It seems that a 6" floor is the way to go to support a 2 post lift.
If the lift manufacturer says so, yes. If they spec a thinner slab, you may be causing more issues...
The main issue you have to contend with is cracking of the slab. All slabs do this due to the volumetric changes in the concrete (it shrinks) as it cures. Thicker concrete builds up a greater amount of tensile stress, which requires more reinforcement and/or more closely spaced control joints to control the shrinkage cracking.
The limiting factor of a floor for a two post lift is not the vertical forces in the slab - its the bending forces. An uncracked thin slab could be much stronger than a thick cracked one for a two post lift.
I'm wondering if I can save a few bucks by only having 6 inches (or more) in the lift area and 4" everywhere else.
This actually makes things (engineering wise) much more complicated and I'd suggest having an engineer experienced with industrial floors detail the reinforcing or put an expansion joint at the transition to isolate the slabs. Unless you have a special rebar design at the transition, you'll get different tensile forces built up there, which could crack the slab. (This is why the lift manufacturers spec a simple uniform slab. They are idiot proof and very simple to design. Once you get in to depth changes, "footers," etc. the design/engineering gets complex.)
Also...Can you prep the pad for installation of a lift? Maybe staging bolts/studs and pouring around them? This would avoid drilling holes. Is this ever recommended? It might be really difficult to get right in the middle of a giant pad.
You should be able to contact the lift manufacturer to confirm. (Depending on depth, spacing, etc. embedded J bolts could be weaker than anchors drilled in after the pour.) If you do this, you'll need to build a jig to hold everything in the correct position. Simply planning out your rebar plan ahead of time could get most of your headaches out of the way.
My concrete guy has been in the business for 40 years and had poured alot of garage slabs for lifts.
Experience is very important, especially in finishing the concrete. The best contractors will have pictures of their work or will give you a tour of satisfied customers and slabs they have done in the past. If you can't get a portfolio or (even better) tours of uncracked slabs
just like yours, you shouldn't trust experience - trust the results. An owner can be very happy with the work for the week after a pour when they lay down a check, but pissed off a month later when a slab cracks.
One thing that may or may not make a difference is your location. Here in Tennessee the ground dosn't freeze like it does up North. This may be something to consider. I simply don't know.
As long as the base under the slab is uniform and the concrete doesn't loose contact with it, heave won't be an issue. If there is differential movement or if voids form underneath, that will create weak points where cracks can occur.
My guy isn't even charging any extra to do mine that way and he is also using 4000psi mix.
This is a great way to get a stronger floor - without needing to do with a thicker slab.
