Concrete and low temperatures

[nickk]

Set 10 posts in for some handrails,between 1.30pm and 4.30pm yesterday,the two that were concreted at 1.30 have gone off,the later ones are still soft,these were checked at 9 am today ,I had covered the concrete with cotton bed sheets,folded over several times,obviously not enough,is there any chance they will cure with another day or two?
Thanks for reading.

 

[Shox Dr]

Has it dropped below 4*.

If so the water can expand, and will do below freezing. The concrete will crumble

 

[Screwdriver]

So long as it doesn't freeze. Don't forget the metal posts will be acting like highly conductive cold bridges. With them sticking out in the breeze they will certainly reach freezing temperatures and draw all the heat out of the concrete. I'd wrap them too if I were you.

 

[Erie Fred]

When I have had formwork poured, we stripped within an hour or so.

 

[Shox Dr]

It might set, but its still "Green" for 3-7 days. Achieving Full strength in 21 days, it gets a little harder after 28 days and thereafter but not much.

 

[premmington]

Cement, from memory, should set between 60 (or 100?) minutes and 600 minutes. It is all in the relevant British Standard (was BS 12, but may have been changed in the last 35 years). That is at a standard temperature of around 20 Celsius. BS 4550 (IIRC) covered concreting practices/requirements.

Concrete needs to achieve a compressive strength of about 5 Newtons per square mm in order to resist frost damage.

Planting concrete in relatively small amounts (section) in freezing conditions is fraught with danger of failure without precautions. - heated aggregate/water, accelerant, insulation, etc.

If you had used ‘postcrete’, there would likely have been no problem?

+1

 

[nickk]

Has it dropped below 4*.

If so the water can expand, and will do below freezing. The concrete will crumble

Yes,0-2 degrees last night.

 

[Shox Dr]

Yes,0-2 degrees last night.

its ******** mate. sorry. The water expanded while the concrete was going off. Its been hydraulically forced apart while curing.

(So I can Bugger but not be Bugger ed)

 

[Agrismith]

We had rapid set today that wouldn't set either it's just to cold

 

[Shedendman]

Been using this for years,always used it in winter

https://www.amazon.co.uk/Everbuild-ACC5L-Accelerator-Frostproofer-203/dp/B0012MN1IS/ref=sr_1_4?crid=111ZI53U45Q5S&keywords=concrete+admixture&qid=1704824056&sprefix=concrete+admixture%2Caps%2C69&sr=8-4

 

[Agroshield]

This might or might not be an option, depending on the location, but keeping the bags of postcrete somewhere warm before you use them and using warm water will give it enough heat for the initial set to occur before mother nature takes over overnight.

Mixing another 1/4 bag of cement into each bag of postcrete will help, as well as not making it too wet.

We are bad in the UK at this, because we are neither too hot nor too cold. In Saudi, all the aggregate is kept under cover to keep it cool, and they inject liquid nitrogen into the mixer truck to reduce the heat of hydration. In Sweden, the aggregate is kept under cover to keep it warm and they use hot water in the mix to promote the heat of hydration.

 

[nickk]

Thinking I’ll wait for a warm period.

 

[julianthegypsy]

In Canada I remember seeing them use insulated shuttering, which was backfilled and left in place after pouring and heated concrete. I suppose they had to come up with something when the temperature often doesn't get above minus ten for several months!

 

[Craig-SM]

I’ve used Sika Wintermix before to set concrete in freezing conditions not sure what the science is behind it but neighbour who is a council ground worker gave me some to add to the water for mixing. Posts are still solid now so guess he knows what he’s doing.

Sika® Wintermix

Sika® Wintermix is air entraining chloride free liquid admixture for mortar and concrete which also acts as an accelerator for work in sub - zero temperatures. Improves workability and freeze/thaw resistance in brick laying mortars and eliminates the need for a separate frostproofer.

irl.sika.com

 

[Erie Fred]

In Canada I remember seeing them use insulated shuttering, which was backfilled and left in place after pouring and heated concrete. I suppose they had to come up with something when the temperature often doesn't get above minus ten for several months!

Yes, the stuff I saw look like large thick bubble wrap.

 

[Anguz]

So does cement that for the last few years comes in a plastic bag go off and unusable if unopened?
Sorry for the slight tangent

 

[premmington]

Thinking I’ll wait for a warm period.

So we declare concreting a summer sport

 

[addjunkie]

So we declare concreting a summer sport

No it goes off too quick if its baking hot…….or worse before youve got it how you want it! You get surface cracking too if you dont wet the surface too to slow it down!

 

[Smallfry]

No it goes off too quick if its baking hot…….or worse before youve got it how you want it! You get surface cracking too if you dont wet the surface too to slow it down!

Indeed. I learned this the hard way. Raft foundation and floor for my old workshop. 10 inches thick floor with 24 X 24 inches round the edges. Was summer and I couldnt believe how hot it got. The aim was to have the floor a finished, smoothish surface. Ended up looking like a ploughed field as it went off far too quickly. I had to screed it after the building was finished.

Probably will not matter for fence posts though .......

 

[Screwdriver]

It is my understanding that freezing is the problem with winter concrete.The water freezes, expands and blows apart the concrete as it cures.

Is this not the case then?