and walls of different types of building and for decorative and other purposes.
Table 15.1. Dimensions and Tolerances of Corrugated Sheets
|All Dimensions in millimetres|
|S.No.||Type of Sheets||Depth of Corruga- tions||Pitch of Corruga- tions||Overall width||Effective width||Nominal Thick- ness||Length of sheet|
|(iii)||Tolerances on Dimensions||+3
For details refer to IS : 459-1970.
Asbestos cement boards.
These are made by moulding under pressure mix of ordinary Portland cement and asbestos fibre. The boards are usually 1.22 metre wide 3 mm to 6 mm in thickness and up to 2.44 metre long. It has dense hard surfaces. The exposed surface is finished smooth. The sheets may be in the colour of plain cement or any other colour, with plain finish or with tiles cut on its surface or with marble finish. The sheets can be sawed and nailed with ease. These provide a nice wall covering with fire resistant properties. Asbestors is used for electric insulation and heat insulation.
It is a thermosetting plastic obtained by polymerisation of phenol (obtained from coal tar) and formaldehyde. It is of three categories :
As amber coloured substance called bakelite A is obtained in the form of a liquid. Mixed with inert materials called fillers it is used for forming different materials. It is soluble in alcohol and scetone. On cooling Bakelite A solidifies to form Bakelite B. Further heat treatment of bakelite B gives the final product called Bakelite C. It is dark brown in colour and melts at a temperature 50ºC and is soluble in spirit. It is widely put to use in various electrical insulation, decorative fittings and for bearings in roller mills, properller shafts, pumps and machines.