Hairline Cracks in Post Tensioned Slab 1

[Y3392]

Here's the condition:
8" post tensioned slab with 1/2" 7-wire strands, undonded.

Hairline cracks x 8" long consistently happen at uniform tendons dead end location at BOTTOM of slab. The cracks are perpendicular to the uniform tendons direction. Found at almost every (maybe 50%) location of at the uniform tendons anchorage end.

Initial thoughts: The end anchorage was constructed higher than the mid depth, therefore produce a curve up situation - and causes cracks at bottom of slab. This looks like a local bending to me, because it only happens right at the location where the dead end anchor is.

What do you think would be the cause of this? Have you seen this condition before? Could you please kindly give some direction on how to remedy this situation?

Your expert input and advices on this would be deeply appreciated.

Thank you in advance.

 

[flamby]

Use of post tensioning on a 8" slab is highly questionable- you hardly get any advantage of PT and yet expose yourself to such dangers like crack on slightest misalignment.

What is this slab for? How big?

Ciao.

 

[wyanko]

flamby: Are you kidding me? You think that an 8" thick post-tensioned slab is questionable? An 8" thick post-tensioned slab is used practically in every residential high rise building in the southeast United States. The benefits are enormous, economically:
8" PT slab: ~1.1 psf mild+0.75 psf PT
versus
8" Non-PT slab: ~5 psf mild

Y3392: As for the original question, it really depends on where the cracks are occuring. They may show up near the anchorages if they were not placed at cgs (4") of the slab (+/- 1/4"). If they were no, then I agree with you. It sounds like a curling problem due to the cgs misallignment (flexural cracking).

Other thoughts:
How was the quality of the concrete placement (could be an issue in areas of high congestion of slab rebar, masonry sleeves, and pt anchorages)? Did you put top rebar at the edges (say #5@18", 4' long) parallel to uniforms? That usually helps with any cracking at the slab edges.

 

[Y3392]

Wyanko
the cracks are parallel to the slab edges, and the slab edges are cantilvered free ends. The uniform tendons are perpendicular to the slab edges. the cracks are at the bottom of slab and almost everywhere at anchorage location

both shop dwgs and contract dwgs show the anchorage @ center. But, if the tendons were misplaced at higher location than the slab center, then a secondary local moment will occur which will cause local cracking at bottom of slab. BUT - if this is the case, how could the contractor be so stupid to make the same mistake @ almost every place? That's one of the question I have
What do you think?

Again, thanks for your professional opinios.

 

[flamby]

I just don't have any experince in high rise. Sorry for my wrong post above.

Ciao.

 

[rapt]

To get bending induced cracking from eccentricity of the anchorages would normally require a fairly large eccentricity as it has to produce a tension stress to overcome both the P/A and the tensile strength of the concrete. I do not think you could get this with single strands widely spaced. This should be easy to calculate.

What was the actual concrete strength at the time of stressing?

How far into the slab from the anchor face are the cracks?

What reinforcement is in the bottom of the slab and at the anchorages?

 

[wpeg]

For the stress due to P*e to overcome the P/A compression stress, the tendons would have to be placed outside the middle 1/3 of the slab (this hold true for all rectangular shapes) which would be outside of 1/6 * slab depth of the cgs. For an 8" slab this would mean 1.33" of eccentricity. And in order to cause cracking an even larger eccentricity would be req'd.

 

[Ron]

Likely cause is curling in the cantilevered section due to shrinkage. Top is shrinking more than the bottom and exacerbated by the cables pulling a bit more at the top than the bottom.

 

[Y3392]

Thank you all for your valuable comments and advices.

I kinda agree that it would take quite a bit of eccentricity to produce a local moment to crack the bottom of the slab.

The concrete strength at stressing was about 4000 psi.

At first, I was suspecting that the curing may have caused the cracks; however, the slab has already been stressed for about 2 to 3 months already before we found the cracks. Be aware that there is no real load other than selfweight applied to the slab yet since it is still under construction. This is what confuses me - If it's shrinkage cracks due to curing, then it would have happened earlier.

I would not sleep well until I find out the cause.

 

[rapt]

Ron,

I would doubt that shrinkage would induce much stress in a cantilever. The curling has to be resisted by some form of restraint to cause stress on the concrete or there has to be a severe discontinuity in the amount of shrinkage over the depth of the slab. This is not likely in an 8" slab.

As there is no restraint to curling in a cantilever then there is no stress induced by the curling.

 

[arnel123]

to Y3392

Have you considered in your design the flexural reinforcement distribution? this will be affected due to reinforcement spacing, concrete covers.. try to check the crack limit then if the calculation of your design for cracks satisfy the interior and exterior crack limits then nothing to worry about...

 

[wyanko]

Y3392:

Strength sounds good (4 ksi). Elongations? Perhaps the cables were understressed.

It could also still be shinkage and/or creep issue with the slab (which should be a factor for about a year). I think Ron has a point with the drying shrinkage of the top versus the bottom of the slab. We had similar problems on a pt slab in a garage structure. Cracks formed due to inadequate curing methods by the contractor. We sent an inspector there to make sure the proper curing was done in a particular set of bays. These bays had no cracks, indicating to us a curing and thus drying shrinkage problem.

Perhaps the cracks will close once you put your SDL on the slab.

Let me know what you end up doing. Hairline cracks don't seem like a big issue for this case if the cracks cease to open more.