Concrete Slab Crack Repair

Safety Tip: Wear gloves when repairing concrete, and goggles when breaking up a damaged surface.

Surface Preparation. Remove damaged concrete and all dirt, debris and standing water, and keep the area damp for several hours — preferably overnight.

Filling Cracks. For cracks up to 1/8 inch wide, use a latex or epoxy patching compound. Force it into the crack with a putty knife or a mason’s trowel and smooth it level with the surrounding concrete. Mend larger cracks with mortar prepared without lime. You can also use a latex or epoxy patching compound and apply it in a similar way.

Spalling. Surfaces on which the concrete has flaked off in thin scales — a condition called “spalling” — are best patched with an epoxy patching compound.

Curing. A mortar patch must cure slowly and in the presence of moisture. Let the patch set for about two hours, then cover it with a sheet of plastic. On a horizontal surface, the cover can be secured by bricks or rocks; for vertical surfaces, use tape. Over the next three days, lift the cover daily and sprinkle water on the patch. If a vertical patch cannot be covered conveniently, moisten it twice a day. For latex and epoxy compounds, check the product label for curing instructions.

Repairing a Chipped Edge

Clear the Damage. With a cold chisel and a ball-peen hammer held horizontally, chip off the damaged concrete all the way across the edge of the step.

Undercut the Groove. Holding the chisel at an angle, chip away enough of the edge to make a V-shaped groove. Clean away the debris and keep the edge damp for several hours, preferably overnight.

Rebuild the Edge. Cut a board to the length and height of the riser and set it against the step as a form board. Hold it in place with bricks or concrete blocks. Mix 1 part Portland cement with 3 parts masonry sand, then add just enough water to make a mortar paste that holds its shape. Coat the groove with cement paint, then immediately fill it in with the mortar, using a steel trowel to shape it and smooth it flush with the step and form board.

Finish the Job. After the mortar is thumbprint-hard, round the edge of the step with an edger, then carefully remove the form board. Let the mortar cure for at least three days, then avoid stepping on the edge for a few more days.

How to Fix Broken Concrete Steps

Glue the Chip. Brush particles of dirt and cement from the broken piece and the corner of the step. Mix a small batch of epoxy patching compound for concrete; then, with a mason’s trowel, spread some onto the chipped part of the broken piece. Hold the piece firmly in place until the compound hardens; you can prop a board against the piece to hold it in place.

Complete the Job. After the compound has set, use a putty knife to scrape away any excess that has oozed out between the piece and the step. If a small crack remains around the repair, pack patching compound into the crack with a trowel, then smooth the patch level. Avoid touching the repaired corner for at least 24 hours.

Masonry Restoration: Rebuilding a Corner With Glue

Shape a Replacement Piece. Clean the corner and keep it damp for several hours, preferably overnight. Mix 1 part Portland cement with 3 parts masonry sand and add just enough water to make a mortar paste that holds its shape. With a mason’s trowel, apply the mortar to the damage, roughly shaping it. Let the patch harden until it’s firm enough to hold a thumbprint.

Finish the Corner. Finish and smooth the corner flush with the steps with a steel trowel. Let the mortar cure for at least three days, and avoid putting weight on the corner for a few days afterward.

Filling Cracks in Concrete

Remove Damaged Concrete. With a cold chisel and a ball-peen hammer, chip away all cracked or crumbling concrete to about 1 inch below the surface.

Undercut the Edges. To provide a better bond and keep the patch from heaving upward after the job is done, undercut the edges of the crack: Chisel at an angle to make the hole wider at the bottom than at the top. Remove all rubble and dirt. Soak the crack with water for several hours. If possible, run a trickle from a garden hose over it overnight.

Prepare the Crack. Prepare the mortar by mixing 1 part Portland cement and 3 parts masonry sand, adding enough water to make a paste stiff enough to work with a mason’s trowel. Make a small batch of cement paint by adding water to Portland cement until it has the consistency of thick paint. Coat the edges of the crack with the cement paint, then proceed immediately to complete the repair before the paint dries.

Mend the Crack. Pack the mortar firmly into the crack with a mason’s trowel, cutting deep into the mixture to remove air pockets. Level the mortar with a steel trowel. Let the patch stand for an hour, then spread it evenly across the surface, sliding the trowel back and forth with its leading edge raised.

Treating Palled Surfaces

Use an Epoxy Mix. Break up large areas of scaling concrete with an 8-pound sledgehammer. (Don’t slam the tool against the surface; let its own weight provide the force.) A rotary hammer with a bush hammer head is an alternate tool for the job. For small areas, a ball-peen hammer and a cold chisel will be adequate. Sweep up dust and debris, and dislodge small fragments with a stiff wire brush. Soak the damaged area with water and keep it wet for several hours, preferably overnight. The area should still be damp when you apply the patch. Prepare a commercial epoxy patching compound for concrete and apply it with a steel trowel. Bring the new layer level with the surrounding concrete, and feather it thinly at the edges. Let the patch stand for 24 hours before letting it support any weight.

This excerpt has been reprinted with permission from Masonry: The DIY Guide to Working with Concrete, Brick, Block and Stone by John Kelsey and published by Fox Chapel Publishing, 2012.