Timely concrete repairs may prevent water intrusion and extensive damage, enhance the appearance of surfaces, and correct potential safety hazards. Concrete patches are hard to disguise, but see How to Improve the Look of Repaired Concrete for a solution.
Preparing the surface
Whether you're repairing a walkway, patio, driveway, basement or other surface, remove cracked and loose materials with a stone chisel and/or wire brush.
Use a grinder with an abrasive masonry wheel to widen hairline cracks for filling.
Clean the surface with a hose and nozzle, or a pressure washer.
Use a shop vacuum to clean out cracks and holes.
Dampen the surfaces with a hose or spray bottle, and brush off any standing water.
Filling a small crack with pourable grout
Cut the tip off the grout container to create a hole a little smaller than the width of the crack. Pour in the grout to fill the crack.
For deep cracks, pour in the grout one layer at a time and allow complete drying between layers.
Filling a large crack
For cracks wider than about 1/2 inch (12 mm), use a mason's hammer and chisel to cut the crack so it is wider below the surface than at the surface, a process called undercutting. Undercutting prevents the patching material from being forced out in response to temperature changes.
Mix vinyl concrete patcher with water as directed. Press the material into the crack with a mason's trowel in layers no more than 1/4 inch (6 mm) thick. Allow each layer to dry completely before applying the next.
Using a steel trowel, wood float (a wood block with a handle), broom or brush, finish and texture the surface to match the surrounding area. A steel trowel gives the smoothest surface; hold it flat and use a swirling motion.
Repairing a broken step
Apply a concrete bonding adhesive to surfaces you'll be covering with new material. Use an old paintbrush to work the milky liquid into the surface. This extra precaution is worth taking, considering what could happen if the patch were to fail and break off underfoot. The primer or adhesive will stain adjacent surfaces, so mask them with tape.
Wash the brush immediately after use with soap and water, or discard it.
Mix an already fortified concrete mix (typically a nonsagging formula designed specifically for this type of repair) with precisely the amount of water indicated by the manufacturer. If you'll be applying the materials in layers, mix only what you can use in each layer. Or use a latex primer or additive in lieu of water if you choose a standard sand or concrete mix. Measure quantities carefully. Otherwise, concrete can fail.
For shallow or small repairs, use a mason's trowel to apply the patch mortar. Press firmly to work the initial layer into the surface for a good bond.
Allow each layer to dry completely before applying the next one.
For large repairs, screw together a wooden form and hold it in place with wooden braces, bricks, concrete blocks or similar heavy objects. Then apply the fortified concrete in layers as directed by the manufacturer until the form is filled (see A). Again, work in the initial material well to eliminate all voids.
When the patch is firm but not yet dry, remove the form by taking out the screws and carefully lifting off the pieces.
Finish or texture the repair to match, and feather it into the surrounding area with a wood float or stiff brush (see B). Use an edging tool in a back-and-forth sanding motion to round over the corners, if that's how the existing nosing on the tread is finished.
Cover the repair with plastic to retard evaporation and slow curing, which helps prevent cracking. Keep the surface damp for the next two or three days by misting it with water and keeping it covered.