Slab pour in cold weather with night freeze.

[Renze]

The blankets help. I’d put a few inches of straw on top of the blankets too.

I would rather put some tarps over it, a few inches off the ground so the air insulates. With the heat from the curing cconcrete, its going to get comfy under the tarp

 

[BlueKnife]

I would rather put some tarps over it, a few inches off the ground so the air insulates. With the heat from the curing cconcrete, its going to get comfy under the tarp

The blankets that the contractor has are basically two layers of tarp with an insulation layer sandwiched between. They're used regularly at temperatures much colder than what the OP is talking about.

 

[RayIN]

When my basement floor was poured, the contractor had some sort of anti-freeze added to the mix on site. That was 58 years ago, floor is still smooth without major cracks; which I suppose is due to all the old worn-out stone mill saw blades,woven wire and re-bar I had him use first.
Any concrete contractor who tells you his pour will not crack should not be trusted IMO.

 

[EddieWalker]

I'm putting a 12x16 addition on the south side of house. Foundation is poured, 2" EPS insulation, 6 MIL poly, mesh and PEX (for radiant heat) is in place.

However, the weather has turned here in NY with forecast for daytime temps in the mid/upper 30's and lows (nights) in the mid 20's.

Contractor, said to do a lot of commercial as well, says the slab pour will be fine, as he has insulated blankets that can stay in place for a few weeks and the concrete will generate enough internal heat from the curing reaction to prevent any freeze damage. Forecast for a week after the pour, if it happens, includes a couple of rainy/snowy days. Which may very well freeze those blankets in place.

Feeling a bit uneasy about this. While I do fully trust the concrete guy, my natural inclination is to wait for a break in the weather, just to be safe. I understand he is motivated to get this done now (for final payment, if nothing else), so feel it may be unfair to press him to delay this until, well Spring, the way the forecasts look.

Any experience with such things?

As a Contractor, my experience with Concrete Contractors puts them into 2 categories. Hacks and Pros. Almost all of the residential concrete Contractors that I've dealt with are hacks. If they do commercial work, they are required to follow the Engineers instructions, and do it right. The difference is night and day.

In my opinion, the easiest way to tell the difference between a Pro and a Hack is looking at the rebar. Hacks will try to use wire instead of rebar, and if they do use rebar, they try to leave it on the ground and not put it on chairs. The one thing every Hack will say is that they will lift the wire or rebar while they pour it, and it will remain in the middle of the concrete while they walk on it spreading and floating the concrete. This is the most common lie told in all construction!!!

If your Contractor has rebar on chairs, odds are pretty good that he is a Pro.

The second thing to watch for is on the day of the pour. The concrete has to have slump to it. That means the concrete will pile up on itself and not spread out. Slump is measured, the more you have, the stronger the concrete will be. Hacks will add extra water so the concrete is easier to spread, easier to float and gives them more time to work on it. Water creates volume. When the excess water evaporates, the loss of volume creates cracks in the concrete. These cracks will show up in the first week of curing.

A Pro will not add water to the mix. Sadly, the homeowner needs to be there during the pour to take pictures of the concrete as it comes out of the truck, and piles up. The homeowner needs to tell the crew not to add water. You have to be very direct with them, and you have to stay on top of it, because they will want to add water!!!

Concrete is poured in cold conditions all the time. It's not an issue for a Pro. It's not something to be worried about if the guy knows what he's doing. If he's a Hack, you are in big trouble no matter what the weather is like. If they do the basics right, odds are good they will finish the concrete right.

 

[rScotty]

As a Contractor, my experience with Concrete Contractors puts them into 2 categories. Hacks and Pros. Almost all of the residential concrete Contractors that I've dealt with are hacks. If they do commercial work, they are required to follow the Engineers instructions, and do it right. The difference is night and day.

In my opinion, the easiest way to tell the difference between a Pro and a Hack is looking at the rebar. Hacks will try to use wire instead of rebar, and if they do use rebar, they try to leave it on the ground and not put it on chairs. The one thing every Hack will say is that they will lift the wire or rebar while they pour it, and it will remain in the middle of the concrete while they walk on it spreading and floating the concrete. This is the most common lie told in all construction!!!

If your Contractor has rebar on chairs, odds are pretty good that he is a Pro.

The second thing to watch for is on the day of the pour. The concrete has to have slump to it. That means the concrete will pile up on itself and not spread out. Slump is measured, the more you have, the stronger the concrete will be. Hacks will add extra water so the concrete is easier to spread, easier to float and gives them more time to work on it. Water creates volume. When the excess water evaporates, the loss of volume creates cracks in the concrete. These cracks will show up in the first week of curing.

A Pro will not add water to the mix. Sadly, the homeowner needs to be there during the pour to take pictures of the concrete as it comes out of the truck, and piles up. The homeowner needs to tell the crew not to add water. You have to be very direct with them, and you have to stay on top of it, because they will want to add water!!!

