Pouring new concrete over existing heaved concrete can be a way to increase the lifespan of your concrete sidewalk, driveway or patio. With the proper preparation, the concrete overlay will be as strong as the ordinary application of concrete. Tree roots, land settlement, and the formation and melting of ice beneath the concrete surface often cause concrete heaving. Although nothing may be done for the cause of the heaving, there is no reason that the concrete should be removed from the area.
Determine the amount of new concrete you need. The exact amount depends upon the amount of heaving there is in the existing concrete. The new layer of concrete should be at least 1 to 2 inches thick over the highest portion of the heaved concrete. This may require more than the minimum amount of concrete in other areas.
Clean the surface of the heaved concrete thoroughly with a high pressured water hose and a stiff broom. Scrape loose areas of concrete with a shovel to remove them. Also remove any large chunks of loose concrete from the area. Large holes need to be filled in with the new concrete.
Dig a 6-inch wide trench around the entire heaved concrete surface with a shovel. Position 2-by-4 form boards around the concrete surface. Hold them in place by hammering wooden stakes against the outside of the boards at 3-foot intervals and at each joining location of the boards.
Locate the existing expansion control joints and mark their location on the form boards. The joints in the new concrete should be made in the same places. Paint the inside of the form boards with vegetable oil to keep the new concrete from sticking to them.
Apply a coat of concrete bonding adhesive to the heaved concrete surface with a paint roller. Adhesive should ideally be the thickness of a coat of paint. The new concrete can be poured as soon as the adhesive is dry, which can take from two to six hours.
Pour the dry concrete mix into a wheelbarrow and slowly add approximately 6 pints of clean water, mixing with a shovel as you add the water. The proper consistency of the concrete is almost stiff, but still workable and mouldable.
Spread the prepared concrete over the heaved concrete surface, pressing down on it with a shovel to pack it into the corners of the form boards and into the cracked and heaved areas of the existing concrete.
Cut the expansion control joints while the concrete is still wet, but is firming up. Pull a chalk line at the locations of the previous joints and use a concrete grooving tool to carve the new joints. Hold the grooving tool at a 45-degree angle and press it into the wet concrete to a depth that is approximately 25 per cent of the total thickness of the new concrete.
Allow the concrete to dry for four to six hours before walking on it. The concrete fully cures within 72 hours, when the form boards can be removed.