Cold Weather Concrete

[bubs88]

Pouring: 24x24 4inch thick with 6inch curb
Concrete mix: 4000psi + Hot Water + Calcium (2bags)
Poured at 8am with ambient temperature 37f
Power trowel off slab by 8pm. ambient temp dropped to 32f
Concrete guy said it will be fine without tarps(wasnt impressed). I came home around 9pm and covered as much as i could with tarps but 6" curb did not get covered.
Concrete guy showed up next day and took off forms and cut slab.
i put tarps back on

My question/concern: Am I screwed? How can i tell if the slab didnt start to cure? Will it start curing when the temp shoots back up to 53f 4 days later? I don't believe it froze the first 24hours. Looks perfectly fine from the naked eye but who knows could crumble when garage goes on it
Research tells me I'm in a sticky situation. concrete guy tells me "you'll be fine".

 

[theoldwizard1]

Power trowel off slab by 8pm. ambient temp dropped to 32f

If the overnight low did NOT go below 30F, I think you will be OK.

 

[bubs88]

Thanks for reply, definitely cut er close!

 

[Rusted Nut]

Concrete will usually not freeze on the first night unless really cold, it makes enough of its own heat. After day two as long as it’s not getting below 30 and staying there you‘re fine.

 

[bubs88]

Ok, so what would happen if concrete did get below 30f in the first few days?
Does it fall apart when a load is on it? or just not as strong as it should be?

 

[bubs88]

If the overnight low did NOT go below 30F, I think you will be OK.

Checked under the tarps with heat gun the surface of concrete was reading 40f .. the curb was closer to 32 because it was left exposed .. these are surface temps.. internally it should be higher

 

[wssix99]

The Heat of Hydration is why you are NOT screwed: https://pavementinteractive.org/ref...ion is,increased, heat of hydration increases.

I would keep the concrete covered with tarps for a few days. There's no dire danger to the pavement, but this should help keep a more common temperature profile through the cross section.

 

[bubs88]

The Heat of Hydration is why you are NOT screwed: https://pavementinteractive.org/reference-desk/materials/portland-cement/heat-of-hydration/#:~:text=The heat of hydration is,increased, heat of hydration increases.

I would keep the concrete covered with tarps for a few days. There's no dire danger to the pavement, but this should help keep a more common temperature profile through the cross section.

Thanks I’ll do that

 

[wssix99]

Here's an expert resource on the topic: https://www.cement.org/learn/concrete-technology/concrete-construction/cold-weather-concreting

Looks like temperature is only a major concern for the first 24 hours and the plastic you are using now is most helpful for preventing the concrete from drying out too quickly in the cool dry air.

 

[theoldwizard1]

Ok, so what would happen if concrete did get below 30f in the first few days?
Does it fall apart when a load is on it? or just not as strong as it should be?

YES !

 

[wssix99]

Check out the article I posted in #9 above. Once the concrete achieves 500psi of strength in the first 24 hours of cure, that is enough to hold it together and counteract any expansion from free water still in the concrete.

One thing to remember is that in that first 24 hours a lot of the water in the mix is gone - already chemically reacted with the cement.

 

[bubs88]

Check out the article I posted in #9 above. Once the concrete achieves 500psi of strength in the first 24 hours of cure, that is enough to hold it together and counteract any expansion from free water still in the concrete.

One thing to remember is that in that first 24 hours a lot of the water in the mix is gone - already chemically reacted with the cement.

Thanks, I definitely made it through the first 24 hour without freezing

 

[jkuro]

Did the concrete supplier warranty the mix? Did your concrete guy warranty the pour? My guess is, they did not. Good luck, you need 28 days to fully cure. Don't let it freeze in those 28 days. Some turn out fine, some not so much.

 

[bubs88]

Did the concrete supplier warranty the mix? Did your concrete guy warranty the pour? My guess is, they did not. Good luck, you need 28 days to fully cure. Don't let it freeze in those 28 days. Some turn out fine, some not so much.

Thanks for the reply, From what I’ve gathered the freezing issue is only in the first 24 hours approximately or until the concrete reaches 500psi ..I understand curing time slows down the colder it gets. Are you saying ppl tarp/heat their concrete for a month after the pour? I can see why that is beneficial but can’t see many projects put on hold for a month to keep tarps on

 

[jkuro]

There is still some water, if it freezes it won't be good.

 

[ConCretin]

I agree with most of the advice you've received including the opinion that your slab is very likely fine given the conditions you describe but I would offer a couple of additional points;

You don't need to protect the slab from freezing for 28 days. You just need to maintain it at 50 degrees or better for a few days so it gains enough strength to resist physical damage. If that isn't possible, just be aware the concrete is gaining strength very slowly and is susceptible to damage.