Concrete is poured in cold conditions all the time. It's not an issue for a Pro. It's not something to be worried about if the guy knows what he's doing. If he's a Hack, you are in big trouble no matter what the weather is like. If they do the basics right, odds are good they will finish the concrete right.

Eddie, that's a great post. Thanks.

 

[joea99]

As a Contractor, my experience with Concrete Contractors puts them into 2 categories. Hacks and Pros. Almost all of the residential concrete Contractors that I've dealt with are hacks. If they do commercial work, they are required to follow the Engineers instructions, and do it right. The difference is night and day.

In my opinion, the easiest way to tell the difference between a Pro and a Hack is looking at the rebar. Hacks will try to use wire instead of rebar, and if they do use rebar, they try to leave it on the ground and not put it on chairs. The one thing every Hack will say is that they will lift the wire or rebar while they pour it, and it will remain in the middle of the concrete while they walk on it spreading and floating the concrete. This is the most common lie told in all construction!!!

If your Contractor has rebar on chairs, odds are pretty good that he is a Pro.

The second thing to watch for is on the day of the pour. The concrete has to have slump to it. That means the concrete will pile up on itself and not spread out. Slump is measured, the more you have, the stronger the concrete will be. Hacks will add extra water so the concrete is easier to spread, easier to float and gives them more time to work on it. Water creates volume. When the excess water evaporates, the loss of volume creates cracks in the concrete. These cracks will show up in the first week of curing.

A Pro will not add water to the mix. Sadly, the homeowner needs to be there during the pour to take pictures of the concrete as it comes out of the truck, and piles up. The homeowner needs to tell the crew not to add water. You have to be very direct with them, and you have to stay on top of it, because they will want to add water!!!

Concrete is poured in cold conditions all the time. It's not an issue for a Pro. It's not something to be worried about if the guy knows what he's doing. If he's a Hack, you are in big trouble no matter what the weather is like. If they do the basics right, odds are good they will finish the concrete right.

Thanks. Post came a few days after the pour happened.

The fellow says he does mostly commercial work, or did, and came recommended by someone that did right by me in the past. When I discussed concrete suppliers, he said he would not use the outfit I first mentioned as their stuff was cheapest, by a lot, but often never cured properly or pass strength tests.

The walls did have rebar and steel panels were used as forms. The rebar was not the minimum size, but I forget now what number it is. Was 4000 psi mix.

The slab did have "chairs" but mesh was used, again said to be over minimum. The concrete did not slump all that much, to my eye, but did need to be raked out and "coaxed" into corners. Said to have used hot water and cold weather additives.

He put down insulated blankets, and my own "billboard tarp" over those. I pulled those back yesterday and did not see any obvious cracks and the slab felt quite warm to the touch. Also noticed the pressure on the PEX test gauge was a few PSI over what it had been, which only means it got hotter. Not that unexpected I guess.

In a week or so may pull all those off, but it snowed about 7 inches last night, so, weather dependent.

 

[bigtiller]

The second thing to watch for is on the day of the pour. The concrete has to have slump to it. That means the concrete will pile up on itself and not spread out. Slump is measured, the more you have, the stronger the concrete will be. Hacks will add extra water so the concrete is easier to spread, easier to float and gives them more time to work on it. Water creates volume. When the excess water evaporates, the loss of volume creates cracks in the concrete. These cracks will show up in the first week of curing.

Eddie,
If I am reading this correctly, you're saying the higher the concrete piles up on its self, the stronger the concrete will be.

But it reads in my mind just the opposite...the more slump the stronger the concrete.

People who don't know much about concrete might misunderstand.

 

[Renze]

Eddie,
If I am reading this correctly, you're saying the higher the concrete piles up on its self, the stronger the concrete will be.

But it reads in my mind just the opposite...the more slump the stronger the concrete.

What exactly are we talking about ? How much water is added? Untill it self levels ?

My father kept spraying fresh cast concrete 3 times a day in summer, to prevent cracking from too fast drying in hot weather.

 

[Renze]

As a Contractor, my experience with Concrete Contractors puts them into 2 categories. Hacks and Pros. Almost all of the residential concrete Contractors that I've dealt with are hacks. If they do commercial work, they are required to follow the Engineers instructions, and do it right. The difference is night and day.

Residential concrete contractors, you mean guys pouring driveways and walkways, and commercial concrete contractors, you mean construction companies ?

 

[Streetcar]

What exactly are we talking about ? How much water is added? Untill it self levels ?

My father kept spraying fresh cast concrete 3 times a day in summer, to prevent cracking from too fast drying in hot weather.

Keeping the finished concrete wet helps it cure. It is bad to add water to concrete prior to finishing it