How to Fix Cracks in Concrete Slabs

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Few things are more distressing than finding a crack in your concrete driveway, patio, garage floor or other slab surface. Not only are cracks an eyesore, they can lead to even worse damage if left untreated.

Fortunately, most cracks in concrete can be easily repaired if you use the right patching materials and application techniques. In some cases, concrete crack repair can be a simple DIY project. In others, you may need to hire a professional to correct the damage. Here are how-to tips for fixing both wide and narrow cracks in concrete, along with advice on choosing the best tools and repair materials for the job.

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What causes cracks in concrete?

There are many reasons why a concrete slab might crack. But in most cases, the cracks can be prevented by properly mixing and installing the concrete.

The main cause of cracking is shrinkage, which occurs when the freshly placed concrete dries during the curing process and the excess mixing water evaporates. Adding too much water to the concrete mix and inadequate curing are both factors that increase the likelihood of shrinkage cracking.

A lack of control joints is another common cause of cracking in concrete slabs. When properly installed, these joints help to prevent cracking by allowing for the movement of the slab caused by temperature fluctuations and drying shrinkage.

Concrete slabs may also crack due to subgrade settlement, which can occur due to improper compaction of the subgrade or soil erosion. Usually cracks caused by settlement will require removal of the concrete and stabilization of the underlying soil.

Find out more about why concrete cracks and what you can do to prevent cracks from forming.

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Are all concrete cracks worth repairing?

In most cases, yes. Not only for aesthetic reasons, but also to maintain the longevity of your concrete and prevent bigger, more expensive problems in the future.

Although narrow hairline cracks in a slab are usually just a cosmetic issue, if water gets into the cracks and freezes, it will cause internal pressure that may force the cracks to widen. Over time, the expansive pressure from repeated cycles of freezing and thawing can break away the top surface of the concrete, resulting in more extensive concrete repairs.

Of greater concern are deep cracks or cracks over 1/2 inch wide, which may be the result of underlying structural issues. While filling these cracks can be done, the fix is often temporary unless the cause of the cracks is addressed first. Depending on the extent of the problem, it may be more cost-effective to remove and replace the concrete rather than try to repair it.

Tip: If you have extensive cracking in your concrete slab, or the cracks continue to grow or show signs of movement, consult a professional before trying to repair the cracks yourself. An experienced concrete contractor can often identify the root cause of the cracking and recommend the appropriate repair strategies.

How to fix narrow cracks in concrete

If you’re dealing with minor cracks in concrete (no wider than 1/4 inch), here are the basic steps for repairing them using a concrete crack filler.

1. Clean out the crack. Use a wire brush to loosen debris in the crack, then sweep it up and use a wet/dry vac to remove any residual dust.
2. Fill the crack. Working from one side of the crack to the other, squeeze the crack filler into the void until it fills the entire space. With some types of crack fillers, you may need to use a caulking gun to dispense the material. Others are pourable products that flow into the crack and self-level.
3. Smooth the filler. Before the filler begins to cure, use a putty knife or margin trowel to press the material into the crack and to smooth it evenly over the surface. Allow the filler to cure completely before returning the concrete to service.

Keep in mind that while minor cracks in concrete are usually easy to repair, it may not always be possible to disguise your patchwork, especially if you are repairing decorative textured or colored concrete. If aesthetics are a concern, an experienced concrete repair contractor will know how to repair the cracks so they blend in seamlessly with the surrounding concrete surface.

How to repair wider cracks in concrete

If the width of your crack is 1/2 inch or wider, the process of repairing it is more involved and will often require additional tools and materials.

1. Prepare the crack. Use a hammer and chisel to undercut the edges of the crack and form an inverted V-shape. Then clean away loose material as described above. Undercutting the crack to make it slightly wider at the base helps the patching material gain a stronger foothold.
2. Install a backer rod. For cracks over 1/2 inch deep, insert a flexible foam backer rod into the crack to help support the repair and minimize the amount of patching compound you’ll need to apply.
3. Apply a bonding adhesive. Apply a thin layer of latex bonding adhesive to the repair area using a paint brush. This adhesive helps to ensure a strong bond between the existing concrete and the new repair material. Alternatively, you can use an acrylic latex concrete patching compound that doesn't require the use of a bonding agent.
4. Apply the concrete patching compound. Mix the concrete patching compound according the manufacturer’s directions, then use a trowel to fill the crack. Make sure to press the patching material into the crack to eliminate any air pockets. For deep cracks, you may need to apply multiple layers of patching compound. Once the crack is completely filled, use the trowel to smooth the compound evenly over the surrounding concrete surface and achieve a seamless finish.
5. Allow sufficient dry time. When repairing larger cracks, the patching compound can take several days to cure, depending on the product you’re using, the depth of the crack, and weather conditions. Always allow the compound to dry for the full time recommended by the manufacturer before walking or driving on it. To provide extra protection from moisture infiltration, consider applying a concrete sealer over the repaired area once it’s completely dry.

Tips for choosing a concrete crack filler

Selecting the appropriate concrete patching material for the type and size of the crack you’re filling is crucial to achieving a long-lasting repair. Here are tips for choosing one that best suits your needs:

• For narrow cracks, pourable concrete crack fillers that flow into the crack and self-level are easy to use and will ensure that you fill the crack completely. These products often come in ready-to-use in tubes or bottles with a nozzle for precise application. One to try is Quik Fix from Deco-Crete Supply, a fast-setting, high-strength concrete repair material that hardens in as little as 15 minutes.
• For wide or deep cracks, consider using a concrete patching compound reinforced with vinyl resins and fine sand, which adds strength and flexibility to the repair, making it more able to accommodate slab movement than a thinset repair mortar.
• When you shop around for concrete patching compounds, you’ll typically have a choice between a premixed product or a dry mix that requires the addition of water or a bonding agent. The benefit of a premixed product is ease of use because it’s ready for application with no mixing required. A dry-mix patch will require the addition of water, which is less convenient, but you will be able to adjust the consistency by adding more or less water.
• Most concrete patching compounds are formulated for both indoor and outdoor use. But be sure to check the manufacturer’s guidelines for temperature or weather conditions when applying the product outdoors.

Note that while some concrete crack fillers will also work to repair foundation walls and floors, they may not be suitable for leaking cracks. Often epoxy injection is a better remedy for preventing water intrusion and ensuring a permanent fix.

How much does it cost to repair cracks in concrete?

The cost of repairing cracks in concrete depends on a number of factors, including the extent of the cracking, the type of repair material being used, and whether you do the work yourself or hire a professional. Generally, you can purchase a concrete crack filler or patching compound and the required application tools for under $100, which is a small price to pay to prevent further deterioration of your concrete and potential structural issues. However, if your slab has widespread, deep cracking and settlement has occurred, you’re better off spending your money on resurfacing or replacing the concrete, since patching will only be a short-term fix.

More resources:
How to Fix Cracks in a Concrete Driveway
How to Fix Cracks in Stamped Concrete
How to Fix Cracks in Concrete Foundations
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