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Unread 10-08-2019, 09:11 AM   #1
Jaymi Cross
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uncoupling over stress core concrete??

hey there! I'm wondering if anyone has advice or experience installing over stress core concrete. Its 4 ft wide by various length slabs with a 2.5 inch concrete "top coat" on top. The first section of the job is on the second floor of the building, there is a few places its already cracked. The builder said that's to be expected since they had scissor lifts running around up there before it was fully cured. He doesn't anticipate further cracking but he did say the floor as a whole does have "a little give".

I was planning on using ditra to uncouple the tile from the concrete just to make sure the tile doesn't crack in the future. Do you guys have thoughts on whether this unnecessary precaution or worth the time and money for underlayment.

It seems like there's conflicting info about setting tile directly onto concrete, I've always recommended using a uncoupling membrane but I'd like to be more informed about how often cracked concrete will actually cause issues, particularly this slab stress core stuff.

Thanks!
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Unread 10-08-2019, 09:21 AM   #2
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Welcome, Jaymi.

Are you talking about pre-fab hollow core concrete panels made specifically by StressCore, Inc. or something else? I'm not familiar with the term "stress core concrete" is why I ask.
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Unread 10-08-2019, 09:37 AM   #3
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Thanks! I don't know why I haven't taken advantage of this platform sooner.

Yes thats the stuff! I hadn't heard of it or installed on it before either.
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Unread 10-08-2019, 09:59 AM   #4
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per the TCNA Uncoupling is-- not for use over above-ground structural slabs and other floors subject to movement and/or deflection.(page 102)

ok go.
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Unread 10-08-2019, 10:05 AM   #5
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Are you the tile installation contractor or the "design professional" on this project?

The TCNA Handbook has a number of Methods listed for above-ground concrete installations, some having a higher degree of tolerance than others. All of them include a paragraph of Limitations that always included the following:

"Requires additional consideration by design professional to accommodate movement and/or deflection. Setting materials with improved bond strength and deformability are required."

That would apply especially to the direct bonding to the concrete with thinset mortar (F113A) where an A118.15, or better, mortar is required.

The Handbook lists a handful of mortar bed installation methods and a number of thin-bed methods, including those using Crack Isolation Membranes and others. I don't recall any Methods for that application that specify "Uncoupling membranes," I think probably because there is still no industry standard for those membranes. But A113A does indicate that an uncoupling membrane can be specified, but cautions that you need to "[c]heck with membrane manufacturer for recommendations and requirements." I would want that in writing.

The TCNA's preferred recommendation for such installations is the unbonded mortar bed Method, F111.

My opinion; worth price charged.
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Unread 10-08-2019, 10:19 AM   #6
Jaymi Cross
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Wow that's very helpful thank you! I haven't done a unbonded mortar bed before but I'll do a bit of research and see what I think. I'll contact Schluter as well and get their opinion. I would assume Ditra would qualify as a crack isolation membrane. Probably just semantics whether its called "uncoupling membrane" or "crack isolation".

You said you'd like to get Schluters advice in writing? Or have the builder sign off on the decision we make?

I am the tile contractor not the design professional. I don't know if they have a design professional lol. Unless that would be the GC in charge of the build.

Thanks again this is a 10,000 sq ft total job and I really want to make sure its done right. It's my first job this big.
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Unread 10-08-2019, 12:08 PM   #7
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Ditra is not a crack isolation membrane nor does it claim to be.

It is uncoupling.

the difference is not just semantics.



https://terrylove.com/forums/index.p...rane-do.56540/

From the TCNA handbook:

Uncoupling membrane: An uncoupling membrane is a plastic membrane system geometrically configured to provide air space between the tile and the substrate to allow independent movement between the two and limit the transfer of stresses.

From the TCNA Handbook:
Crack Isolation Membrane: Crack isolation membranes (ANSI 118.12) for thin-bed ceramic, glass, and stone installations act to isolate the tile from minor in-plane substrate cracking.

Uncoupling and crack isolation membranes have two very different purposes. An uncoupling membrane protects the installation from differential expansion/contraction between the substrate and the tile relieving almost all shear stresses.



Uncoupling and crack isolation membranes have two very different purposes, and comparing them, or grouping them in the same category shows lack of understanding of their purpose and application. An uncoupling membrane protects the installation from differential expansion/contraction between the substrate and the tile relieving almost all shear stresses. It is for this reason that there is NO need to bond the tile with a modified thinset. The fact that porcelain and glass tile installations have survived intact for centuries with an uncoupling layer (in the old buildings, it was a compacted sand layer) and the tile held down with just an ancient, unpure version of modern cement says something, if you'll listen...uncoupling membranes work, and unmodified thinsets work on top of them.
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Unread 10-08-2019, 03:27 PM   #8
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Thanks for the information. Much appreciated.
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Unread 10-08-2019, 06:18 PM   #9
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My experience from the good ole days as a GC says your greatest risk is going to be at the joints between the panels, particularly if the concrete topping is not reinforced.
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Unread 10-08-2019, 06:46 PM   #10
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Quote:
Originally Posted by Mike
Uncoupling and crack isolation membranes have two very different purposes, and comparing them, or grouping them in the same category shows lack of understanding of their purpose and application. An uncoupling membrane protects the installation from differential expansion/contraction between the substrate and the tile relieving almost all shear stresses.
Mike, if you actually know a way to differentiate between those categories of membranes and a method to test that difference, the ceramic tile industry wants to hear from you. I'm quite serious about that.

