why we do the curing to concrete

Need for curing

a) The primary aim of curing is to prevent the loss of moisture from fresh concrete due to evaporation and to maintain the immature concrete moist for a suitable time until the concrete has reached the strength and hardness desired.

b) As concrete dries, it shrinks and if the drying occurs before the concrete has gained strength, cracks will result. Also , since drying occurs first on the surface , if ample moisture is not provided over the surface, the cement will be present as a dust coating having no strength to hold the aggregate particles together.
c) To lessen eveporation, the fresh concrete should be protected from direct sunlight and from drying wind. Alternate wetting and drying can do more damage than no curing at all.

Curing period:

a) Cure concrete for a period of atleast 7 days and preferably 14 days when ordinary Portland cement is used.
b) Rapid hardening cement require less time ( about half) and pozzolana cement require more time than ordinary portland cement.
c) When low heat cement is used , curing must extend to 21 to 28 days.
d) Delay in starting the curing results in a decreased strength of the concrete.

Curing procedure

Curing is done in two stages, namely
(1) Initial curing and
(2) Final curing.

• Initial curing is done to prevent the loss of moisture from the concrete surface due to evaporation . It should be started immediately after the final finishing of concrete(say, within 3 to 6 hours after placing) and should be continued atleast over night.• Final curing is done to provide or preserve moisture over the concrete surface for a suitable time till the concrete reaches the desired strength and hardness. It should be followed immediately after the initial curing and before the concrete has dried. • For Horizontal members like pavements, sidewalks, canal lings, small docks and building floors, the initial curing may be done by covering the concrete with two layers of jute cloth kept continuously wet. The final curing may be done by ponding or by covering with 2 inch thick moist earth or sand or 3 inches of moist hay or grass or straw uniformly over the concrete and kept wet by spraying. • For vertical members like walls, columns, small piers, dams and abutments, the initial curing may be done as in the previous case but should be continued for 4 days. The final curing may be done by continuous spraying of water directly on the concrete or by covering the concrete with water proof paper or by applying curing compounds over the concrete to prevent the loss of moisture due to evaporation. • For mass structures like large piers, large dams, large abutments and other massive concrete sections, the initial curing may be done by applying a curing compound over the exposed surface or by suspending a covering above the concrete surface to prevent water loss prior to setting. The final curing may be done by keeping the outside surface continuously wet for about 2 weeks. Also, a liquid membrane forming compound may be applied to the newly exposed surface, soon after the forms are removed. • Oiling and wetting the forms before concreting may also be helpful for initial curing since this prevents the absorption of water concrete by the formwork.