Ref. ACI 117-06 Specifications for tolerances for Concrete Construction and Materials
Section 2.1 gives tolerances on reinforcing steel fabrication:
Per Fig. 2.1(a) For stirrups - (+/-) 1/2" for bar sizes #3, #4, #5, and with gross length < 12'-0"
(+/-) 1" for bar sizes #3, #4, #5, and with gross length > 12'-0"
(+/-) 1" for bar sizes #6, #7, and #8
Section 2.2 gives tolerances on reinforcement location: