Cracks in Existing Concrete slab with PT beams

[AK4S]

I was looking at the roof of an existing Building which has Post Tensioned Concrete Beams with 5" thick concrete slab spanning between the beams. I see cracks at the bottom of the slab. The attached sketch shows the crack pattern observed. I am trying to find the cause of the crack before I decide if injecting it is the best repair.
I don't think these are flexural cracks (I ran the slab design and it is stressed to only 70% of its capacity at max. loads). Also the cracks seem to originate at the free edge of the slab and propagate inward.

The slab has only mild steel reinforcement (no post tensioning) and the building was constructed in 1960's.

I don't have much experience working on PT structures and was wondering if these cracks are due to any issues associated with the original Post Tension on the beams. If the sequence of Post Tensioning of the beams or in general the compression at the beams could have led to tension at the free edge of the slab, leading to these cracks.

Any insight is appreciated.

 

[r13]

Tomfh,

Thanks for the explanation. I know it has been done this way, and have no doubt that this matter has been thought through, and been well taken care of. However I just couldn't get the picture, that the slab was dragged during beam tensioning, out of my head. It does make sense for the wall in your example, a gravity system, thus everything moves in the same direction, albeit differ in stress. But for the slab/beam...well, I do need to think more. Thanks again.

 

[rapt]

Yes, there will be stresses caused as Kootk indicated but they are fairly small and easily controlled by normal slab reinforcement as discussed above previously by people who are experienced in the area and have done the calculations.

But without knowing and having calculated the effects, comments like "much more damage done when stressing only one part of a concrete monolith" are simply misleading and inflammatory.

 

[r13]

rapt,

I welcome positive comments/suggestions to my lack of knowledge in this field, but your citation is in itself a distortion of my words. Let's be civil towards each other, please.