New slab over old slab?

[Redlands Okie]

Lots of variables on why the old slab cracked and if its a good idea to pour over the old. If your going to pour over the old slab then mesh and rebar as you plan on is still very important. Mesh and rebar do two SEPARATE types of jobs in the concrete. I agree with RNeumann suggestion on keeping the two slabs separate is very important. A couple inches of sand could work. A layer of closed cell styrofoam will also work even better, tape the seams and slow down and be careful to not let the new concrete get under the foam and float it up. Foam is common under slabs for a variety of reasons and is far from a new idea. Sometimes we will use a bit of sand to fill in bigger voids or cracks and to help prevent air pockets under the foam that we install.
You need the isolation barrier between them to help keep the existing cracks from transferring themselves into the new slab.

 

[rScotty]

Any advice may or may not be sound advice we have no idea what was under the old slab ( clay, gravel, sand ) wet area dry area cracks from settling or just poor concrete with no rebar. If it's on solid ground just pour over it with rebar I'd forget the fiber glass reinforcing fiber.

What we need here is some more information on why the old one cracked. If the concrete was poured on expansive clay, or if there is a wet area, or poor reburying, or no compaction..... then the solution is way different than if it cracked due to weight from machinery above. In the first cases, those are common enough mistakes and a new pour needs the old removed and decoupled from the underlying soil. If weight was the problem then simply do a good job with rebar and pour right over it. It's a common engineering call & probably worth your time to take a small soil sample and find why it cracked. Or check around with your neighbors about their soil. This is a common problem - and if it is the soil then they know about it.
Or check to see if your neighbors include a retired civil engineer or concrete guy. It would take him (or her) no time at all to check it out. Or maybe there is a county agent who does that sort of thing.
Sure sounds like a nice place you have there. .
rScotty