New Concrete Slab - Dilemma

[cornish_crofter]

Hugo... Hi mate..


Wow quite an issue hey!..

Ok.. Slurry coat the new concrete base.. then screed.. leave till able to walk on and then stick down Dural CI with an S2 slow set adhesive.. leave to dry.. this will cope with any shrinkage and curling, then the Dural Ci will do the vapour equalisation and waterproofing if you tape the seams as well..

Same with the former, slurry coat the base and then set in in place and cover with Dural Ci..

Now I'm confused!

Dave - I don't actually understand most of what you said!

Slurry coat - with what? Cement slurry, PVA slurry? I assume a cement slurry

Screed, I think I can manage that 4:1 sand/cement mix

Dural CI - is that similar to the schluter Ditra matting? I can get the Ditra for just over £10 per metre. Ditra is what BAL suggested.

S2 slow set adhesive. OK that leaves out the fast setting. Would the BAL Greenscreed fit that description and can you give other examples please.

I'm sure I'm capable of doing what you advice - I just don't understand it 🙂

Finally, is my approach of sticking down the former with the BAL Greenscreed sound, or should I use a different adhesive for that? I don't mind leaving it a week or so. The concrete is taking time to go off completely because of the temperature. It's above the 2 degrees here but not much, so its initial dry is slow - should be hard in a few days but can sell the customer the idea of waiting a bit.

Thanks for taking the time to answer.

 

[cornish_crofter]

I swore I'd never do another wet room last time

I can now remember why!

 

[DHTiling]

Soz mate tiler head on..:lol:

OK..

Firstly BAL will say Ditra.. kind of recommend each other but Dural is just as good and cheaper.. but Ditra will serve the purpose.

Ok Slurry coat.. SBR mixed with portland Cement or Rapid set.. screed onto the tacky coat..

S2 slow set.. Mapei Ultra flex mono is one but yes the green screed will suffice to bond the membrane with.

Then you can tile with any flexi adhesive on top of that..also using the membrane will keep efflorescence away from the tile adhesive bed and grout joints.

If using the ditra then use the kerdi band for the seams..( joints).. owt else just ask.

 

[Ajax123]

Bin drinkin the Birthday wine so won't respond tonight but will do so tomorrow. The answers are probably straight forward but typin after the wine is not.........

 

[cornish_crofter]

Thank you Dave for clearing that up 😉

I'm no stranger to SBR, wonderful stuff! I use it as a bonding agent for render etc sometimes.

Going back to the former, am I OK to go with the proposed method of bedding it down with Greenscreed after the concrete's been down for a week?

Also, I was going to backfill the channel with normal fast setting adhesive. It's wider than it should be, really because I didn't have the fomer or the tray when the concrete went down. It's a good job to as the drain out of the trap is offset, not central to the trap. Had I gone narrow I would have had it in the wrong place :lol:

Furthermore, Am I right that the Dural Ci also tanks the floor? I am told that the Ditra does. This would then eliminate my having to buy a tanking kit for the floor.

Finally, although I've been steered towards the Greenscreed, one assumes the Mapai Ultra is cheaper? I've been quoted about £23 for the Greenscreed 15kg

Thanks for your help so far guys - really appreciated 😉

 

[Ajax123]

Mapei are wrong and Hanson are right in this instance. Concrete does shrink similarly to screed but the mechanical interlock associated with the coarse aggregate tends to help to prvent curling and cracking provided it is laid correctly. Also the bay size : depth ratio is far too small for it to cause a problem.

My time scale would be

Lay concrete using mix design suitable for the application (Gen 1 concrete) Apply a trowel and then brush finish.Cover it for 7 days by covering with a polythene sheet. Do not traffick it in this period. After 7 days lift the polythene and either

Apply a coat of acrylic primer to the surface of the concrete and whilst still tacky apply the screed (I assume you are talking 50mm ish otherwise it changes slightly) directly to the concrete. If you use a brush finish there will be a very good mechanical key so no need for bonding coat or slurry. Make sure your screed is not too dry otherwise you will not be able to compact it properly. I would tend to pop to the local builders merchatn and buy some mortar plasticiser and add this to the screed during mixing. Cover the screed for 7 days with polythene the same way as you did the concrete. Curing will significantly reduce cracking and curling issues.

Lift the polythene after 7 day and then use Dural or similar uncoupling prior to tiling. I would use uncoupling due to the problem with efflorecnece as the system will not be fully dried due to the quick turn round time.

 

[DHTiling]

Top advice alan, i would still slurry bond rather than acrylic primer, stronger bond IMO.... Hugo could also use SBR in the screed mix to help strengthen.. and will add some flexibility ... but as Alan has stated, curling and shrinkage is what you need to combat..


Yes the Dural Ci along with seam tape will tank the floor.. don't forget to seam tape floor to wall transitions as well..

As for the former.. back fill with screed..

 

[cornish_crofter]

Could I reduce the week down time for applying the matting onto the screed to 24 hours if I used the Bal Greenscreed?

Alan, the concrete is already down and it's too late to brush coat it. However it has been tamped down with a straight edge so there is plenty of key on the top. You are dead right - it is Gen 1 concrete. 1/2 a cumic metre picked up from the works in my trailer and onto site! It's taking a while to go off properly because of the low temp but they have got the heating on just outside that room, which is good.

I suspect that the former will have to wait a week before going down. I'm just going to have to brave it to the customer. In the mean time I can backfill the channel as much as poss with a screed mix. If the daytime temp isn't too bad I may be able to finish the other job I've got on at the moment, which would be great.

The only problem I can see with what you suggest for this job is leaving the screed to dry out for a week, which is why I was hoping I could use the Green screed (Bal) within 24 hours to at least put the ditra/dural down.

Thanks again guys

 

[DHTiling]

BAL say you can use it as soon as the screed can be walked on.. but concrete is 7 days .. So you want to use it on screed..?

Key features: Allows fast-track installations at 20ºC
– on new concrete after 7 days
– on new cement:sand screed after 24 hours

 

[Ajax123]

Could I reduce the week down time for applying the matting onto the screed to 24 hours if I used the Bal Greenscreed?

Alan, the concrete is already down and it's too late to brush coat it. However it has been tamped down with a straight edge so there is plenty of key on the top. (SBR/Cement slurry is optional in my opinion unless the top of the concrete is disty) You are dead right - it is Gen 1 concrete. 1/2 a cumic metre picked up from the works in my trailer and onto site! It's taking a while to go off properly because of the low temp but they have got the heating on just outside that room, (if you had covered it with polythene it would have gone off quicker but too late now) which is good.

I suspect that the former will have to wait a week before going down. I'm just going to have to brave it to the customer. In the mean time I can backfill the channel as much as poss with a screed mix. If the daytime temp isn't too bad I may be able to finish the other job I've got on at the moment, which would be great.

The only problem I can see with what you suggest for this job is leaving the screed to dry out for a week, which is why I was hoping I could use the Green screed (Bal) within 24 hours to at least put the ditra/dural down. Unfortunately what ever you put on top of it the screed will still want to do what screeds do which is shrink curl and crack. I am not going to counter advice given by BAL in terms of adhesives cos they are the experts i that bit but it is not something I would use personally knowing the screed bit as I do.

Thanks again guys

My response in Bold