It sounds hollow
Tap it or walk it and listen for a drummy, echoey note instead of a solid one. This is the most reliable sign.
Traditional mudjacking in Northern Colorado since 2003 · 20,000+ slabs lifted
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Services / Void filling
Filling the space under a slab that is still at the right height. The cheapest work we do, and the work that saves you the most.
There is a stage before settlement where the base under a slab has washed out but the concrete has not dropped yet. It is spanning the gap, held up by nothing but its own strength, and it will keep doing that until a load or a freeze finally breaks it.
Filling that space is a fraction of the cost of lifting the slab afterwards, and it is the single most useful thing you can do for concrete in Northern Colorado. There is no lift to pay for, less material, and no risk of the slab cracking on the way up because it never has to move.
Most people find out about a void by accident. The slab sounds hollow, or a corner rocks slightly, or water disappears somewhere it should be pooling. If you have noticed any of that, the concrete is telling you something useful.
You can check most of these yourself in a couple of minutes.
Tap it or walk it and listen for a drummy, echoey note instead of a solid one. This is the most reliable sign.
Step on a corner or an edge and feel for movement. Concrete sitting on compacted base does not move.
Runoff that used to pool now disappears quickly at the slab edge, because it is running underneath instead of across.
A visible gap or erosion channel along the side of a slab, usually near a downspout or a sprinkler.
A crack that has opened but where both sides are still level often means the slab is spanning a void and has not dropped yet.
If the concrete next to it has gone down, the same base problem is very likely under yours.
The economics here are unusually clear. Filling a void under a slab that is still at grade means less material, no lift, no risk of cracking the concrete, and a shorter job. Waiting until the slab drops means all of those costs plus the lift itself.
The other thing you avoid is the damage that happens in between. A slab that drops takes its slope with it, which is how water ends up running at your foundation, how trip hazards appear, and how a crack that was cosmetic becomes structural.
This is also the service that competitors selling deep foam injection and soil stabilization are talking about. It is the same underlying problem. What differs is the material going into the space, and we are happy to walk you through that comparison honestly.
What an open void looks like, and what it looks like filled.
Before: pavers dropped away from the step with an open void beneath.
After: base restored and the pavers reset tight to the concrete.
Before: an open void running underneath a concrete step.
After: a level paver walkway beside the step, fully supported.
Erosion opening a channel along a foundation, the usual cause.
Irrigation infrastructure, one of the main reasons soil moves here.
If the slab has not moved, why spend money on it?
Because it will move, and lifting it afterwards costs considerably more than filling it now. You are also protecting the slope, the joints and the concrete itself from the damage that happens when a slab finally drops.
How do I know for certain there is a void?
The hollow sound is the strongest indicator you can check yourself. During an estimate we can confirm it, and if there is nothing under there we will tell you so and not sell you a fill you do not need.
Is this the same as soil stabilization?
It addresses the same problem, which is an unsupported slab. Deep foam injection systems are marketed for stabilizing weak soil at depth. Ours fills the cavity directly beneath the concrete with dense material. Different approaches to the same symptom, and worth comparing.
Can void filling stop concrete that is already cracking?
It can stop it getting worse by giving the slab support again. It will not close a crack that has already formed or bring back a section that has already dropped.
How long does a void fill take?
Usually less time than a lift on the same slab, since there is no raising stage. Many are a few hours.
Free estimate, and an honest answer if there is nothing under there to fill.
Most of it can. Get a free estimate and a straight answer about what your slab needs.