Notes
When Concrete Sounds Hollow
A hammer, a stick of chalk and ten minutes will map the problem before anyone quotes it. What a drummy patch usually means, and the one case where it means something else.
Most people find it by accident. A dropped tool bounces with a dull thud instead of a crack, or a heel strikes one part of the garage floor and it sounds different from the rest. Then they start tapping, and discover a patch where the concrete sounds as though there is nothing behind it.
That sound is one of the most useful clues a slab offers, and you can read most of it yourself before calling anyone. Doing so makes any estimate faster and far harder to pad, because you will already know how big the affected area is. This page walks through the check, what the results normally mean, and where it fits with our concrete repair work.
What is usually going on
The common cause has a name: delamination. The National Ready Mixed Concrete Association's sheet on it describes a thin layer of the surface separating from the concrete underneath, typically an eighth to a quarter of an inch thick. The separated area can be anything from a few square inches to many square feet.
It forms during finishing. Freshly placed concrete keeps pushing water and air upward for a while as the solids settle. If the top is closed off by troweling while the concrete below is still doing that, the water and air collect under a sealed skin. The skin hardens, but it is never properly joined to the slab beneath it. Sun, wind, a cold base or a mix that sets slowly all make that timing harder to get right.
The sheet notes that it is more common on hard-troweled floor slabs, which is why garage floors feature so often. Outdoor flatwork that is broomed is less prone, though not immune, since any finishing method that densifies the top too early can do it.
There is a second, less common source. Steel reinforcement sitting close to the surface can rust, and rust takes up more room than the steel it came from, so it pushes the concrete above it off. The sheet notes that these separations are typically thicker. They are often the ones you can see as well as hear, with a rust stain or a crack line following the steel.
The ten-minute check
You need a hammer, some chalk, and a phone for photographs. The inspection standard the industry uses, ASTM D4580, is built on exactly this idea: tap or drag, and listen for the change in sound. On large areas professionals drag a heavy chain; on a garage floor or a stoop a hammer is fine.
- Start on concrete you trust. Tap somewhere well away from the suspect patch so your ear knows what a solid ring sounds like on this slab.
- Work in a loose grid. Tap every hand's width or so, moving toward the suspect area. You are listening for the moment the note drops and goes dull.
- Draw the boundary. Each time the sound changes, make a chalk mark. Join the marks and you have the outline of the separated area.
- Check the levels. Lay a straight edge or a spirit level across the hollow area and past it. Note whether anything has dropped, tilted or stepped.
- Photograph it. One picture of the chalk outline, one of the whole slab for context.
Reading what you found
| Result | Most likely | Kind of job |
|---|---|---|
| Drummy patch, surface level, slab not moved | A separated top layer | Surface repair |
| Hollow line following a crack or rust mark | Steel corroding near the top | Deeper repair, steel treated |
| Whole slab dull, and it has dropped or rocks | A gap in the ground below | Not a surface problem |
| Hollow spots scattered over most of a floor | Widespread delamination | Grinding and an overlay |
The row that matters most is the third one. A thin surface separation sounds hollow over a patch and the slab around it stays put. A slab sitting over a void sounds dull across a much wider area, and it has usually moved: it has settled, tipped, or rocks slightly underfoot. Those are two completely different problems, and treating one as the other wastes money in either direction.
What the repair involves
For a separated surface, the published method is straightforward. The loose layer is taken off, the sound concrete beneath is cleaned properly, and the area is patched. Where the separation covers a large area, the sheet describes grinding and overlaying a new surface instead. Where corroding steel is the cause, the rust has to be removed from the steel by blasting before any repair material goes back.
The key word in all of that is removal. A separated skin will not stick itself back down, and a sealer or paint over the top does nothing to reattach it. Anything that treats a hollow patch as a surface to be coated rather than a layer to be removed is going to come off with it.
Where the overlay route is on the table, the conditions it needs and the figures are in what resurfacing costs.
What we do and do not take on
Surface separations, rust-driven spalling and slabs that have broken are all repair work, and they are the kind of job this crew does.
A slab that has dropped as one piece over a void is a different trade. Filling underneath or lifting a slab back to level is not something we offer, and we would rather say so here than after a visit. If your tap test and levels point that way, we will confirm it and tell you what kind of firm to call. If the slab has cracked and broken as well as settled, then replacement of the failed sections is on the table and we can quote it.
A word on garage floors
Garage floors deserve a mention because they sit awkwardly between indoor and outdoor concrete. The NRMCA sheet warns against using air-entrained concrete under a hard trowel finish, because entrained air raises the chance of delamination, and it recommends no more than 3 percent air for troweled floor slabs. A garage floor that sees winter slush is exactly where those two aims pull in different directions.
We are not going to tell you which way to resolve that from a web page. It is, however, a very good question to put to anybody quoting a new garage floor: what finish, what mix, and how they are handling the tension between the two.
Before anyone quotes
Send the photographs with the chalk outline. It turns a vague description into something a contractor can size, and it makes it immediately obvious if a quote covers far more area than your tapping found. A drummy patch the size of a doormat does not need a whole new floor.
Our free repair visit starts where your chalk lines stop: we confirm the boundary, check what is underneath it, and tell you plainly which of the rows in that table you are looking at.
Sources
- National Ready Mixed Concrete Association, CIP 20: Delamination of Troweled Concrete Surfaces - the definition and eighth-to-quarter-inch thickness, the size range, how and why it forms during finishing, corrosion-driven separations, detection by hammer or chain drag under ASTM D4580, the repair methods, and the air-entrainment caution with its 3 percent limit for troweled floors. Sheet revised 2018, read 24 September 2026.