How to Repair Cracks in Concrete

The wear and tear from foot traffic and the elements takes its toll on walkways and driveways. Concrete can start to crack and crumble, looking unsightly and presenting a potential tripping hazard. Follow these steps to repair cracks in your concrete walkways this weekend.

Step 1: Assess the Damage

Carefully examine the structural conditions of your walkway. In most cases, any damage you can see goes deeper than the surface.

Step 2: Prepare Concrete Cracks for Repair

Thin cracks can be easily patched, but hairline cracks cannot be filled. Wider cracks require that you prepare them for repair. Use a hammer and chisel to chip away at soft or crumbling concrete in and around cracks or crumbling areas. The goal is to undercut the edges of the crack at an angle to make it wider inside than it is at the surface. This helps make the patch more secure so that it lasts longer.

Clean the surface and inside the crack with a stiff brush to remove all loose particles. Try an air compressor or a can of compressed air to blow out particles and dust. Then rinse the concrete well, making sure the surface is damp but free of standing water.

Helpful Tips

Use gloves, goggles and a dust mask when chipping away damaged concrete.

Don’t use a wire brush when cleaning concrete. Bits of wire left behind can cause rust stains.

Step 3: Apply Patching Compound

If you have thin cracks, use a patching compound or masonry crack filler. Apply the crack filler as you would apply caulk, moving along the length of the crack. Use a putty knife or trowel to apply patching compound, following the manufacturer’s instructions. Press it firmly into the crack to force the initial layer into all nooks and crannies. Allow drying time between layers.

Or, make your own patching material. Mix one part cement and three parts fine sand in a small bucket, with enough water to make a stiff paste. In a separate container, mix a small amount of cement with more water — enough to make a cement ”primer.” Paint the inside of the crack with the thinner cement mixture. This will act like a primer or bonding additive. Then pack the paste mixture firmly into the crack with a putty knife or trowel. Level the mixture with a straight-edged concrete-finishing trowel. Let the patch sit undisturbed for about an hour, then go over the entire surface with a float or trowel in a circular motion, blending it well with the surrounding surface.

Helpful Tips

The primer will stain, so avoid getting it on adjacent surfaces. Wash it off promptly if it does.

When applying the patch, make sure you cut deeply into the mixture with your trowel to remove air pockets.

Note that there will almost always be differences in color between the existing concrete and the patch.

Step 4: Patch Large Concrete Cracks

If the cracks in your walkway are larger than 1/4” wide and 1/2” deep, repair the crack in stages. Depending on the concrete patching compound being used, it can take several applications.

Use a putty knife or trowel to apply concrete patching compound, pressing it firmly in to the crack. After smoothing the patch with a trowel, let it set for a couple of hours. Then cover it with a sheet of plastic, weighing the plastic down at the edges with bricks or rocks. Keep the area covered for a week, making sure to give it a little water each day to keep it moist. The patch will set during that time and be ready for resumed traffic.

Step 5: Repair Crumbling Edges

Walkways in exceptionally bad shape often have sections with crumbling edges. A patch won’t work on edges. You’ll need something that will support a patch on an edge. Build a form to hold the patch in place. Find a piece of scrap lumber just long enough to span the size of the crumbling area and tall enough that its top edge is flush with the walkway’s surface. Set the board lengthwise against the edge of the walkway and stack several bricks against it at each end, holding the board firmly in place.

First, remove broken concrete and debris from the damaged area with a chisel and hammer and then a brush. Pre-dampen the area, but don’t leave any standing water. Apply vinyl patching compound with a trowel. Tamp the compound down to remove air pockets. Then smooth out the compound with the trowel, using the top of the form as a guide.

Cover the patch with plastic to slow down evaporation. This process, known as slow curing, prevents the patched edge from prematurely cracking. Dampen the surface a couple of times a day and recover with plastic for the next two to three days. When the patch has set for the time recommended by the manufacturer, remove the form.

Well done! You’ve repaired and improved the function and appearance of your concrete.

Project Shopping List

Here’s what you’ll need to complete this project successfully.