It looks about right for 75 psi. As a rough guideline, structural floor slabs (not SOG) have an average compression stress of about 200 psi and SOG should be about 1/3 of this (as a rough start for preliminary design) You need to understand the loading the slab is subjected to and the elastic moments developed. If the slab is uniformly loaded to say 75 psf, the only (or main) flexural stresses are due to minor variations in slab thickness, moments within the slab due to tensioning, tractions from the sub-base, and variations due to uneven bearing. I would place the strand above the centroid of the slab...but below the top kern, this provides a greater compression in the top fibre. Minor cracking of the bottom of the slab isn't visible... Friction losses can be assumed to be 0... even for slabs, I assume the losses to be 0 for most cases, the poly sheath has such a low coef of friction. An oversight on an earlier project I had 300' long strands tensioned from both ends. When they stressed from the opposite end, with the initial overstressing, there was almost no loss.
Care should be taken to ensure that the slab is not restrained and that the thickness is uniform.
There are ways of minimizing cracking and treating control joints without resorting to PT.
Dik
