Biscuit screed drying time

[Alex Carr]

I'm getting an ASHP & wet underfloor heating installed on Tuesday and Wednesday and an 8:1 biscuit screed laid over the top to a depth of 47 - 50mm. Solid answers to the three questions I have would be really helpful:

1) How beneficial would it be to add polypropyline fibres to the mix?

2) And in what proportions?

3) How long will it take for the mix to dry sufficiently to float 14mm engineered wood flooring over the surface?

[CalvinHobbes]

Sorry can't answer your question and you likely know this but yesterday someone reminded me to ensure a temp probe is in the wet room floor before screed and not stuck outside in the hall as an afterthought.

[saveasteading]

I'm a fan of fibres but no need in your case. It won't  crack noticeably unless you have too much water. 

Drying time varies. But if you get the water content right it nearly all disappears in the chemical reactions. The screed will be dry in days but give it 2 weeks. 

Do you have a mix recipe including the water content?

Using wet sand can mess this up.

[Alex Carr]

Hi - Well, no, I don't have a mix recipe - I'm not sure I know what you mean. How is the water content gotten right? I mean, the contractor goes to the builder's merchants, buys his sharp sand in bags, ditto cement; 8 shovels of the sand into the mixer + 1 of cement. How is there more to it than this? I mean, how does my contractor avoid buying a load of wet sand? Thanks for the pointer on the fibres; I won't add them. Apropos drying time: I thought I would use a hygromenter to measure the humidity level of the rooms where the screed is going to be laid; this will give me some indication of how much moisture is coming off the screed, though I'm nore sure how to leverage that information.

 

[Alex Carr]

2 hours ago, CalvinHobbes said:

Sorry can't answer your question and you likely know this but yesterday someone reminded me to ensure a temp probe is in the wet room floor before screed and not stuck outside in the hall as an afterthought.

I don't get what you mean, sorry. If you have a moment, please would you explain.

[saveasteading]

34 minutes ago, Alex Carr said:

8 shovels of the sand into the mixer + 1 of cement. How is there more to it than this? 

That is the recipe. I haven't checked if that is appropriate.

On the cement bag it should also say how much water to add. Most contractors will ignore this and put in a lot as it makes it much easier for them  to mix and to handle and float.

This is based on the sand bring dry. If it's wet then add less water.

Too much water and it has to evaporate, leaving that much spare space in your mortar. 5% surplus water ends up as 5% aero bubbles. Thus the screed is weaker and also will shrink and crack a lot.

 

And for precision don't  use shovels as volume guides. Use whole or half bags, or fill buckets.

 

Your builder may say nonsense to the above: most manage to avoid  knowing about the subject. It will then be your decision. 

 

An easy hygrometer substitute is an upturned glass on the screed. If it steams up it is still too wet.

 

It would be great if you can report back in 2 weeks.

[JohnMo]

2 hours ago, CalvinHobbes said:

someone reminded me to ensure a temp probe is in the wet room floor before screed and not stuck outside in the hall as an afterthought

You just don't need them in a well insulated house, unless you are logging something. They are required for heat sieve houses, that foolishly install UFH and have to flow stupidly high temperatures. A simple low hysterisis room thermostat is all you need - just one for the whole house.

[Alex Carr]

1 hour ago, saveasteading said:

That is the recipe. I haven't checked if that is appropriate.

On the cement bag it should also say how much water to add. Most contractors will ignore this and put in a lot as it makes it much easier for them  to mix and to handle and float.

This is based on the sand bring dry. If it's wet then add less water.

Too much water and it has to evaporate, leaving that much spare space in your mortar. 5% surplus water ends up as 5% aero bubbles. Thus the screed is weaker and also will shrink and crack a lot.

 

And for precision don't  use shovels as volume guides. Use whole or half bags, or fill buckets.

 

Your builder may say nonsense to the above: most manage to avoid  knowing about the subject. It will then be your decision. 

 

An easy hygrometer substitute is an upturned glass on the screed. If it steams up it is still too wet.

 

It would be great if you can report back in 2 weeks.

That's interesting: I will insist to the contractor that he be precise in creating the mix, and as we have been working well together for the past 12 months I don't expect issues. What bothers me is my ignorance about how to assess the moisture level of the the sand - which, I presume, is determined by how it has been stored; if it's outside in a big pile then the weather becomes a factor, surely? I exited the building trade more than 30 years ago, so maybe these days sand comes already bagged up - is it or not, do you know? Buckets: good idea. I ordered a hygrometer today because half of our 200 year-old Edinburgh property is a basement that was soaking a year ago because of decades of v. poor ventilation: we stripped everything back to the bare brickwork and the stone of the external walls and lined every surface with Permaguard joined up at the base with a DPM; fingers crossed on that. Too we have PIR insulation throughout in the various build-ups. The air feels dry, but I'm very keen to get an accurately monitor ongoing relative humidity levels. The upturned glass is a delightful idea. I'd be happy to report back. Thanks for taking the time to write.

[saveasteading]

1 hour ago, Alex Carr said:

how to assess the moisture level of the the sand

Absolutely. The ready-mix companies take a reading every day and adjust the water accordingly. but they have fancy kit for it.

 

If you wanted to be scientific about it you take a kg of sand , then dry it in the oven then weigh it again.

 

If the sand is in sealed bags and feels dry I would say it is dry.

If it comes in bulk, assume it is wet and deduct 5% of the water otherwise specified.

 

When it is just right it will be annoyingly dry for the builder to lay and float. That's great that you get on well.

[saveasteading]

I googled and found this . Seems sensible. No too wet and not too dry!

 

https://www.alphasand.in/blog/what-is-the-moisture-content-of-sand

 

Maybe someone else knows better than I, but I have seen a dry screed laid and it needed compaction (banging down with a batten) before floating. 

A bit wetter than that seemed optimum to me.

[Super_Paulie]

I thought biscuit mix had zero added water? The only water in it is what was in the bag when you hoyed it in the mixer.

I'm doing a biscuit mix myself and was planning on giving it a few weeks after compacting it in, but I'm defo not adding any additional water.