What Is Curing In Construction?
Curing is the maintaining of adequate moisture content and temperature in concrete at an early age so that it could develop properties the mixture was designed to achieve.
Curing begins immediately after finishing and placement so that the concrete can develop the desired durability and strength. Without an adequate supply of moisture, the cementitious materials in concrete can’t react to form a quality product.
Drying can remove the water needed for this chemical reaction called hydration, and the concrete won’t achieve its potential properties.
Temperature is an important factor in proper curing, since the rate of hydration, and therefore, strength development is faster at higher temperatures.
Generally, the concrete temperature should be maintained above 50°F (10°C) for an adequate rate of strength development.
For exposed concrete, relative humidity and wind conditions are also important; they contribute to the rate of moisture loss from the concrete and may result in cracking poor surface quality and durability.
Protective measures to control the evaporation of moisture from concrete surfaces before it sets are essential to prevent plastic shrinkage cracking.
Why Is Curing of Cement Concrete Required?
Predictable strength gain:
Laboratory tests show that concrete at a dry environment can lose up to 50% of its potential strength in comparison to similar concrete that’s moist cured.
Concrete placed under high-temperature conditions will gain early strength quickly, but later strengths may be reduced.
Concrete placed in cold weather will take longer to gain strength, delaying form removal, and subsequent construction.
Well-cured concrete has a better surface hardness and can withstand surface wear and abrasion.
Curing also makes concrete more watertight, which prevents moisture and water-borne chemicals from entering into the concrete, thus increasing durability and service life.
Better serviceability and appearance:
A concrete slab that’s been allowed to dry out too early will have a soft surface with poor resistance to wear and abrasion. Proper curing reduces crazing, dusting, and scaling.
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The Right Time of Curing of Concrete Depends On:
Initial Curing – Bleeding of Concrete:
The rate and duration of bleeding depend on many factors such as concrete mix properties, thickness or depth of concrete, method of compaction of concrete, etc.
This bleed water begins to evaporate from the surface. When all the bleeding water has disappeared from the surface, the drying of concrete starts, then the initial curing of concrete must minimize the moisture loss and prevent plastic shrinkage cracks to the concrete before and during finishing operations.
The initial of concrete could be done by techniques like fogging or using the evaporation reducers, or by providing the sunshades and windscreens.
Intermediate curing is done when the concrete surface finishing operations were carried out before the final setting of concrete.
This happens when the required surface texture of a concrete member is achieved rapidly or if the setting of concrete is delayed.
After the concrete is finished after the final setting of concrete, the final curing of concrete should be done.
This helps to prevent surface drying of concrete because the loss of moisture in the concrete surface occurs immediately.
Methods of curing
Moisture retention can be enhanced by several methods, including moisture addition, moisture-retaining covers, and liquid membrane-forming curing compound.
Water curing by ponding, sprinkling, or fogging is practical only for slab areas without joints or where the water is positively confined by dams to prevent flooding the base course or saturating the subbase/subgrade.
This is necessary to limit potential slab curling due to moisture gradients and to preserve compaction of the soil-support system.
Water used for curing should be within 20 °F (7 °C) of the concrete temperature to avoid thermal shock. Continuous wetting should be maintained to avoid isolated dry spots.
Water curing of wet covering should be used for shrinkage-compensating concrete slabs (ACI 223). The wet covering is generally the more practical and satisfactory method of water curing.
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When properly applied and maintained, burlap and other wet coverings provide a continuous supply of moisture uniformly distributed on the slab surface.
Burlap has been the most commonly used wet covering; wet burlap tends to reduce the temperature of the hydrating concrete slabs.
Moist hay, straw, earth, or sand have been used, but their use is usually too labor-intensive for large projects and can discolour the surface.
If sand or earth is used, it should be applied at least I in. (25 mm) deep and kept continuously wet during the curing period.
Wet coverings should be laid over the concrete as soon as finishing operations are complete, and surface marring can be avoided.
Exposed concrete edges should be carefully covered. The coverings should be kept wet so that a film of moisture remains continuously in contact with the concrete throughout the curing period.
Burlaps are available that resist rot and fire or that reflect light reducing heat absorption from sunlight or a combination thereof.
Coverings with burlap on one side and polyethylene on the other are also available; the polyethylene is helpful in keeping the moist burlap longer, but it makes rewetting more difficult.
Other polyethylene-backed fabrics are also available. These fabrics do not stain concrete like some burlaps and are often lighter and more durable than the burlap-backed product
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Moisture retaining coverings
Although not usually as effective as water curing and wet coverings, moisture-retaining coverings are widely used due to their convenience.
Polyethylene film and other plastic sheet materials are available in clear, white, or black and are easily handled; the white is especially good for covering fresh concrete subject to sunlight.
These films avoid leaving a residue that can prevent the bond of new concrete to hardened concrete or the bond of resilient floor coverings to concrete.
Plastic films are particularly effective for curing the base slab of two-course floors. They can, however, leave blotchy spots on the slab and should not be used for coloured concrete or where the appearance of the slab surface is important.
The sheets should be spread as soon as possible after finishing operations without marring the surface finish.
Edges of sheets should be lapped a sufficient distance to prevent moisture loss and sealed with tape, mastic or glue, or held in place with wood planks or sand. Construction traffic should be restricted because the film can be extremely slippery
The water-Proof paper has the same advantages and disadvantages as plastic film, except that discoloration is less likely.
It should be light in colour; the edges should be lapped and sealed and left in place for the duration of the curing period. Tears caused by construction traffic should be repaired to maintain proper moisture retention.
Liquid membrane-forming Curing Compounds
The application of liquid membrane-forming curing compounds is the most widely used method for curing concrete.
Advantages are relatively low in-place cost, early access to the floor, elimination of the need to monitor the curing process, and the opportunity for longer uninterrupted cure.
The membrane should be protected from damage due to construction traffic. Disadvantages include the potential for insufficient and uneven coverage, conflict with regulations on the release of volatile organic compounds, interference with a bond of surfacing materials, and variability of quality and solids content.
Liquid membrane-forming curing compounds should be applied as soon as finishing operations are complete while the surface is still damp but without free water.
Machine spraying is preferable, but manual spraying is acceptable if accomplished with sufficient care to ensure uniform and complete coverage.
The manual application should be accomplished by either spraying or rolling and by backrolling with a wide short-nap paint roller.
This can ensure full coverage without ponding of the curing compound in low spots. White-pigmented or fugitive-dye compounds help ensure even coverage and can be considered to reflect light and heat for floors exposed to sunlight.
Generally, the curing compound should meet or exceed the minimum moisture retention requirements of ASTM C 309 or ASTM C 1315.
Reference: ACI 302.1R-04