How to Fill a Void Under a Concrete Slab Foundation

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If you’re looking for a how to fill a void under concrete slab foundation method, you may have come to the right place. There are various methods, including urethane foam, sand, or a cement grout mixture. We’ll discuss each strategy below. To begin, you should pump the material through the holes at a pressure of 10 psi. Next, switch hoses and repeat the process until the void is filled.

URETEK’s polyurethane foam fills voids

Polyurethane foam is a highly effective method for stabilizing concrete slab foundations. Injecting this material through tiny holes in the foundation walls, the foam expands to fill the voids and seal them. This process is also beneficial for discrete sections of damaged slabs. While it’s possible to repair a gap by breaking up the concrete over the affected area, the void is very likely to reform.

Another method of lifting concrete is called mud jacking. It involves strategically drilling holes in the concrete slab foundation and injecting the material into the voids. The process typically takes 24 to 48 hours and costs 25% to 50% less than replacing the concrete slab foundation. This method is also significantly less expensive than URETEK’s polyurethane foam in filling voids under the concrete slab foundation. However, it is slower and requires more cleanup than foam. In addition, it creates one to two-inch holes in the concrete slab, which means that weeds can grow through it.

Sand can be placed under concrete

To fill a void in a concrete slab foundation, you must first measure the void’s volume. Then, mix a mixture of cement, fly ash, and sand. Use a special additive to expand the variety. Then, place the sand mix under the slab foundation and compact it. Repeat this process for the remaining sections of the slab.

When filling a void, choosing the right kind of sand is essential. You may want to select sand that will blend in with the surrounding yard. While sand is not a very aesthetic material, it can serve its purpose – providing drainage. However, if it is dirty, it may not look good and become a maintenance issue.

The mixture can be mixed with water to form a concrete void filler. Sand-cement grout is the best choice for a void filler. It is cheaper than pea gravel and compacts more easily. However, the material takes a long time to cure, so you should plan accordingly. When mixing the mixture, make sure to add an expander additive, which prevents the mixture from shrinking during the curing process. Next, add water to the mix until it reaches the consistency of thick grout.


A traditional method of repairing a void under a concrete slab foundation is to inject high-density polyurethane foam into the gap. This material does not degrade or leach chemicals into the soil underneath the slab. Once injected, the foam will stabilize and compact the ground below the slab. In addition, the concrete lifting foam is waterproof, so it will not cause the soil beneath the slab to erode.

A common problem with a concrete slab is tree roots. While this isn’t the cause of your void, it is another factor. Roots can absorb moisture from the soil and push the concrete slab up against the tree. Over time, this can cause the concrete slab to shrink, causing the void. To fix this, mud jacking contractors inject cement slurry under the slab. This cement slurry is usually a mix of soil, water, and cement. The process is simple and can be completed in as little as a few hours.

Sand-cement grout mixture

If your home’s concrete slab foundation has a void, you should consider using a sand-cement grout mix to fill the void. The mixture is much easier to work with than pea gravel and allows for cost-effective void penetration. Because it takes a long time to cure, adding an expander additive to the mixture is essential to prevent it from shrinking during the curing process. Then, mix in water until the mixture reaches a thick grout consistency.

A pump with a seven cubic foot capacity and a ten-horsepower engine is necessary to apply the grout mixture. The pump should be able to deliver the grout to the desired depth and fill the void evenly. The pump should also come with a masonry bit that enables you to drill holes into the slab and push the grout mixture into place. The number of holes you need depends on the size of the slab. A smaller slab may only need two or three holes.