Residential post tensioning of slab not done - now cracked
Residential post tensioning of slab not done - now cracked
(OP)
Hello all,
My in-laws are having a home built that is under construction, with framing, plumbing & roof completed & windows & doors installed. It was noticed during a 3rd party inspection that it appeared that post-tensioning of the slab was never done. In digging into it, this was confirmed. A couple of days ago my wife was doing a walk-through and noticed a significant crack. She posted this issue here, which includes pictures. She went back today and swears that the crack has gotten wider
She's made calls today trying to line up an independent PE to provide an analysis, not trusting the one that the GC will line up. The GC is rushing to get this behind him and continue on and let her know this evening that he has lined up tensioners for tomorrow. Obviously time is of the essence for us, and I wish I had posted this here earlier, but things are moving very quickly, so better late than never.
Primary question: Can this oversight be properly corrected? What issues might arise in the future if the GC gets his way with tensioning it now and continuing on?
thanks.
My in-laws are having a home built that is under construction, with framing, plumbing & roof completed & windows & doors installed. It was noticed during a 3rd party inspection that it appeared that post-tensioning of the slab was never done. In digging into it, this was confirmed. A couple of days ago my wife was doing a walk-through and noticed a significant crack. She posted this issue here, which includes pictures. She went back today and swears that the crack has gotten wider
She's made calls today trying to line up an independent PE to provide an analysis, not trusting the one that the GC will line up. The GC is rushing to get this behind him and continue on and let her know this evening that he has lined up tensioners for tomorrow. Obviously time is of the essence for us, and I wish I had posted this here earlier, but things are moving very quickly, so better late than never.
Primary question: Can this oversight be properly corrected? What issues might arise in the future if the GC gets his way with tensioning it now and continuing on?
thanks.
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
Yep, as evidenced by the pocket former still in place and the stressing tails not cut-off those tendons have not likely been stressed. To be 100% certain, remove the plastic pocket former and view inside the pocket at the anchorage cavity to see if there are a pair of wedges present, AND if so, check for gripper-marks on the strand tails about 8" from the slab edge.
Judging by the cracked/desiccated soil, it looks like you have some very dry conditions there, and hence a likely cause of the cracking over the part 8 weeks.
I do not see any harm in stressing the tendons now - the tendons are you only reinforcement system in these foundation type - but I would ask that the stressing tails NOT be cut off until ALL construction is completed.
I would insist that the GC setup crack monitoring, i.e. check crack widths BEFORE stressing, then checked AFTER stressing to see if the cracks widths reduced. Additionally, I would keep the crack monitors in place for as long as possible to monitor for widening/closing. Please all this on the GC.
RE: Residential post tensioning of slab not done - now cracked
I'll have her start measuring gaps with my machinist ruler.
RE: Residential post tensioning of slab not done - now cracked
Crack gauge monitors like the above work well, and you can 'date mark' the cracks width measurements, etc., and it gives you a simple visual reference for movement too.
RE: Residential post tensioning of slab not done - now cracked
The cable company will mark the cable ends with spray paint (white in our area) prior to stressing. Then they will stress the cables and use the spray painted marks as way to measure the elongation.
In our area, it is standard practice for 1 crew to spray-paint and stress all the cables. They do NOT cut the cable ends at that time.
The cables are not cut until the slab engineer signs off on the stressing as being acceptable. This could be as short as a week later; or it could be a month later depending on how busy the engineer is.
If your in-laws have tile areas right where the cracks are, then I would make sure you note the cracks and their location.
Even though the cracks are closed up, you can have moisture move through them and cause the tile thinset to release right above the crack; which can result in the slab crack telegraphing through to the tile.
RE: Residential post tensioning of slab not done - now cracked
Fortunately, the area above the crack will be covered with vinyl-plank flooring, so that's a plus.
How much will the soil preparation below factor in with this issue? I have word that corners are frequently cut in track housing developments on that front, and that the soil prep itself is likely somewhat deficient.
I'm asking this in regards to the future, not whether the soil prep could have contributed to the crack. I'm thinking that this crack would not have happened the post tensioning had been done properly, even if the soil prep was marginal, within reason.
RE: Residential post tensioning of slab not done - now cracked
With that being said, the geotech is aware of the the soil conditions when they specify the beam depths and slab design.