Secondly it's risky to assume that 24 hours is long enough for a slab to reach 500 psi. Concrete sets very slowly below about 50 degrees or so so it could take much longer to reach 500 psi.

And finally, the 500 psi rule assumes that by the time the concrete has reached that point, enough mix water has dissipated from the capillaries to minimize expansion and resulting damage if it freezes. This might not be true if there was excessive mix water i.e. slump and/or water was applied to the surface during finishing.

We place concrete year round and after tens of thousands of yards placed, I've never had to rip any of it out. It's pretty resilient stuff and with modest amount of planning and attention to detail, it's pretty easy to manage in cold weather.

 

[bubs88]

There is still some water, if it freezes it won't be good.

I agree with most of the advice you've received including the opinion that your slab is very likely fine given the conditions you describe but I would offer a couple of additional points;

You don't need to protect the slab from freezing for 28 days. You just need to maintain it at 50 degrees or better for a few days so it gains enough strength to resist physical damage. If that isn't possible, just be aware the concrete is gaining strength very slowly and is susceptible to damage.

Secondly it's risky to assume that 24 hours is long enough for a slab to reach 500 psi. Concrete sets very slowly below about 50 degrees or so so it could take much longer to reach 500 psi.

And finally, the 500 psi rule assumes that by the time the concrete has reached that point, enough mix water has dissipated from the capillaries to minimize expansion and resulting damage if it freezes. This might not be true if there was excessive mix water i.e. slump and/or water was applied to the surface during finishing.

We place concrete year round and after tens of thousands of yards placed, I've never had to rip any of it out. It's pretty resilient stuff and with modest amount of planning and attention to detail, it's pretty easy to manage in cold weather.

Thanks for the advice. the first 5 days the temp hung around freezing and below for periods in the night. The next couple days are supposed to pour rain and be in the 50's. should i keep it covered for the rain?

 

[Rusted Nut]

Thanks for the reply, From what I’ve gathered the freezing issue is only in the first 24 hours

You gathered incorrectly. Concrete has enough of its own heat for the first 24 hours. It the next 48 hours that are critical. However, you original post said dropping to 32 and up to 50 during the day, if that’s the case - relax you are fine!

 

[bubs88]

You gathered incorrectly. Concrete has enough of its own heat for the first 24 hours. It the next 48 hours that are critical. However, you original post said dropping to 32 and up to 50 during the day, if that’s the case - relax you are fine!

So for the critical days after the pour the concrete continues to cure and strengthen in cold temps very slowly correct? Does it speed up with increased ambient temp?
We are moving into a period of heavy rain with temps in 50s .. I’m wondering if I should take tarps off or keep them on

 

[ConCretin]

So for the critical days after the pour the concrete continues to cure and strengthen in cold temps very slowly correct? Does it speed up with increased ambient temp?
We are moving into a period of heavy rain with temps in 50s .. I’m wondering if I should take tarps off or keep them on

Your first two statements are correct. Once the concrete has had a few days at 50+ degrees or an extended period below 50 but above freezing, it no longer requires protection. If concrete temps were a lot higher during the initial curing period, you'd want to ease the temps down sgradually before exposing the concrete to freeze/thaw.

The only other consideration is curing, which ironically involves trying to maintain the moisture in the concrete long enough for hydration to complete its work. Since curing a slab isn't really necessary given your weather conditions, it's not an issue.

All that to say, the tarps aren't doing any harm but there's no real need to keep them on either.

I'd just add for general consideration that concrete's ability to resist freezing depends on its mass, exposed surface area and the ambient temps. A 4" slab might be able to resist freezing for 24 hours at a calm 25 degrees but it will freeze solid at a breezy 15 degrees. On the other hand, a 12" deep footing would probably fine in those conditions.

 

[bubs88]

Your first two statements are correct. Once the concrete has had a few days at 50+ degrees or an extended period below 50 but above freezing, it no longer requires protection. If concrete temps were a lot higher during the initial curing period, you'd want to ease the temps down sgradually before exposing the concrete to freeze/thaw.

The only other consideration is curing, which ironically involves trying to maintain the moisture in the concrete long enough for hydration to complete its work. Since curing a slab isn't really necessary given your weather conditions, it's not an issue.

All that to say, the tarps aren't doing any harm but there's no real need to keep them on either.

I'd just add for general consideration that concrete's ability to resist freezing depends on its mass, exposed surface area and the ambient temps. A 4" slab might be able to resist freezing for 24 hours at a calm 25 degrees but it will freeze solid at a breezy 15 degrees. On the other hand, a 12" deep footing would probably fine in those conditions

That's all-great info, I appreciate your response. This cold weather concrete science is pretty interesting. Many different factors contribute, and many different opinions.