A concrete slab is one of the most stable and long-lasting floor surfaces there is, but over time, the slab will inevitably develop small cracks and chips. These minor problems are entirely natural, and they generally do not indicate any structural problems that require resurfacing or major repair. Still, is best to tend to cracks as soon as possible. Once a small crack or chip appears, future wear and tear will only cause it to expand.
The method described here is suitable for filling cracks 1/4 to 1/2inch wide. Larger cracks or holes require a more substantial repair process.
Before You Begin
There are a variety of products available for fixing cracks and other small flaws in concrete slabs, ranging from simple caulks containing calcium and silicone additives, to vinyl concrete patch that contains special additives designed to help the patch material adhere. A self-leveling concrete sealant can be used to repair cracks and yields a nice finish when applied correctly. Our method, however, is a time-tested repair that uses ordinary sand mix concrete, plus a brush on latex bonding liquid that helps the concrete patch adhere to the old concrete.
What Is Sand Mix Concrete?
There are many dry concrete mix products available at home centers, often sold in 40- or 60-pound bags that contain a mixture of Portland cement powder, mineral aggregate, and sometimes additional additives to give them unique performance properties. They are generally categorized according the coarseness of the aggregate they contain. "Sand mix" uses moderately fine particles of sand as the mineral aggregate, falling midway between very fine masonry mortar used for bonding bricks, and much coarser concrete gravel mix that is used to form structural slabs and steps. The relatively fine consistency of sand mix allows the wet concrete to be pushed into fairly small cracks to form a good bond with the surrounding concrete.
Equipment / Tools
- Wire brush
- Shop vacuum
- Scrub brush (if needed)
- Masonry chisel
- Safety glasses
- Margin trowel
- Concrete bonding adhesive
- Sand mix concrete
- Concrete cleaner or dish soap (if needed)
Clean the Crack
To ensure your repair work holds well, the area must be very clean. With a wire brush, clean out the crack. Sweep away any loose debris, then vacuum the crack. If grease, oil, or dirt remains in or near the crack, clean it thoroughly with a commercial degreasing product, or with ordinary dish soap and a stiff brush.
Undercut the Crack's Edges
Once the area is clean, use a masonry chisel and hammer to slightly widen and undercut the edges of the crack. This will allow you to force the patching material down into the crack and under the edges, which will help hold the patch in place as it dries.
Be sure to wear eye protection while chiseling concrete; small particles can easily break free and fly through the air.
Apply Bonding Adhesive
With a small paintbrush, coat the crack surface with a thin coat of concrete bonding adhesive. Allow the adhesive to dry completely; you will know it's dry when it transforms from a whitish liquid to a clear coating. Make sure bonding liquid in the depths of the crack has dried fully. This can take at least an hour.
Prepare the Sand Mix
Mix a suitable amount of dry sand concrete mix with water as instructed on the bag. The consistency should be very pliable, but not runny.
Fill the Crack
With a pointed trowel, force the patching concrete firmly down into the crack, making sure to fill it as fully as possible. Screed off the excess concrete with the edge of the trowel, then smooth the surface with the flat surface of the trowel.
Let the Patch Dry
Allow the patch to cure undisturbed as instructed on the bag. This generally takes a full 24 hours, but there are also fast-setting formulations that set up in less than 1 hour.
Seal the Concrete (Optional)
Despite their apparent hardness, concrete slabs are actually more porous than you think, and over time, water can penetrate the slab to cause more cracks to form. You can reduce cracking by periodically recoating the entire slab with a clear concrete sealer or heavy-duty polyurethane coating. If you don't want to tackle the entire slab, then sealing just the patched cracks will help them resist moisture penetration.
In high-traffic areas, plan on applying at least three coats of sealer, allowing each coat to dry fully before applying the next coat.
Painting the slab with concrete-rated floor paint also creates a waterproof barrier.
When to Call a Professional
Occasional cracking is common in nearly all concrete slabs as they age, and such cracks are usually not a cause for great concern. However, cracks that create an obvious vertical offset, where one side of the crack is noticeably higher than the other, may be a sign the the base layer beneath the slab is settling unevenly. If you notice this, you may want to call in a concrete professional to examine the slab. In some cases, injections of slurry or foam beneath the slab can halt the settling and prevent more expensive repairs in the future.