How to fill voids under concrete slabs

When you get a void under a concrete slab – whether it’s under your driveway, patio or footpath – it’s a problem that needs addressing as soon as possible if you want to avoid a surface becoming unstable and creating a potential trip hazard. Otherwise, it’s only going to get worse over time as more water gets in.

In this article, we’re going to discover how these voids appear, how to identify a potential issue, and how to go about fixing it.

What causes voids to form under concrete slabs?

Voids under concrete slabs can appear for a number of reasons, including:

• Soil erosion
• Pests and rodents
• Poor drainage
• Poor soil compaction during construction
• Soil settlement
• Tree root ingress or decomposition

Signs you have a void issue beneath a slab

Things to look out for that could indicate you have a void under your concrete slab include:

• Cracks in the surface: These could appear as a result of the slab being unsupported where the void exists
• Hollow sound when tapped: A hollow sound suggests that there’s a gap beneath the slab where there shouldn’t be.
• Uneven sections: Where parts of the slab are uneven or, in some cases, sunk – suggesting that there’s a gap they can move into.
• Pooling water: Water will gather in lower sections of a slab where it has sunk as a result of a void.

Common methods to fill voids under concrete slabs

So, the important next question if you’ve detected a void under a concrete slab is what to do about it. Fortunately, there are a number of practical solutions.

• Mudjacking: This term refers to the technique of injecting a slurry mix of cement, sand and water through holes drilled into the slab. It’s relatively quick and inexpensive as a solution, but it shouldn’t be used if the void is a result of the soil shifting, because adding heavy concrete will probably only make it worse. 
• Polyurethane foam injection: Drill holes into the concrete and inject polyurethane foam, which will quickly expand and harden to fill the gap and provide future support for the slab. It sets far quicker than mudjacking slurry, but can be harder to control, meaning you can easily overlift the slab. The foam is also less environmentally friendly than mudjacking. 
• Partial or full slab replacement: Where damage to the slab is more substantial, it may just be easier to go for a partial or even a full replacement rather than a fix. The downside to this as a solution is that you’ll have to wait much longer for the fresh concrete to achieve full strength and be ready, but it does give you the opportunity to address underlying issues more thoroughly.

Preventing future void issues

Of course, there’s no use filling the void if the underlying issue hasn’t been addressed. One of the biggest causes of voids under concrete slabs is drainage issues, so make sure you’re preventing any more excess water from reaching the site by cleaning nearby gutters or ensuring the water is being channelled efficiently.

You should also remember that when undertaking any construction or landscaping project involving concrete, it’s important that you do the necessary preparation to ensure the ground is ready before you start pouring. You also need the right concrete mix for the job, and the best way of doing that is to source it from experienced local experts like the team at Stewart Brothers Concrete.

Get in touch with us today to find out more, ask for advice or place an order for your next delivery.

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