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How to Fill a Void Under a Concrete Slab or Sidewalk

Filling a void under a concrete slab or sidewalk is critical to the structural integrity of the concrete. Concrete is strong and durable, but it does require consistent support or it will likely crack and/or sink.

Source: apluslifting.com

For this reason, concrete professionals use rebar, compacted gravel, and fiberglass to strengthen a slab as it is poured. Voids can form under a concrete slab if the erosion of the soil occurs, which is usually due to uncontrolled water.

Today we will discuss common reasons why voids form under concrete and how to correct them.

Is Having a Void Under My Concrete Slab Bad?

The short answer is, maybe. Cracks in sidewalks are not generally a major problem, but water leaks and erosion under a concrete slab can cause serious foundation issues. Concrete will usually crack (and even sink) under its own weight in an area that is unsupported, such as above a void. Cracks in the floor of a basement, for example, can allow groundwater to collect and pool in the basement, causing mold and mildew.

Why Is There a Void Under My Concrete Slab?

There are a few reasons why there may be a void under a concrete slab or sidewalk, but in most situations, the void is caused by soil erosion. Erosion is often caused by uncontrolled water flow, such as poor grading and draining, or a leak in an underground pipe. Here we will describe the most common reasons a void appears:

  • Uncompacted Substrate

What does uncompacted substrate mean? An uncompacted substrate describes the loose condition of the base material supporting the concrete slab. This can be dirt, gravel, crushed stone, or other materials commonly used as a substrate. Building codes generally require an independent inspection of any substrate that will support the concrete.

This applies to footings, slabs, sidewalks, porches, and any other concrete form. These substrates are inspected by a building code officer by probing for weak spots using a blunt metal rod. These weak spots are caused when air is trapped within the substrate, such as in disturbed, back-filled dirt.

In building parlance, this typically means the substrate must be compacted back to the density that occurred naturally before any digging was done. Since the air trapped in back-filled dirt will eventually escape, the void left by the escaped air will cause a void under the slab. 

  • Soil Erosion

Soil erosion causes the vast majority of voids under concrete and is often the result of poor drainage or underground water leaks. For example, most building codes require the ground to slope away from concrete by at least 5 percent to drain away rainwater.

This applies to masonry foundations, as well as sidewalks and other concrete structures, but often this slope is lacking or slopes the wrong way. Leaking underground pipes are also commonly to blame for soil erosion, such as those used for geothermal heating, under-slab plumbing pipes, and hydronic radiant heat systems.

Residing in or under the slab, when these pipes leak the water will find the path of least resistance, which is usually the soil, causing erosion. 

Can I Fill a Void Under My Concrete Slab or Sidewalk?

If you are an experienced do-it-yourselfer, you can probably repair a void. However, some modern techniques require special materials and equipment, such as foam injection, and should probably be left to the pros.

The tried and true methods for correcting a void are generally possible by DIYers, as long as the appropriate safety gear and techniques are used. These techniques can include drilling, cutting, and working with cement. Common tools used by the pros often include masonry saws, hammer drills, and cement mixers, so if a do-it-yourselfer has experience with these tools, often a repair is possible.

How To Fix a Void Under a Concrete Slab

The appropriate solution will often depend on the severity of the problem. Here we will describe a few common problems and the common method used to correct it. However, there are no hard and fast rules. Generally, if a solution works well for the problem, it is used.

Problem: Broken Corner Caused By Erosion

A common concrete problem is when a corner breaks off from a slab. This is usually a result of inadequate backfill intended to contain the substrate that has eroded away, taking the substrate with it. These cracks are often very evident and can result in a small or large section of the slab breaking away. Depending on how much support remains, this can allow the slab to fall several inches.

Solution: Lift the slab and Replace the Support

If the broken section is still intact, often the pros will simply lift the slab and replace the missing substrate. This is typically done with either expanding polymer foam, inflatables, or jacks. Some companies prefer the expanding foam, as it may be the easiest, but it does require special tools and training. This is done by drilling small holes all the way through the slab and directing liquid foam into the void. As the foam cures, it also expands and applies upward pressure to the slab, lifting it. This method also uses the foam as a substrate, so often no other support or backfill is required. After the foam cures, the surrounding topsoil is appropriately sloped away from the slab to prevent the problem from recurring.

Problem: Underground Pipe Leaks Under the Slab

As mentioned previously, leaking pipes are a common cause of erosion leading to voids under concrete. Some home designs require that plumbing supply and drain pipes be located under a concrete slab, or even in it. This is often done using PEX pipe, which can be directly buried without the use of conduit. These pipes are very durable, but impacts and improper connections can eventually fail, leading to water running or collecting under the slab. 

Solution: Cut Away the Broken Section and Replace it

When this type of leak occurs, it can cause a crack towards the center of the slab, while the perimeter remains intact. This can cause dips and sways in the slab, in addition to crumbling.

Obviously, the leak will also need to be repaired, so often the pros will demolish the area around the crack to locate the source of the leak. After the repairs have been made, the concrete must be repaired. This can be done using new concrete, cement (concrete without the gravel), or flow fill. 

Flow fill is a semi-liquid material made from cement and other additives. Flow fill is less expensive than concrete and is generally used as a filler only, meaning it is not designed to be finished like concrete. Flow fill is typically pumped through a hose and allowed to “flow” into the void, displacing any trapped air. Flow fill is a good option for concrete floors that are covered with laminates, carpet, or other finished floor material.

Problem: Broken Sidewalk

Sidewalks tend to be close to the home’s foundation. This often means that landscaping drain pipes coming from french drains or gutters run under a sidewalk to divert water away from the home. Unfortunately, sometimes the incorrect pipe is used and the volume of water causes deterioration of the pipe, allowing water to escape. This usually results in the erosion of the substrate under the sidewalk, causing a void.

Solution: Lift the Sidewalk and Replace the Substrate

This problem can usually be repaired any number of ways, depending on the size of the crack and condition of the sidewalk. Most of the time sidewalks can be excavated, allowing access underneath. In these situations, often the best option is to excavate under the low side of the slab and place jacks under the slab. These jacks are then raised simultaneously, one turn at a time, until the slab is in the correct position. Then new support and substrate is returned back into the void and the topsoil is replaced.

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