Curing is one of the processes that help to prevent moisture from the concrete. It is carried out after achieving the initial setting of concrete and striking off of formwork. Curing helps to keep the surface wet and moist in order to gain the desired strength of concrete. It shall be done to maintain the temperature of freshly placed concrete. Curing not only controls the moisture from the surface but also helps to reduce the plastic shrinkage and long term permeability of concrete.
The American Concrete Institute (ACI) recommends a minimum curing period corresponding to attaining 70% of the compressive strength of concrete.
Why curing is important for structures?
As we know that water is important for mixing the concrete. Without water concrete mixing is not possible. Water is required to take part in a chemical reaction with cement which is known as Hydration reaction. It takes 28 days to get complete strength after casting of concrete.
It is important to retain the water inside the concrete until it achieves its full strength. The strength of concrete can be reached quickly when admixture is used at the time of mixing. If we cure the concrete for a longer period of time, permeability gets decreased and the strength of concrete gets increased.
Once concreting work is finished, the formwork is removed from the RCC member such as Columns, Shear walls, etc. After the De-shuttering of Formwork concrete gets exposed to the environment. The water from the surface starts to evaporate.
Loss of moisture from concrete surface occurs due to ambient temperature, wind velocity, and Relative humidity. Due to the loss of water from the surface, concrete get affected in terms of strength and durability. When the wind blows with high velocity, it dries the concrete surface that means concrete stops hydration.
To get a clear ides about the importance of curing, I would like to give an example of tress. As we know that when we plant the trees of mango, it requires nurturing the tress for its growth so we add water, insecticides, pesticides, and fertilizers.
Let’s us know how the Evaporation occurs in Concrete.
As we know that Hydration reaction is an exothermic reaction that means heat is released outside. Due to this heat, the temperature of concrete increases and causes the evaporation of water inside the concrete.
Conclusion: It is, therefore, necessary to do the curing because it helps to prevent evaporation of water and to keep control of the temperature of concrete.
Advantage of Curing of concrete
The advantages of curing of concrete are as follows.
- It increases the strength and durability of Reinforced concrete members.
- It prevents the loss of water due to evaporation.
- Curingcontrols the temperature inside the concrete.
- It increases the resistance to freezing and thawing.
- Curing helps to continue the process of hydration.
- It prevents the concrete from shrinkage and cracking.
What happens if Concrete is not cured?
If the concrete is not cured properly, Plastic shrinkage cracks and thermal cracks are formed. Due to a lack of curing, concrete doesn’t get strength and durability.
Methods of curing concrete
There different methods of curing in order to retain the water in the concrete surface during the early stages of strength development. The method of curing depends upon the availability of curing material, Types of structural members, size, shape, and age of concrete. The method/ techniques of curing are as follows.
This method of curing is applicable for horizontal surface members like a concrete slab, deck slab rigid pavement slab, etc. Rectangular or square ponds are built with the help of bunds over surface members. Bunds are prepared with a mixture of cement, sand, and water.
Water is then filled up to a height of 25 mm to 50 mm inside the rectangular or square ponds at least two times a day. In this technique usage of water is huge. The height of bunds shall be less than 4 inches.
In this method, small precast units such as a concrete cube, cylindrical cubes, etc shall be fully immersed in curing tanks for a certain period of time. This curing is generally done in the laboratory in order to do the curing of test specimens.
Spraying or Fogging Method.
In this method, curing is done by sprinkling water with the help of nozzles or sprayers. This method is applicable for members such as masonry walls, columns and shear walls, slabs, etc. In
Wet Covering or Moist curing method
The moist curing method is one of the conventional methods of curing concrete in the construction field. In this method, external covering material such as Gunny Bags, Hessian cloths, jute matting, sawdust, etc is used to prevent the moisture from the concrete surface. This type of method is used for vertical members such as columns, shear walls, etc.
The curing compound consists essentially of waxes, resins, chlorinated rubber, and solvents of high volatility. The formulation should be capable of providing a seal shortly after being applied and must not be used on the surfaces that are to receive additional concrete, paint, or tile that require a bond, unless the membrane can be satisfactorily removed, or can serve a base that can provide a bond.
Curing compounds must be applied after the free water on the surface has disappeared and no water sheet is seen, but not so late that the compound will be absorbed into the surface pores of the concrete.
On formwork concrete surfaces, curing compound should be applied immediately after stripping the formwork. In such application, if the surface is dry, water should be sprayed and it should be allowed to reach a uniformly damp appearance with no free water standing when the compound is applied.
Minimum Curing Period of Concrete according to IS code guidelines.
As per IS 456-2000, 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 cement are used.
The curing period 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 cement are used, it is recommended that the above minimum periods may be extended to 14 days.