Concrete curing

Concrete curing is mandatory. As a matter of fact, the higher the environmental temperature is, the more meticulously it has to be done. In every case, concrete’s surface has to be moistened throughout the entire day for at least the first week after casting; however the curing process 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 pe-rimeter of the slab right after concreting. We fill this dam with water thus creating a pond and we make sure to replace water losses due to vaporization. The circumferential dam may be constructed with bricks cut in half or simply with fast curing cementious mortar. This solution has two disadvantages: it is not economical and it stops all 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. Thiscan be achieved only with the use of a mechanical screed board that compacts the concrete as it vibrates. Without doubt, concrete casting must be done with the best possible conditions i.e. very early in the morning or late at night, with concrete being the ‘coolest’ possible, aggregates stored in shade etc.

The simplest of the above ways of curing, is the third one, as long as the surface of the concrete is completely flat, meaning that the surface should not have the ridges, which are created by hand floats. A vibrating screeder should be used in order to achieve the completely flat concrete surface. What is more, all water from the “bleeding” procedure should be completely removed.

Concrete curing in frost:

Normally concrete must not be cast in extremely low temperatures, however when this is un-avoidable e.g. sudden drop of the temperature below zero, its exposed surface must be covered with concrete curing blankets. These are made out of thermo insulating materials like rolls or plates of rockwool, glasswool with aluminum covering, polystyrene boards later used in insula-tions. 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. straps and ties. 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 oil.

In areas exposed to extremely low temperatures, the use of air–entraining admixtures or addi-tives is mandatory in order to protect the concrete from the catastrophic results of frost.