Curing is keeping freshly placed concrete moist and at a reasonable temperature for its first days, so the cement keeps hydrating and the concrete reaches the strength and durability it was designed for.
Concrete doesn't dry to get hard. It gets hard because cement reacts with water. Let the surface dry out early and the top of the slab, the part that takes traffic and salt, ends up weak, dusty and prone to scaling and cracking.
How long. ACI 308.1 calls for keeping the moisture and temperature up for at least 7 days, or until the concrete reaches 70 percent of its specified strength, whichever comes first. That applies when the daily mean temperature is above 40°F. Cold weather stretches it out.
How. There are three common ways:
- Curing compound. A liquid membrane sprayed on once the finishing is done and the surface water is gone. It's the usual choice on flatwork. Cover the whole surface evenly, and check that it's compatible with any sealer or coating going on later.
- Plastic sheeting. Laid flat and weighted at the edges. It can leave discolored spots on colored or decorative work.
- Water. Wet burlap, sprinklers or ponding, kept wet without letting it dry in between.
Cold weather. ACI 306 says concrete protected from freezing until it reaches about 500 psi won't be damaged by a single freeze-thaw cycle. Blankets, heated enclosures and accelerators get you there. Don't pour on frozen ground.
Hot, dry or windy weather. The surface can lose water faster than it bleeds, which causes plastic shrinkage cracks before you've even finished. Evaporation retarders, sunshades, windbreaks and starting curing right away all help.
Put the curing method on the proposal, and price the blankets or compound as a line. The concrete flatwork guide covers the finish-out.