Sealing the concrete surface soon after is has hardened ensures that water is not lost through evaporation, thereby making it available for cement hydration.
The hardened concrete surface is first thoroughly wetted before the sealant is placed. This is the water that will be available over the next several days thereby enabling hydration to continue, as there will be adequate moisture. The convenience and lower labor requirements of these methods have led them to displace the more traditional water-curing methods in many instances.
ASTM C171 gives the specifications for the impervious paper to be used in curing concrete. The paper consists of two sheets of kraft paper cemented together by a bituminous adhesive with fiber reinforcement. This paper is placed over the wetted concrete and the paper edges overlapped about 6" and tightly sealed with sand. To avoid moisture escaping into the space between the sheet and the concrete, the sheets need to maintain close contact with the concrete surface during the entire curing period. Any convenient material (such as sand) can be used to achieve this.
The paper can be reused once tears are repaired, or double layers may be used. In hot weather, light-colored paper is better as it reflects the heat. In cold weather, black paper is better as it attracts heat thus keeping the concrete warm.
Plastic sheets such as polyethylene film are lightweight and easily applied to both complex and simple shapes. They are therefore more versatile than burlap. The plastic is applied in the same fashion and according to the same specifications (ASTM C171) as impervious paper.
A disadvantage of the film is that it can cause discoloration when it gets wrinkled. This will be more pronounced when the concrete contains calcium chloride and has been finished by hard-steel trowelling. When uniform color is important, other means of curing should be used. Discoloration may also be prevented by flooding the surface before applying the covering.
Combinations of burlap and plastic will retain water well and eliminate the need for continuously keeping the burlap wet.
Membrane-forming curing compounds
Liquid membrane-forming compounds are formulated from resins, waxes or synthetic rubber dissolved in a volatile solvent or emulsified in water. Concrete should normally be wet when the membrane is applied. Soon after application over the concrete by spraying, the solvent or water evaporates and leaves behind an impermeable film of sealant. The compounds do not form a barrier that is as watertight as plastic or waterproof paper. They have however become very popular in the curing of concrete pavements, floors and even on some vertical surfaces. Often two coats are needed to ensure an effective vapor barrier. In this case, the second coat is applied at right angles to the first one.
Pigment is added to the compound during manufacture to make it visible after spraying and thus judge whether complete coverage has taken place. White or dark pigment may also be added to control heat reflection or absorption depending on the weather.
Curing compounds are not good for surfaces on which an overlay or topping is to be laid, as they will interfere with the bonding properties of the two materials. They should also not be used during construction of pavements that will be exposed to de-icing salts as the membrane retard air drying, which is needed to improve the salt scaling resistance of the surface.
Membrane specifications are given in ASTM C309.
Forms left in place
Forms may also provide an effective vapor barrier if they are left in place after casting. In this case the exposed surface of the concrete has to be kept wet using, say, a soil-soaker hose. The forms should also be kept moist by sprinkling, especially during hot weather.