Delamination
Separation of a thin top layer from the concrete beneath, creating hollow-sounding areas that later break loose. Caused by sealing the surface while it is still bleeding.
What does delamination mean?
Trapped bleed water and air collect under the closed surface and destroy the bond between the skin and the slab. Air-entrained concrete finished with a steel trowel is the classic setup, which is why exterior air-entrained flatwork should be floated and broomed instead. The damage is invisible when new and shows up as hollow sounds under a chain drag or as sheets popping off months later.
In practice
If a finisher taps the slab and hears a drum sound, the surface has delaminated and more troweling will only enlarge it.