Various committees and subcommittees of the various technical arms of the industry have been trying for years to come up with a standard for uncoupling membranes without success. Most recently (back when I still regularly attended such committee meetings) they decided initially to determine the difference in application of crack-solation and uncoupling membranes and to the best of my knowledge have not yet even managed to establish that. Not even the claim of the profiled membranes being better suited to installation over green concrete proved accurate.

A metric ton of energy and dinero has been spent in this effort by a lot of well-meaning representatives of a lot of notable companies over the years and we're still left with nothing more than advertising claims. The only agreement to date that I'm aware of is that the uncoupling membranes now must be bonded to the substrate with a shear bond strength of at least 50psi. That was the result, as I understand it, of a very large and expensive failure of one such membrane where the entire membrane came unbonded from the substrate and the manufacturer said in his defense that it uncoupled the tile installation as it was designed to do.

Establishing a standard for the uncoupling membranes is a complicated problem and if anyone out there has a good idea how to do it, his input would be most welcomed by some very serious folks.
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Unread 10-09-2019, 08:57 AM   #11
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Unread 10-09-2019, 01:49 PM   #12
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An unbonded mud bed is the safest bet for this situation. You may get an uncoupling membrane manufacturer to sign off on it, but I wouldn't have back when I was the one doing the signing. The problem is the deflection and the chance for out of plane movement. You really need to isolate what's above from what's below.

As for the test methods for uncoupling vs. crack isolation, there has been some good work done by the committee recently that involves building an assembly and then subjecting it to lateral stresses on the Instron machine that can measure the amount of movement before failure and/or the amount of movement back (recovery) once the force is removed. For a typical, flat crack isolation membrane the amount of lateral force it can withstand is much lower than an uncoupling membrane. I left just before the round robin testing began but the preliminary data I saw from Schluter and Laticrete seemed encouraging. Still, neither would be my preference in this situation. Good luck with your installation!
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Unread 10-09-2019, 05:00 PM   #13
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Dan, is that particular Instron machine the one where there were only something like three in the world and they cost about a brazillion dollars and can't hardly anybody afford to have any such testing done?

I recall some discussion of such a testing device at the last meeting I attended (MMSA?), which was now four years ago or so, and only a few people in the room had ever seen or heard of such a machine. You got any specifics on it? I'd like to try to look it up online and see just what it's supposed to do.
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Unread 10-10-2019, 11:05 AM   #14
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They aren't for the home enthusiast, they are pricey machines but the concept is pretty simple. They push or pull a sample and measure the amount of resistance the sample provides to the pushing or pulling and how far apart the top and bottom plates have moved from each other. If you clamp a piece of silicone and pull it apart, it will stretch quite a bit with minimal force before it breaks. If you try the same thing with a tile, it won't stretch and the force will go up and up until ultimately the piece breaks. Here is a link to the company page: https://www.instron.us/en-us/product...0aAo53EALw_wcB

For uncoupling membrane, the idea is to bond tile to the top and concrete to the bottom at an offset so the only thing being stretched is the membrane. By pushing them together, the membrane moves. It either continues to deform, or comes unbonded.

TCNA has an Instron as do MAPEI, Laticrete, Schluter, and I'm sure several other companies in the setting material and membrane game. It's used for a variety of testing (such as bond tests for A118.15 mortars) so every company needs access to one to check their products as they are being developed. The discussion you were probably in on was in regards to the size of the Instron. This testing requires a fairly large machine which not everyone has.
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Unread 10-10-2019, 11:23 AM   #15
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Quote:
Originally Posted by Dan
The discussion you were probably in on was in regards to the size of the Instron. This testing requires a fairly large machine which not everyone has.
That's most likely correct, Dan. Instron makes a whole gaggle of testing machines and without a specific model number or description of the tool in question it's difficult to research any further. I see nothing on your linked pages that at all resembles the verbal descriptions of the machine discussed at the meeting in question.
Quote:
Originally Posted by Dan
For uncoupling membrane, the idea is to bond tile to the top and concrete to the bottom at an offset so the only thing being stretched is the membrane. By pushing them together, the membrane moves. It either continues to deform, or comes unbonded.
Not to belabor the point, but that sounds very much like a unidirectional version of the test used for crack isolation membranes.

How would that help differentiate between the uncoupling membranes and the crack isolation membranes?
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