Post Tensioned Beam Low Breaks - Changing Class T to Class C 1

[RFreund]

I have a one-way slab (non-PT) spanning over PT beams. The concrete strength was either poured wrong or is coming in way low (4,000psi vs 6,000psi spec'd). When I go back to look at the design with the lower concrete strength, I get 18.3.3 failures (i.e. tensile stress at service load failures) f.t is greater than 12*sqrt(f'c). If I change the class, from class T to Class C, then I get a design that "passes". I think ultimately beam deflection will be an issue since we will need to use cracked section instead of gross section properties. However, if deflection checks out is there anything else that you see being an issue here? It just feels wrong that I can simply change the class and everything is "OK".
If deflection is an issue, I'm thinking we may need to come up with some sort of external post-tensioned repair. Any other ideas or direction/examples on external PT repairs?

 

[KootK]

The consequence of switching classes is simply that you are accepting a greater degree of cracking in your design. Depending on the situation, that might matter from the perspective of durability, aesthetics or, as you intimated, poorer deflection performance.

Given the above, the only point of a reinforcement scheme would be to restrain cracking. In my mind, that pretty much necessitates an external post tension repair of some kind. Other, passive solutions would require more cracking in order to mobilize the reinforcing. And, in a non-corrosive environment, this begs the question: will the owner and/or occupants find the cracking or the external reinforcing more objectionable?

There's endless content here and elsewhere online with respect to external PT repair of beams. You'll have a deviator or two, some interesting anchorage conditions, a nifty jacking procedure, answers to fire proofing etc...