Residential post tensioning of slab not done - now cracked

[timsch]

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.

 

[Tomfh]

Start measuring and photographing the crack at key locations. Crack width gauges are cheap. Even a scale ruler will do.

 

[Ingenuity]

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.

 

[timsch]

The soil does look dry, but we're not in drought conditions. We actually had several inches of rain last week with a hurricane coming through. When it gets really dry around here, there'll be a gap around the foundation 1/2 to 3/4" where the clay soil has pulled away.

I'll have her start measuring gaps with my machinist ruler.

 

[Ingenuity]

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.

 

[JoelTXCive]

The cracks should close back up when they stress the cables.

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.

 

[timsch]

If the cracks are clean, I can see them closing, but with all of the construction debris around, I'd expect that something will prevent the closing.

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.

 

[JoelTXCive]

Prior to being an engineer, I used to build production residential homes in the Houston area. There was little to no soil prep prior to the foundations being constructed. The builders are supposed to scrape the top ~6 inches of soil off to get rid of any vegetation, but they often only scrape the top 2 inches. As for the soil itself; 90% of the time, it's just the in-situ clays. Sometimes you have built up lots, but the built up soil is usually just the in-situ soils from a nearby detention pond that the developer needs to construct.

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.

 

[hokie66]

If the slab has beams, don't they restrain the slab and create cracking in spite of the tensioning, especially if the PT is unbonded?

 

[JoelTXCive]

Hokie -

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.)

 

[timsch]

Thank you everyone for the responses. We have requested the foundation plans for review; I'll provide more info when I receive them.

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?

 

[timsch]

We have received the PE's report on the slab. He is calling it a shrinkage crack.

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?

 

[jayrod12]

Get your own PE out there.

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.

 

[JoelTXCive]

Yes, definitely possible to be a shrinkage crack.

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.

 

[timsch]

Joel, if the majority of shrinkage cracks occur within hours/days, how is it that there were none found after that short period of time, but only this one that occurred at least 6 weeks afterwards?

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.

 

[hokie66]

A drying shrinkage crack is actually the only possibility. Concrete always shrinks, and if it is restrained, it cracks. The soil and/or thickenings restrained it, and it broke. There was no reinforcement to distribute the crack, so it broke where the stress was highest, at the middle. Not a plastic shrinkage crack...those are generally not full depth and occur before the concrete hardens. Drying shrinkage occurs over time, more at first, but it continues for years.

 

[timsch]

Thanks hokie. You saying that it continues for years. The general concensus I found was that after roughly 1 month the concrete was fully cured. What happens from one month on to years?

 

[jayrod12]

Similar to concrete strength gain, the bulk of it happens in the first month, and then slows to a crawl for the remainder of the structure's life.

 

[rapt]

Jayrod12, If you said the majority of shrinkage happened over the first 12 months I might agree with you. Probably about 70% after the first 12 months, but it varies with depth and other conditions and may be longer for the 70%.

 

[hokie66]

This might help in understanding drying shrinkage. There are a lot of variables, the most influential being water/cement ratio. By the way, there is no such thing as 'fully cured'.

https://www.ccaa.com.au/documents/Library Documents/CCAA Datasheets/DS2002DryingShrinkage.pdf