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Residential post tensioning of slab not done - now cracked

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.

RE: Residential post tensioning of slab not done - now cracked

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

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

(OP)
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.

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 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.

RE: Residential post tensioning of slab not done - now cracked

(OP)
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.

RE: Residential post tensioning of slab not done - now cracked

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.

RE: Residential post tensioning of slab not done - now cracked

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?

RE: Residential post tensioning of slab not done - now cracked

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.)

RE: Residential post tensioning of slab not done - now cracked

(OP)
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?

RE: Residential post tensioning of slab not done - now cracked

(OP)
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?

RE: Residential post tensioning of slab not done - now cracked

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.

RE: Residential post tensioning of slab not done - now cracked

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.

RE: Residential post tensioning of slab not done - now cracked

(OP)
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.

RE: Residential post tensioning of slab not done - now cracked

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.

RE: Residential post tensioning of slab not done - now cracked

(OP)
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?

RE: Residential post tensioning of slab not done - now cracked

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.

RE: Residential post tensioning of slab not done - now cracked

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%.

RE: Residential post tensioning of slab not done - now cracked

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%20Docume...

RE: Residential post tensioning of slab not done - now cracked

Timsch,

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

I was taught "18 months to shrinkage of concrete", and similarly that significant modifications to existing heritage homes will likewise have an 18 month window for all the secondary stresses to work themselves out.

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

(OP)
Thank you all very much for your input; it has been most helpful. I'm appreciative and humbled.

RE: Residential post tensioning of slab not done - now cracked

Correct me if I'm wrong, but wouldn't you want to have the slab tensioned prior to building the structure? Once the structure is built, the structure may act against the tensioning of the tendons. Was a vapor barrier used? I would typically require a vapor barrier where a flooring system that relies on adhesives such as glue are used to hold down the flooring. I ask because it was mentioned a vinyl plank flooring is used and depending on how it is attached it could have future issues should water penetrate the cracks.

RE: Residential post tensioning of slab not done - now cracked

Every PT project I've seen requires the tendons be stressed within 4 days of the pour. If the concrete hasn't reached stressing strength by then, the stressing is done to 50 percent. Isn't this a code issue?

Best of luck on this

RE: Residential post tensioning of slab not done - now cracked

(OP)
TG, that was the point for my post. I felt and still do that they screwed up. A crack might have happened, but the separation and the dropping of one side wouldn't have if it had been tensioned properly. Unfortunately, with contract wording being what it is, we've got a weak hand. Shallow pockets don't help.

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

My brothers foundation in Dallas has a similar issue. They waited over a month to tension the cables. The house was fully dried in but no brick or sheetrock yet. The crack was probably over 1/4" to 3/8" wide. They stressed the tendons and the crack only minimally closed. They sent the builders engineer out and he had the crack filled with epoxy and then they put self leveling grout to bring the foundation surface to level as best they could. They put a membrane down prior to placing the flooring. You're supposed to tension the cable within 3-8 days of pouring slab after the concrete has reached around 2000 psi. The soils in the Dallas area have a high PVR, so this un-reinforced slab was loaded with framing and sat un-reinforced supporting load. Then they tensioned the cables. They haven't closed on the house. The builder did not follow proper procedures placing the slab. Basically, even after tensioning the cables, the deflection from the settling soils and load just locked in the deformed shape in my opinion. No way tensioning under load did the foundation regain its camber. It was already +50% loaded. I told them to request a PE sealed engineering assessment report with repair recommendation.

RE: Residential post tensioning of slab not done - now cracked

(OP)
s23, I hope that your brother has better luck with the resolution that my in-laws did, who were basically told to pound sand, suck it up and accept the house as-is. The builder did have a PE to sign off on it, but his report was less than impressive in my opinion. The city inspector was concerned about the issue, and he did post a stop-work order on the front door, but that didn't stop the builder from going in that same day and tensioning the slab, which did pull the gap together more than I'd have anticipated. No ramifications from even going around the posted stop-work order. It's apparent who has the teeth in these agreements, and it's more than sad that it's not those who would enforce proper, industry approved procedures.

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