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Slab Repair: How To Fill A Void Under A Concrete Slab

Table of Contents

1. Causes Of Voids Under Concrete Slabs
2. How To Fill A Void Under A Concrete Slab: Foam Vs Mudjacking
3. How To Fill A Void Under A Concrete Slab: Other Methods Of Slab Repair
4. How To Prevent Damage To A Concrete Slab

Looking for information on how to fill a void under a concrete slab? If so, you’ve landed on the right page. Voids under sidewalks, driveways, pool decks, and other slabs are common and can often lead to cracks and unlevel concrete. Voids are usually caused by problems with the soil under the slab. In this article, we will talk about why voids happen, how to fill a void under a concrete slab, and how to prevent voids from forming.

Causes Of Voids Under Concrete Slabs

Voids under concrete slabs are caused by various things including:

  • Erosion – Excess water around a concrete slab leads to soil erosion and then voids. Two common causes of erosion are drainage issues and leaky plumbing.
  • Soil settlement – Soil settlement is another cause of voids under a slab. Soil needs adequate compaction before you place something heavy on top of it. If it’s not compacted, the soil will compress unevenly after something heavy – like a concrete slab – is placed on it. This, combined with poor drainage, is a sure recipe for void formation. Settlement can also cause problems with drainage.
  • Freeze/thaw cycles – Seasonal temperature changes cause freeze/thaw cycles that contribute to erosion and void formation.
  • Clogged gutters and short downspouts – Excess water under the slab can lead to erosion if it cannot adequately drain off. Two causes of excess moisture are clogged gutters and downspouts that are too short and therefore unable to channel water away from the slab.
  • Drought – Soil will shrink if it dries out, and soil shrinkage often leads to voids. For more information see California Drought And Your Home’s Foundation.
  • Hydrostatic pressure – When too much water under the slab can’t drain off, hydrostatic pressure will build up and push against the slab. This can cause it to crack and become unlevel.
  • Poor maintenance – Poor maintenance practices can also cause voids. For example, if you don’t fix cracks in the slab right away, rain will get under the slab and this could lead to erosion.
  • Burrowing critters – Even burrowing animals can contribute to void formation.

How To Fill A Void Under A Concrete Slab: Foam Vs Mudjacking

Void filling foam

Polyurethane foam injection is an excellent way to fill a void under a concrete slab. As soon as it’s injected under the slab, it spreads out and rapidly expands to fill all voids. This void filling foam also consolidates all soil, rock, and any organic material under the slab. Polyurethane void filling foam supports, lifts, and levels the slab

The procedure involves first drilling penny-sized holes into the slab. The voiding filling foam is then injected through the holes and under the slab. As the foam expands and spreads out, it not only fills any existing voids but lifts and brings the slab to level like conditions as well.

Unlike the cement slurry using in mudjacking, polyurethane foam won’t erode, shrink, or settle over time, and soon as the repair is done, you can start using the slab again. No downtime.


Mudjacking is an older method for filling a void under a concrete slab and uses a cement slurry (cement, sand, water) instead of polyurethane foam. It’s not as popular as it once was but is still being used today.

The mudjacking procedure involves drilling quarter-sized holes (compare with the penny-sized holes used in polyurethane foam injection) in the slab and then injecting the slurry under the slab via the holes.

While mudjacking is less expensive than polyurethane foam injection, it doesn’t quickly expand to fill all the voids under the slab. So, you can assume that there will still be some voids left after the repair is complete.

Another drawback to mudjacking is that the concrete slurry needs time to cure, approximately 2-3 weeks. Therefore, you won’t be able to immediately start using the slab again once the repair is done. Compare with polyurethane injection, where there’s no downtime.

How To Fill A Void Under A Concrete Slab: Other Methods Of Slab Repair

You can also fill a void under a concrete slab by either digging up and replacing part of the slab or replacing the entire slab.

Replacing part of the slab

This method of repairing a void under a concrete slab involves tearing up one or more sections of the slab to access the voids. The voids are then filled with a mix of sand and concrete, and then new sections of the slab are poured.

Replacing the entire slab

If there’s a lot of damage, it might be necessary to dig up and replace the entire slab. While replacement is one of the most costly ways of repairing a slab, this may be the last resort option when there’s extensive damage.

Of course, digging up and replacing a slab is a time consuming procedure because freshly poured concrete needs around 2-3 weeks to dry and harden. During this time the slab will be out of commission. You won’t be able to use it.

How To Prevent Damage To A Concrete Slab

After the slab is repaired (or replaced), you can prevent future problems by ensuring good drainage in the soil under the slab. Water needs to drain away from the slab, not toward it. You can make sure this happens by regularly cleaning gutters, installing downspouts that are long enough to direct water away from the slab, and by regrading your yard, if necessary. You might also possibly consider installing a French drain to prevent water from pooling near the slab.

Voids under concrete slabs such as sidewalks and driveways can lead to cracks and other problems. Fortunately, voids under a slab can be filled in just a few hours using polyurethane foam injection.

If you have a cracked, uneven sidewalk, driveway, pool deck, or some other type of concrete slab and you’re in our Northern California service area, contact us today for a free inspection and estimate.

Steve Egloff

Steve Egloff

Founder of Bay Area Underpinning

Steve is the Founder of Bay Area Underpinning, a foundation repair company serving the San Fransisco Bay Area, California. Bay Area Underpinning was founded in 2005 with the goal of providing a cost-effective, engineered solution to foundation settlement problems with an emphasis on educating customers to make them feel comfortable with the various methods of foundation repair.

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Disclaimer: “Concrete leveling” means the process by which cracked, uneven concrete is stabilized, and in many cases lifted, by means of PolyRenewal™ polyurethane foam. Bay Area Underpinning does not guarantee that PolyRenewal™ can make your concrete perfectly level.

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