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Use of Polyfibres in Construction |
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Recently, use of polyfibres in concrete and mortar has seen a significant rise. Many Government departments have accepted them as an essential ingredient for concrete works such as floorings, roads, bridges, pavements, plastering and water retaining structures. This sudden acceptance of polyfibres by most of Government departments necessitates a look into its behaviour and usefulness towards the properties of concrete and mortar.
What are Polyfibres? Polyfibres are light weight, fine diameter, polyester fibres of small lengths varying from 6mm to 12 mm and having a triangular or circular cross- section. One kg of these Polyfibres contains 120 to 240 million fibres. These are supplied in small pouches of 150 gm each. Triangular shaped polyfibres have better interlocking properties than circular fibres and are thus preferred for addition to concrete and mortar. Earlier polypropylene fibres were imported in India but now big industrial houses like Reliance have started their production in India. In-home product being cheaper than the imported one has captured the construction industry in a phenomenal manner.
Effect on strength of concrete : Manufacturers claim that addition of polyfibres to concrete increases its strength. Even if there is an increase in strength, it can’t be taken into account for the structural design of concrete members. Though use of polyfibres may enhance the toughness of concrete, reduce shrinkage cracks and reduce permeability, economizing by assuming impart of structural strength can’t be permitted. More research is required to define the exact role of polyfibres towards strength of concrete.
Effect on workability of concrete : Addition of polyfibres to concrete or mortar reduces their workability thus making their placing difficult. More water has to be added to concrete or mortar to maintain the required slump. Thus to maintain water-cement-ratio, more cement has to be added. Other alternative is to avoid addition of water or cement but to add plasticiser to concrete. Both the options mean an increase in cost of concrete.
Test results : Test results conducted on a sample of M20 concrete show a workability of 120 mm with 0.5% plasticiser when no polyfibres are used. On adding polyfibres, workability reduces to 85 mm even after making double the quantity of plasticiser. Addition of more plasticiser increases strength of concrete by a few percent and this increase can’t be attributed to polyfibres. Addition of plasticiser also raises the cost of concrete.
Use of Polyfibres : Use of polyfibres in industrial floors, concrete pavements and parking lots may give better results but their use in plaster work needs to be kept under keen observation. Plaster being 12 mm or 20 mm thick only, polyfibres may appear on the plastered surface, much to the dismay of the user. Only 6mm long polyfibres should be tried in plaster work. Prescribed dosage of polyfibres is 900 grams per cubic-meter of concrete or 125 grams per bag of cement. Their use in water retaining structures such as underground water reservoirs, overhead tanks, sumps and tanks may be considered as these help in holding the mass of concrete together and avoid hair cracks in it thus avoiding seepage of water or corrosion of reinforcement.
Portland Pozolana cement (PPC) releases less heat of hydration in comparison to OPC causing less shrinkage cracks. Thus PPC is preferred for use in water retaining structures. Dosage of polyfibres should therefore be defined separately for OPC and PPC after experimentation.
Designating polyfibres as micro or secondary reinforcement may mislead the user and veil their good uses. Assigning such designations should be avoided by the manufacturers. Concentration should be on their real and proven benefits. Often, the manufacturers suggest taking into account increase in strength of concrete due to addition of polyfibres. This is a dangerous suggestion and must not be accepted.
Cost factor : Addition of 1 pouch of polyfibres per bag of concrete increases its cost by about 8%. However polyfibres being a totally different material than concrete ingredients, their long term effect on life of concrete needs to be studied.
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Avoid providing a socket in the bathrooms. If you wish to provide a heater in the bathroom, choose a ceiling fitting model that combines light and a circular heater.
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