How to Fix Cracks in Concrete
Concrete is a strong construction material that can bear a great deal of weight, but it can crack over time as a consequence of shifting temperatures. Read our guide below for tips on how to repair cracks in concrete slabs that comprise your concrete driveway, sidewalk, concrete patio, garage floor, or basement. We’ll also help you determine when it’s time to call in the pros for concrete crack repair.
What Causes Concrete to Crack?
As concrete dries, water evaporates from the mixture, causing concrete slabs to develop cracks. Cracking is usually minimal if a professional has mixed and poured the concrete. In this case, slabs will probably only develop small hairline cracks or narrow cracks. If you notice large cracks, there may be issues with the entire slab.
Concrete expands in heat and shrinks in cold, especially in climates that experience significant temperature fluctuations. Tree roots, soil pressure, and settlement can also cause cracks.
Why Should I Fix Concrete Cracks?
If water pools in a concrete crack and freezes, it will expand and further damage the entire slab. Water can also cause the rebar inside a slab to rust. Homeowners should repair the concrete as soon as they notice an issue.
How to Repair a Concrete Crack
Fixing cracked concrete involves determining the size and severity of the crack, getting the concrete ready, filling the crack, and allowing the filler to cure.
Assess the Crack
Examine a concrete crack closely to determine how you should proceed and whether you need professional repair. If you notice the crack is a half-inch wide or smaller, you can consider a DIY approach. Larger cracks will require additional tools and time. If the cracks are very wide, or if there are many cracks, you’ll probably need to call in a concrete contractor.
If you spot a horizontal crack in your foundation, you should call a professional to help with repairs. Foundation cracks are a sign of a larger problem, such as foundation bowing or shifting, and catching them early will help preserve your home’s structural integrity.
Choose Repair Method
The width of the crack will typically dictate the repair method you’ll need to fix the crack. Small cracks are narrower than a one-eighth inch (the thickness of a quarter), moderate cracks are one-eighth inch to a half-inch, and large cracks are wider than a half-inch.
Hairline cracks typically only require sealant to fix, while moderate cracks need to be filled with some type of backer rod. You may need to chisel the space surrounding the crack to remove damaged or loose concrete. Large, wide cracks call for masonry patching compounds that a professional can apply with a bucket and trowel instead of a tube or caulk gun.
You’ll need the following materials, no matter the size of the crack:
- Mason’s trowel or putty knife
- Wet-dry vacuum cleaner
- Wire brush
Here are the size-specific materials you’ll need to fix your concrete crack:
- Small: Epoxy or concrete crack filler, caulk gun
- Moderate: Concrete crack filler, concrete backer rod, masonry chisel, hammer, caulk gun, screwdriver
- Large: Concrete patching compound, bucket, masonry chisel, sledgehammer, sandpaper, masonry paint
Prepare the Concrete
If the cracked concrete is outdoors, wait for a dry day on which the temperature is higher than 50 degrees Fahrenheit. Prepare the concrete surface by sweeping it clean of large debris, then use the wire brush to loosen debris and dust from within and around the crack. Vacuum up the debris with the wet-dry vacuum cleaner.
You may need to widen moderate or large cracks with a chisel to access the deeper parts. Chip down to a depth of about one inch for moderate cracks. Create a V-shaped valley in large cracks so you can lay the patching compound without air bubbles. When you’re done chiseling or chipping, use the wire brush to dislodge the debris, then vacuum again. Errant dust will prevent the filler from adhering properly.
Fill the Crack
You can fill hairline cracks with epoxy once you’ve cleared them of dust. The first step to fill a moderate crack wider than one-quarter inch is to use your finger or the tip of your screwdriver to plant a backer rod. The backer rod will take up space in the crack, which means you’ll need less filler. Apply masonry crack filler with a caulk gun, and push it down into the crack with a putty knife. Use a knife or trowel to scrape off excess epoxy or filler, and make the solution level with the surface concrete.
Large cracks require concrete patching compound, which you must stir or mix with water in a bucket. Mix according to the manufacturer’s instructions, and apply the compound into the crack with the trowel. Use the back of the trowel to press the compound down as you proceed, and use the tip to poke the wet compound to release air bubbles. When the crack is full, smooth the compound surface with the trowel, and feather the edges so that the material blends in with the surrounding concrete.
Let the Filler Cure
Some epoxies dry and harden in as little as four hours, but some concrete patching compounds take several days to fully cure. Read the manufacturer’s instructions, and keep the area dry and unoccupied during curing.
Concrete patching compound in large cracks may shrink as it dries. Use sandpaper to roughen the surface of the cured compound if this happens, and add more of the compound. Once the crack is filled to your satisfaction and the compound has dried, you can cover it with masonry paint to further disguise the repair and seal it against moisture and stains.
Always consult the manufacturer’s instructions when it comes to safely applying sealant and compound materials. Wear safety glasses if you’re using a hammer and chisel to widen a crack, and wear a respirator mask if you think you may be sensitive to concrete dust. Ensure your workplace has good air circulation.
How to Prevent Concrete Cracks
The best way to fix a crack is to prevent it from developing at all. If you’re preparing and pouring the concrete mix yourself, don’t add too much water to the mix or allow it to dry too quickly. You may need to spray the fresh concrete with water to slow down the drying process or cover it with an insulation blanket. Add control joints to driveways and patios, and use wire mesh or rebar for large slabs.
Plant trees and shrubs away from the concrete so that their roots don’t disturb it, and ensure your sprinklers aren’t pointed toward driveways or sidewalks. Apply sealer every three to five years to keep out excess moisture. Finally, ensure the elements of your home’s drainage system, such as gutters, downspouts, and yard grading, work properly to keep water from pooling on or under concrete.
You can and should fill small concrete cracks yourself to prevent them from getting wider. Fixing large cracks and repairing a cracked foundation should be left to the professionals, though, as these jobs frequently require demolishing the existing concrete and pouring fresh material. The cost of a new concrete slab may be high, but it’s worth it for the stability it adds to your home.
Fixing Cracks in Concrete FAQ
Can cracked concrete be fixed?
Yes, small and moderate concrete cracks can be fixed without needing to pour new concrete.
What is the best material to fill cracks in concrete?
The best material to fill concrete cracks depends on the size of the crack. You can fill small to moderate cracks with epoxy or masonry filler, but large cracks require patching compound.
Should you fix cracks in concrete?
Yes, you should fix cracks in concrete as soon as possible, as they can widen if you don’t quickly fill them.