NEC 250.50 Exception:
"..where the steel reinforcing bars or rods are not accessible for use without disturbing the concrete."
If the Ufer grounding was not well prepared before concrete pouring one has to "disturb" the concrete in order to connect it with the general grounding electrode.
IEEE 665/1993 ch.5.7.4 Station reinforcing steel states:
"When reinforcing steel is connected as part of the ground grid, it should be recognized that there may be some risk of damage to structural concrete for high values of ground current."
That means a large ground fault current flowing through the rebar would heat the steel and the concrete adjacent evaporating the concrete moisture and destroy the concrete.
For your edification and amusement search mineral accretion, Wolf Hilbertz, and biorock. Your interest is in AC but if you move to DC, concrete can be repaired by mineral accretion or structures even grown from rebar in seawater. A ground rod inserted in concrete does make the best ground.
