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 light and wind velocities.

Different Methods of Curing of Concrete

There are different types of concrete curing methods. The most common of them are mentioned below.

• Water Curing
• Chemical Curing
• Electrical Curing
• Infrared Radiation Curing
• Steam Curing

Water Curing

One of the oldest and most effective methods of concrete curing is water curing. Water is used to keep the concrete members moist in this procedure.

To keep the concrete moist for a long time, gunny bags are used to cover the surface and then water is poured on top of it. Water is stored in squared size sections on horizontal concrete surfaces such as slabs, which is known as ponding.

To keep vertical members moist, they are sprinkled with water at regular intervals.

Chemical Curing

Chemical membranes are used to cure concrete in this type of curing technique. Sodium silicate is one of the most commonly used chemical compounds. The use of this compound slows the evaporation of moisture from the concrete and fills the existing pores to seal the surface voids.

The application of this compound should be done immediately after the concrete placement has been completed. When the thin layer of water on the finished concrete surface has vanished, it should be applied.

If you apply this membrane curing compound on a finished concrete surface with a thin water layer, the water on the surface may be absorbed by the concrete, causing the membrane to break. In most cases, two coatings of this membrane curing substance will enough, however one coat will suffice if necessary.

Electrical Curing

This method of curing involves electricity to cure concrete objects. In this procedure, high and low alternate currents are carried through electrodes in the form of plates that cover the full area of two opposite concrete sides.

Water evaporation from the concrete is stopped by installing an impermeable rubber membrane on top of the concrete surface. If this method of concrete curing is used, the 28-day strength can be achieved in just three days.

The process of this concrete curing method may appear to be quite amazing, but the actual cost of this curing procedure can be very expensive, which is why it is rarely adopted.

Infrared Radiation Curing

With the use of infrared radiation curing technique, you can build strength considerably faster than you can with steam curing. The ultimate strength is unaffected by the rapid initial temperature rise.

This method of curing is most suited for the production of hollow concrete items, where the heaters are located within the hollow areas of the product.

Steam Curing

Curing can also be achieved with the use of artificial heat while the concrete is kept moist. Both of these conditions can be met by using steam curing technique. Because a faster rate of strength development can be achieved, this form of curing is also known as rapid curing.

The temperature of the concrete can be boosted by immersing it in steam, hot water, or by running an electric current through it. When calcium hydroxide is generated during the hydration of cement at higher temperatures, it combines with finely divided silica in the coarse and fine aggregates to form a strong and relatively insoluble compound with greater strengths.

Steam is used to heat the concrete members at 93°C, either at low or high pressure. In low pressure steam curing, roughly 70% of the 28-day compressive strength of concrete may be achieved in 16-24 hours, however in high pressure steam curing, 28-day strength can be achieved in only 24 hours.

Also Read : Compaction of Concrete
Also Read : Admixtures for Concrete
Also Read : Types of Concrete

Stages of Concrete Curing

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.

Initial Curing

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.

Intermediate Curing

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.