Concrete can be defined as a mixture of cement, sand, aggregate and water in a definite proportion. To increase the efficiency of concrete; the application of water is done to support the hydration. Such an application of water is a type of curing.
“Curing of concrete refers to the process of maintaining the temperature and the moisture of concrete within the acceptable range for the hydration reaction such that the concrete develops into a hard mass with desirable properties”.
Curing is not only the application of water over the concrete surface; preventing the loss of moisture is also the type of curing. Two different ways of curing are:
1. Application of water to the surface
E.g. Spraying, ponding, immersion, damp sand, etc.
2. Preventing the loss of moisture
E.g. Continuous wetting the surface, using different compounds, using wet jute bags, etc.
The primary objective of the curing is to reduce the degree of permeability of the hardened concrete thereby increasing the strength and the durability. The essential parameters that need to be considered during the curing process include moisture, heat, and time.
Objectives of Curing of Concrete
The major objectives of the curing of concrete can be listed as follows:
i. To mitigate the shrinkage of concrete.
ii. To enhance the desirable properties of concrete such as impermeability, strength, durability etc.
iii. To maintain the desired temperature for the hydration reaction.
iv. To maintain the required moisture content for the complete hydration of the concrete.
v. To prevent the untimely or pre-mature drying out of concrete due to wind, sun etc.
Parameters Required for the Process of Concrete Curing
The essential parameters that are necessary for the curing process of concrete can be listed as follows:
The appropriate time for the curing of concrete can be selected based on the rate of evaporation of moisture from the concrete. Further, the rate of evaporation is dependent on a number of factors such as wind, solar radiation, sunlight, humidity, the temperature of concrete, etc. The appropriate time for the various phases of concrete depends upon the following:
a. Initial Curing
The placing and the compaction of concrete are followed by the bleeding of water. The water then rises to the concrete surface on account of the settlement of the concrete. Such bleeding of concrete depends upon a number of factors such as mix proportion of concrete, method of compaction of concrete, the thickness of the concrete, etc. The water that rises up to the surface of the concrete slowly begin to evaporate from the surface of the concrete.
After the complete evaporation; the drying of concrete begins and thus this is the right time for carrying out the initial curing of the concrete. This is because the initial curing of concrete can minimize the loss of water from the surface of the concrete thereby preventing the premature drying out of the concrete. This further prevents the plastic shrinkage cracking of concrete prior to the commencement and during the finishing works.
b. Intermediate Curing
The right time to carry out the intermediate curing is after the completion of the finishing works and before the final set of the concrete.
c. Final Curing
The right time for the final curing of the concrete is when the final set of the concrete has been completed. The final curing helps to minimize the drying of concrete due to the immediate loss of moisture from the surface of the concrete.
In general practice; longer the duration of the curing, the higher will be the strength and durability of the concrete. The major factors that affect the duration of concrete can be listed as follows:
a. The purpose of curing.
b. The size of the concrete structural member.
c. The rate of hardening of the concrete.
d. The grade of the concrete
e. The prevailing temperature and moisture conditions of the surrounding.
f. The existing exposure conditions of the concrete.
g. Specification of the concrete.
Codal Provision for Duration of Curing of Concrete
According to the American Concrete Institute (ACI) Committee 301;
The curing period for the concrete that attains 70% of the specified compressive strength must be greater than 7 days.
According to the Indian Standard IS 456 – 2000;
The duration of the curing must be greater than 7 days for the concrete in which Ordinary Portland Cement has been used as the binding material and greater than 10 days for the concrete in which admixtures have been added.
In case, the surface of the concrete is likely to be exposed to the extreme climatic condition such as hot and dry weather condition, the curing of concrete must be done for a period of at least 10 days (when admixture is not used) and at least 14 days for concrete in which admixture has been used.
Importance of Curing of Concrete
The main importance of curing of the concrete can be listed as follows:
i. It is an essential process for the development of strength and hardness in the hardened concrete.
ii. It is necessary to ensure that the concrete maintains its performance ability and durability.
iii. It provides more resistance to the abrasion of the surface of the concrete.
iv. The microstructure of the concrete is improved due to the curing process.
v. It makes concrete more resistive towards cracks, vegetal growth, chemical attacks, etc.
