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All about concrete curing
What is the kind of care one has to take during curing to achieve the desired
strength and hardness in concrete? |
 CURING IS the maintenance of a satisfactory moisture content and temperature in concrete to achieve the desired strength and hardness. Drying removes water needed for hydration. Without adequate water and insufficient hydration, concrete tends to be weak. Temperature is an important parameter in proper curing. In outdoor concreting temperature, humidity and wind velocity contribute to evaporation, that is, loss of water. Prevention of loss of moisture from concrete is important not only from the point of view of strength development but also to prevent plastic shrinkage, decrease permeability and improve resistance to abrasion.
The loss in the 28-day strength seems to be directly related to loss of moisture during the first three days. Five per cent loss in moisture in the first three days leads to nearly 75 per cent loss in strength of concrete.
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Duration of curing
The curing should start early and should be continued till needed. The period of curing to be adopted will depend on a number of factors such as severity of temperature, drying condition, humidity and prevailing wind conditions. In case of concrete made with ordinary cement, it should be kept wet for at least seven days from the date of placing concrete under normal weather conditions. This should be increased to 10 days in dry and hot weather conditions prevailing in Chennai. When concrete is made either with mineral admixtures or blended cements, curing period should be extended to 14 days. Curing should be continuous. Evaporation of water on the surface of concrete depends on:
- Relative humidity of surrounding air
- Temperature
- Velocity of wind
If unfavourable conditions because of the above exist, then the period of curing should further be increased to suit the site conditions.
In this method, exposed surface of concrete is kept continuously in a damp or wet condition by covering it with sacks, canvas, Hessian or similar water-absorbent materials and kept constantly wet for the periods mentioned above. It is important to note that the absorbent material used should not be allowed to dry. The drying of these materials can be further prevented by covering it with another layer of plastic sheet. The purpose of moist curing is to prevent evaporation of the water added originally to fresh concrete and preserve it for the purpose of hydration. The quality of curing water used should be as good as mixing water. Vertical surfaces such as walls and columns lose curing water fast and hence it is preferable to retain the formwork longer or use very frequent wetting. Moist curing prevents loss of moisture from concrete, as well as maintains a satisfactory temperature regime. The curing regime prevents the development of high temperature gradients within the concrete. |
Membrane curing
Where there is water scarcity curing compounds may be used. Curing compound should be applied to all the exposed surfaces without exception. All moisture from concrete may be lost if partial covering is done through the holydays (openings) left. Membrane curing is a more skilful operation than conventional moist curing. Impermeable membranes such as plastic sheeting covering closely the concrete surface can also be used to provide an effective barrier against evaporation.
Membrane curing does not control temperature regime and hence this has to be kept in mind when such methods are used.
The prevention of loss of moisture from concrete at early ages is particularly important in modern high strength concrete where water cement ratio is low. The 53-grade cement used nowadays has a high rate of strength. The rate of strength development at the early ages of concrete made with super plasticisers is significantly large. In these circumstances curing regime becomes utmost important. In such cases, curing should start early without allowing the concrete to dry. The rate of strength development at early ages of concrete made with super-sulphated cement is significantly reduced at low temperatures. Super-sulphated cement concrete is seriously affected by improper curing. For concrete containing Portland pozzolana cement, Portland slag cement or mineral admixtures like fly-ash curing period should be increased.
When to start curing
Curing of all exposed surfaces should be started as soon as possible after the concrete has set. This will depend on the type of cement, admixture used and prevailing weather conditions and best decided based on field inspection. It should be remembered that early age curing is most important.
It is difficult and expensive to alter or repair concrete once placed and set. Improperly cured concrete has been the cause for early deterioration and reduced service life of concrete structures. Such concretes are porous and have reduced durability and strength.
Hence, constant and strict supervision of all items of curing is necessary during construction. The strength gained by concrete at the time of stripping of formwork should be adequate and this depends on how well the concrete is cured. Therefore let us not neglect curing.
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