What are the optimal conditions for sand and cement screed to cure?

[Adsibob]

Our sand and cement screed was poured 5 days ago (about 55mm to 60mm worth). I noticed yesterday two things that made me wonder how screed cures and whether the conditions on site are okay for it to cure nicely without cracks etc:

1. the builders are storing many boxes (tiles, toilet frames, other stuff) on top of some wooden battens that are resting on the screed; and
2. big industrial heater is blowing hot air onto part of the screed in another room.

I didn't think much of observation (1) as the wooden battens, although only about 2cm high, are probably allowing enough air to circulate and I'm not even sure screed needs air to cure - doesn't cement cure underwater? We are expecting many more deliveries in the coming week, and the builders are very behind schedule so the delivered items are unlikely to be used in the next month or two. Could covering more and more of the screed with boxes be an issue for its curing? What about the area of screed directly covered by the battens, that will have very little air.

 

As for (2), this is potentially concerning. It's not particularly cold on site (we're in the South East and the last week has been mild) so I'm not sure why heating would be required at this early stage in the screed's curing. Whilst I can't imagine heating will do much harm if it was uniformly applied, but given there is only one industrial heater and about 80m2 of screed it is unlikely they will be able to apply the heat evenly, even if they move it round a lot. I asked the foreman what the heater was for and he said it was to move the air around in the ground floor a bit "to help with drying".

 

 

Should I ask the builders to make any changes or am I worrying unnecessarily?

[Mr Punter]

17 minutes ago, Adsibob said:

Could covering more and more of the screed with boxes be an issue for its curing?

 

No issue.  It will cure under water.  Rapid drying can lead to weakness and cracking.  With temperatures above 5 degrees there will be no problems.  They obviously want some of it to dry more quickly, hence the hot air.  It still needs a really long time before you lay finishes.

[Adsibob]

3 minutes ago, Mr Punter said:

 

No issue.  It will cure under water.  Rapid drying can lead to weakness and cracking.  With temperatures above 5 degrees there will be no problems.  They obviously want some of it to dry more quickly, hence the hot air.  It still needs a really long time before you lay finishes.

Thanks, very reassuring. Do you think it will be ready for floor finishes by early to mid December (so 60 days after pouring) or is that a bit tight?

Edited October 14, 2021 by Adsibob

[markc]

+1 on the above. Curing and drying out are not the same thing.

60 days is short for drying time

[Adsibob]

32 minutes ago, markc said:

+1 on the above. Curing and drying out are not the same thing.

60 days is short for drying time

So how long would you allow for 55mm to 60mm of screed to dry. Let's call it 60mm. Is 70 days enough?

[markc]

1 minute ago, Adsibob said:

So how long would you allow for 55mm to 60mm of screed to dry. Let's call it 60mm. Is 70 days enough?

“Accepted” time is 1day per millimetre for first 50mm then 2 days per millimetre after that.

75mm screed really needs 80-100 days

[saveasteading]

11 minutes ago, markc said:

“Accepted” time is 1day per millimetre for first 50mm then 2 days per millimetre after that.

75mm screed really needs 80-100 days

In real life I have always found it to be less than half that. BUT it depends on the screed having been laid with the right (low) amount of water so that there is little free water after curing.

Curing in ambient temperatures takes about 28 days, but continues thereafter. The chemistry of curing takes water into the matrix and it does  not register as free water/ dampness.

If they have added water for ease of laying (normal I am sorry to say), or allowed it to get wet then this is different, and it needs to dry

 

Ventilation is essential, more than extra heat.

 

If the heater is gas then it is pointless, as the combustion creates water.

 

A surface dampness monitor can be hired (for accurate measure) or bought quite cheaply for 'near enough' guidance.  You could insist the contractor proves it before covering.

 

Beware of carpet/vinyl contractors wanting extra to seal the slab. 1. it is better to be dry than sealed in . 2. they charge a lot and it is a good earner, often pointless...Also I have had them fiddle the readings.. or perhaps they didn't really understand.

[Declan52]

Keep the windows open fully of possible during the day and on the latch so still partially open over night to get the air circulating through the house.

[PeterW]

Dehumidifier is better than a heater but only after the first 14 days as you’ll pull water from the curing process otherwise. 

[Mr Punter]

If you gaffer tape a 400x400 piece of clear polythene on the screed where it won't get damaged you can see if any vapour condenses on the underside, or dark patches on the screed, indicating the floor is not dry.