Concrete curing is mandatory. As a matter of fact, the higher the ambient temperature and the wind velocity are, the more meticulously it has to be carried out. In every case, the concrete surface has to be hosed as needed to remain moistened throughout the entire day for at least the first week after casting. The curing process however will last for 28 days.
Concrete curing in extremely high temperatures can be done in three ways:
(a) Right after finishing the concrete’s surface, we cover it with special sheets (burlaps). These sheets must be kept wet 24 hours a day for at least one week. Special attention must be paid to hold them down and prevent them from being blown away.
(b) By ponding. We form a dam a few centimeters high (4 to 5 centimeters) around the perimeter of the slab right after concreting. We fill this dam with water thus creating a pond and we make sure to replace water loses due to vaporization. The circumferential dam may be constructed with bricks cut in half or simply with fast curing cement mortar. This solution has two disadvantages: it is costly and it hinders works on the top of the slab for at least 7 days.
(c) We spray the concrete’s wet surface with a special chemical fluid that becomes a membrane thus preventing concrete from drying out.
This is the simplest procedure but in order to be effective, the concrete’s surface must be free from grooves created by a manually operated screed board. This can be achieved only with the use of a mechanic screed board that compacts the concrete as it vibrates. Also, “bleeding” water should be removed. Evidently, concrete casting should be done under optimal conditions i.e. very early in the morning or late at night, with concrete being as ‘cool’ as possible, aggregates stored in shadow etc.
Concrete curing in frost:
Normally concrete must not be casted in extremely low temperatures, however when this is unavoidable e.g. sudden drop of the temperature below zero, its free surface must be covered with concrete curing blankets. These are made out of thermo insulating materials like rolls or plates of rockwool, glasswool with aluminium covering, polystyrene boards to be later used in insulations. In that way, we can make use of the concrete’s own heat. The blankets must be secured from wind up-turnings with e.g. rafters and balks. If the temperature drops too low, heaters like the ones used in out-door coffee shops may be used with their reflectors in an inverted position. In the past, barrels with fire were usually placed underneath the formwork; they contained sand wet with diesel oil.
In areas exposed to extremely low temperatures, the use of air–entraining admixtures or additives is mandatory in order to protect the concrete from the catastrophic results of frost.