New screed problems - part of the floor moving.

[RedHerring2]

I hope someone can offer some advice. Sorry about the long story.
We recently (beginning of July) laid the concrete slab in the house, about 60 square meters, 6 cubic metres of concrete. ( A renovation project in France).
I was going to lay slabs straight on the concrete but due to the unsatisfactory levels I thought a screed and tiles would be a better but less expensive job.

I'm confident of doing about 1 cubic metre a day but the screed would have been about 2 cubic metres and having asked around, an acquaitance, who is English, working here in France as a mason said he could do it in one day, I would do the mixing and shifting, and he would want 150 Euros.

Well, half way through the afternoon he took a phone call and he had to rush off for a delivery of materials on another job. So I finished off the bay that he had started, which meant that just over half the floor had been done.
He returned at about 5 PM and suggested that he'd return tomorrow to finish it. The whole idea of paying someone else was to do it all in one day.

He returned the next day at about midday and we completed the rest of the floor by about 6 PM.

It looked OK and I paid him. I damped it down each day for about 3 days before walking on it.

Now 1 week later the join of the two halves is very obvious and along this joint the two halves flex under foot differently. There's about 2 square meters of the second half that is not adhered to the concrete subfloor and about 1 square meter of the first half not adhered.

I realise I'm going to have to lift these sections and try to redo them.
The prospect of redoing them doesn't bother me other than it's annoying but advice on procedure would be appreciated.

 

[theoldun]

Your screed must be only 33mm thick. Bit thin. Never mind.
What we would do.
Cut back to solid with a saw cut straight edge. Not feathered edge.
SBR mixed with Portland cement for slurry We use Febond. Read instructions on tub for mixing ratio, especially about not to slurry more than you can cover in 15 minutes. Well coat the cut edge, lay screed over in normal manner. Damp down over couple days. Should stick like s**t to blanket.
oldun

 

[RedHerring2]

Thanks 'Oldun. The screed ranges from about 25mm to about 50 mm. Hence the reason to go with screed and tiles as against slabs.

The straight cut edge is no probs. I have a 9" cutter and the screed will lift easily, it sounds hollow and gives when someone walks on it. So finding a sound part should not be difficult.
Could I use PVA instead of SBR? I can get PVA here easily but never seen SBR. I might have to get that from UK.
There is a DPM in the concrete slab so no reason for it to get wet again in the future.

The slurry is no problem I've done that before on semi-engineering bricks before rendering. Should I coat the floor as well, just for good measure?

Will it help if I stagger or 'castellate' the cut edge, like cut 'V' shapes into the old screed rather than have a straight-line edge?

Thanks again.

 

[theoldun]

Be very surprised if you can not get SRB in France. It is a universal product.
SRB and PVA are two different families
PVA is designed to be mixed with water to provide a sealing and bonding coat.
SRB is designed to be mixed with water and Portland cement to give a sealing and far superior bonding coat..
Years back all we used was a cement slurry for bonding just prior to laying.
Don’t cut at an angle, cut straight and give edge a real good brushing with the SBR mix.
Cutting at angle weakens the edge by 50% where you knit in. Think about it
Keep her well damped down with poly over top for week. She won’t shrink out on you then.
No need to seal her prior to tiling, providing you use correct addy.
If you can not source SBR, then use PVA.
oldun

 

[RedHerring2]

Just an update.
The new section has been down a week now with no problems.

I cut round the hollow sounding part of the old screed with the 230mm cutter.
Once I started to lift the old section I could easily slide a shovel under it and prise it up in large sections. A few taps with the hammer and it broke up into pieces. There was no bond to the concrete beneath at all.

I painted a slurry of PVA and cement (1 part PVA , 4 parts cement) onto all cut edges and the floor, peaking it up with the brush. (The instructions said 5 parts cement but it would have been rather thick to paint on. Troweling on would have been more likely)

The following morning it was hard as steel. There was no way it would come off the concrete. I know the old fella said only cover as much as you can do in 15 minutes but I thought that if the PVA was going to revert to liquid in contact with water then what's the harm in that.

Then just re-screeded as normal, although, due to the 'primer' the concrete slab didn't soak up any water at all, so the screed seemed a little too wet, even though it was a semi-dry mix.

I just hope the rest of the floor is going to be OK now 'cos there's no way I'm going to be able to lift the new section and I've calculated and cut the stringers for the stairs, which I'm going to be constructing over the next couple of days.