Can it be done? Are my slabs too big! Help!

[Ajax123]

Sounds like a reasonable solution to me Dave...

Just need to make sure the risks of delamination are managed cos adding the level of moisture in a wet bed mortar to the surface of the screed (assuming it is gypsum) will make it difficult to maintain a bond between the cement and the screed. Would need epoxy type primer to prevent sulphate attack on the cement interface (don't think acrylic would work with that level of moisture). If you go down the route of wet bed it would be a good idea to use a water reducing plasticiser to minimise the amount of mixing water needed.

You are right about the efflouresence both short term and long term with the action of the underfloor heating. Don't think the gypsum would cause efflouresence but it might take a long time to dry sufficiently to allow the slabs to be sealed. You can use the gypsum as a grout as well which would allow the system to "breath" so moisture could egress through the grout joints.

Would you normally seal sandstone slabs - would this prevent efflouresence in the long term?

(When I say you Dave- i mean it figuratively - I know it is not one of your jobs)

 

[DHTiling]

It's the salts coming through the joints that concerns me with wet bed fixing... even if it's a gypsum base and you wetbed with portland then it this that is the the possible cause...

Anyway, lets see what flowable screed it is and then take it from there...

 

[Perry]

or Use PTB and backerboard under the thinner ones

 

[g cambrensis]

Many thanks for all the replies.

Liquid screed was gypsum based i think and as such we used an acrylic sealer over the top.
Ufh has been commissioned and screed had been dried out fully prior to sealing.

Have spoken to Mapei and Nocros technical and they say that their flex adh can be bedded up to 25mm which is about the limit with some of the slabs.

Not to sure about the grouting yet but that will have to be flexible as well i'd guess

 

[DHTiling]

I don't know of a mapei that will go 25mm...P4 goes to 20mm...?

Also you say you have primed....Have you removed the laitence first..?

 

[g cambrensis]

I don't know of a mapei that will go 25mm...P4 goes to 20mm...?

Also you say you have primed....Have you removed the laitence first..?

sorry, your right 20mm the mapei goes to. Any idea of any that can deal with a deeper bed. the Nocros brand goes to 25mm

Washed floors down, dried them out, and hoovered the lot before sealing.
Another tiler called in today. See what he comes back with price wise. Any ideas on what would be reasonable on 80 sqm?

Thanks

 

[DHTiling]

Have you removed the laitence first????? it won't wash off...it needs to be scraped or scabbled off..

 

[g cambrensis]

Have you removed the laitence first????? it won't wash off...it needs to be scraped or scabbled off..

How do you scabble the floor? Sorry Complete novice on your hands!

 

[Gazzer]

How do you scabble the floor? Sorry Complete novice on your hands!

Various ways from a good stiff floor scraper and loads of effort to hiring a scabbling machine. The main thing is that the top (soft ) layer shoould be removed as this is the laitance which will cause the problems later on.
When scabbled or scraped use a hoover to remove the left over laitance and prime floor as nescessary

 

[Ajax123]

YOU DO NOT SCABBLE A FLOOR SCREED!!!!!!

Scabbling is used for removing laitance from a concrete floor in order to expose the coars aggregate within the surface matrix. This provides a suitable mechanical key when bonding levelling screeds or structural toppings. It is far too aggressive to use this method on a gypsum scred or any other screed for that matter. If you scabble it you run the risk of turning it into crazy paving or even to gravel. additionally if you scabble it you are likely to remove between 5 and 15mm of the screed. As these types of screeds require a minimum 25mm cover to underfloor heating pipes this could also lead to movement cracking during the operation of the heating as there is not enough screed left. You also run the risk of damaging the underfloor heating pipes.

DO NOT SCABBLE IT!!!!

Firstly you need to determine if there is a skin on the surface of the screed which is not a physical part of the screed. This will be apparent if the screed is the old fashioned anhydrite type. It is not present in either the new generation low laitance anhydite screeds or in alpha hemi hydrates. If you let me know which screed supplier was used I will be able to tell you which it is although if there is a skin it will be immediately apparent. To remove this the best method is to use a rotary floor scarrifier with a 60 grit carberundum sanding disk. It depends how long the skin has been left in place as to how difficult this will be as it hardens with age. If this proves difficult a copper disk can be used although you need to take a bit of care as the copper disk is quite aggressive. If it is old fashioned anhydrite and the skin is not removed the tiles WILL fail.

If it is the new generation low laitance anhydrite this is designed so as no skin forms on the surface. This means that you only need to sand the surface in order to rough it up and form a key for the primer and adhesive, rather like sandiong woodwork before painting. This will also have the benfit of removing any construction contamination such as plaster and mortar debris. Again a rotary floor scarrifier with a 60 grit paper or a carborundum block sander is suitable for this application. Make certain that you vacuum the dust up.

Scraping can be adequate in some curcumstances but the manufacturers of the screed usually recomend sanding as the best method.

DO NOT SCABBLE THIS FLOOR!!!

I do not rate most acrylic primers on these screeds although as with all things some are better than others. The key is to ensure that the primer forms a chemical barrier between the cement based adhesive and the screed. If this barrier is insufficient the cement will react with the sulphates in the screed which migrate through the primer during the normal aquadynamic operation of the floor. It is even more important with heated screeds. I have unfortunately seen many instances where tiles have failed as a result of inadequate priming. I would suggest several coats personally if you must use this type of primer. A much more robust solution would be to use a water dispersible epoxy primer which will act as a barrier to the aforementioned sulphates and is not affected by moisture and does not soften when heated.

Even better still would be to use a gypsum adhesive so you have like for like materials. I am awaiting a call from someone to let me know if it can be used at 25mm deep. I have seen no failures to date where this type of adhesive has been used.

Alternatively use a decoupling mat which wil also serve to divorce the tile adhesive form the screed. I have also seen very few failures where decoupling mats have been used.

Just in case you did not hear me YOU DO NOT SCABBLE FLOOR SCREEDS!!!!