How To Fill Void Under Concrete Slab

How To Fill Void Under Concrete Slab
  • Author: Amanda Arnold
  • Posted On: May 17, 2022
  • Updated On: August 21, 2023

Many factors can generate voids in concrete slabs, from sliding soil beneath the slab being displaced due to a lack of adequate compression to water erosion progressively washing away the soil over time. Voids can cause a slab weakness, resulting in surface dips, tilting, fractures, and breaks.

You may look into do-it-yourself alternatives, talk to local foundation professionals, or put off the work. Try to fix voids soon if you know what happens and cannot solve foundation issues. This article will explain the origins of voids under concrete slab foundations and how to repair them.

Why Is It Important To Fill Voids Under Concrete Slabs?

The obvious truth is that voids beneath concrete slabs are quite hazardous. At the absolute least, avoiding this might result in uneven flooring, bowls, and dips. However, if things go wrong, it can result in tilting or leaning building walls, foundation disruption, and even collapse of the walls.

Big holes under large concrete slabs can, in the worst-case scenario, cause entire buildings to collapse. If you notice any signs that there may be voids beneath your concrete slab, you must move quickly to fix the problem.

What Are The Reasons For Voids Under Concrete Slab?

Moisture is the primary cause of voids beneath a slab foundation. These factors are unavoidable in some situations. You can, however, help prevent foundation damage in some circumstances.

Poor Drainage Channels

If your slab foundation wasn’t designed to drain water away from it, there’s a higher potential of topsoil erosion, which can lead to a void beneath it. The loss of structurally significant foundation soil can be caused by poor drainage systems and inappropriate foundation slopes.

This frequently results in voids forming beneath concrete slab foundations. The growth of voids can also be aided by faulty under-slab plumbing. Soil erosion will be exacerbated by water accumulation from leaking pipes beneath building structures.

Soil Or House Settling

Cracks in your walls and ceilings might be caused by normal house settling, but they can also be signs of more significant issues. Significant soil settlement might result in voids beneath concrete slabs. The erosive nature of improperly drained water hastens the development of soil settlement.

If there isn’t enough drainage, excess water under the slab can cause erosion. Clogged gutters and downspouts that are insufficiently short of draining water away from the slab are two common sources of excessive moisture.

Steps To Fill Voids Under Concrete Slab

Let’s get started by going over how to fill a void under a concrete slab in detail, step by step. Remember that we’re dealing with small to medium-sized slabs in this scenario. If you discover a void under your house foundation or another structural slab, you should get professional assistance.

Collect Needed Materials

Assemble the following items: a drill, a masonry drill bit, a slab jacking pump, cement, fly ash, sand, and concrete mortar. To complete this mission, you’ll need these resources and things. If you don’t have these materials in your collection, rent out all of them from a store instead of buying them.

Drill Holes In Concrete Slab

Drill at least four holes in the slab with a drill equipped with a masonry bit. Depending on the size of the slab, the number of holes may vary. A smaller slab must have at least one or two holes in the middle. On the other hand, larger slabs may require three holes.

The distance between your holes is also affected by the slab thickness. The holes are usually spaced three to eight inches apart and are roughly similar in size.

Prepare Filling Material

Use a sand-cement grout mixture to fill voids. Because sand is easier to compact than pea gravel, it is the ideal choice. This is also the most. Although this is a cost-effective fill material, it has the disadvantage of allowing affordable void penetration and requiring a large amount of time for the filled concrete to cure.

In a wheelbarrow, mix the fill material with a shovel. You can use an expanded to keep the material from shrinking during the curing process.

Fill Jacking Pump

All that’s left is to load the slab jacking pump with filler material. Set the slab jacking pump to 10 psi. Pump the filler material into the voids beneath the slab using the slab jacking pump and the attached hose.

Switch the hose between the numerous holes you drilled in the second step while continuing to turn the slab jacking pump off. Continue alternating between the numerous drilled holes to fill the vacuum as evenly as possible.

Once the filler material reaches the slab’s bottom, the job is more or less finished. When the filler material reaches the bottom of the drilled holes, you know the void is full.

Level The Filled Voids

Finally, combine a small amount of concrete mortar with water to fill the drilled holes. To make the surface look new, use a trowel to level it out. The task should now be completed.


Cracks and other issues can result from voids under concrete slabs such as sidewalks and driveways. Fortunately, gaps beneath a slab can be filled with polyurethane foam injection in just a few hours.

To avoid voids, make sure your concrete has a good drainage system and that the supporting soil is compacted well. This will stop water from absorbing into the concrete and causing voids.

Everything you need to know about filling a hole beneath a concrete slab: again, voids under concrete slabs are quite dangerous, especially if there are walls and additional levels above; therefore, you must address the problem as soon as possible.

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Author: Amanda Arnold

Amanda has been working with ConstructionHow since 2021. Her experience spans over 5 years in the creative niche such as home decor and trends, landscaping, renovations, and custom architectural values. As a home designer expert, she has a keen eye for the latest home improvement trends with accurate facts that readers find impossible to ignore. Being invested in home-building trends is how she has gained her lucrative expertise exploring more to bring a positive ambiance for all homeowners (and even tenants!). Currently, she lives in a beautiful beach home, a source of fascination for her.