4:1 screed with fibre

[spooner68]

Does anyone have any idea on the drying time for a 70mm 4:1 screed with PP fibres? I know that the BS is 1 day per mm up to 50mm and then 2days for everything above 2mm but the info I have says that the fibres reducevthe drying time but dont say by how much.

The builder hasn't a clue, we asked him to use a fast drying screed as the new floor runs up against an existing one making the whole room 52m. I tiled the old area yesterday so that at least most of the new kitchen can be installed otherwise it would be weeks?

Any info gratefully accepted

[Knackered Knees]

Too many variables to give advice. Save yourself any further guesswork and invest in a quality moisture meter. It will pay for itself in no time.

[jay]

Ajax 123 would be the person to talk to
if it was mine id play it safe and wait the recommended time (recommended for normal screed)

[Ajax123]

Fibres have absolutely no effect whatsoever on drying time. They are simply there to reduce the screeds propensity to crack due to shrinkage. That is their only function and they do nothing more. Unless there is a specialised plasticiser to reduce the overall water content then you are stuck with BS drying times.

Additionally there will need to be a joint in your tiles where the two slabs meet.

You could speed things up by using an uncoupling membrane or possibly a surface damp proof membrane to trap the moisture in the screed.

Drying time for 70mm would be in the region of 90 days to full dry.

[spooner68]

I told the customer 90 days yesterday. The supplier didnt even know. No way I'm risking doing it earlier unless I get a written ok from the supplier and getthe client to sign a disclaimer.

Funny that several supplier/manufacturer websites say it will speed the drying time but don't say by how much.

[Ajax123]

disclaimer not worth the paper it is printed on. If you do the job knowing it may fail and then it fails legally you will be responsible I am sorry to say. Typically with a 1:4 that has been properly compacted and cured would be suitable to tile onto after about 6 to 8 weeks. 90 days will give you plenty of comfort zone. Which suppliers say that PP fibres will speed drying. I will check it out cos unless they are some sort of special fibre it is untrue.

[spooner68]

disclaimer not worth the paper it is printed on. If you do the job knowing it may fail and then it fails legally you will be responsible I am sorry to say. Typically with a 1:4 that has been properly compacted and cured would be suitable to tile onto after about 6 to 8 weeks. 90 days will give you plenty of comfort zone. Which suppliers say that PP fibres will speed drying. I will check it out cos unless they are some sort of special fibre it is untrue.

Fair point, thanks. I have pencilled it in for 6 weeks as its a sunroom and very warm with good ventilation. Will do a moisture check the week before and see what shows.

As for the drying time, I did a Google search
"4:1 screed with fibres drying time"

and even in the results summary it says it. Unless of course I'm mis-reading stuff. If you follow some of the links it does say that it strengthens the mix and shortens the drying time. Either way none of them say what the reduction is.

[Ajax123]

Polypropylene fibres do not add compressive or flexural strength to a screed. As I said before they they simply add tensile resistance to the surface matrix of the screed and help to prevent or control cracking due to shrinkage. They can add resistance to freeze thaw spalling in concretes but this is not a requirement for a screed. They do not reduce the drying time in any way - simple as that. If someone is making a claim that they do I would like to see evidence and research based test data to support the claim. Fibres are often incorporated into fast drying screeds but that is as close as I can get you technically speaking. In fact all floating and unbonded screeds should be reinforced with PP fibres or D49 steel mesh as a crack control reinforcement.

[spooner68]

D49 steel mesh as a crack control reinforcement.

Aha, chicken wire.

This might explain why the supplier was a bit vague with the builder. I'm a bit miffed (and so is the client) that they left the screed so late and didnt use a rapid set. Also it needs SLC over a large part of it as its seems to have slumped or been set low to start with . About 15mm. I'm not too happy about the whole thing and have planned for a joint on the margin of the two areas already. Client not happy about that either but I said I wouldnt do the job if they insisted on not using it. Pointed them to the BS to shut them up.

[Ajax123]

Do you know Spooner the more I read this one the more I see it a fraught with danger. Sand cement (semi dry) does not slump like that so it has either been laid too thin which means it could become unstable and crack or it has curled badly and you are seeing the effect of it dishing........I would guess the former rather than the latter.

Is the screed hard through its full depth. You might want to check because if the screed installer has done finish badly it is entirely possible that the mix is sub standard or the level of compaction inadequate.

Over insulation you need a minimum of 65mm depth of sand cement screed (typically 75mm over underfloor heating). You would be wise to break out a small chunk (maybe 2 or 3 square inches) just to check the structure of the screed. If you do and are not sure post some pics on here an I can advise accordingly.

Sorry to put a downer on your job but you would not want to put the tiles down and then have a problem due to the screed would you.

[spooner68]

Funny, but I quizzed the building foreman today when I was finishing the first half (limestone by the way). The surface was breaking up a bit here and there and there were indentations where the plasterers had put their staging. Clearly the screed was very fresh when they did this, probably less than 48 hrs old.

Whats the best way to break out a sample? Just a hammer and chisel? There was so much crap on it when I started you couldnt see but I cleared everything off and the "dishing" is clearly visible.

If its not sound how do I prove it and how do I make sure that my client knows I'm not being arsey and ensures that the builder puts it right?

Thanks for the thought Ajax.

[whitebeam]

I laid a ready mixed screed a few weeks ago which was dropped outside the house on a plastic sheeting, it could still have used the next day.

[Ajax123]

hammer and chisel is easiest. If it is not well compacted it will show relatively solid at the top of the screed and as you go down into the screed body you will see more loosely packed and more friable material.

[jay]

take it theres no ufh involved in this job

[spooner68]

take it theres no ufh involved in this job

thankfully not. One complication less eh?