Pouring concrete with overnight frost?

[flanagaj]

This week is looking nice and dry with daytime temperatures in the region of 14c-18c, but with low temperatures overnight.  If the concrete is poured in the morning and I cover the trenches with a concrete frost protection blanket, will that suffice and ensure that the concrete doesn't suffer irreparable damage. 

[markc]

In trenches you won’t have any problems (unless you are pouring into permafrost) as the ground will be several degrees above freezing.

[SteamyTea]

From my limited understanding of the chemistry involved, once the water is added, the reactions start.

So if warm(ish) when poured in the morning, a frost should not bother it.

 

[Russell griffiths]

For footings, pour it in get it level go to pub, that is it. 
do not cover, waste of money. 
slab, that’s a different matter, finished floor that’s a very different matter. 
 

[Oz07]

Yeh trenches be fine. I'd like to think now we are past the stage of hard ground frosts. I'd like to think we're past the window scraper stage tbh

[saveasteading]

Agreed.

But as I say to everyone.  Do not add any water to the supplied mix. That spoils the concrete but also remains as liquid water and might freeze.

If using a builder tell them this strictly and also the concrete driver. No added water. 

If requested the driver insists on a signature to absolve his company.

 

Your concrete will be safe after about 4 hours.

 

@flanagaj you haven't set out the building, let alone dug the trenches. Ambition is great but assume there will be challenges. You may hit obstructions.

How are you going to control the concrete level?

[flanagaj]

6 hours ago, saveasteading said:

Agreed.

But as I say to everyone.  Do not add any water to the supplied mix. That spoils the concrete but also remains as liquid water and might freeze.

If using a builder tell them this strictly and also the concrete driver. No added water. 

If requested the driver insists on a signature to absolve his company.

 

Your concrete will be safe after about 4 hours.

 

@flanagaj you haven't set out the building, let alone dug the trenches. Ambition is great but assume there will be challenges. You may hit obstructions.

How are you going to control the concrete level?

I'm going to bang some vertical rebar into the centre line of the trenches and level the tops with my laser level.  I'll spray the tops red to aid visibility.

 

I'm also hiring an inline pump so it can be poured quickly.

[saveasteading]

42 minutes ago, flanagaj said:

rebar into the centre line

OK that works. Lethal to fall on so be ultracareful.You can alternatively bang them in the sides, even using big nails or lath.

[Russell griffiths]

Side wall of trench is better. 
if you bang them in the bottom and go over by 10 mm you will never find them. 
if in the side the concrete tends to build up around the bar like a dam and is more visable. 
 

do you have a rotary laser and a receiver 

if so you fix the receiver on to a chunk of wood with a flat t shape on the bottom, tamp the concrete and put the receiver on it and wait for the beep beep noise. 
10-20 mm out of level from end to end over 15m is perfectly acceptable. 
and will be taken out with the first course of blocks. 

[Oz07]

Also I think you have to specify a pump mix so they dont just water it down. Best bet is to hire pump and concrete off same firm or at least 2 who regularly work together. 

[flanagaj]

37 minutes ago, Russell griffiths said:

Side wall of trench is better. 
if you bang them in the bottom and go over by 10 mm you will never find them. 
if in the side the concrete tends to build up around the bar like a dam and is more visable. 
 

do you have a rotary laser and a receiver 

if so you fix the receiver on to a chunk of wood with a flat t shape on the bottom, tamp the concrete and put the receiver on it and wait for the beep beep noise. 
10-20 mm out of level from end to end over 15m is perfectly acceptable. 
and will be taken out with the first course of blocks. 

I bought a Topcon rotary last year.  Invaluable piece of kit.

[Nickfromwales]

On 16/03/2026 at 07:29, saveasteading said:

Lethal to fall on so be ultracareful

Yup. These should be bent over and sprayed white. Then mark the depth with a fat permanent marker. 
 

There’s a high risk of someone slipping and falling into the trench, and if they land on a rebar it’s game over. 🪦