screed drying times and UFH

[madge1]

Morning all

Got a question about screed drying times...seems to be confusion as to how long to wait until we can put the UFH on.

It's a normal sand/cement screed, about 18sqm - about 95mm thick

The builder says 1inch per week. The heating engineer says 1mm per day.

So one is saying [roughly] 4 weeks, the other 13 weeks. A big difference! We've had about 7 weeks so far.

There seems to be two British Standards going on; the heating engineer appears to be following BS 8204-1, and the builder BS 1264-4:2001 (which rather says 21 days, and most screed should be 75mm thick. So the inch per week).

We have a couple of rads in the kitchen as well - obviously their pipes go under the screed - they've been on - to counter the cold, and there's no cracking from them.

Don't really mind either way, but would prefer it on as I think it's going to get colder. Also want to test it fully before it gets too hot in spring!

We've a flooring man coming around on thursday to check - would he be able to give us a definitive answer with a hygrometer do you think?

So - basically - what's the answer? One inch per week, or 1 mm per day? Or the longer the better? Or something else?

ta

 

[dcawkwell]

After 4 weeks concrete/screed will have hardened plenty.

You can drive cars over a concrete drive after only 1 week so I don't see a problem. Set the ufh on low to start with.

 

[Agile]

If I had been mine then I would have put it on to warm up the speed the curing screed albeit on very low at about 20 C and increase a degree per day!

When I worked in the middle east it was amusing how reinforced concrete floors were laid one day and the shuttering taken down the next day. But then the daytime temps were about 35-45 C

Tony

 

[madge1]

Thanks people

I'll have a go at turning it on (low setting etc).

Just don't want to scupper things for the sake of a couple more weeks

 

[Dan Robinson]

If I had been mine then I would have put it on to warm up the speed the curing screed albeit on very low at about 20 C and increase a degree per day!

When I worked in the middle east it was amusing how reinforced concrete floors were laid one day and the shuttering taken down the next day. But then the daytime temps were about 35-45 C

Tony

When I lived in Saudi Arabia it was amazing how many buildings had large chunks falling off.

 

[Agile]

That's because they skimp on the amount of cement to use!

Due to the buckshish arrangements the value of saved cement can be quite considerable.

On Nigerian TV I saw this Nigerian Chief called Dermot Ryan ( or similar ) who was a roads contractor being interviewed about why a bridge that he had built had collapsed.

Chief Ryan looked straight at the interviewer and replied "well some bridges just collapse" as in a well practiced doorstep drive tarmac seller!

 

[Blagard]

There seems to be some confusion in the thread about drying and curing.

I would expect to be able to apply heat after 7 days almost irrespective of thickness as after 7 days cement will have cured pretty well - it will continue to for a long time after that and continue to gain strength. Applying heat too soon does affect curing so you want to give the mix time before working on the drying out

Drying on the other hand does usually depend on thickness but with UFH I would expect this to be accelerated somewhat but only after the initial curing period!

Next time you pass the Holiday Inn at Brent Cross bear in mind that as a twelve storey hotel a complete floor was cast each week so the strength of the concrete at 7 days was the issue. To achieve this time span a higher cement content concrete was used than would otherwise be necessary to ensure the concrete had cured to sufficient strength to allow construction to proceed. For controlled curing test cubes are kept in a tank of water and are usually tested at 7 days (As a check on development) and at 28 days to check if design strength has been reached. Traditionally in a hot summer it is customary to cover concrete and keep it wet while the initial cure is taking place.

 

[madge1]

There are many underfloor heating websites http://www.nu-heat.co.uk/s.nl?it=I&id=573 which suggested a millimetre per day for screed. To quote "In other words for a 65mm screed there is a recommended drying time of 80 days. "

 

[Agile]

The confusion continues!

The curing is the cement becoming hard. To do that it needs plenty of water. Curing depends on the temperature! That's why in Saudi they can take shuttering off the next day. At cooler UK temperatures its well cured in 5-10 days but continues to gain hardness over several years.

Drying is another aspect and primarily affects the next building stage like decorating or laying wood flooring. This is where the long times get quoted and depends on ambient humidity and temperature. This is where using the UFH to speed drying is useful. But it has to be applied gently from a low starting temperature to avoid cracking.

 

[Blagard]

There are many underfloor heating websites http://www.nu-heat.co.uk/s.nl?it=I&id=573 which suggested a millimetre per day for screed. To quote "In other words for a 65mm screed there is a recommended drying time of 80 days. "

Great on a building site where the UFH is not switched on until much later. That is the natural drying time without heat being applied. It's purpose is to ensure that an impermeable finish can be safely applied or an adhesive that may be sensitive to dampness.

As for hot conditions - True a faster cure , but protection is essential.

http://www.cement.org/for-concrete-.../concrete-construction/curing-in-construction

 

[Richardthe3rd]

On no account should UFH be used to cure a concrete screed!! Concrete can take twenty years to cure, so I'd give the screed as long as possible to cure.

With UFH you have plastic pipes encased in concrete, this is not a bridge, building or anything else. It's a thin screed & if it cracks you'll get major leaks, so the last person I'd listen to is some hairy ar.ed builder. Just ensure Bob the Builder has enough expansion joints in his screed & if there's any pipes in the expansion joint, make sure they're sleeved.
HTH

 

[muggles]

Having seen the effects of trying to cure screed with UFH, or turning the UFH on too soon after screed has been laid, whichever way you want to look at it, I'd recommend leaving it as long as possible. I've seen an entire floor having to be carefully dug up again and re-laid after screed was warmed up too quickly.

The best people to ask will be the screed manufacturers, they will know the properties of their particular mix better than anyone. Whatever you do though, start cool and very very gradually increase the temperature over a week or two

 

[ALEC1]

the best method to cure the concrete is to use a screed drying programme that comes with most controls. Vaillants rises a degree a day then peaks for about 3-5 days then drops down a degree a day...it works an absolute treat.

Viessmann gives you all sorts of profiles too...so on the basis what works in one country (germany) works in others, it is absolute twaddle to say that UFH should not be used to cure concrete!

 

[Richardthe3rd]

the best method to cure the concrete is to use a screed drying programme that comes with most controls. Vaillants rises a degree a day then peaks for about 3-5 days then drops down a degree a day...it works an absolute treat.

Viessmann gives you all sorts of profiles too...so on the basis what works in one country (germany) works in others, it is absolute twaddle to say that UFH should not be used to cure concrete!

I completely disagree Alec. You've quoted details from boiler manufacturers only??!! When have boiler manufacturers known anything about concrete? Also, why should advice from a boiler manufacturer be trusted? They can't manufacture a boiler to last more than a few years, yet they are experts in concrete?

 

[Norcon]

28 days is recommended before heating is turned on.

When they built hoover dam they had to run cooling pipes through the vast mountains of concrete to slow down the setting process!