Curing plays an important role in strength development and durability of concrete but often is not employed long enough or with the correct technique, says Bryan Perrie, CEO of Cement & Concrete SA (CCSA).
Perrie says newly-cast concrete must be properly cured to ensure that hydration continues until the full potential strength of the hardened concrete is achieved and to minimise the tendency to crack. The concrete should be kept damp and not allowed to freeze during this time. Adequate curing ensures that concrete becomes stronger, more impermeable, durable and resistant to abrasion.
He says correct curing of concrete involves:
* Covering the surface with a water-retaining material such as sand, earth, straw or hessian kept continuously damp;
* Sprinkling or spraying with water often enough to keep the concrete continuously moist;
* Covering with plastic sheeting of waterproof paper. The covering must be held in place at its edges in a way that does not damage the concrete or allow wind to get under the sheeting, and be sufficiently overlapped at joints;
* Applying membrane-forming compounds. Such components are supplied in liquid form and applied by spray, roller or brush. Curing compounds must be applied as soon as possible without marring horizontal surfaces, or within two hours of formwork removal on vertical surfaces; and
* Leaving formwork in place and covering any exposed concrete surfaces.
Perrie adds: “If freshly placed concrete is exposed to hot sunshine or drying winds over the coming summer, evaporation should be prevented by covering with plastic sheeting immediately after placing and before finishing. If there is concern that plastic sheeting could damage the surface, a water-filled atomiser spray of the type used for spraying insecticides on fruit trees should be used to produce a mist over the fresh concrete until the surface is hard enough to permit one of the curing methods mentioned.
“It may also be necessary to provide adequate windbreaks for the concrete. In cold weather, newly placed concrete must be protected from frost by covering it with insulating material such as sacking or straw. The temperature of placed and compacted concrete should not be allowed to fall drastically until the concrete has attained a strength of 5 MPa. Curing must continue for at least five days after placing concrete, even longer than a week in cold weather,” he adds.