Scaling, flaking, or spalling—whatever the name, you see it often in an old garage or driveway slab exposed to freeze-thaw weather and de-icing salts. The top surface of the slab flakes up, revealing coarse gravel aggregate and sometimes even reinforcing mesh or bar. Preventing weather-related spalling requires using air-entrained concrete, curing well, and applying a water-repellent sealer after the slab is cured.
But you occasionally see “delamination” of the slab surface even when there’s no freezing and thawing, early in the slab’s life. Sometimes a “drumming” sound betrays the hollowness under the surface while construction is still ongoing.
In those cases, the cause may be traceable to improper finishing: typically, the finisher hard-troweled the slab surface while bleed water was still trying to escape. The trapped water creates a weak layer between the top surface and the strong slab below. The repair, unfortunately, will be the same as for freeze-thaw damage: chip or grind away the weak surface, clean thoroughly, and patch or re-surface using a polymer-modified cement compound.