Following two factors are considered while selecting any mode or method of curing:
(1) The temperature should be kept minimum for dissipation of heat of hydration.
(2) 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:
(1) Ponding with water.
(2) Covering concrete with wet sand, saw dust, etc.
(3) Covering concrete with wet sand, saw dust, etc.
(4) Covering concrete with water-proof paper or polyethylene sheets and holding it I position.
(5) Intermittent spraying with water and continuous sprinkling of water.
(6) Applying curing compounds.
The methods (1) to (5) 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 earthern 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 (3) and (4) 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 had dried out. It results into harmful effects
(1) For members retained in the formwork, the concrete should be wetted thoroughly before spraying or painting of curing compounds.
(2) If the concrete has lost some surface water, it is advisable to fog down the surface before the spraying.
(3) The bleeding water should be allowed to evaporate.
(4) The curing compound containers should be shaken before use.
(5) 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.
(6) They should not be applied when standing water is present.
Following are the advantages claimed for the curing compounds.
(1) 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.
(2) 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.
(3) They are available in the white and other solar reflective colours to achieve reduction of surface heat by reflectance.
(4) They are indispensable at places where there is acute shortage or non-availability of water.
(5) They are most suitable for large exposed areas like concrete roads, bridges, runways, etc.
(6) 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.
(7) They can be employed in case of overhead repairs where water curing is not feasible and cn also affect the working conditions at its base.
(8) They prove to be economical for industries, manufacturing precast concrete pipes, poles, girders, tiles, etc.