Curing is a term which means to control the temperature and moisture in concrete for certain period after placing of concrete so as to cement gains strength and hardness because of the chemical action between cement and water. Because this chemical reaction requires favourable temperature, moisture and time referred to as the curing period and this depends on the type of cement and nature of work. For OPC (Ordinary Portland cement) curing period is about 7 to 14 days but if rapid hardening cement is used it can be considerably reduced.
Purpose of curing –
curing is done to protect the concrete from loss of moisture due to sun and wind. Because the presence of water is essential to cause the chemical reaction which leads to hardened the cement. Generally, sufficient water is mix at the time of mixing to cause hardening of concrete. But it has to retain the moisture until the concrete has fully hardened. As we know that concrete increase the major part of the strength in early stage (about 3 weeks) is contributed by the clinker compound C3S and partly by C2S and the later strength contributed by C2S is gradual and takes long time. If curing is done efficiently then the impermeability of concrete is increases and shrinkage is reduced.
Methods of Curing –
Water retain in concrete may be a number of ways. Mainly two ways to curing the concrete either supplying additional moisture to concrete by ponding, spraying, sprinkling, etc. or by preventing loss of moisture in form of evaporation from concrete by sealing the surface of concrete by membrane formed by curing compounds. Two factors are considered while selecting any mode of curing – (i) The temperature should be kept minimum for dissipation of heat of hydration. (ii) The water loss should be prevented. Following are some of the prevalent methods of curing.
Water Curing –
- Water curing is very easy and effective process and it is mostly used in India. The horizontal surfaces are kept wet by ponding or damp by gunny bags, straw etc.
- By sprinkling of water is done at intervals, but in this method, care must be taken that the concrete does not to dry out between applications, otherwise may be possibility of crazing (The fine cracks that may occur in the surface of new concrete as it hardens).
- By covering concrete with wet jute bags, wet sand, wet saw dust etc.
- Applying Curing compound – Many numbers of curing compound presence in market like acrylates, chlorinated rubbers, resins, sodium silicates, bitumen, linseed oil and wax etc. When applying a curing compound, a membrane is created on surface by coating a layer of compound to prevent loss of moisture by evaporation. However, the concrete here may not achieve full hydration as in moist curing.
Curing of concrete can be also completed by generating heat which promote to the reaction between cement and water while the concrete is maintained in moist condition for availability of water. To create these conditions can only be fulfilled by the use for steam curing. As providing artificial heat leads to increase the rate of strength, so that This method of curing is also known as accelerated curing.
Curing by Infrared Radiation:
This method is also a process to rise of temperature. As the rapid rise of temperature at initial stage does not affect the ultimate strength, so we increase the temperature by infrared radiation. This method is used in very cold climate conditions where hydrate rate of concrete very low. A much more rapid gain of strength can be obtained with the help of infrared radiation than even with steam curing.
Electrical curing is another way to use in cold climate conditions. In this method is done by passing the high ampere alternating current having low voltage (generally 30V-60V potential difference adopted) through electrodes. These electrodes form of plate cover the entire area of two opposite faces of concrete. Evaporation is prevented by using an impermeable rubber membrane use to cover the top surface of the concrete to avoid the evaporation. Using this method (electrical curing), concrete can attain the normal 28-days strength in just a period of 3 days.
Effect of improper curing –
- The cracks may develop due to plastic shrinkage, drying shrinkage and thermal effects.
- The compressive and flexural strengths not fully attained.
- The chances of intake chlorides and atmosphere chemicals are very high.
- The durability decreases due to higher permeability.
- The frost and weathering resistances are decreased.
- The rate of carbonation increases.
- The surface is layed with sand and dust and it leads to reduce the abrasion resistance.
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