Curing

Why curing is needed

Curing is designed primarily to keep the concrete moist, by preventing the loss of moisture from concrete during the period in which it is gaining strength. Protection from premature drying is necessary so that the strength development of concrete is not affected by water removal.

The consequences of too early water losses are:

• Lower strengths in part near the surface.
• Tendency to dusting.
• Higher water permeability.
• Reduced weather resistance.low resistance to chemical attacks.
• Occurrence of early age shrinkage cracks.
• Increased risk of all forms shrinkage cracking.

The curing period may depend on,

• The properties required of the concrete.
• The purpose for which it is to be used.
• The ambient conditions, i.e. the temperature and relative humidity of the surrounding atmosphere.

Compressive strength at 180 days as % of continuously moist sample

Effect of duration of curing on properties of concrete

As may be seen in this example, concrete allowed to dry out immediately achieves only 40% of the strength of the same concrete water cured for the full period of 180 days. Even 3 days water curing increases this figure to 60%, while 28 days water curing increases it to 95%. Keeping concrete moist, therefore, is a most effective way of increasing its ultimate strength.

Methods of curing

There are two distinct ways of curing.

1.By retaining water in the concrete, in the early hardening period

Methods used in this process are:

• Ponding, which is done by a dammed wall of sand around the concrete formation and then flooding with water.
• Sprinkling water on to the formation. This method is achieved by continuous spraying of water on to the slab with garden sprinklers or hand held hose.
• Use a layer of sand of gunny bags that holds and retains sprayed on water. The sand cover or gunny bags need to be kept wet at all times for the duration of the curing period.

2.By preventing evaporation of water (Sealing the exposed surface)

• Use a clear polythene sheet laid on top of the slab to stop the evaporation process. This is one of the most efficient methods to cure concrete, especially in sloping areas and horizontal surfaces.
• Leave form-work used to create the concrete formation. The framework itself, if left in place, underneath a suspended slab, or around a concrete column will stop concrete from drying out too quickly which helps the curing process.
• Use concrete curing compounds. These compounds now come in a variety of types. Use water soluble waxy emulsions that can be sprayed on the fresh concrete with a hand pump.

When to start ‘Curing’

It is always better to start curing as soon as the concrete is hardened to a point that the concrete is not damaged when the water / curing agent is added. Curing can be started after 2-3 hours of concrete placement.

Effects of not having good curing conditions

• Plastic shrinkage cracks occur when the surface of the concrete dries rapidly and shrinks before it can gain sufficient strength to resist cracking.
• When ambient conditions and concrete temperatures combine to produce conditions that create a high evaporation rate, it creates increased chances of surface drying and shrinkage cracks forming prematurely.
• Formation of plastic shrinkage cracks could be reduced by adopting the curing process early.