Learn me how to pour a concrete pad for MaxJax?

My new house came with a 29x29 garage (yay!), but unfortunately the concrete pad is only about 3.25 thick (boo!). MaxJax requires 4.25 and 3000psi so it looks like I need to pour a new concrete pad. I couldn't find this info on the MaxJax website-does anybody have advise on:

1. Dimensions of the pad(s)? 

2. Do I need to tie the new pad into the old pad with rebar, or should I attach them using some other method?

3. What kind of rebar/reinforcement do I need for the new pad?

I've never poured concrete before and will probably outsource this job. Having said that, I'm trying to learn enough to make sure I can tell the contractor exactly what I need.

No personal experience, but Tom's series on building a backyard shop might have some guidance: How To Build a Backyard Shop

Sounds like the first step is finding a concrete contractor willing to take the job.  "Should" just be a matter of saw-cutting a couple of holes in the existing slab, digging out to the required depth, then pouring the concrete, maybe with some re-bar tying into the existing slab.  Cost seems to vary wildly depending on where you live.  You might be able to save some money by doing the demo work yourself, but that'll depend on what you can work out with the contractor. 

In my recent experiences, just finding a contractor willing to even look at the job and offer a "yes, we actually want to do this job" price can be the most challenging part of the project.  Good luck.  

The MaxJax instructions should tell you what to do, and how big to make it. You could also ask MaxJax.  They should have done the engineering.

A pad for the hoist would key under the existing slab.  Dig deep regardless.  When I had my whole slab poured, my 2-post called for min. 4", and I just left holes in the styrofoam under where the posts would go to make that portion 6".  Except the ground wasn't level enough (and all rocks), so it ended up being 8" under the hoist.

Do you know if that slab is reinforced with rebar and highway mesh or if its a post tensioned slab?

Is the house on piers/pilings or is on grade supported?

If post tensioned cutting holes in it could be catastrophic.    If the slab is on piers or pilings that is another problem for load transfer.

I did 4' x 6' and 8" deep when I replaced the cracked concrete under mine. That was the width of the control joints.

I drove the rebar under the existing slab 5"-6" as well as poured the new concrete under the old slab a couple of inches to key it all together. It seems to be holding well.

On their instructions it calls for a 3'x3' hole. It also calls for it to go under the existing slab about 3 to 4 inches. It showed a couple pieces of rebar in the lower section of the hole. Which looked to be about 10" deep. 

I'm a over do kind of guy when it comes to these things. My floor has a grid of rebar on 2' centers and wire mesh tie wired to that. I attached it to the existing slab of my house with rebar epoxied into that slab. Inspector asked if I planed to park a tank in my garage.

Granted I was working on a job site were they were throwing away 10' 1/2" rebar so it was cheap for me to do the overkill.

I'm in a similar/same situation. Slab was made 4" by turning a 2x4 on its side. Anchors wouldn't hold.

Bendpak's documentation says 3'x3' pad 12" thick, 1/2 rebar at 6" spacing and lapping into the adjacent concrete. I'm going to go ahead and tie the two sides together for a couple reasons:

1. Strength - having both on the same pad is a bit stronger

2. Flexibility to put another set of anchors in a more narrow position. Current plan is to put the anchors at the max recommnded width, so if I end up with a vehicle in the future than needs them closer, I'm fine to put another set in the new slab.

Also, instead of using the provided anchors and having to drill the new concrete, I'm making some templates to hold the bolts/sleeves to be embedded in the concrete after it is poured, but before it has the finish applied. Piece of plywood with holes to match the lift bases, with threaded sleeves and a bolt at the bottom. This will embed a bolt into the concrete to hold the sleeve that the provided bolts will go into once the concrete is set. I also plan to tie the two sides together so the distance apart is set correctly. I'll try to remember to add pictures here.

Depends on the manufacturer. Bendpak's requirements at much higher than Rotary's. MaxJax is really low- low enough that I don't trust it. 
 

I typically cut 4'x4'. Since MJ's spec says 3', I would go 18" outside of the centerline of each post and connect them as as 1 slab (4' x the width of the lift plus 18" each side). Make the new thickness 8" deep. Undercut the slab 4" on all sides, and drill 1/2" rebar into the edge of the slab in a new grid pattern 12" oc in both directions. 

3000 psi concrete with fiber reinforcement.  Full cure before use (28 days).

THATS what I'm comfortable with. 

If you are in a seismic area, you MUST hire an engineer. 

The reason the MaxJax specs are lower is because the posts are short. Shorter posts= less leverage for the weight. 
 

Doesn't matter to me. It's still a 2 post lift with several thousand pounds over my head. I'd build it like any other 2 post lift. 

Where I feel that the 3'x3' is fine is the under cut of the original slab. With a 3000 PSI that's some heavy twisting the lift would have to do to break the original slab. I will say if you have the room I'd probably go with the 4'x4' because OverKill of course.

I'll probably die soon but my 9000lb extra wide asymmetrical two post in in 4" of concrete with heavy fiberglass, no steel.