How you find it before you can see it
Delamination usually announces itself with sound before it shows with your eyes. Drag a chain, a piece of rebar, or the edge of a hammer across the surface and listen for a hollow, drummy sound where a solid slab should ring flat and dull. That hollow patch is trapped air or water sitting between the top skin of the slab and the concrete underneath it, a separation that started during finishing and only becomes visible later, when the thin surface layer finally cracks, blisters, or breaks loose under traffic or a hard freeze.
What causes the surface to lift free
It usually starts with the timing of the finish. Troweling or power finishing a surface before bleed water and entrained air have had a chance to work their way up and out seals a thin skin over moisture and air that has nowhere left to go. Air entrained mixes, common on flatwork built for freeze thaw, need more patience at the trowel than a plain mix does for exactly this reason. A slab finished too early, too fast, or in conditions that make the surface skin over before the interior catches up is the common thread behind most delamination.
Where it turns up on a real job
Delamination shows up most on power troweled surfaces, since that finish packs the top layer the tightest and gives trapped moisture the least chance to escape on its own. A few places worth checking on a slab that concerns you:
- A garage or shop floor with a hard troweled finish, especially one built with an air entrained mix
- A patch or repair poured over an existing slab, where the bond line itself can trap moisture underneath the new layer
- A spot that got extra trowel passes to chase a smooth, glossy finish
- An area that sat in direct sun and skinned over while the rest of the slab was still workable