Pouring concrete in the fall

[71blackcheyenne]

Thinking about getting my shop floor poured this week, its is supposed to stay above freezing at nights, daytime highs of 15-20C for the next couple of weeks, but after that who knows. Is it going to affect the floor if the temp drops below 0deg C at night during AFTER having set up for the first couple weeks? The floor is inside of the building, but keeping the building warm isnt an option since i dont have a furnace set up yet, and im not going to be home to turn on a construction heater.

 

[brownbagg]

cover it with hay or dirt. like a blanket. we pour all year round, construction doesnt stop because its cold

 

[Sureshot]

Don't sweat it. You have another month or more to get it done. The cement heats as it cures so it will keep itself warm. As long as you are not pouring over frost you will be fine.

 

[NUTTSGT]

cover it with hay or dirt. like a blanket. we pour all year round, construction doesnt stop because its cold

Straw not hay.

 

[Cryptic1911]

Yeah it'll be fine if you cover it.. we had our front porch area poured, and it snowed that night

 

[ConCretin]

Standard practice for a slab in cold weather is to maintain the concrete at a minimum temp of 55 degrees F for 3 days. After that it won't be negatively affected by freezing temps.

Since concrete generates heat during hydration, covering it with hay, insulated blankets, etc will protect it from temps as low as 20 degrees or so.

Just keep in mind that concrete doesn't gain much strength when it is below 50 degrees so don't load it too early.

 

[Kevin54]

You shouldn't have too much to worry about yet as far as covering it. The ground is still warm, so the chances of a slab freezing is pretty slim. We're getting frost now but nothing drastic. Once it starts getting down in the low 40's during the day and below freezing every night, then you would need to cover it with straw or something similar to help keep the warmth in.

 

[REPO]

If the shop has doors just close them. It might have a bit of frost outside, but the interior of that shop will easily stay above freezing overnight.

 

[Angelfire]

I've poured while there were 2 ft of snow on the ground. No problems. If you are in very cold temps, you can always add an accelerator and/or mix with hot water as well but it doesn't sound like you need that.

 

[theoldwizard1]

Just keep in mind that concrete doesn't gain much strength when it is below 50 degrees so don't load it too early.

In other words, wait a full 30 days !

 

[jvitez]

We had our attached garage slab placed in ~-20C. I had to pay for hoarding/insulation/heating as an additional expense. They put up the hoarding and insulation, used kerosene heaters for a couple of days to heat the ground (not sure how effective that is, but they said it's usual practice for this builder), placed the concrete, and kept it heated for 3 days with the same heaters. They put some type of sealer on top after it cured. No control joints were used.

So far 6 years later, there are some full thickness cracks that allows some efflorescence to come up, but no big cracks, no spalling, and looks like new. Except for oil and fluid stains....

 

[willymakeit]

Depending on size and budget, rent concrete blankets. It is the second 24hrs where damage can be done. Check the ACI specs for your temps and area.
Your sub should know also. Google ACI, there is a lot of good info.

 

[ConCretin]

They put up the hoarding and insulation, used kerosene heaters for a couple of days to heat the ground (not sure how effective that is, but they said it's usual practice for this builder), placed the concrete, and kept it heated for 3 days with the same heaters.

Even though it's been done for years, most commercial jobs prohibit the use of non vented heaters on slabs on grade. The CO2 from the heaters can supposedly cause the surface to be weak and chalky. If I recall correctly, the CO2 that causes a chemical reaction called carbonization.

 

[ConCretin]

It is the second 24hrs where damage can be done. Check the ACI specs for your temps and area.
Your sub should know also. Google ACI, there is a lot of good info.

Gotta disagree with you on the second 24 hour thing. Concrete is the most susceptible to freeze damage early in its strength development. ACI 306R88 is the spec for cold weather concrete. It says concrete that reaches 500 psi can withstand freeze thaw. At concrete temps above 50 degrees, this can occur in the first 24 hours or shortly thereafter.

Depending on the size and final exposure of the structure, ACI recommends a protection period of 48 to 72 hours at 50 to 55 degrees. I'd wager that 99 out of 100 residential concrete contractors don't have a clue what ACI says.

 

[6768rogues]

With no freezing temps predicted in the next few days, do it. It is a great time of year to do concrete work. After a couple of weeks, it is ok to let it get cold. If you are concerned, have them put in one or two percent calcium or pay a little more to have the concrete made with heated water or both, as they accelerate the hydration process and will make the concrete create heat faster and cure faster.

 

[71blackcheyenne]

So am I completely crazy for having a brand new slab poured in the fall and then sitting all winter in an unheated and uninsulated building? I think i might be just asking for trouble, concrete guys not sure, he thinks there is a 50/50 chance of heaving and bad cracking and would rather wait till spring, but of course now there is rebar already layed out, making the building completely useless for the winter.

 

[willymakeit]

Gotta disagree with you on the second 24 hour thing. Concrete is the most susceptible to freeze damage early in its strength development. ACI 306R88 is the spec for cold weather concrete. It says concrete that reaches 500 psi can withstand freeze thaw. At concrete temps above 50 degrees, this can occur in the first 24 hours or shortly thereafter.

Depending on the size and final exposure of the structure, ACI recommends a protection period of 48 to 72 hours at 50 to 55 degrees. I'd wager that 99 out of 100 residential concrete contractors don't have a clue what ACI says.

The first 24 hrs is when it is building heat, so less likely is what I should have said. I agree on the ACI. However strange things do happen no matter what anyone thinks or says.

 

[ForceFed70]

To be honest. This is one of the best times of year to pour.

You won't need blankets, etc unless the temp suddenly starts to drop and is getting close to freezing (and only if this happens within a few days of the pour)

 

[Sureshot]

I am not that far from you and poured mine in December. I covered the spot with plastic then straw bales until the crew could get here. Was no frost at all under the plastic. We put up the pole building then poured the slab inside. I did heat it though as soon as I could get the lines tied in and it was attached to a heated building we put a door way too right away.
Why not just insulate and heat once the slab is poured?

 

[bobscogin]

Even though it's been done for years, most commercial jobs prohibit the use of non vented heaters on slabs on grade. The CO2 from the heaters can supposedly cause the surface to be weak and chalky. If I recall correctly, the CO2 that causes a chemical reaction called carbonization.

True. I think the carbon dioxide reacts with calcium hydroxide in the cement to form calcium carbonate on the surface. Looks like chalky white patches on the slab.

Bob

 

[jvitez]

I know there are a bunch of chemical additives you can add to concrete. Is there something that helps in below freezing pours?

 

[superdutymike27]

True. I think the carbon dioxide reacts with calcium hydroxide in the cement to form calcium carbonate on the surface. Looks like chalky white patches on the slab.

Bob

Eflouresence? Did I spell that correctly?

 

[ConCretin]

I know there are a bunch of chemical additives you can add to concrete. Is there something that helps in below freezing pours?

You can ask the plant to send along some calcium, which will accelerate the set time and heat development. This is an obvious benefit in cold weather.

There are also a number of 'non chloride' accelerators that are more commonly used on commercial jobs where calciums corrosive affect on reinforcing steel is a concern. Polarset is the generic term used around here.

These products have their limits however. Slab placements in below freezing temps are tricky and risky. If your not familiar with all the variables, wait for better weather.