New Concrete Slab - Dilemma

[cornish_crofter]

Apologies for the length of this. I know it’s a bit on the long side but....

I may have backed myself into a corner with this small wetroom that I am undertaking. But I’m looking for your views on what I’ve typed below please.

The whole area is approximately 2.7m long and 1.3m wide. The former will sit at the far end meaning that the screeded area will be 1300m wide x 1800mm long. The back wall of the room isn’t square, hence the former is going to be cut to shape and will ‘fill’ the otherwise useless area at the back of the room.

Having poured the concrete yesterday (Friday) and learning today that Mapai recommend leaving 7 weeks before tiling onto concrete, I have had to consider my options.

My original plan was:
1) Pour the concrete leaving a channel for the wetroom former waste to travel until it goes under the concrete. This has now been done. The slab is 5 inches thick.
2) Fit the former using a good flexible fast setting adhesive directly onto the concrete. Use the same material to back fill the channel for the waste as illustrated in the former instructions.
3) Screed the remainder of the floor and apply a tanking solution – I was thinking of the Weber kit for £40 odd at CTD
4) Tile the former then the floor.

However

1) Mapai are adamant that before tiling onto concrete I should wait 7 weeks. This apparently is due to the thermal shock that concrete undergoes during the curing process from the end of week 1 to the end of week 7. Failure to comply with this risks cracking of tiles. However I have been told by Mapai that tiling onto concrete before the first 5 days are up after laying the slab would beat the start of the thermal shock, thereby allowing the adhesive to fully cure before and (so far as I understand) hence be immune to the thermal shock. My view is that it could work for the former.

2) Mapai were also concerned that laying the screed onto concrete before the 7 weeks are up could also cause me problems with the screed decoupling with the concrete. They also quoted about 1 day per mm of screed for complete curing and drying times. For a 3 inch screed that’s 75 days!

3) BAL on the other hand were excelling the virtues of their Greenscreed. This is apparently a slow setting flexible tile adhesive. Apparently this can be used on one week old concrete, or 24 hour old screed. The only issue that the BAL guy seemed to see was the potential movement between the concrete and the screed during the first few weeks.

4) Hanson, who supplied the concrete, don’t seem at all perturbed about laying a screed on new concrete, even when I raised the issue about the curing process of concrete. The technical rep was of the view that screed on top of concrete slab was the way to go, and it is best to do this before the concrete dries out too much, which to my mind goes against waiting 7 weeks for the concrete to dry before laying a cement based adhesive onto it (since screed is also cement based). When I raised the issue of movement between the screed and concrete he was of the view that applying the screed as early as possible (but not while the concrete is green) was the best way to go, and differential shrinkage shouldn’t be an issue over such a small area of screed. He also reckoned that wetting the concrete or adding a bonding agent before the screed went down would help if it were too dry.

Hence, having taken all this on board I reckon I have a few options

1) Stop the job and wait 7 weeks to lay the former, then screed and wait longer to tile. I think the customer will think I’m taking the mick!

2) Just carry on regardless, after all it’s only a small slab and the screed is only 2 m square or so.

3) Lay the former quickly, before the first 5 days are up to get in before the thermal shock using a good flexible tile adhesive, then concentrate on laying up the screed across the rest of the room, tank then tile. Again it’s a small area and I am unlikely to get any problems with the screed cracking. The use of a mesh in the screed would further reduce that likelihood.

4) Similar to above, but wait a week and use the Greenscreed for the former, backfilling the waste channel with ordinary fast setting tile adhesive to strengthen the floor under the former. Then screed the rest of the floor and use the Greenscreed to tile the whole floor a day later. As far as waterproofing the floor is concerned, I could
a. Use SBR in the screed mix, making it a waterproof screed
b. Apply the Weber tanking kit to the new screed
c. Use the Ditra matting. I can get it for £10.50 plus VAT a metre. I understand that it is a waterproof decoupling membrane

Finally, am I worrying about this too much? After all it is only a small area. It’s not like it’s a 5m x 5m room.

What do you think?

[jay]

hi is it going to put anyone out if you leave the slab to dry

[cornish_crofter]

hi is it going to put anyone out if you leave the slab to dry

For 7 weeks, yes it will - big time.

Otherwise I would be happy to do that. During our own property renovation that is something I can accomodate. Unfortunately, long story, they have been waiting a while for this to be done.

[jay]

shame you didnt pour it all in one go but moisture is the problem the norm here is leave slab about 30 days before rendering ajax might be the one to talk to

[Dan]

I think the quickest safest route out of all your options is to get the greenscreed from BAL and if you have issues in the future you've got it in writing off them that they advised it, maybe?

[cornish_crofter]

Couldn't really pour it in one go as I am putting in a former.

The idea was to put the former in then screed up to it. Would have probably worked fine if the slab wasn't new I suppose

[jay]

I think the quickest safest route out of all your options is to get the greenscreed from BAL and if you have issues in the future you've got it in writing off them that they advised it, maybe?

might be your safest option drop Ajax a pm might help

[kilty55]

dave was talking about a vapour membrane a while ago,dont know exactly its spec but i think you can tile quicker using it on screeds,give him a nudge crofter he may have the answer

[Dan]

might be your safest option drop Ajax a pm might help

Of course! Ajax will know what to do here! (I think?!) Send him a PM.

[Dave]

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..

[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!

[Dave]

Soz mate tiler head on..

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

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.

[Dave]

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

[Dave]

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

[cornish_crofter]

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..?

Basically, the plan was always that the former would go down directly onto the concrete. The screed would be laid on the area of concrete that the former doens't occupy, butting up to the former. Bearing in mind if it were an existing concrete floor I would have removed the screed in the area that the former would be going in so that it sits level with the screed.

