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Curing is the process to control moisture loss during hydration of cement. Hydration takes time – days, or even weeks rather than hours. To achieve its potential strength and durability, curing needs to be done for an ideal period of time.

Purpose of Curing

The reaction between cement and water is called hydration. It is an exothermic reaction (which releases heat). After adding water to the concrete mix, hydration starts, which tends to dry the concrete quickly. Hence concrete is kept moist by curing, to stop it from drying out before attaining its maximum strength.

The factors required for curing of the structural elements are:

1. Specified Strength of Concrete
2. Grades of concrete
3. Temperature – The chemical reaction between cement and water in concrete releases heat, which requires constant adding of water to complete hydration. In summer fifty percent of water is evaporated. So, more water is needed during sunny days.
4. Size and Shape of the Concrete member
5. Economy
6. Material availability
7. Labor force
8. in situ versus plant concrete production
9. Aesthetics

Curing Methods

Curing methods can be classified into three basic methods

• Water Addition Method

This method keeps the surface of the concrete moist by ponding, spraying/sprinkling, fogging, misting, wet burlap, and other water absorbent materials

• Water Retention Method

This method prevents loss of moisture from the concrete by covering it with polythene sheeting or leaving the formwork in places

• Temperature control Method:

This involves live steam, heating coils, electrically heated forms or pads

Considerations for selecting a curing method:

a) The type of construction such as those involving large horizontal surface areas as in roads, floors, and airfields, or, those involving formed concrete in walls, columns, beams, cantilevers, and arches, etc

b) The place of construction, whether indoors and damp situations (as inside a building) or outdoor

c) The weather conditions where concrete is being laid in cold climates or in dry and hot weather

The methods in detail:

Water Addition Method

Water addition is considered as the best method of curing of concrete as it satisfies all the requirements of curing – absorption of the heat of hydration, promotion of hydration, and elimination of shrinkage. Water curing is carried out by adding water to the surface of concrete to ensure that it is kept continuously moist. The water used for this purpose should not be more than about 5°C cooler than the concrete surface. Spraying warm concrete with cold water may give rise to ‘thermal shock’ that may contribute to cracking.

Water curing can be done in the following ways:

i. Spraying or Fogging.

ii. Ponding.

iii. Wet covering.

iv. Immersion.

i. Spraying of Water:

This method is ideally suited for almost all types of construction in most conditions. Vertical reclining walls, plastered surfaces, concrete columns, etc. are cured by spraying water. It involves spraying water with the help of house pipes connected to the main water supply lines. However in tall structures, water spraying at top levels may hamper work on the lower floors.

ii. Ponding of Water:

Ponding is a quick, inexpensive and effective form of curing when there is a ready supply of good ‘dam’ material (e.g. clay soil), a supply of water, and the ‘pond’ does not interfere with subsequent building operations.

In this method, pavement slabs, roof slabs, etc., are kept under water by making small ponds. It is the next common method of curing of concrete suited best for horizontal surface areas such as roads, floors, and slabs.

Small ponds, not more than 5 cm deep, are made over the surface by raising temporary barriers.

These ponds are kept filled with water for several days. In hot weather, ponding is the ideal method of curing of concrete. It has the added advantage of helping to maintain a uniform temperature on the surface of the slab

iii. Wet Coverings:

In some cases, wet coverings such as wet gunny bags, jute matting, and straw are wrapped to the vertical surface for keeping the concrete wet.

For horizontal surfaces, sawdust, earth or sand are used as wet coverings to keep the concrete in a damp condition for a longer time.

Another method, suitable for flat, columnar, and vertical surfaces (after the formwork is removed) is covering the surface with straw, burlap, hessian or jute soaked in water. These are kept moist for the entire period of curing. Fabrics are particularly useful on vertical surfaces since they help distribute water evenly over the surface and even where not in contact with it, will prevent the surface evaporation from within the concrete and supply the additional water required for hydration.

iv. Immersion:

The precast concrete items are usually immersed in curing tanks for a specific duration.

Water Retention Method by Curing Compounds(Membrane Method)

This may be broadly described as the chemical method of curing of concrete.

In this method, some suitable chemical compounds dissolved in solvents are sprayed over the fresh concrete to be cured. The solvent evaporates, leaving behind a thin film of the chemical compound spread over the concrete surface.

This film prevents evaporation from the concrete.

Thus, if enough water has already been added at the time of preparation of concrete, it will set and harden nicely without much addition of water. The thick film of chemical compound starts peeling off after some time (2-4 weeks) leaving behind the properly cured concrete. Most curing compounds consist of different types of resins.

A major disadvantage of this method is that continuous concreting is not possible. The cured surface will not bond firmly with a new layer unless the chemical is completely removed from it.

Still, on flat surfaces of single thickness, the membrane method is used extensively.

Temperature control Method

The development of the strength of concrete is a function of not only time but also of temperature.

When concrete is subjected to a higher temperature, it accelerates the hydration process which results in faster development of strength.

The exposure of concrete to higher temperature is done in the following manners:

i. Steam curing at ordinary temperature

ii. Steam curing at high temperature

iii. Curing by infrared radiation

iv. Electrical curing

 i. Steam curing at ordinary temperature

This method is mostly adopted for prefabricated concrete elements; application of steam to construction will be a difficult task. For steam curing, the concrete elements are stored in a chamber. The chamber should be large enough to hold a day’s production.

The door is closed, and steam is applied, either continuously or intermittently. Accelerated hydration takes place at this higher temperature and concrete attains the 28-day strength in about 3 days.

In large prefabricated factories, they have tunnel curing arrangements. However, concrete subjected to a higher temperature at the early period of hydration is found to lose some of the strength at a later stage.

Very fresh concrete should not be subjected suddenly to high temperature. A certain delay period after casting the concrete is desirable.

ii. Steam curing at high temperature and pressure

Unlike ordinary steam curing, this curing is carried out in a closed chamber. The super-heated steam at high temperature and high pressure is applied to the concrete. This process is also called ‘autoclaving.’

The following advantages are derived from this process:

a. In one day or less the concrete develops the strength that normally cured concrete gains in twenty-eight days. Also, it does not lose strength at a later stage.

b. This concrete shows higher resistance to sulfate attack, freezing & thawing action and chemical action.

c. High-pressure steam cured concrete shows lower drying shrinkage and moisture movement.

 iii. Curing by infrared radiation

Curing of concrete by infrared radiations has been practiced in very cold climatic regions of Russia.

 iv. Electrical curing

Another method of curing concrete, which applies mostly to very cold climatic regions, is using electricity. This method is not likely to find much use in ordinary temperatures due to economic reasons.

What is the Right Time for Concrete Curing?

The right time for concrete curing depends on several factors. Like, temperature, the evaporation rate, moisture content in the air, etc.

According to ACI-308, three phases of concrete curing should be done.

(1) Initial Curing.

This process is also called Bleeding of Concrete.

After placing of concrete, the water starts bleeding from concrete and rises from its pores. After rising of water, evaporation takes place, and the water starts disappearing from the surface due to evaporation.

Bleeding of concrete depends on many factors like thickness, length, temperature, etc. So, to reduce the loss of water and prevent shrinkage, initial curing of concrete is required. Evaporation reducers can be used for this process.

(2) Intermediate Curing of Concrete.

This process is done when finishing work is finished before the final setting of cement. It is required as water plays an important role in the strength of concrete.

(3) Final Concrete Curing.

After the final setting of concrete, it should be cured so that one can prevent the loss of more water and increase the strength of concrete.