Debonding tiles on anhydrite screed

[jonnyc]

Just been to look at a problem floor laid by another tiler.
Anhydrite screed.
Very chalky latence not removed before tiling.
Screed not primed
dont know how dry screed was before decoupling membrane went down.
Back of alrge format porcelain not backed with adhesive.
Spot bedded with approx 40-50 % coverage to back of tile before adhesive debonded compltely from back leaving tile clean.


So plenty of mistakes , but im wondering what is the overriding factor for current problem.
Incidentally i cut through joints around tiles ,right through grout/adhesive and decoupling membrane.
Stuck a marble slab sucker on tile and pulled the whole sandwich of mixed fillings clean off the screed.

Me thinks that decoupling laid in flex adhesive will not stick to anhydrite if not dry or sealed or latence removed but logically i cant think that this debonding would physically detach adhesive bed from underside of tile.
There seems to have been some considerable pressure to make adhesive debond clean off back of tile , working on assumption right flex adhesive used.
This suggests to me that the voids in adhesive bed have caused some kind of uneven heat dispersion that may have popped the tiles.
I have heard of this before but never actually seen a case as such.
Only reason to post this is that i think this brings up the question in my mind about how one should fix a tile over underfloor heating.
The great unsaid word is that we all know that with rectified edges you cannot just bed a tile in a notch.
We have to overcome this problem by bedding out to suit.
Can be done in various ways but ultimately there will be a bed underneath that may not be full enough and what is that percenatge.

 

[DHTiling]

If the screed was not primed then there is your answer... as to why they have not stuck.. Ettrignite failure between the Anhydrite and the cementious adhesive...

You also need to know how the screed has been down as if there i s moisture still within then sulphates will migrate also causing it to debond with reaction....

Either way it deffo needs to come up... do you know the brand of Anhydrite screed.?

 

[jonnyc]

Dave thanks for response.
The point i am trying to make is that even if decoupling membrane debonded from a still wet/non latence removed and non primed anhydrite screed would this debonding cause a clean
separation from underside of tile .
I guess and i am not an adhesive manufacturer chemist that even if whole floor was debonded from anhyrdite substrate it would not cause enouh pressure to cleanly remove adhesive from back of tile.
I think there is a mat on market from memory few years back that just gets loose laid on substrate, thereby being a floatiing membrane.
This was german that always suggests there has been some considerable thought in thier process of coming to market.
Slightly off the point recently i worked for an english client who was a ships engineer.
He now sorts out major ship salvage problems around the world and he told me an interesting story, concerning the fact that whenever he went to germany he always got a room upgrade just because he was an engineer. He could have been any engineer although in this case highly specialized.
It is the same recognition in switzerland that finds proper tradespersons finding themseleves paid as well as other middle management workwrs in public sectors.
True specialists in our trade are just not appreciated in pecunary terms or professional recognition as in other countries.

 

[Spud]

Jonny the major problem with anhydrite floors is that on the continent where they have been used for the last 30 years part of the screed installers responsibility is to sand off the laitence, over here for some reason the screed installers dont do it ,sanding of the laitence is normally done within three days of laying which aids in the drying process and my opinion is that it should be the screed companies responsibilty to provide a floor which is ready to use for floor coverings not one which still needs work to take a covering, it just adds to the resistance to use of anhydrite from the following trades

 

[jonnyc]

thanks gary.
i understand the problem of anhydrite screeds .
this tiler and incidenatlly it was not me and i dont want to suggest who it might be, has made a number of errors of judgement or rather lack of product knowledge and we all need this in a profession that is becoming increasingly complex.
my only point and maybe i am not being clear enough is that i think maybe a tile could debond from adhesive just because the bedding is not quite as solid as it should be.ie 40 -50% as found in this instance.
I hear that the heat variation in bedding can cause excessive stress and debond a tile.
iN THIS INSTANCE IT IS A GIVEN THAT TILE IS STUCK TO DECOUPLING MEMBRANE.
THERE ARE TWO FAILURES .
ONE COUPLING TO SUBSTRATE .
SECOND ADHESIVE TO UNDERSIDE OF TILE.

how are we to know exactly what percentage bed we should have.
we all bed tiles in our own way.
For myeslf and my employees we always back the porcelain tile and for that matter stone before we solid bed in to laid out bed of adhesive.
this is having already laid a thin bed of adhesive to substrate.

 

[DHTiling]

If the tile was spot fixed..? then there is insufficient tensile strength of adhesive to hold them in situ and if UFH is in the screed then that will evidently be the cause of debonding the tile.. but apart from that it is really down to the screed prep etc that the installation failed.

 

[Spud]

I always thought that the tiles debonded when there was airpockets under the tile because when air is heated it expands and thats where the stress comes from but thinking about it logically its probably that porcelain tiles rely on a chemical bond in the adhesive rather than the mechanical grip you get in a ceramic tile and in stone when the adhesive hydraulically sets so thats why you need the full coverage as the greater the coverage the more of the surface area is chemically bonded
, on heated floors i tend to use 2 part these days as I have seen so many porcelain tiles come cleanly up off of heasted floors with spf adhesives even when back buttered

 

[Ajax123]

Catalogue of mistakes and any one of them could have caused the failure but in reality it is likely a combination of several.

It depends on which suplier it was as to why it is chalky on top but this does not sound like "laitance" in the form that it should appear but sounds more like segregation which is a screed defect.

 

[tommyzooom]

I have seen a few tiles come up clean from the addy (mainly large format porcs with nanolight). In my opinion some of these flexy adhesives just dont have enough polymers in them to cope with todays super dense porcelain