Anhydrite screed with UFH - Help please

[creulworld]

Hi,

I am after some advice on my recent project of self build house.

I have recently tiled my kitchen floor which was a anhydrite screed with wet type UFH fitted. The floor has been down for over 12 months or more now and fully dried out.

I have used the following for my floor after seeking advice and using Mapei technical.

10mm Porcelain tiles
Mapei Primer G to prime the floor firstly
Mapei Mapeker adhesive using 10mm half moon trowel applied to the floor only and not to tile back. All tiles were twisted when appling to ensure good bond. This was mixed with water and not Mapei latex plus.
Mapei Grout

The floor has been down for a couple of weeks now and UFH system on.
I have noticed the odd couple of very small cracks in the grout and after tapping tiles near these cracks I can hear that part of the tile is not bonded either to the floor or the adhesive.

I had hoped I had covered all bases with this and the floor looks great. However I fear that the problem could get worse and any advise would be helpful.

 

[DHTiling]

Hello and welcome..

What make of screed is it..?

Did you remove laitence..?

How did you prime.. method..?

Was the heating commissioned first .?

What was the process of turning the UFH on after tiling ..?

 

[Richard Edwards]

Interesting post and welcome to TF

I will await your ans to Dave's Questions and see what transpires

Thanks

 

[Rich]

Im afraid it will only get worse if it is not sorted. Dave has listed all the things we need to know before any advice can be given...

 

[White Room]

Hi and welcome

 

[The Legend; Phil Hobson RIP]

Sounds a bit like thermal shock to me, hope I'm wrong:yikes:

Sorry welcome.

 

[creulworld]

Hi guys thanks for the replies,
Further to Daves questions

What make of screed is it..?

Screed was Lafarge Agilia

Did you remove laitence..?

Not sure exactly what this means guys.

How did you prime.. method..?

Primed floor with Mapei Primer G via brush and left overnight to dry.

Was the heating commissioned first .?

Yes heating had been commissioned for 12 months or so and has been in use ok. Its just that I have got round to Tiling now.

What was the process of turning the UFH on after tiling ..?

UFH was used again approx 2 days after Tiling..

----------------------------------------------------------------------

Just to note also after an inspection tonight I think the Tile has come away possibly from the adhesive rather than the adhesive from the screed.

Also its not all tiles but say 5-10% I am not happy with.

 

[DHTiling]

The laitence is a milky residue left on the surface of the screeds and needs to be sanded off prior to tiling.

And you need more than 1 coat of primer..

When you turned the UFH on , did you do it in stages or just turn it on..?

 

[creulworld]

Any residue has been removed with general traffic in last 12 months, there was none visable.

I gave it 1 coat of primer and then again within a couple of hours.

The UFH was switched on as normal which runs at about 40 to 48 degreesC

 

[DHTiling]

The residue will not just walk off with foot traffic i'm afraid.

So you just turned the heating straight on to required temp..?

You are supposed to turn it on low and then increase in small increments so you do not shock the adhesive and cause a failure.

 

[creulworld]

Ok cant say I had seen any residue then.

regarding the heating this was switched on as normal but left for 48 hours before. Any idea how long before the adhesive goes off fully then ??

One point the tiles seem fine near the flow and return from the pump set where the water would of been warmest...

If some of the adhesive has let go im hoping that just some of the tiles may need relaying ?? will this be the case ??

 

[DHTiling]

Without actually seeing it i cannot say....but if the prep was incorrect then you could have bigger probs on your hands.. fingers crossed it is just a small area and not just the start of them de-bonding.

 

[creulworld]

Dave,

Thanks for the replies. If the worse happens what method would you recommend the re-lay the floor and methods please..

 

[Ajax123]

there are a few things this could be but my first impression is that the underfloor heating was turned on too soon after tiling. Typically you would leave it for 4 weeks for the adhesive to gain it's full strength.

In terms of screed prep regardless of how long it had been down it would be best to run a sander over it before tiling. This removes any remaining surface laitance assuming there is any and also removes any dirt, debris etc which WILL have built up over a 12 month period particularly if it has been trafficked.

The use of acrylic primers on anhydrite screeds does present a risk of chemical reaction occurring between the cement in the adhesive and the screed itself. This is a well documented reaction and my own preference would be to either replace the cement with a gypsum adhesive or where this is not possible use a water dispersible epoxy primer.

Primer G is quite a high viscosity primer and in order to get good penetration into the screed surface the sanding needs to be carried out and then the primer needs to be diluted. some say 1:1. I have had better results with this primer with a much higher dilution i.e 3 or 4:1. This obviously necessitates extra primer coats. I would say 2 to 3 coats minimum. The issue is then compounded becasue whilst you say your screed was dry the primer is water based as presumably is the adhesive.

Ettringite failure would usually take several weeks to manifest itself however so I don't really think this is the main issue.

The fact that the adhesive has come away from the tile and not from the screed supports the thermal shock scenario as the expansion generated when you turn on the heating too quickly will cause a failure at the weakest point which in the case of porcs will be between the adhesive and the tile. If it were ettringite I am absolutely 100% certain that the adhesive would part company with the screed. There is likely to be some ettringite but as I say it is in my opinion too ealry for this to have caused a failure.

Remedials unfortunately are likely to be painful unless you can recover and reuse the tiles. Where I have seen this before it is likely that a patch repair to those tiles already failed is unlikely to be a long term solution and eventually you will end up replacing the whole tile area.

my suggestion would be

1. Remove the tiles and adhesive and if possible recover the tiles for re-use.
2. Run a sander over the screed surface to remove any traces of cement, ettringite and primer
3. if this leads to screed damage to the extent that a leveller is required then use step 5 otherwise ignore step 5
4. Prime the screed using 2 coats of good quality penetrative acrylic primer and allow 24 hours for this to dry
5. Place a gypsum based levelling compound and allow for it to dry. Typically a couple of days is sufficient.
6. Prime the surface of the levelling compound if required by the manufacturer (this would not normally be done but one or 2 manufacturers would ask you to do so.)
7. Re apply the tiles using a gypsum based tile adhesive.

There are 2 schools of thought with regard to the underfloor heating. Some sya it should be switched off but I am personally happy for it to be left running at a maximum of 20oC flow temperature whilst tiling.

Mapei do have a gypsum based adhesive available but as it is imported it would be relatively expensive. there are a few others available in the market if you PM me an e-mail address I can let you have some details of others which may be a bit cheaper.

 

[TrumpersUnited]

I would have used Keraquick rather than Mapeker