The carbonation of concrete or the chlorides penetrated in the concrete may lead to corrosion of the rebar and in a later phase to a reduced bond between rebar and concrete, cracks and to spalling of small to larger pieces of concrete. This can have a negative effect on the durability, serviceability and safety of concrete structures. The reason for cracking and spalling of concrete is the volume of the corrosion products which is larger than that of the originally un-corroded metal.

In course of condition evaluations of concrete structures (underside of bridges, walls etc.) the question of whether or not there is a risk of spalling (for third parties like car drivers, trains) is raised quite frequently. There are very similar questions related to insufficient durability of rehabilitated structures (patch repair). There are only a few investigations and no recommendation on these topics.