Tough as it is, concrete doesn’t hold up forever, especially if abused. When it does break, it’s usually along an edge or at a corner, areas that are exposed to the most impact and wear.
Fortunately, edges are easy to fix, and if you go about the job correctly, your repair should be as strong as the existing concrete. Small repairs: If your repair is fairly small – something like a chipped corner – you won’t need to use any forms or any special reinforcement. Start by chipping away all loose concrete. Use a cold chisel, a hammer and an aggressive attitude. Make sure you remove anything loose or flaking, all the way back to solid concrete. You can use a special patching cement if you like, or just mix up a batch of mortar, made of three parts sand and one part portland cement. The smaller the repair, the more sense it makes to use special patching cements. Small repairs made with ordinary cement tend to dry out too quickly and thus lose most of their strength. Special patching mixes contain synthetic additives that largely eliminate this problem. If you use the patching cement, simply follow the directions on the label.
If you mix your own mortar, use just enough water to make a firm paste that will hold its shape. Set this aside for a minute and mix up a creamy slurry of pure portland cement and water. Dampen the exposed surface of the old concrete with water, and then brush on a thin coat of the cement slurry. This will act as a bonding agent and help assure a strong repair. Quickly, before the slurry can dry out, trowel on your mortar, shaping it to match the original corner. Press the mortar down firmly to assure good contact, but don’t worry about shaping things perfectly at this point. Let the mortar set from one to five hours – until it retains a firmly pressed thumbprint. Now, use your trowel to do the final shaping. Let the mortar cure for at least a week, keeping it damp the whole time. If the mortar dries out prematurely, it will lose much of its strength. So sprinkle it lightly every hour or so for a few more hours; then, cover with damp rags and keep these soaked for a full week.
Larger repairs: These will probably require both forms and reinforcements like those shown in the sketch. Start as above, by chipping away all loose concrete. Then lay out one or more reinforcements. One-quarter or 3/8-inch lag bolts or L-screws (like those shown) work well. Space them every 10 inches or so. To set these, drill into the existing concrete with a carbide-tipped bit just a bit larger than the diameter of your reinforcement. Drill in to a depth of about 2 inches, but don’t place the reinforcements quite yet. Instead, set your forms. These need not be elaborate. A piece of 3/4-inch scrap pine or plywood, secured with a couple of stakes, will do the job. Coat the face of the form with some oil before you place it.
For larger repairs like this, ordinary concrete mix is cheaper and stronger than mortar. Mix this with water, following the directions on the sack. There’s always a temptation to use too much water so the mix is easy to pour and work with, but you’ll get a stronger mix if you use only as much water as needed to make a workable mix. When your concrete is ready, mix up a batch of the creamy cement-water slurry described above. Work some of this into the holes you have drilled in the slab, dip the ends of the reinforcers into the mix, and them tap them into their holes. Dampen the edge of the slab slightly, then brush slurry onto the exposed edge to serve as a bonding agent. Quickly pour your concrete, scrape it down level with the rest of the slab and trowel lightly. Let the repair set up until it will hold a thumbprint, as described above. Then trowel again, trying to match the texture of the surrounding concrete. A wooden trowel or float will give a fairy rough texture. A magnesium trowel produces a smoother surface, and a steel trowel creates the smoothest surface of all. For a striated surface, brush lightly with a broom. Again, keep your repair damp for at least a week. Then remove the form and the job is finished. Avoid any stress on the repair for another week or two.