Cracking at the base of a waterproof foundation 2

[CELinOttawa]

Hello All,

We've got a foundation we waterproofed with swelling waterstops (bentonite clay style).

The stops were placed in the classic location, behind the reinforcement bars. The bars were epoxied in for this project, a first time we allowed epoxy instead of cast in place.

Please see the attached photo. This cracking is occurring at the base of the wall, and I am at a loss to explain. I keep thinking that maybe the waterstops swelled prior to the concrete achieving sufficient strength, but I don't know. That thought is persistent as I have been involved in anchor testing before, and this looks like tension failure, with the classic pattern of thick at point of load, tapering to paper thin at the edge of a "volcano" cross section. The issue here is I personally reviewed the waterstop placement before the pour, and it was central with good cover (beyond the manufacturer's recommended). If this shifted during pour, that could perhaps explain the issue, but it looked to be reasonably well placed and the contractor had the correct "glue" to temporarily position the product.

Overall I am most worried about performance, and will likely insist that this be reviewed by the manufacturer of the swelling waterstop. I don't think I can sign off on this in the current condition.

All thoughts are appreciated; a real head scratcher. With the exception of the epoxy installation of the bars, this is a known and proven detail, with a novel issue presentation.

 

[dik]

I've attached some of my 'standard' foundation details for anyone's use. I often use bentonite rod for waterproofing for decades and never had an issue. There have been a couple of containment areas where I've gone to PVC because of the chemicals involved. Bentonite is good for water... some other salts, not so good. I've also had a few problems with PVC waterstops collapsing due to improper tying.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1632284953/tips/StripFooting_urnzuz.dwg[/url]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1632284954/tips/StripFooting_ifn7bw.pdf[/url]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

I think it's a matter of looking at the issue on site and determining what concrete cover there was to the material, specially the vertical stuff. It is also a matter of determining what the concrete was like. I wouldn't have thought that the concrete would spall like that if the product were installed in accordance with the manufacturer's printed instructions.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Sheer Force Engineer]

Hi CELinOttawa, that's an interesting photo.

More out of curiosity than anything else, is this in a basement? The wall formwork looks a bit rough, was this an off-form finish?

Are you sure that this spalling isn't caused by poor concrete placement or similar? Adjacent wall "panels" appear to have void pockets evident, the darker wall which is spalled looks especially rough.

Can bentonite develop enough pressure to blow out concrete like that? Even if close to the face of the element? Must admit I haven't seen or heard of this happening before (but that doesn't mean it can't)

For the latest insights into the art of Structural Engineering

http://www.sheerforceeng.com

 

[Enable]

More out of curiosity than anything else, is this in a basement? The wall formwork looks a bit rough, was this an off-form finish?

Are you sure that this spalling isn't caused by poor concrete placement or similar? Adjacent wall "panels" appear to have void pockets evident, the darker wall which is spalled looks especially rough.

The forms are so-called Advance Bar forms, which are predominately used in residential. You can tell by the 24" widths and nubs at all the tie-locations. Residential guys have to be so competitive that these things are used well beyond their service life by 100s of pours. What you are seeing is not at all atypical for this type of system, and does not immediately suggest anything that terrible with the pour/concrete.

Note: They are sometimes used on commercial jobs but usually non-exposed and non-waterproofed elements

CELinOttawa: Is the issue only in this single location or is it pretty much everywhere +/-? If it's only a few locations I might think those were trough areas in the footing and water accumulated in the forms overnight. This would lead to expansion of the waterstop but more so just weak concrete at these areas. Other than that I have few ideas. Might be worth coring through (if outside hasent been backfilled) to take a look at what's inside.

I'll pass this to some old guys that do material science for a living and see what they say.

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[LittleWheels]

What does it look like when you pull out the spalled concrete?

 

[CELinOttawa]

Thank you all for your responses. I'll try to be brief.

@Dik: We'll know tomorrow morning what's on site with respect to concrete cover and, generally, the quality. We use Schmidtt Hammers and will rebound the wall both high and low. I expect that there is to be some variation found, and not the natural/normal better at the bottom than the top.

@Sheer Force Engineer: This is indeed a basement, per what Enable has said.

I've been involved in several waterproof structures (genuinely waterproof, unlike the "waterproof" basement BS most contractors and home owners misunderstand), and never seen this failure. It is, however, why the manufacturers require a minimum cover of concrete, to the best of my understanding. I also have wondered at when water is "okay", ie: Tensile strength of concrete is so poor [roughly 0.15 of SQRT(fc') ] that I have been surprised a minimum strength is not more prominently required.

@Enable: Good suggestions! The site visit tomorrow should be very interesting. I am hopeful that we can get some samples; I am very, very tempted to send some of the concrete for petrographic analysis. I too suspect understrength concrete, by mix design, batch plant error, or by placement. I am, gratefully, not responsible to figure out which. I know the detail, I know the material, and the failures appear to be near self-evident. From here, it should be "Why", but on someone else's dime.

@LittleWheels: We'll know tomorrow! My money is on "volcano" cross section, interrupted by where it passes across an open joint (ie: The wall joins to the raft slab).

I want to add a little something: We admit fault. If we make a mistake, I will *SAY SORRY*, quite up front and frankly. If I lose my house for doing the right thing, so be it. I will sleep at night, I will be human, I will say when something went wrong because I was wrong. I can look at these Contractors tomorrow and simply state "This isn't on us; Someone has screwed up", and they will know, really KNOW, that I am being open and honest.

Will try to keep the tread up to date; Thank you so much for your replies!

 

[CELinOttawa]

Well we attended to site this morning, and a few issues of note were discovered:

- There appears to be little to no bond between the concrete of the two pours. I am betting that the bottom of the formwork was not dry at time of pour.

- In all areas where failure has occurred, the swelling waterstop breaches the manufacturer's minimum required clear cover.

- The bentonite clay waterstop in the failed areas has completely deteriorated; we're talking soft, fractured, mud.

So: It is our belief that the waterstop was insufficiently secured to remain in place, shifting during the pour, and the combination of a constant source of water (little dewatering being noted on site, with OSB boards placed as walking surfaces in the basement being waterlogged and flaking apart) and insufficient cover resulted in the failure of the system.

Luckily, the repair seems simple, and the Contractor understands that this isn't our fault.

Outcome: We have developed a simple design for a repair using steel plate with anchoring, replacing the waterstop with new, and patching with repair mortar extended 40% by weight with pea stone.

Thank you all again for your contributions.