Voids Under Concrete Slabs

Concrete slabs are designed to bear on continuous ground. Take some of that ground away and the slab does not immediately fall; it spans the gap like a bridge deck, which hides the problem. But concrete's strength is compressive. In bending, the kind of load a bridging slab carries, an unreinforced four-inch slab has little to give. The void grows, the span grows, and eventually a wheel load or just the slab's own weight exceeds what the span can carry.

What Happens When There Is a Void Under a Slab?

Three outcomes, in rough order of frequency. The slab cracks over the void and the broken section settles into it, producing the classic dropped, cracked panel. Or the slab settles gradually as a unit, tilting into the soft zone, producing offset joints and reversed drainage. Or, worst and rarest, the slab punches through suddenly under load, which is how a car ends up with a wheel in a driveway hole. Every one of these costs more to fix than the void did before the slab moved.

How Do Voids Form?

• Washout. Moving water carries soil fines out from under the slab, grain by grain. Downspouts, irrigation, plumbing leaks, and joint pumping under traffic are the usual pumps. Detailed on the washout page.
• Clay shrinkage. Panhandle clays lose volume as they dry. In a long Amarillo dry spell the soil contracts away from the slab bottom, and the resulting gap does not close evenly when the rain returns. Background on why concrete sinks here.
• Fill consolidation. Loose backfill compresses under its own weight for years. The soil surface drops; the slab, briefly, does not, and the space between them is a void.
• Decomposition. Buried organic material, stumps, construction debris, breaks down and loses volume. More common than anyone would like under older additions.
• Burrowing and erosion channels. Animal burrows and old root channels give water a highway under a slab, accelerating all of the above.

How Do You Detect a Void Without X-Ray Vision?

The slab tells you, if you check. Tap or chain-drag the surface: supported concrete sounds solid, bridging concrete sounds hollow and drummy. Watch the edges after storms for sediment fans, the soil that used to be under the slab, deposited beside it. Look for new cracks with no offset, a bridging signature. Feel for panels that rock or click under load. On commercial floors, joints that pump water or fines when a wheel crosses are announcing the cavity below. Where it matters, a small drilled pilot hole or probe confirms depth and extent directly.

How Are Voids Repaired?

By filling them before the slab fails, ideally. Expanding polyurethane foam is injected through small ports and seeks out the cavity's edges, restoring contact between slab and ground; large exterior cavities sometimes take flowable grout for bulk with foam finishing the contact. If the slab has already settled, filling and lifting happen as one operation. If it has not, the job stops at restored support, which is the cheapest version of this repair you will ever be offered. The service pages cover the residential and commercial versions: void filling and commercial void filling.

Can You Just Ignore a Small Void?

A small, dry, stable void under a lightly loaded slab is not an emergency. The problem is that voids under Amarillo concrete are rarely stable: the same water path or shrink cycle that opened the cavity keeps operating. The practical rule: find the cause, stop the water if water is the cause, and get the cavity assessed. Sometimes the honest answer is monitor it. Sometimes it is fill it this month. What it never is: hope the ground grows back.