Basic Concept of Curing Of Concrete
The process in which the moisture is maintained inside the freshly casted concrete to continue the hydration process to achieve the desired property of hardened concrete is called as curing of concrete. Before curing of concrete one should know the behavior of freshly casted concrete. It gives us better idea about when to start the curing on concrete.
when we add water in dry concrete mix, hydration of cement will start immediately and heat will be generated inside the concrete. Without proper hydration of cement, concrete will never gain the design strength.
When the concrete is placed and compacted cement slurry rises through the surface of concrete due to the settlement of concrete. In technical term this phenomenon is called as bleeding in concrete. The rate and duration of bleeding depend on many factors i.e mix proportion, thickness, depth of concrete, concrete compaction method, water- cement ratio etc.
Factors Effecting Curing of Concrete
- Chemical Composition.
- Fineness of cementitious materials.
- Water cement ratio.
- Mixture proportions.
- Aggregate characteristics.
- Chemical and mineral lab mixtures added.
- Temperature of the concrete.
During initial setting, bleed water will evaporate from the surface of the concrete. The rate of evaporation of bleed water will depend on so many factors i.e air, temperature and relative humidity, concrete temperature, radiant energy from the direct Sun and wind velocities.
Types of Curing of Concrete
Due to above described factors curing is carried out in three phases and the length of time depends on the concrete and environmental conditions here we have discussed these three stages.
During the initial setting, sometimes bleed water start evaporating from the surface faster than it is rising out from the concrete. If evaporation happens faster than usual time then the only factor that effect is the high temperature. when all the water gets evaporated from the surface you need to do some initial curing to minimize the moisture loss. During this period if you do not start the curing it may lead to plastic shrinkage cracks in concrete. The initial curing of concrete can also be done by fogging.
Sometimes intermediate curing is necessary and is carried out when finishing is completed, but before the concrete has reached its final set. During this period, evaporation may need to be reduced but the concrete is not yet be able to tolerate the direct application of water or the mechanical damage resulting from the application of plastic sheets. In this stage liquid forming membrane compound can be used effectively to reduce evaporation loss.
Final Curing :
final curing is carried after final finishing and after the concrete has reached its final set. Final curing can be done by application of wet covering i.e ponding method, saturated burlap, spraying water, curing compounds etc.
Quick Note (Summary) :
The normal final setting time of cement is 6-10 hours. In short the curing should start after minimum 6 hours (after final setting time of cement) and not less than 24 hours. Thus when concrete curing has to start it varies and depends on when the surface of the concrete begins to dry. Curing should be continued for minimum of 10 days for OPC and 14 days for blended cement.
Curing should be started before concrete surface loses its water and undergo shrinkage. This is very important for concrete with low to very low water cement ratio which does not have much water to bleed to the surface. Concrete gains its 100 % strength after 28 days of proper curing. Proper curing of concrete is great for its strength, volume stability and wear resistance.