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.

Water leaks into cracks and cervices when its warm and then freezes when its cold. Since the ice expands and concrete has negligable tensile strength the cracks and crevices expand even further and the material begins to chip away and crumble.

Freezing water expands in little cracks and makes them bigger.