Here in MA an electrical permit must be obtained, an electrician must make the connection, and finally an electrical inspection before the pour.
In other States the rebar installers turn up a section of rebar that the electrician hooks onto later.
We have always cad welded the conductor to the rebar but mechanical fasteners are available.
By the way the GEC to a concrete encased electrode is never required by the NEC to be larger than 4 AWG and can be smaller. It could be 8 AWG CU for a 100 amp service.
