I poured a slab in two sections once. I cut a notch just wide enough to allow the rebar to fit, in the bottom of the 2x4 frame at the joint. That allowed me to have the rebar in place when I poured, the 2x4 could be removed easily and it worked well. As to the use of an expansion joint I believe you should still use it: however, I am not an expert.
I thought I'd throw this idea out about the rebar. I would like some follow up by someone with some expertise, since I will be pouring a gar floor & driveway in 4 or 5 sections this summer totalling 30 yards concrete.