In our Northern Illinois work, we often find that concrete can look solid from above even when support is changing below it. When soil settles, washes away, shrinks, or loses compaction, an empty space can form between the slab and its base. The concrete may bridge over that void for a while, giving few obvious clues—until it starts to move.

We use concrete void filling to restore contact beneath unsupported concrete and help stabilize the slab. When settlement has already occurred and the concrete is still suitable for lifting, polyurethane can also raise the slab toward its intended elevation.

What Causes Voids Under Concrete?

Voids are usually a symptom of a support or water problem, not a random hollow spot. Common causes include:

• Poorly compacted fill beneath the original slab
• Water washing soil through open joints, cracks, or slab edges
• Downspouts, irrigation, plumbing leaks, or drainage concentrated near concrete
• Soil shrinkage during dry conditions
• Freeze-thaw and seasonal moisture changes
• Settlement above utility trenches or disturbed ground
• Erosion near retaining walls, foundations, culverts, or poorly supported edges

When we evaluate a suspected void, we consider both its location and the reason it formed. Filling the space provides support, but correcting an active water source or drainage failure may be essential to the long-term repair plan.

1. The Slab Sounds Hollow

A slab with full, consistent support tends to produce a more solid response when tested. A noticeably hollow or drum-like sound in one area can suggest an air space beneath the concrete.

We do not diagnose a void by sound alone. Slab thickness, reinforcement, surface coverings, and surrounding construction can change what you hear. But a hollow response—especially near cracks, settled corners, or eroded edges—is a useful reason to investigate further.

2. The Concrete Rocks or Moves Under Load

If a sidewalk panel shifts when stepped on or a driveway slab moves as a vehicle passes, the concrete may be pivoting over inconsistent support. That movement can widen cracks, damage joints, and accelerate deterioration.

We recommend evaluating rocking concrete promptly. Repeated motion can turn a support problem into a broken-slab problem, reducing the repair options available later.

3. A Slab Has Settled or Tilted

Visible settlement is one of the clearest signs that support has changed. One corner may drop, a sidewalk joint may become uneven, or a driveway may sink at the garage or curb. A patio or entry slab may begin directing water toward the building instead of away from it.

The void may not sit directly under the lowest visible point. Concrete transfers loads across the slab, and unsupported areas can extend beyond the obvious settlement. We plan injection locations around the full support pattern.

4. Cracks Keep Growing or Reappearing

Concrete can crack for many reasons, including shrinkage and temperature changes. But a crack that widens, changes elevation, or returns after surface patching may indicate that the slab is still moving over unstable support.

Filling the crack addresses the visible opening; it does not fill a hidden cavity. If movement is the cause, the support condition below the slab needs to be evaluated before cosmetic repairs are expected to last.

5. Water Disappears Beside or Beneath the Slab

Watch what happens during rain or when a downspout discharges. Water that repeatedly flows into an open joint, drops below a driveway edge, or vanishes beside a sidewalk may be carrying soil away. Mud pumping from a joint under traffic is another warning that water and fine soil are moving.

Correcting the water source is critical. Otherwise, erosion can continue outside the filled area or create a new unsupported path.

6. There Is a Visible Gap at the Slab Edge

Erosion or settlement sometimes exposes an open space beneath a driveway, patio, step, or sidewalk edge. The visible gap may look small, but the void can extend farther under the slab than you can see.

Do not assume that packing material into the edge will support the full area. Loose fill may not reach deep cavities or make uniform contact, and it can wash out again if drainage is not corrected.

Why an Unsupported Slab Is a Problem

Concrete is strong in compression, but a slab spanning an empty area is being asked to behave like a bridge it was not necessarily designed to be. Loads from people, vehicles, equipment, or the slab’s own weight can concentrate stress at the edges of the void.

Over time, unsupported concrete may:

• Settle farther and create trip points or rough transitions
• Crack as the slab bends or loses bearing
• Rock under traffic and damage adjoining joints
• Collect water because the surface no longer drains correctly
• Become too fragmented for a simple lifting repair

Early evaluation can preserve more options. A structurally sound slab may be stabilized before major breakage makes replacement necessary.

How We Identify a Concrete Void

The diagnostic approach we use depends on the structure and the stakes. We may combine visual inspection, elevation measurements, sound testing, probing at accessible edges, review of drainage patterns, and response under controlled load.

For larger commercial, industrial, municipal, or engineered projects, we may recommend additional investigation. Ground-penetrating radar, core information, geotechnical review, or other testing can help define hidden conditions. The repair scope should match the risk and complexity of the slab.

How Polyurethane Concrete Void Filling Works

Our Smart Lift System® uses application-specific polyurethane material placed through small holes drilled in the slab. Once injected, the material expands through open spaces and creates a lightweight, water-resistant structural foam.

1. Evaluate the slab and surrounding conditions. Our technician identifies settlement patterns, likely void zones, drainage issues, and lift constraints.
2. Plan the injection points. Dime-sized holes are positioned to reach unsupported areas while allowing controlled material placement.
3. Inject in measured stages. Polyurethane flows and expands beneath the slab, filling gaps and restoring support.
4. Lift when needed and feasible. If the slab has settled and remains liftable, additional controlled injections can improve its elevation and alignment.
5. Patch and verify. Ports are removed or finished, holes are patched, and the repaired area is reviewed before reopening.

Why Polyurethane Is Well Suited to Void Filling

• It reaches irregular spaces as the material expands
• It is lightweight compared with cement-based slurry
• It cures quickly, helping reduce closure time
• It is water-resistant after curing
• It can be placed through small injection holes
• It supports controlled lifting as well as stabilization

We select materials and injection techniques for the slab, load, void size, and site conditions. Different projects may require different polyurethane formulations and placement strategies.

Void Filling, Slab Lifting, and Soil Stabilization Are Related—but Not Identical

Void filling focuses on restoring contact beneath unsupported concrete. Slab lifting adds controlled pressure to improve elevation. Soil stabilization may involve treating weak or loose material below or around the void. A project can require one or more of these objectives.

That distinction matters. A warehouse floor that is still level but unsupported may need stabilization without visible lifting. A sunken sidewalk may need both void filling and elevation correction. An active washout may require drainage or site work before injection is the right next step.

What Concrete Void Filling Cannot Fix

Polyurethane is a powerful repair tool, but it is not a substitute for every type of work. It cannot restore a severely crumbled surface, reconnect concrete that is broken into unstable fragments, replace failed reinforcement, or correct an active plumbing leak. It also cannot prevent unrelated soil outside the treated area from moving in the future.

Replacement or engineered reconstruction may be necessary when the slab has lost structural integrity, the base must be excavated, utilities need access, or the site requires a major drainage redesign.

What to Do If You Suspect a Void

1. Limit unusually heavy loads on a slab that rocks or appears unsupported.
2. Document the location, visible settlement, cracks, water flow, and any recent change.
3. Redirect obvious runoff or stop active leaks when it is safe to do so.
4. Avoid filling only the visible edge gap and assuming the hidden area is supported.
5. Schedule an evaluation with us before the slab breaks further.

Frequently Asked Questions

Can a void exist if the slab has not sunk yet?

Yes. Concrete can bridge over a cavity for a period of time. Hollow sound, edge gaps, or movement under load may appear before obvious settlement.

Will filling a void automatically lift the slab?

Not necessarily. Stabilization and lifting are related but distinct goals. Our technician controls material placement based on whether support only or elevation correction is intended.

Can you fill voids beneath interior floors?

Many interior slabs can be evaluated for void filling, but utilities, floor coverings, load requirements, and access must be considered. Commercial or engineered slabs may require additional investigation.

How quickly does polyurethane cure?

Polyurethane cures in minutes. Actual reopening instructions depend on the slab, load, scope, and site conditions.

Restore Support Before the Damage Spreads

A void under concrete rarely becomes easier to repair after the slab has cracked, rocked, and settled further. If you notice hollow sounds, movement, widening cracks, water loss, visible gaps, or changing elevation, have the area evaluated.

We specialize in concrete lifting and leveling, polyurethane void filling, and slab stabilization in Crystal Lake and across Northern Illinois. Call us at 815-264-2200 or request a free evaluation to learn which repair fits your slab.