As it's a new floor, former, with screed butting up to it was my plan.

I was looking to use the Greenscreed to bed the former down onto the concrete, which I will have to wait 7 days to do even if I use Greenscreed. Then my plan was to put down the screed using your idea of slurry mix or Alan's idea of SBR. Then the decoupling membrane the next day or as soon as time will allow using something like the Greenscreed. Then finally tile on top of the former and screed (all one level) using a flexible fast setting adhesive.

So you're right in a way. 7 days before the former goes down using BAL GREENSCREED, then 24 hours minimimum after laying the screed (which I'll be able to do after laying the former) for the decoupling membrane onto the screed again using BAL GREENSCREED.

[Dave]

That's it..

[cornish_crofter]

My response in Bold

Alan

Thanks for that post.

The only other idea I had was to batten off where the former is due to go (leaving an inch or so to make laying the former easier, which can be backfilled with adhesive later)and lay the screed up to that, then wait until the concrete has been down for 7 days before fitting the former. This would

a) have the screed down earlier, meaning that the screed will be on the way to being down for 7 days before laying the membrane.

b) would reduce the total lead time on the job - screed and cement are curing simultaneously.

For example. The concrete went down on Friday. Hanson reckon as early as poss for the screed whilst there is still moisture in the cement. Screed would go down on Monday up to where the former would go. Then day 1 for the screed curing would be day 4 for the concrete. I would then leave it until Friday to put the former down using the Greenscreed, level with the new screed. By then the screed would be 4 or 5 days old. Then leave it until the following Monday to lay the matting on the screed.
Does that make sense to you?

[cornish_crofter]

OK, now I need to lie down! :LOL:

[Dave]

Can you not screed your own former? then you can do it all in one go..

[cornish_crofter]

Can you not screed your own former? then you can do it all in one go..

I see where you're coming from Dave but I like these formers. Cast your mind back to 2008 when I used one on an existing concrete floor, OK I had to dig a bit of the concrete floor out as the former was thicker than the screed that was down.

The thing with these formers is that the fall is basically worked out for you. It's a Tilux as supplied by Wetdecs - again.

I did have to replace a small area of concrete underneath the floor then. Having said that I seem to remember only using about 1 mixer full. Most of the area under the former was old concrete.

I recently got in touch with the customer I did that job for to ask him how they were getting on with the wetroom. His response was along the lines of "You would have heard from me if there were any problems." They use it regularly. His GFs father, who is wheelchair bound and their 6 dogs benefit.

[cornish_crofter]

With this job starting again tomorrow, I am considering laying the screed before fixing down the former.

The approach would be:

1) Trim and position the former. Mark off where the former would end and the screed would need to begin on the floor. Remove the former and place out of the way.

2) Batten off the floor in the marked position at the correct height.

3) As I'm having to run 15mm pipes in Denso tape down both sides (heating and hot and cold water) use 3x2 battens to facilitate this and to help me level out the screed. One batten each side.

4) Coat the area to be screeded with a slurry mix (SBR and cement 1 to 1 with water). Refresh/add as screed progresses. Backfill former waste channel with screed mix as much as possible at this stage, bearing in mind that some flexibility with the plumbing will be needed for fitting the former.

5) Allow screed to dry and remove battens.

6) At this stage the screed should only be walked on when protected by suitable ply etc. Hence it is feasable to start to lay the plumbing for the towel rail and the hot and cold supplies. All this while the screed is drying out as is the cement slab ready for day 7 for when the former can go down using BAL Greenscreed.

7) After plumbing is in, tested and suitably protected, backfill those channels with flexable tile adhesive (Greenscreed not necessary, just a normal fast setting flexible adhesive will do)

8) After a MINIMUM of 7 days has passed since the original concrete slab was laid, fix the former using BAL Greenscreed. Backfill the channel for the waste and any gaps between the former and the screed with the same adhesive.

9) When this has set lay Dural/Ditra decoupling membrane (decoupler and waterproofer - right?) onto screeded area (including where plumbing is) only. Use BAL Greenscreed for this, even though the screed may have been down for a few days at this point.

10) When the Dural/Ditra has set, I will then need to run some tape on the floor/wall joints either side of the room. This will allow any tanking on the walls to effectively join the floor.

11) After the Greenscreed under the former and the DuralDitra has cured I am now in a position to tile when I am ready - ie after boxing in the plumbing for the shower.

How does that read to you?

[cornish_crofter]

Thanks Dave, I take it you approve

[Dave]

Yep.. happy screeding..

[cornish_crofter]

Well, got on with it today and on the whole it went OK, apart from the sharp sand had frozen!

I didn't use hot water to mix as I was worried about it curing too quickly and not bonding, though afterwards I realised that this probably wouldn't have made any difference and may have helped me.

I managed to do most of the area I had set out to do. Though I did start to wonder why I bothered when there's the plumbing to go down. The only benefit I can see with this approach is that the battens I had laid onto the floor helped me to get a near perfectly flat floor - running towards the position of the former and away from the door.

I had measured and cut the former before I started then left my inch then battened off for the screed. My guess is that I won't be able to do much tomorrow as the screed will still be setting. It's taking a lot longer for everything to set in this weather.

The SBR slurry went down a treat. I also applied neat SBR into the channels I backfilled as well.

When I can next get onto that job I'll be laying the plumbing for the heating and water along the floor, ready to cover and backfill. I'll probably put down a suitable length of DPC, lay the pipes on that then cover with denso tape, sticking that down onto the dpc as well as the pipes. I uncovered a similar arrangement on another job and it seemed to work well for 20 odd years. The pipes were only leaking where they had come into contact with screed or concrete.

It'll be well over 7 days before I can do the former I think - which was part of my plan