What is curing?
are developed up to a sufficient degree to ensure service requirement of finished member.
Objective of curing:
• To keep specimen saturated or retain saturation as nearly as possible until spaces filled with thin fresh cement paste are filled by hydration products of cement particles to a desired extent. In actual, concrete placed at site, effective curing are stopped always nearly long before maximum expected hydration has been taken place.
• To prevent loss of moisture from concrete; this is essential not only because of adverse effect on strength development but also because of formation of plastic shrinkage, reduction of abrasive resistance, increase in permeability.
Thus curing is important for better durability and improved strength.
Plastic shrinkage cracking:
When water comes out from any porous body that is not gained rigidity yet, it gets contracted. Such movement of moisture in the early state of concrete is also observed. Some volume change is associated with formation of hydration products and some are associated with moisture loss due to evaporation from exterior surface of concrete when mix is in plastic state. When proper protection is not taken such loss may occur under the suction of underlying soil or dry concrete. Such contraction is termed as plastic shrinkage as concrete remains in plastic state at the time of contraction.
Again plastic shrinkage depends on environmental condition like temperature wind velocity and relative humidity as they influence amount of moisture loss from surface of plastic concrete. However, rate of moisture loss cannot be used alone to determine plastic shrinkage as it is more dependent on rigidity of concrete mix.
Cracking appeared on surface when amount of loss of water from unit area is more than amount of water that come out on surface through bleeding process and becomes significant. Such cracking is termed as plastic shrinkage cracking. Thus evaporation just after placing of concrete must be prevented completely to eliminate such cracking.
Excessive drying of cement paste will increase permeability of it; this is probably due to shrinkage which may rupture some portion of gel formed in capillaries and open new passage along which fluid can travel. Prolong curing period of concrete (wet-curing) having very high water-cement ratio to 7 days instead of 1 day was reported to decrease permeability of concrete by a factor of five. This permeability was tested for permeability of concrete against water, not other fluid.
A change in surrounding relative humidity from 100% to 94% was reported to increase capacity to absorption water of concrete which is an indication of formation of continuous system of larger pores in concrete. Curing at ambient relative humidity less than about 80% was reported to yield increase in volume of pores larger than 37 nm that are an important issue in durability of concrete.
In case of concrete road or pavement (may be of industrial floor), if top surface is dried out rapidly under the exposure of sun with drying wind, shrinkage stress in plastic concrete is exerted. The resulting dried concrete is weak and cannot withstand large magnitude of stresses due to numerous cracks are developed on the surface. The defective gel structure of concrete will yield weak surface having poor wearing quality.This result less abrasion resistance and under the action of traffic will create dust in dry season and mud in rainy season.
Laitance appeared on the surface also results identical situation, but it is a form of segregation. Good curing is essential to achieve good wearing surface; European Standard (ENV 206) suggested to maintain curing period of two times that of normal curing in this regard.
Concrete gains strength thorough hydration of cement. All particles of cement are not hydrated at a time. This process takes time, though rate is fast at the beginning but hydration continues over a long time; with the increase in duration, rate decreases. The extent of development of hydration product and consequent formation of gel depends on hydration.
Water loss may take place by two ways one is losing water to environment, the other is self-desiccation occur due to chemical reaction during hydration of cement. It is established that hydration of cement only can take place in capillary spaces spaces filled with water. Such self-desiccated water must be replaced with water from external source; this means we have to ensure ingression of water into concrete.
It should keep in mind that only half of water exist in cement paste is effective to take part in chemical combination. This statement is valid even total water present in the mix is not adequate for required chemical reaction.
Strength and permeability relation:
It should be extended that to develop satisfactory strength of concrete, all cement particles must not be hydrated and actually practically it is very rare to achieve this. Concrete quality depends mainly on gel/space ratio of paste. However, if space filled with water in fresh concrete is more than volume to be filled with hydration products, hydration will be greater which will result higher strength and lower permeability.
Management of heat of hydration:
During hydration of cementitious materials considerably high heat is released; such heat of hydration generated within concrete is harmful considering volume stability.If generated heat can be removed by any means, the ill effect of heat can be decreased; in this regard, we can suggest water curing, above figure shows strength development under curing by wet covering and ponding.