I've had this happen several times. The cadillac way to repair it is to remove the whole slab and recast it with Ameron's T-Lock liner. It's pretty easy to line the bottom of a slab (walls and tops of slabs are another matter)and the protection is very good. Maybe if you present it as the most permanent fix, they can figure a place to temporarily put the equipment above.
Or you can thicken the bottom of the slab by 4 or 6 inches. Add the reinfocing in the space. Build falsework and pour from above using access holes cut in the slab above. You can line the formwork with T-Lock. It's tricky, because you're working blind.
The second best fix is to do your structural repairs (replace reinforcing, trowel on repair material, or shotcrete or whatever)from below and line it with a high quality coating done by a qualified applicator (harder to find than it seems). We like Sauereisen's systems, but everyone (Plasite, etc.) has a system and most of them will work for a while. There are PVC liner systems that can be retrofit, but applying it to the underside of a slab seems very difficult.
Whatever you do, carry the lining down the wall. Once the gas is resisted by the liner, it moves on to the next weak link, which will be the walls.
