When joining new concrete work to old concrete work, the joint will always develop a crack and will be a point of expansion and contraction. Over time this expansion and contraction, especially if they occur at different rates due to the sections of concrete being exposed to different temperatures, will work the sections apart, and the crack will widen. One way to minimise these separations is to pin the two sections together with rebar. The rebar, or dowels, should be sized for the job. Very heavy and massive pieces of concrete will use heavier gauge rebar, but for typical driveway, sidewalk and step repair, ½-inch rebar is usually adequate.
Drill 5/8-inch diameter holes six inches deep into the old concrete. Make these holes 12 inches on centre horizontally or vertically, depending upon the predominant orientation of the joint. If the joint is wider than it is tall, place the holes horizontally. If the joint is taller than wide, place the holes vertically. Keep the holes at least six inches in from any edges, to avoid breaking chips out of the old concrete.
Flush the holes with water.
Inject epoxy into the backs of the holes. Use enough to fill the holes approximately halfway.
Insert 12-inch lengths of rebar into the holes, twisting them to ensure an even coating of epoxy around their circumferences and along their lengths within the holes.
Push duct tape over the ends of the rebar, putting holes in the pieces of tape, and slide them against the holes in the concrete to prevent the epoxy from dripping out while curing.
Paint the portion of the rebar extending from the holes with a metal primer. This will help reduce rusting.
Pour the new concrete so it flows around the rebar pins.
Adjust the sizes of rebar and the depth and number of holes to match the size of the job.
Wear gloves, a dust mask and boots when working with concrete. For structural repairs on buildings and in situations where a concrete repair failure will be dangerous, consult with an engineer for the proper design. Follow all local building codes.