You may have seen fresh concrete covered with plastic while it is curing. Covering the curing concrete with plastic keeps it cleaner, but there is a more important purpose behind this practice. Water is mixed into concrete to activate the cement binding agent and as the mix dries, it hardens. The drying, or "curing," should be gradual, otherwise cracking may occur. To prevent cracks, plastic is placed over the curing concrete to trap the water inside and regulate its temperature, ensuring gradual curing.
How to Use Plastic to Cure Concrete
You can purchase an insulated concrete curing blanket from most DIY stores, but it's cheaper to simply use a sheet of plastic. Choose polyethylene plastic that is at least 4 millimeters in thickness. Because temperature impacts the curing process too, use dark plastic if the temperature is below 60 degrees Fahrenheit to absorb heat and keep the concrete warm while it cures; if temperatures are above 85 degrees Fahrenheit, use a white or metallic sheet that reflects light from the sun to keep the concrete cooler. For temperatures in between, use clear plastic sheeting.
While plastic sheeting is highly effective in helping concrete cure without cracks, there is a downside: The concrete will be discolored anywhere the plastic touches it. If you plan on using an opaque stain on your concrete after it cures, this doesn't matter; if not, tent the plastic so it doesn't come into direct contact with the concrete. You can build a framework around the concrete slab with lumber, PVC pipes, or rebar to hold the plastic sheeting. Another method to prevent discoloration is to place dry straw directly onto the concrete, then put the plastic on top of it.
Wet the concrete, then place the plastic sheeting over it. Use bricks, boulders, or stakes to hold the plastic in place and keep small animals out. Once a day, remove the plastic sheet and wet the concrete, then replace the cover. Do this for a week for the best results.
It technically takes 28 days for concrete to dry and cure, but as Aurora Paving points out, "For practical purposes, we recommend that car traffic can generally enter onto concrete slabs after three days and truck traffic after seven days."
Adding Control Joints to Further Prevent Cracking
There is another step to take to help concrete cure without cracks: adding control joints.
Control joints are grooves inserted into a concrete surface to control cracking. Essentially, this groove is an intentional, controlled crack inserted to prevent the concrete from cracking on its own in an uncontrolled manner. By placing control joints in the concrete before it cures, any subsequent stress the concrete is subjected to will not produce haphazard cracks that will be a landscaping eyesore.
To add control joints to your concrete, use a trowel or jointer to cut an even, straight control joint. The joints should be cut to a depth of one-quarter of the slab's thickness (for example, if your concrete slab is 4 inches deep, you'll cut a 1-inch control joint).
By using both control joints and plastic during the curing process, you can minimize cracks in your newly poured concrete.