What Causes Voids Under Concrete Slabs?

What Causes Voids Under Concrete Slabs?

What Causes Voids Under Concrete Slabs?

There are many potential causes of voids under concrete slabs. The most common causes are;


Excess water surrounding a concrete slab causes soil erosion and, eventually, voids. Drainage problems and faulty plumbing are two typical causes of erosion.

Soil Settling

Another source of voids under a slab is soil settlement. Before you set something heavy on top of the soil, it must be adequately compacted. If the soil is not compacted, it will compress unevenly when something substantial, such as a concrete slab, is placed on it. This, along with inadequate drainage, guarantees void development. Drainage issues can also be caused by settlement.

Thaw/Freeze Cycles

Temperature fluctuations throughout the year create freeze/thaw cycles, which lead to erosion and void development.

Clogged Gutters And Short Downspouts

Excess water under the slab might cause erosion if it does not drain properly. Clogged gutters and downspouts that are too short to move water away from the slab are two causes of excess moisture.


When the soil dries out, it shrinks, and this shrinking frequently results in voids.

Hydrostatic Pressure.

When there is too much water under the slab that cannot drain away, hydrostatic pressure builds up and pushes on the slab. As a result, it may break and become uneven.

Poor Maintenance. 

Vacancies can also be caused by poor maintenance procedures. For example, if you don’t repair fractures in the slab quickly away, the rain will get beneath it and cause erosion.

Do I Need To Reinforce The Concrete Slab?

Finally, the answer relies on the type of weight or traffic that the slab is anticipated to withstand. If a slab is just used for foot traffic, such as a walkway, steel reinforcing is unnecessary. Many sources state that the minimum thickness of a slab before requiring reinforcement is 5 inches.

Larger, deeper slabs meant to support some of the weight of a building will require steel rebar mesh to strengthen and disperse the load.

To validate this, it is advised that an engineer be engaged in the design of a slab. By using a rebar guide, the final placement can be determined and the installation of reinforcement can then be conducted to ensure proper support.

Steel reinforcement has been an essential aspect of the concrete building during the last century or so, and it is impossible to fathom what our urban landscapes would look like if reinforced concrete had not become the standard.

While new innovations in reinforcement are occurring at a dizzying pace, reinforcing steel appears to be destined to stay at the core of concrete buildings for the foreseeable future.

How Do You Fill A Void Under A Concrete Slab?

Polyurethane foam injection is a good method for filling voids beneath concrete slabs. It spreads out and swiftly expands to cover any spaces as soon as it is injected beneath the slab. This cavity filling foam also compacts any dirt, rock, or organic material beneath the slab. The slab is supported, lifted, and leveled using polyurethane void-filling foam.

Drilling penny-sized holes into the slab is the first step in the operation. After that, the voiding filling foam is pumped through the perforations and beneath the slab. As the foam expands and spreads, it not only fills any existing cavities but also lifts and returns the slab to level conditions.

Polyurethane foam, unlike cement slurry used in mudjacking, will not degrade, shrink, or settle over time, and soon as the repair is done, you can start using the slab again.

Mudjacking is an earlier method of filling a gap beneath a concrete slab that uses a cement slurry (cement, sand, and water) rather than polyurethane foam. It is no longer as popular as it once was, but it is still in use today.

The mudjacking process entails drilling quarter-sized holes in the slab (as opposed to the penny-sized holes used in polyurethane foam injection) and then injecting the slurry through the holes.

While mudjacking is less expensive than polyurethane foam injection, it does not expand rapidly enough to fill all of the spaces beneath the slab. As a result, you might expect some voids to remain when the repair is completed.

Another disadvantage of mudjacking is that the concrete slurry requires time to cure, often 2-3 weeks. As a result, once the repair is completed, you will not be able to use the slab immediately.

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