Important of Concrete Curing and Methods
Curing is the process of preventing the loss of moisture from the concrete whilst maintaining a satisfactory temperature regime. The prevention of moisture loss from the concrete is particularly important if the watercement ratio is low, if the cement has a high rate of strength development, if the concrete contains granulated blast furnace slag or pulverized fuel ash.
The curing regime should also prevent the development of high temperature gradients within the concrete.The rate of strength development at early ages of concrete made with supersulphated cement is significantly reduced at lower temperatures.Supersulphated cement concrete is seriously affected by inadequate curing and the surface has to be kept moist for at least seven days.
Methods of Curing
Appplication of heat
Exposed surfaces of concrete shall be kept continuously in a damp or wet condition by ponding or by covering with a layer of sacking, canvas, hessian or similar materials and kept constantly wet for at least seven days from the date of placing concrete in case of ordinary Portland Cement and at least 10 days where mineral admixtures or blended cements are used. The period of curing shall not be less than 10 days for concrete exposed to dry and hot weather conditions. In the case of concrete where mineral admixtures or blended cements are used, it is recommended that above minimum periods may be extended to 14 days.
This is the best method of curing as it satisfies all the requirement of curing, namely, promotion of hydration, elimination of shrinkage and absorption of heat of hydration. It is pointed out that even if the membrane method is adopted it is desirable that a certain extent of water curing is done before the concrete is covered with membrane
Types of Water Cuing
Spraying or fogging
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Approved curing compounds may be used in-lieu of moist curing with the permission of the Engineer in-Charge. Such compounds shall be applied to all exposed surfaces of the concrete as soon as possible after the concrete has set. Impermeable membranes such as polyethylene sheeting covering closely the concrete surface may also be used to provide effective barrier against evaporation.
Application of Heat Curing
The development of strength of concrete is a function of not only time but also temperature.When concrete is subjected to higher temperature it accelerates the hydration process resulting in faster development of strength . concrete cannot be subjected to dry heat to accelerate the hydration process as presence of moisture is also essential in concrete.
Types of Heating Curing
Steam curing at ordinary pressure
Steam curing at high pressure
Curing by infra-red radiation
Steam Curing at Ordinary Pressure
This method is often adopted by prefabricated concrete elements. Application of steam curing to in situ construction will be a difficult task. However at some places it has been useful with the help of thick polyethylene sheets. But the rate of development of strength is not beneficial.
Duration of Curing
The concrete should not be allowed to dry fast in any conditions. This conditions should be maintained for 24 hours. The best practice is to keep the concrete in gunny bags for 24 hours and then commence water curing by ponding or sprinkling method. The concrete cured for the long time will show superior strength and show other good properties. However,curing for long time will be a costlier process. The curing period varies for different structures,situation and different atmospheric temperature. So for general it is cured for 7 days.