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Unread 06-11-2022, 05:19 PM   #1
Chidude
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Hairline cracks in SLC

So I have about 220sf of Self leveling I poured 2 days ago and this morning I just started noticing hairline cracking. This is a kitchen, narrow walkway to a foyer. Everything sounds super solid still, but this is bothering the hell out of me. This is over 16"oc joists, 2 layers of 5/8" (to match existing height and new hardwood)plywood. This was already very solid. 2nd layer screwed plenty. I went with 1/4" durock for reasons on top.. its all thinseted down and screwed plenty. All was extremely solid. Used Primer T, then poured a few hours later after it was well dried. between 1/16 and 5/16" depths. Used less than max allowed water per instructions for the novoplan plus 2. I've been in tile for 16 years, setting for 5. All commercial, and never do sidework, other than my couple bathrooms that I didn't need to pour. So this is relatively new to me, not going over concrete the way we always do. What does everyone think? It sounds solid even when smashing with a hammer, is there a chance it could come loose later? Does this happen and after how long could it happen? I have some Aqua Defense I could roll over it as a crack suppressor. Thanks.

Edit: I've had a dehumidifier going since i've had new hardwood for the last few weeks during finish work. It's been around 40-45%. Could it possibly be that it dried out too quickly after the first 24 hours? Maybe I didn't notice the cracks the next night..idk
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Unread 06-11-2022, 06:20 PM   #2
cx
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Welcome, Jon.

To be clear, you poured over a CBU that was bedded in thinset mortar and mechanically fastened?

Your joist structure meets the L/360 deflection requirements?

The minimum allowable thickness for that product is 1/8th of an inch, and even that thickness would worry me a bit, primarily in regards to actually getting it flat, but I can certainly see thinner sections being prone to cracking.

I've only seen that product used in a MAPEI training seminar, and didn't see it two days later, so can't say if there was any cracking. And we poured it in sections a good deal thicker than the minimum 1/8th-inch.

A photo or your results may be helpful for one of our members more familiar with the product.

My opinion; worth price charged.
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Unread 06-11-2022, 06:35 PM   #3
Chidude
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2nd layer of 5/8 plywood is screwed throughout every 8" or so, and yes Durock is embedded in Ultraflex LFT with a 1/4" trowel and mechanically fastened with screws every 8".

Not sure. they're 2X9's 16" OC. 12' span.

We've used the product and have zeroed out the depth with feathered edges regularly with no issues. Did the same here on edges and everything seems well-bonded.
Just really curious for someone that doesn't pour in residential, if regular shrinkage cracks are normal?

The only thing I might not have followed is mixing time, which we rarely mix more than 20-30 seconds after lumps disappear..

I'll work on how to submit pictures.
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Unread 06-11-2022, 07:21 PM   #4
cx
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Use the paper-clip icon above the Reply dialog box to attach photos from storage on your computer, Jon.
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Unread 06-11-2022, 07:46 PM   #5
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Pictures may help.

Do the cracks seem to follow any particular pattern, like the edges of the plywood, or the joists, or are they just randomly throughout?
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Unread 06-11-2022, 08:06 PM   #6
Chidude
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They don't follow joists. They're pretty much a few feet apart, coming from the outcorners and centers of each area kind of. One from a baseboard vent notch.. A couple of them intersect. No hollow spots even at the cracks which are slightly raised when checking with the flat side of a trowel. I tried to chop a bit out in a closet area at a crack and it's stuck really well.
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Unread 06-11-2022, 08:22 PM   #7
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Pictures arent turning out in the lighting. A close-up of one of them plus another if you can see it.. Pic on left there's a horizontal crack running through the center of the circle. Pic on right is a vertical crack from upper left to bottom right.
Attached Images
  
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Unread 06-12-2022, 04:36 PM   #8
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Did you tape the seams of the cbu?
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Unread 06-12-2022, 06:10 PM   #9
Chidude
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Yep. Mesh taped and filled the joints. Dried out for 2 days before I rolled Primer T. Absolutely no flex in this floor at all. Self level is still solid, not hollow. No more cracks. Just looking for anyone to say this is relatively common shrinkage. Talked to another setter friend yesterday that does a lot of residential side work, and says he's had it before with no issues, he always uses 2 coats of Hydroban or AD anyways. He seemed to think it happens sometimes with a bit too much water.
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Unread 06-12-2022, 09:45 PM   #10
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Did you measure the water in your pour?
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Unread 06-13-2022, 08:39 AM   #11
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It seems to me that at least one company has changed their policy on pouring slu over cement board. But I don't know how Mapei, specifically, feels about this procedure.

However, we used to do this sometimes and hairline/spiderweb cracks were normal. They wouldn't follow specific lines but were more haphazard and random.

For the future, I would look into a product like Ardex Liquid Backer Board and install it right over the subfloor- no cement board. That stuff doesn't crack after-the-fact.

Mapei probably has a product that would compete with LBB but I don't know the name of it. Maybe if Holden is still around he can make a recommendation.

Finally, what I've learned is that the right mixing speed, paddle, and length of time make a huge difference in the flow of the slu. With the LBB, I mix it 2 full minutes and it matters.
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Unread 06-16-2022, 01:31 AM   #12
Tool Guy - Kg
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I haven’t used your exact product, but I’ve seen the super shallow random surface hairline cracks often and the tech people aren’t concerned. But call your manufacturer and get back your confidence.

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Unread 06-16-2022, 03:05 AM   #13
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Nothing to be concerned about, especially if your base is solid.
We pored pallets of the stuff and would get them. Rep said it was a non issue, due to surface tension pull as it cures out, humidity and drying rate all affect it.
Looks good in the photos shown btw
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