Methods of Curing of Concrete
The different methods of curing of concrete are explained in brief below.
I. Water Curing of Concrete
Water curing is the most common method of concrete curing used all over the world.
The curing work performed by the application of water to the concrete surface is water curing.
Water curing includes the following:
Ponding is mostly adopted for curing of slabs of roads, roofs etc. In this method, a small pond of water is created for the curing purpose. It is desirable for the horizontal surface and cannot be used for the vertical surfaces.
Immersion is adopted for the curing of precast concrete elements. In this method, the concrete members are immersed in a small tank that is filled with water for a certain period of time.
3. Spraying (or fogging):
Spraying is adopted for curing of the structural elements such as columns, plastered surfaces, retaining walls etc. In this method, curing is done by the application of water through spraying by pipes.
4. Wet Covering:
The wet covering is adopted when sufficient water is not available. In this method, curing is done by covering the concrete members with wet gunny bags, wet jute mats, wet hessian cloth, etc. Usually, sawdust, earth, sand etc are used for curing the horizontal concrete surfaces.
Advantages of Water Curing of Concrete
Some of the major advantages offered by water curing method are listed as follows:
a. Drying of the concrete is completely eliminated due to the application of water frequently.
b. This method also enhances the cement hydration process.
Disadvantages of Water Curing of Concrete
Some of the disadvantages of water curing of concrete can be listed as follows:
a. Sufficient amount of water is required for the curing. Hence, it is not desirable for the areas where there is a scarcity of water.
b. This method of concrete curing is comparatively expensive.
II. Membrane Curing of Concrete
Membrane curing of concrete is used in the areas where abundant or ample amount of water is not available for the water curing of concrete. In this method, the curing of concrete is done by covering the concrete member or surface with a membrane (or sealing compound) such that the evaporation of water from the surface of the concrete is prevented.
The covering of the concrete with the membrane facilitates the required conditions of hydration without using an excessive amount of water. Usually, membrane curing of concrete is done after one or two days of water curing.
Advantages of Membrane Curing
Some of the major advantages offered by membrane curing can be listed as follows:
i. It is highly efficient in the mitigation of evaporation from the
ii. It is highly desirable for curing the horizontal concrete surfaces.
iii. It also helps to protect the concrete surface from weathering.
Disadvantages of Membrane Curing
Some of the disadvantages of membrane curing can be listed as follows:
i. It is quite expensive as it requires the use of costly materials such as bitumen emulsion, wax emulsion, etc.
ii. It is not useful for increasing the strength of the concrete.
III. Heat Application for Curing/ Steam Curing
Another method that can be used for the curing of the concrete surface is the application of heat. The application of heat can be used to boost the hydration process as well as to increase the strength of concrete in a relatively short period of time. This is because the strength developing process of concrete not only depends upon the time but also depends upon the temperature.
In this method of curing, steam is applied to the concrete members directly at a temperature above 22 degree Celsius.
Advantages of Steam Curing
Some of the major advantages offered by steam curing can be listed as follows:
1. It can be completed within a short period of time.
2. It is also suitable for cold weather.
3. It is highly desirable for the curing of small precast concrete members.
Disadvantages of Steam Curing
Some of the disadvantages of steam curing can be listed as follows:
1. It cannot be applied for the large concrete surfaces.
2. It is expensive as it must be carried out at a controlled temperature greater than 22 degree Celsius.
IV. Shading Method of Curing
Shading method of concrete is a simple method which involves the preventing of the exposure of the concrete surface to the sunlight and heat thereby preventing the evaporation of water from the concrete surface. It is mostly used for curing the concrete road surfaces.
Effects of Improper Curing of Concrete
Improper curing leads to the following adverse effects:
a. The decrease in compressive strength and flexural strength of concrete.
b. Formation of cracks on a concrete surface.
d. Increase in rate of carbonation.
e. Decrease in durability due to the increase in permeability.
f. Reduction in the resistive property of concrete towards weathers, chemicals, vegetal growths, etc.
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