Freshly poured concrete requires five to seven days to properly harden, or cure. In the process, it gradually transforms from a wet, slushy mix to rock-solid pavement. Moisture and ambient temperature are critical to the curing process. If conditions are unfavorable, though, covering the concrete can sometimes accelerate the curing process or ensure better results.
It is a common misconception that concrete cures by drying out, like a pie crust that hardens as it bakes. Concrete curing is actually a chemical process involving a reaction between water and some of the ingredients in the mix, so it is vital to keep concrete moist while it cures. Temperature is another key issue. Hot days accelerate evaporation and remove moisture; cold temperatures greatly slow the reaction.
A plastic polyethylene covering retains moisture in the concrete and improves temperature control. If the temperature is below 60 F, use dark-colored plastic sheets to absorb more light and boost the temperature; when temperatures are higher than 85 F, reflective, white-colored sheets help to keep the concrete cool as it cures. Plastic sheets should be thick, at least four-thousandths of an inch; if possible, use sheets that incorporate glass fibers for reinforcement.
Plastic sheets work very well with complex shapes, so they may be ideal for certain kinds of concrete projects. However, they leave marks and discolor the concrete, so choose another option if the final appearance is important to you. The most effective way to cure flat concrete in warm conditions is to spray it with water, to keep it moist, or to apply wet burlap, which must be free from compounds that could potentially cause discoloration.
Plastic will not be your first choice if appearance is critical, but in some situations it is superior to other options. Sprinkling concrete may cost more; because concrete cannot be allowed to dry between applications, it must be continually wetted with sprinklers or wetted at intervals and monitored, which requires more manpower. Fogging concrete with water does not work very well if the temperature is close to freezing. If the temperature is low or your funds are limited, plastic may be the better option. Whether plastic is the best choice depends on the importance of the aesthetics of your project, your budget, the ambient temperature and the shape of the concrete object being prepared.