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]

On top of grout injection, FRP strip strengthening (suggested by many others in varies threads) maybe is a good idea here.

 

[AK4S]

Thanks retired13.Will look into it.

 

[Ingenuity]

I was not able to get to the cracks, observed them from the ground 30ft below.
The cracks show efflorescence and rebar corrosion due to moisture ingress.
So will probably sound the concrete and look at injecting the cracks if no other concrete repairs are needed.

I would certainly do a more definitive condition survey before specifying any repairs...nothing worse than specifying out repairs and having the contractor commence said repairs and find out it is more severe than expected based upon a very preliminary condition assessment...and explaining that to the owner.

1960's structure, partially exposed to elements, T & B rebar with thin slab, existing cracks with efflorescence and (possible) rebar corrosion ==> maybe half-cell corrosion mapping is warranted...and check if the roof membrane is compromised because efflorescence may indicate that there are issues.

 

[phamENG]

Ingenuity - with the half cell mapping, how do they come at the rebar? Do they just go up through the crack or core out an access point? I've never seen it done.

 

[r13]

I don't know the mapping method, but I doubt there is only a single layer of bar mat, and the slab was designed as one way continuous beam. 5" is too tight even for WWF.

 

[Ingenuity]

with the half cell mapping, how do they come at the rebar? Do they just go up through the crack or core out an access point? I've never seen it done.

We usually just do a small concrete chipping probe at a rebar (where a crack is located, or GPR scan) to make an electrical connection. I do another probe to rebar at the diagonally opposite end of the map area and check for electrical continuity across the connected rebars.

Half-cell testing is covered under ASTM C876 Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete - not a perfect test my any means, but supplements a visual and tactile inspection.

 

[Ingenuity]

...but I doubt there is only a single layer of bar mat, and the slab was designed as one way continuous beam. 5" is too tight even for WWF.

There is a double mat of rebar - it was the 60's - what can I say

...per existing dwgs, the 5" Slab is reinforced with #4@8" O.C. @ T&B, 1" cl.

 

[phamENG]

Thanks. I'd heard the test mentioned but I've never seen it done. A good one to store away for later use...

 

[Tomfh]

The beams are stressed, not the slab.

Is the slab isolated from the beams?

 

[r13]

it was the 60's - what can I say

I smell weeds Puzzled as well.

 

[hokie66]

Tomfh,

I don't think the slab is isolated from the beams.

But I hope you were not implying that stressed slabs don't crack in the manner which I suggested.

 

[Tomfh]

But I hope you were not implying that stressed slabs don't crack in the manner which I suggested.

What I said was there is PT in the other direction.
This could be preventing cracking of the slab in the other direction.

 

[hokie66]

I suppose we are misunderstanding each other. Never mind. The OP didn't report cracking parallel to the slab span, which is what I was reporting happens often.

 

[r13]

Not familiar with PT, but I would like to think the beam was pre-tensioned prior to casting the slab. Otherwise the prestress force may cause much more cracks around.

 

[hokie66]

Not according to the OP, retired. He said the beams are post-tensioned.

 

[r13]

hokie66,

Don't you feel there could be much more damage done when stressing only one part of a concrete monolith? Or I need to digest this more!

If that's the case, crack due to end restrain makes sense then.

 

[hokie66]

Sure, that is a factor. But drying shrinkage is normally about 2/3 of shortening, while prestress accounts for the rest.

 

[rapt]

Not familiar with PT, but I would like to think the beam was pre-tensioned prior to casting the slab. Otherwise the prestress force may cause much more cracks around.

Don't you feel there could be much more damage done when stressing only one part of a concrete monolith? Or I need to digest this more!

Why would that be the case?

 

[r13]

I think. Upon tensioning, the beam will shorten but the adjacent slabs don't want to, thus the slabs immediately adjacent to the beams will feel higher stress than in the middle.

 

[Tomfh]

Retired13,

We’ve done slabs like this, where only the beams were post tensioned.

Its analogous to a concrete wall panel on point supports, with point loads on the top edge. The concrete can distribute the stresses.