Concrete is incredibly more porous than you would think. Though it seems rock hard it is actually full of millions of tiny cracks and pours.
Concrete can get damaged in any number of ways but the most common damage occurs from the de-icing salts used by city snow plowing services and freeze/thaw cycles which occur quite often in Calgary during spring warming and fall freeze up.
Salt can sneak into small pours and tiny cracks in your concrete. Here there is an acid-like reaction that eventually creates larger and larger pockets that hold greater and greater amounts of water. It is the water that eventually causes all the damage.
Water expands up to 9X when frozen. The pressure from this expansion grows with each freeze and thaw until eventually it pops the surface of the concrete creating the pitting you so often see in sidewalks and driveways.