Spalling concrete is break away of concrete surface which often extends to the top layers of reinforcing steel. As a result, this area is susceptible to damage. Spalling of concrete happens when rust occupies more volume than the original steel, and the remaining pressure spalls the concrete.

Spalled concrete may be caused by any of the following:

Corrosion of embedded reinforcing steel, because rust occupies more volume than the original steel, and the resultant pressure spalls the concrete
Fire exposure due to which free water in concrete change to stem and create internal expansive pressure in concrete
Freeze and thaw cycling
Expansive effects of Alkali Silica Reaction
Inadequate depth of cover over reinforcement
Low quality concrete cover over reinforcing steel

As soon as spalling is spotted, it should be properly repaired otherwise it compromises concrete surface integrity.