Cracks due to reinforcement corrosion
Steel corrosion products (rust) usually occupy a much larger volume than the un-corroded steel and result in bursting stresses that can cause cracking and spalling of the concrete cover. The cracks that appear on the surface of the concrete will run parallel to the steel.
The reinforcing bars closest to the surface will generally be affected by corrosion first. Thus a typical occurrence of types of cracks due to corrosion will be over shear reinforcement on the sides and soffits of beams in exposed locations.
For further information see Concrete Society Technical Reports 44 and 54.