Curing of Concrete: Purposes, Period, Effects, Factors and Methods | Engineering

The concrete surfaces are kept wet for a certain period after placing of concrete so as to promote the hardening of cement. It consists of a control of temperature and of the moisture movement from and into the concrete. The term curing of concrete is used to indicate all such procedures and processes.

Purposes of Curing:

Following are the objects or purposes of the curing of concrete:

(i) The curing protects the concrete surfaces from sun and wind.

(ii) The presence of water is essential to cause the chemical action which accompanies the setting of concrete. Normally, there is an adequate quantity of water at the time of mixing to cause hardening of concrete. But it is necessary to retain water until the concrete has fully hardened.

(iii) The strength of concrete gradually increases with age, if curing is efficient. This increase in strength is sudden and rapid in early stages and it continues slowly for an indefinite period.

(iv) By proper curing, the durability and impermeability of concrete are increased and shrinkage is reduced.

(v) The resistance of concrete to abrasion is considerably increased by proper curing.

Period of Curing:

This depends on the type of cement and nature of work. For ordinary Portland cement, the curing period is about 7 to 14 days. If rapid hardening cement is used, the curing period can be considerably reduced.

Effects of Improper Curing:

Following are the major disadvantages of improper curing of concrete:

(i) The chances of ingress of chlorides and atmospheric chemicals are very high.

(ii) The compressive and flexural strengths are lowered.

(iii) The cracks are formed due to plastic shrinkage, drying shrinkage and thermal effects.

(iv) The durability decreases due to higher permeability.

(v) The frost and weathering resistances are decreased.

(vi) The rate of carbonation increases.

(viii) The surfaces are coated with sand and dust and it leads to lower the abrasion resistance.

The above disadvantages are more prominent in those parts of structures which are either directly exposed or those which have large surfaces compared to depth such as roads, canals, bridges, cooling towers, chimneys, etc. It is therefore necessary to protect the large exposed surfaces even before setting. Otherwise it may lead to a pattern of fine cracks.

Factors Affecting Evaporation of Water from Concrete:

Following are the four basic factors on which the evaporation of water from the concrete surface depends:

(i) Air temperature;

(ii) Fresh concrete temperature;

(iii) Relative humidity; and

(iv) Wind velocity.

The factors (i), (iii) and (iv) depend on the weather conditions and they indicate the direct environmental influences. Thus, out of four factors, only the fresh concrete temperature can be monitored or supervised by the concrete technologists.

The evaporation of the water in the first few hours can leave very low amount of water in the concrete for hydration. It leads to several shrinkage cracks. Under normal site conditions, the average loss of water varies from 2.50 to 10 N per m² per hour. The major loss occurs in the top 50 mm layer and over a period of 3 hours, the loss could be about 5% of the total volume of that layer.

It is also observed that there is considerable evaporation of water even before the conventional curing process has begun. The concept of early beginning of curing is therefore accepted.

Methods of Curing:

Following two factors are considered while selecting any mode or method of curing:

(i) The temperature should be kept minimum for dissipation of heat of hydration.

(ii) The water loss should be prevented.

Thus all the methods of curing are derived from the basic principle of lowering of the surface temperatures and prevention of water evaporation. However the methods should be employed in proper way and it can only be ensured by the proper quality control and supervision.

Several specialized curing techniques are employed in the modern construction work, but the most commonly employed methods of curing are as follows:

(i) Ponding with water.

(ii) Covering concrete with wet jute bags.

(iii) Covering concrete with wet sand, saw dust, etc.

(iv) Covering concrete with water-proof paper or polythelene sheets and holding it in position.

(v) Intermittent spraying with water and continuous sprinkling of water.

(vi) Applying curing compounds.

The methods (i) to (v) are conventional ones and they all suffer from the common defect of late beginning by which time some harm on the concrete has already been inflicted.

The best of the above methods is ponding. It consists of little earthen dams which are built over the entire surface to be cured. The squares thus formed are then flooded with water to a depth of about 50 mm or so. This is an effective method for flat horizontal surfaces. But it is not practicable on vertical surfaces.

The method of covering of concrete surfaces with wet jute bags has also practical limitations on account of difficulty in maintaining them close to the surface especially in localities having winds blowing at high speed.

The methods (iii) and (iv) of covering concrete surfaces with wet sand, saw dust, water-proof paper, etc. have also certain limitations and they can be used only under specific circumstances.

The intermittent spraying of water is the most common method of curing under Indian conditions. But it is not properly employed at site of work. The basic principle of water curing is to ensure continuous supply of water to the concrete surface.

It is however observed that the intermittent spraying of water takes place after the surface water has dried out. It results into harmful effects as compared to the advantages of curing because the green young concrete is constantly subjected to multiple wetting and drying cycles leading to early deterioration.

The last method (vi) of applying the commercially available curing compounds is a simple operation and it can be brought about by spraying while the concrete is wet. The application is generally carried out by tree sprayers which are employed for spraying of insecticides.

Suitable nozzles should ensure fine spray and they should be regularly cleaned by water or white cement. The nozzle is held at a distance of 0.7 m to 1 m from the concrete surface and it should be confirmed that full area is covered with specified consumption. The pressure should be maintained in the pump to effect fine spray.

The curing compounds are available on different basis like acrylates, chlorinated rubbers, bitumens, sodium silicates, linseed oils, wax and other items.

The precautions to be taken during the application of curing compounds are as follows:

(i) For members retained in the formwork, the concrete should be wetted thoroughly before spraying or painting of curing compounds.

(ii) If the concrete has lost some surface water, it is advisable to fog down the surface before the spraying.

(iii) The bleeding water should be allowed to evaporate.

(iv) The curing compound containers should be shaken before use.

(v) They should be sprayed as soon as possible after casting of concrete and finishing of surfaces and immediately after the disappearance of the glossy shine or brightness of water from the surface.

(vi) They should not be applied when standing water is present.

Following are the advantages claimed for the curing compounds:

(i) It permits early curing as they can be applied within about half an hour of casting after the bleeding water has evaporated and the glossy shine or brightness of water has disappeared.

(ii) They are applied in a single operation and one can note that the entire area is covered. It aids in the quality control of concrete.

(iii) They are available in the white and other solar reflective colours to achieve reduction of surface heat by reflectance.

(iv) They are indispensible at places where there is acute shortage or non-­availability of water.

(v) They are most suitable for large exposed areas like concrete roads, bridges, runways, etc.

(vi) They are useful in places which are either non-accessible or practically difficult to be cured like structures cast with slip forms, tall chimneys, cooling towers, etc.

(vii) They can be employed in case of overhead repairs where water curing is not feasible and can also affect the working conditions at its base.

(viii) They prove to be economical for industries, manufacturing pre-cast concrete pipes, poles, girders, tiles, etc.