What can be done about voids under concrete?

Voids under concrete eventually lead to cracking and/or settling concrete. Voids are created when concrete is partially supported by base soils or through the use of anchors or rebar to foundation walls or through the use of Sonotubes. As the soil settles or is carried away by other means a gap opens up between the soil and the bottom of the concrete slab resulting in a loss of support.

Un-compacted fill dirt, erosion, burrowing rodents, decaying organic material even a loss of moisture in the soil can create voids. Once there, voids are difficult to fill. Smaller slabs such as walks can be packed with gravel or sand to fill voids but this is not practical for larger slabs such as driveways or patios. These need a method that supports but doesn’t lift the slab. Using an expansive polymer is the ideal material for void filling for several reasons such as;

Polyurethane can travel several feet in every direction to fill voids, fissures and holes. It has the ability to consolidate organic material, sand and loose rock. If properly monitored Polymer injection can provide support without excessively lifting or stressing the slab which causes fractures.

A typical void fill application for a residential job would be under a front entry. These are often “pinned” to the foundation wall with rebar. Because fill dirt is often used around the foundation and again because gutters are often not installed on new construction or downspouts are mis-placed, the fill dirt becomes saturated with water this often results in some dramatic settling. If the entry was not pinned to the foundation it will begin to settle. If it was pinned to the foundation a void will open up under the entry. Given enough time, even the entry pinned to the foundation will settle because the rebar will either bend or in time rust and break.

The question with new construction is what should I do? Settling of soil in new construction can go on as long as 10 years so the question becomes do you live with it and replace your broken up concrete at some later time or do you try and preserve it knowing that you may have someone out 2, 3 or 4 times in the next decade to shore up what should’ve been taken care of during initial construction. Immediate replacement of settled concrete or concrete with voids is not a good choice as settling is not yet complete.

Geo-polymer void filling is the best method we currently have for void filling but it cannot affect deeper layers of un-compacted fill so you are left with supporting and densifying through polymer injection.

There are other materials namely soil stabilizers that can be used to solidify deeper layers of soil however due to their cost are impractical for residential work and are primarily used in municipal or commercial applications.

Dealing with voids whether they be stable or dynamic is an important part of maintaining your concrete slabs whether they be residential, commercial, industrial or municipal.

If you see a void opening up under your concrete slabs give the concrete raising and leveling professionals a call we will get you taken care of.

1-855-752-2522

www.SlabjackGeotechnical.com

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4 Replies to “What can be done about voids under concrete?”

  1. I think it is pretty cool that if there are voids under concrete slabs that it can be fixed. Like sand and gravel for small ones, but what I really like is the big stuff a polyurethane that can penetrate and move the slab into place. That would be cool to see, and I wonder how long it would last.

    1. Aging studies done on structural polyurethane have shown no degradation of the material over decades. If it re-settles it’s not the material breaking down, it’s the underlying soils.

  2. I’m hoping someone knows what i should do with void under my three car garage slab that is six inches thick. it does not appear to have moved but i don’t want it to. In winter the void was about an inch and a half deep space about 10 feet around. Now in spring the void is only a quarter inch to almost a half inch deep by the 10 feet around. It seems that the ground under the slab is moving up and down as the slab has not moved. Any help on what to do would be greatly appreciated Thanks

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