I wouldn't pour another thin slab over it, especially so if the old slab is cracking apart. At that point you'd want the new cap to not bond to the old slab anyway and you'd have to reinforce it separately with a bond breaker on the old slab. It's not impossible but it's a bit more work if you ever had to fix the plumbing since it's now in concrete and IMO not needed.
The framing , and the plywood really, should have been pressure treated. Anytime the framing touches concrete, you need pressure treated.
If it was me I'd rip out the old framing and either seal the floor with plastic making sure it's absolutely tight or paint the floor with waterproofing , build the floor with pressure treated, seal the framing with plastic and lay plywood above that. If you are insulating it, use foam board as well.
It's not ideal to seal the framing in moisture rich environment though it'll work if everything is pressure treated.
Well after some more demo I fear the fix will be rather major. Floor is no where near flat, several low spots and the floor "joists" were placed directly on the concrete, so most of them are rotted badly. There are also cracks all over, not big ones but a couple where the rot was the worst. Here is one of the cracks:
And the worst area of rot in front of the door. A cutting edge and light illustrate the issue.
The worst is the wall where the garage door was. I suspect a hole or two in the pour under the sill and would not be surprised if the sill is not pressure treated. we had a big mouse issue years ago, I dug outside the foundation and poured several bags worth of cement into what looked like openings. I also made up metal plates to go around the hot water lines going thru the floor. Instead of a half inch gap it was metal with about a 32nd. The floor under the sill was pretty loaded with mouse sprinkles.
Regardless, time to call some contractors for quotes and plans to fix it. Currently 1:00 PM and I need a strong drink!
Dig out all of the old crap concrete and form up the old garage door opening bottom and place a new concrete floor to the top of the foundation wall and using Plasticizer additives (SuperPlasticizers.com: CONCRETE ADDITIVES) to use less water and help self level. Don't forget to cut contraction joints every 8 feet 1/5 the depth of the concrete thickness within 24 hours of the pour.
We still don't know if your house has a basement or is slab-on-grade. Now will be your chance to install some 4" pvc piping in the garage for future radon gas remediation.
In reply to VolvoHeretic :
Sorry, it is slab on grade. I am not touching the repairs, my skills are limited to demo on this. I do have a great contractor locally, will get the bad news in a week or two. I may do a radon test in that room. Last radon test was 30 years ago and it found nothing.
The concrete cracks don't look too bad, they didn't gibraltar apart so there's at least steel in it. I don't see much leeching around the cracks either so I don't think there's a bunch of water coming thru.
I think the sealing the concrete/pressure treated lumber/sealing the framing is still your best option
