Post Tensioned Slabs - Max precompression

[SE80]

ACI 318 18.12.4 sets the lower limit of precompression force in a post tensioned slab at 125 psi, but I don't see any mention of the upper limit anywhere in the code. I read somewhere that the recommended maximum precompression force is 300 psi for slabs. Does anyone know if this is true and if it only a recommeded value as opposed to something that needs to be followed strictly.

 

[Lion06]

It's more of an industry standard, but I've seen (and gone up to) 400 psi, locally, for slabs around 8", and even close to 400 psi more globally on certain strips for thicker slabs.

 

[Teguci]

The strict interpretation is given under axial design for concrete (chapter 10). In practice, a PT slab stress requirement above min is more of an economic and long term servicablility discussion.

 

[Lion06]

I've found that end spans of a strip along a building edge often needs juiced up PT to meet deflection requirements that are consistenst with acceptable caulk joints and slip allowances from facade suppliers.

 

[SE80]

So, what are the implications of having a higher precompression other than economic issues?

 

[Teguci]

Higher long term creep/volume change.
Higher moment required to overcome the compressive stress and start cracking the concrete (servicability against corrosion and water retention structures).
Lower deflections available.

 

[asixth]

For post-tensioned floor plates, a high precompression can lead to shortenting issues and excessive cracking depending on the level of restraint present.