Curing and Protection
Curing and protection tests whether you can keep moisture in and temperature within limits long enough for the concrete to develop its design properties, then protect it from traffic, freezing and damage.
Core rules
- Moist curing is typically 7 days for normal portland cement concrete and 3 days for high-early-strength, or until the specified strength fraction is reached. Cold weather, low humidity and wind all call for longer.
- Methods: ponding, fog spray, wet burlap or soaker hoses; plastic sheeting kept flat and weighted at the edges; or a curing compound meeting the specified standard, applied at the label rate in two perpendicular coats.
- Do not use a curing compound where a bonded topping, overlay or floor finish will be applied, and do not spray it over standing bleed water.
- Keep concrete at or above about 10 degrees Celsius for the curing period. In cold weather use enclosures, heaters and blankets, and vent combustion heaters, because exhaust gases carbonate a green surface. Concrete must gain strength before it freezes.
- In hot weather follow ACI 305 practice: cool ingredients, shade and fog, windbreaks, retarders and night placing to reduce evaporation and plastic shrinkage cracking.
- Protect edges and arrises, keep traffic off until the specified strength, and do not strip supports early.
Worked example
A 500 square metre slab is to be cured with compound at 0.2 L per square metre per coat, two coats. One coat needs 0.2 times 500, which is 100 L; two coats need 200 L. Supplied in 20 L pails, that is 10 pails, and with a 10 percent allowance for overspray and uneven application, order 11. Apply the first coat as soon as the surface is free of bleed water and will not be damaged, then the second at right angles once the first is dry.
Traps
Curing compound used on a surface that must later receive a bonded topping. Plastic sheeting laid before bleed water finishes, causing mottling. Judging cure by a day or two of dampness. Unvented heaters carbonating a fresh floor.