It's called a concrete encased electrode, and lives at 250.52 (A)(3) in the 2005 NEC.
That's the simple version... no that's the only version
Well, it's not really the only acceptable version.
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......"or consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 4 AWG."
From the period 1942-1960, H.G. Ufer tested 24 buildings in Arizona every 2 weeks for resistance values. The installations consisted of 1/2" steel reinforcing bars encased in the bottom of the footings. As a result of these tests, Ufer recommended that a #4 or larger copper conductor be encased in the bottom of the footings, and further testing be performed. The results of the further testing impressed the members of CMP-5, and the concrete encased electrode was accepted.
The term "Ufer Ground" truly only applies to this copper electrode, but has generally been used for any concrete encased electrode.