And, a post-tensioned foundation is supposed to 'float' on top of the ground. A little seasonal movement is okay, differential movement is what causes problems.
Having uniform moisture conditions around the perimeter of the slab goes a long way towards preventing problems. A tree drinking all the moisture on one side of a slab; and a sprinkler head leaking on the other side of a slab is the type of scenario that poses increased risk.
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
We can't tell from the photo if that is a cable over a pad; or a beam cable. I'm thinking it's a cable over a pad because most post tensioned slabs today have a 2nd draped cable in the beams. If it was a beam cable, we would see the draped cable a couple inches below the exposed cable.
Engineering-wise, my limited post tensioned experience was in grad school. I thought you balanced the stress between the top and bottom cables to keep both sides below the cracking stress. (which sounds silly as I write it because the beam has already cracked prior to stressing)
If it's a single cable system with only cables on the top and rebar in the bottom of the beams, then you would make sure the tension on bottom half of the beams also stays below the cracking stress.
(I should also note that even though 90% of the slabs in Houston are post-tensioned, I'm not a fan of the system. Messing with the cables is not something most homeowners are aware of when they launch into remodeling projects. I do love the pre-stressed bridge beams though.)
RE: Residential post tensioning of slab not done - now cracked
The GC will try to push past this as fast as he can, and we're a bit stressed without having a clearer answer to my question, "Can this oversight be properly corrected? What issues might arise in the future if the GC gets his way with tensioning it now and continuing on?" I feel that if the proper procedure was not followed, it is a breach of contract and my in-laws have an out.
I know that the information I've provided is minimal, so answering that definitively will be hard. My wife pose the question "Would you have your parents move into a house with this issue?" (assuming tensioning is finally done) to an inspector. What would you answer?
RE: Residential post tensioning of slab not done - now cracked
If this were a shrinkage crack, would you have one and only one the full width & depth of the slab, or would there be multiple such cracks throughout the foundation, assuming it was a single pour?
Would a shrinkage crack occur only 6-8 weeks after the pour?
RE: Residential post tensioning of slab not done - now cracked
To answer your question, yes I have seen a single shrinkage crack be that large and full depth. However it was a unreinforced basement slab in a house that was likely poured with concrete slurry instead of true concrete. It was a house that was a perfect rectangle twice as long as it was wide. Popped open at the dead middle of the slab and shrunk into two almost perfect squares.
RE: Residential post tensioning of slab not done - now cracked
The majority of plastic shrinkage occurs in immediate hours/days after a pour.
Until the cables are stressed, its effectively an un-reinforced slab; so there is nothing to restrain the shrinkage stresses.
RE: Residential post tensioning of slab not done - now cracked
It seems to me that the cables reinforce is primarily for loading stresses rather than shrinking stresses. That being said, tensioning would usually happen after shrinkage would have occurred according to your timeline.
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
https://www.ccaa.com.au/documents/Library%20Docume...
RE: Residential post tensioning of slab not done - now cracked
No way is shrinkage finished by 6 weeks. Maybe 50% or thereabouts. After a few years it will be 90%+ complete.
RE: Residential post tensioning of slab not done - now cracked
It is entirely correct that most cracks form within hours (I've been taught 6 hours for this one), but they open over time. That's because the initial shrinkage is rapid thanks to plasticity, and following the initial set slows considerably. It doesn't, technically, stop, but becomes imperceptible and negligible.
I've not been at this anywhere near as long as many around here, but after twenty years greatly focused on existing structures I have yet to see a perfect construction. This building has many other flaws. They all do.
I have a simply answer to your wife's question: Yes. I would move into this house with confidence. I would also watch for the secondary issues, and make sure I understand the limits of the builder's warranty.
I do not mean to sound callus, but you have a significant advantage knowing the issue. The next house will have issues as well, it becomes another roll of the dice as to whether you find out, how you find out, and what the consequences will be.
As an aside:
- Surprised to see a home owner walking around the site, and around here I would be careful about photos of feet on site without steel toes.
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked
Best of luck on this
RE: Residential post tensioning of slab not done - now cracked
I agree with the others above that it'll likely be alright, meaning it'll last probably as long as any other built with our present level of quality....
RE: Residential post tensioning of slab not done - now cracked
RE: Residential post tensioning of slab not done - now cracked