Curing is defined as the process in which freshly cast concrete is kept in sufficiently wet conditions or humid conditions for certain period after compaction. Curing is done to complete the hydration reaction. Usual method of curing requires at least 28 days.
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Curing can be done by following ways
- SPRAYING WATER: This is the simplest method for curing. In this method, after removal of shuttering or the framework, water is sprayed on the concrete through a hose or by bucket, a number of times during each day. This method is suitable for small works. It requires great care in supervision, and as soon as the concrete dries, water has to be sprayed.
- WET GUNNY BAGS: This method is very useful for vertical surfaces of concrete like sides of column. Gunny bags (BORI IN HINDI) are made from jute and wrapped around the concrete and are kept continuously wet. Gunny bags absorb water and retain moisture for long time. Gunny bags can be used for many times and hence this method is very economical.
- CURING BY INFRA-RED RADIATIONS: This method is used in Russia and it is claimed that loss of strength due to rapid rise in temperature does not occur in this method. This method is practically suitable for hollow concrete products where the infra-red heaters are kept in the hollow portion of the pre-cast components. This method is used in cold climatic region as it shows more rapid gain of strength than the steam curing and the rapid initial temperature does not cause decrease in ultimate strength as that of steam curing.
- MEMBRANE CURING: Membrane curing is used in areas of acute water scarcity. Water mixed while preparing fresh concrete is generally sufficient for the entire hydration reaction. In this method a sealing membrane is applied over concrete which will trap the water inside and avoid its escape through evaporation. Membrane may be actually a sheet of polythene or formed by application of chemicals. Different sealing compounds used in membrane curing are:
- Rubber latex emulsions.
- Bituminous and asphalt emulsions.
- Emulsions of paraffin or boiled linseed oil.
- STEAM CURING: Steam curing is used for pre-cast concrete products. Due to steam, the components are heated uniformly and the strength is gained at a very fast rate. Steam can pass and penetrate through small gaps of stacked pre-cast concrete products and strength is achieved evenly from all sides. Steam curing can be done in 2 ways:
- LOW PRESSURE STEAM CURING: In this method the steam curing is done at atmospheric pressure and about 70% of the 28-days strength can be achieved in 16-24 hours.
- HIGH PRESSURE STEAM CURING: In this method steam curing is done at a pressure of 8 atmospheres that is 800KN/m2. Main disadvantage in fast curing is that rapid rise of temperature cause loss of strength. This difficulty is minimised with application of high pressure. High pressure steam curing gives high durability to concrete.
- PONDING METHOD: Ponding method is suitable for large slab and for road pavements. On top of the slab, small ponds are prepared from clayey soil, and water is stored up to a depth of 50mm for 28 days. Water used in ponding should be of good or potable quality (water fit for drinking).