Water penetrates the surface causing the surface of the concrete to flake off and eventually rusts the encased rebar which expands as it rusts which eventually

Water penetrates the surface causing the surface of the concrete to flake off and eventually rusts the encased rebar which expands as it rusts which eventually cracks the concrete from within. The Gardiner Expressway in Toronto is a good example of this - it seems as it's permanently under refurbishment.