Spalling and delamination of concrete

Spalling is the breaking away of pieces of concrete from the surface in the form of flakes or chunks, leaving a rough, shallow cavity. It is caused by corrosion of reinforcement (expansion of rust), freeze-thaw, fire, impact or excess local stress.

Delamination is the separation of the concrete cover into thin layers parallel to the surface, at or near the level of the reinforcement, without the layers falling off yet. It is caused by rust expansion of bars, poor finishing (over-trowelling), or fire. The area gives a hollow sound when tapped with a hammer, and if it is not repaired it ends in spalling.

 Section through the cover (surface at top, rebar below)

 (a) Sound         (b) Delamination   (c) Spalling
 +-------------+   +-------------+    +----+     +----+
 |    cover    |   |    cover    |    |   \     /   |
 |             |   |- - - - - - -|    |    \___/    |
 |     (o)     |   |     (o)     |    |     (o)     |
 |    rebar    |   |    rebar    |    |   exposed   |
 +-------------+   +-------------+    +-------------+
                    crack parallel     cover has fallen
                    to the surface     off; the rusted bar
                    at bar level       is exposed

Delamination (layer parting at bar level) comes first, then spalling (loss of the layer). Repair: remove the unsound concrete, clean the bar, coat it, and patch.