Concrete Curing is the process of maintaining the moisture content of concrete till it gets adequate strength. When water is added to cement, it starts hydrating. This hydration continues till there is a presence of moisture in the mixture.
Concrete gains it maximum strength in 28 days, so it is required to keep it moist till it gets its maximum strength. In case, it loses moisture before gaining strength, cracks will appear on the surface resulting in failure in acquiring sufficient strength. Proper curing of concrete not only gives sufficient strength, but also abrasion resistance, water tightness, and freeze-thaw resistance.
Curing of concrete depends on three factors:
There are different techniques for curing depends on the above factors:
1. Wet Coverings
This is the widely used method of concrete curing. Moist jute bags are wrapped around the surface of members such as beams, columns, walls etc. These wraps are continually moist time by time to keep it wet. Cotton mats or rugs are also used for this purpose. A wet covering is preferable for vertical members of the structure.
2. Left-over Formwork
Formwork act as a covering surface that keeps the moisture from evaporating or reduce the rate of heat of hydration. In the case of dry weather, these forms need to be wetted time to time. Formwork curing may cause non-uniform concrete curing.
3. Membrane Covering
The use of waterproof membranes such as polythene bags, rubber sheets, wax or bituminous emulsion retains the moisture content of concrete from evaporating. This method can be used where there is an inefficient supply of water and slope surfaces.
Ponding is used on horizontal surface such as slabs, pavements, and runways etc. Rectangular ponds are made using sand on the surface and water is filled. This method required sufficient supply of water. Ponding is advantageous for large surface area, as there is a greater rate of hydration.
5. Immersion to water
This technique is used in the laboratory to test concrete specimen. Prior to the destructive test, the cube or cylinder specimen will be cured. Concrete cube or cylinders are placed in a pool of water. The water in a pool should be clean to get the accurate result.
Water can be sprinkled on the surface by mean of sprinkler or manually. Uniform and regular sprinkling are required to get adequate strength.
7. Compound Curing
Chemicals or admixtures are sprayed on the concrete surface to get curing in areas where the temperature is low. These chemicals are salts such as calcium chloride, which has affinity for water. They absorb moisture from atmosphere and retain it on the surface. In this way, the water content in concrete does not evaporate quickly.
Fogging is used in the cold areas, where water can be freeze if used for curing. Fog increase the humidity in atmosphere thus reduce the rate of evaporation.
An area where quick strength is required, the rate of hydration is increased. Since the accelerated hydration may cause the development of crack; it is required to damp the surface during the process. Steam curing can help achieving strength in 4-5 days. Pressure can also be increase while steam curing. It will give highly durable concrete.