I'd buy a 2" core drill bit [about $100 on ebay] and use my 1/2"
drill. [*not* in hammer mode] The bit I have is about 4". I'd
drill till it bottoms- chisel the waste and do it again.
Hitting re-rod might create a problem- but to do 8" of concrete would
be a 10 minute job-- and you'd end up with a nice hole exactly where
you want it.
If you're determined to rent- I'd ask what they have in core drills.
There might be something that will plow through re-rod & 8" of
concrete in short order.
Personally- I'd rather own the tool to use another day.
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