Curling screed

[Mathew Newton]

I am currently being an extension to increase the size of our lounge and have had tradesmen in to the job, which included laying an unbonded 60-80mm fibre-reinforced sand and cement screed to finish flush with the existing lounge floor and a concrete threshold running along a set of doors. Here it is:

This was done 6 weeks ago and, as per the screeder's instructions, it was left covered with a polythene sheet for a week to help the initial cure and has been left uncovered since at a room temperature of ~16c - not for any particular reason; although I was mindful that the slower it dries the less issues I would likely have.

Unfortunately I noticed today that the slab has risen ~7mm along the join with the door threshold as seen here:

(The black sheet on the right is the separating barrier between the screed and the concrete slab underneath but it is very thin and so not affecting the measurement as much as it might appear).

From what I have read, unbonded screeds can be prone to curling as a result of the top surface drying out quicker than the bottom and so whilst it might not be that surprisng that this has happened I am keen to get it sorted. It's not just the height differential that concerns me (some self-levelling compound to bring the lower levels up could likely sort that) but more the fact that you can feel movement in screed if you jump up and down there. There's no cracking (yet!) but it is definitely flexing.

The screeder is coming out on Thursday to have a look but I would welcome anyone's thought on what options I have, noting that I'll be covering it with a 15-18mm engineering wood floor eventually.

Thinking aloud I thought I/we could:

1. Do nothing - not really a viable option (for me at least!) as it'll obviously compromise the final floor covering
2. Bring the threshold height up with some form of self-levelling compound - this still leaves a poorly supported screed in the affected areas which may cause issues a later date (although I imagine engineered wood flooring itself provides some strength?)
3. Drill holes and force some sort of resin in/under the screed and then bring the threshold height as required - don't know anything about this as a technique; it just sounds like a possibility to me.
4. Try and get the screed to settle back down by jumping on it and fill what I assume would be a resulting crack
5. Cut and remove/replace the screed as required

I did read that sometimes the curl can be reversed by putting a polythene sheet back over the affected areas for a week to move the residual water balance. I am surprised that this could work but I've seen a few mentions of it in what appear reputable places. Does anyone know anything more about this? I suppose it's got to be worth a try before Thursday just to see if anything changes.

Grateful for any thoughts!

[Mathew Newton]

Really should proof-read my posts. This should say:

I am currently *building* an extension to increase the size of our lounge and have had *tradesmen in to do some aspects of* the job [...]

[John Rumm]

You are probably over thinking it! Leave it to settle as much as its going to, and then pack out the low bit when it comes time to lay the final floor.

[Mathew Newton]

You know what John; you've just summed me up to a tee! I can't seem to help it.

I've been reading a but more about curling (including in BS8204 'Screeds, bases and in-situ flooring' - if reading something like that is not overthinking it I don't know what is!) and it seems that the curl might actually reverse as the screed continues to dry, particularly if I cover the affected areas to help distribute the moisture more evenly.

The screeder is coming Thursday anyway but, in line with what you suggested, I may well take the position of seeing how things go. It's not like anything can really be done right now if the curling hasn't stopped (we are 42 days in to an at-least 120 day complete drying time).

[The Natural Philosopher]

My screed shrunk cracked and went wobbly - I simply poured about 1/2 gla PVA down the cracks. After a few days it was all bound together nicely and I tiled over it.

[harry]

I could never see the point of unbonded screeds. Maybe too thin or the wrong mix.

[Mathew Newton]

As your link says, for a 50m+ thickness like mine it needs to be unbonded. I suspect it is because the extra thickness results in a greater differential in drying time throughout the depth hence greater potential for movement (as in this case!).

[Mathew Newton]

[...]

An update following the screeder's visit today:

I was all prepared for the inevitable 'it'll be fine' and 'must be something I've done' lines but I needn't have been so cynical as there was none of that! The screeder was really disappointed by what has happened and whilst he couldn't say exactly what the issue was (possibly a slightly-too-wet mixture causing it to shrink more than it would otherwise done) he is keen to get it sorted.

He came up with two options: 1) Cut out the bad corners and re-screed them, or 2) remove the whole lot and redo it. We agreed that option 2 was probably better even if it is more work, although I did offer to assist with the removal. He also said he would use Tarmac's 'Truscreed' screed this time which has an admixture requiring less water and therefore giving faster drying times. This will not only avoid delaying my project (it will actually be dry before the original would've done) but the shorter drying time also means less chance of drying-related issues. Truscreed is also suitable for bonding even at 80mm thickness and so that's what he'll do this time.

So, whilst I am very disappointed that we have this problem I can't fault the screeder's response and so this is a net positive in my book. If there was any negative it's that I discovered I paid more than I could've done given he was actually a subby to the firm I contracted and they added on a decent margin. Still, at least they quoted (and delivered) whereas most companies didn't even return my calls!

[Mathew Newton]

The screeder came on Saturday to dig the old screed up; I helped him with it partly as I did feel sorry for him having to rip out his own work but also didn't want to let him loose with a breaker (my breaker I hasten to add!) so near to the new doors without supervision!

He came again today and put the new screed down:

It's a modified sand and cement mixture so will be fast(er) drying and therefore less prone to issues. Time will tell!

[John Rumm]

Had the previous screed moved any more since last discussed?