I think most of the problem lies with us since none of us have any experience with this type of concrete slab construction. Any information you might be able to send our way would be greatly appreciated.
Thanks in advance for your help,
D.
Panam City, FL
You are talking about an Architect, but I really hope that there is a structural engineer involved in the design of the slab. The code requires such plans be signed by a licensed engineer.
In my recent experience in California I have not seen post-tebsion slabs-on-grade designed for less than 5", and recently these are usually 6" and up. The reason here is the nature of the soil that is very expansive. A flexible slab will deform with soil movement and crack. Even the 6" slabs are stiffened with "grade beams" under the slab.
If you do not have expansive soil issues, they may have specified post-tension so they can pour the slab as one large section instead of creating joints every 10-15 feet. Again, based on my experience, I doubt a 4" slab will guarantee crack free surface. By the way, the cracking is not the result of the tension. Concrete cracks because it shrinks when it dries. The cables are actually supposed to minimize the cracking.
Another problem with such a thin slab is the location of the cables within the concrete. In order to be effective the steel needs to be located near the center of the section. It is going to be difficult to do with such a thin one.
I suggest you contact the engineer who signed the plans (his stamp should be on them) and raise your concern. Get a second opinion from a structural engineer who specializes in post-tension construction.
The cost difference between 4 or 5 inch slabs should be minimal.