concrete pour dilemma

[copmagnet82]

Hello all,

I am finally building a 10' x 18' shed. It will actually be built more like a mini garage versus a typical shed one would pick up at a big box store. The shed will be built on a monolithic slab. My forms and rebars are done. I wanted to pour the concrete this coming week, but the temperatures seem to be in the low 30's at night and 50's during the day for the next 2 weeks. In addition to the low temperatures, rain is predicted for 10 out of 14 days. What do I do? Do I pour the concrete before the winter, or do I tarp up the forms and wait till April/May of 2021.

Thoughts?

 

[corvette43056]

You can always rent concrete blankets to cover it after you pour for a week to keep it from freezing

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[copmagnet82]

I was thinking about that, too, but I'm wondering if the concrete strength would be compromised despite the blankets, since the curing conditions wouldn't be ideal.

 

[LOW1]

Where are you located?

I have seen pourings in much worse conditions than that. I would pour this fall

 

[copmagnet82]

Chicago suburbs.

My old house was poured in February, so I know what you mean, but then, it is 2020, so anything goes.

 

[LOW1]

Chicago suburbs.

My old house was poured in February, so I know what you mean, but then, it is 2020, so anything goes.

Do it now and take advantage of 2020s cheap lumber prices. Said no one ever.

 

[matt_i]

My contractor said he's good to 28F overnight.

The concrete will generate its own heat from the exothermic curing reaction.

Rain will not hurt you as long as its not a diriving downpour right after you place & screed it.

If its about to pour and you just steel troweled it then you can put a piece of plastic over it.

After the plastic is applied you can use "organic insulators" like straw shredded out of bales or pile of leaves.

The timing is up to you, but seeing that your forms are ready, I'd pour, otherwise its going to be a muddy mess in the spring.

 

[ProfessionalAmateur]

Do it now and take advantage of 2020s cheap lumber prices. Said no one ever.

 

[The Cobbler]

I would pour . depending on where you live they may already add winter heat to help the cure. just keep it protected after it has hardened with (mentioned above)

 

[Firebrick43]

I would pour as well. Have them do 4000 or 4500 psi concrete which has more cement=more heat and the rain is a plus.
Actually curing a lower temp *as long as it doesn’t freeze hard* is a plus. Elevated temps are bad. Really big pours they have to run chilled water though to keep the temps in check.


The kicker is are you just screening and floating or are you wanting a smooth troweled finish??? If you want it troweled you will need some accelerator at those temps or it will be in the middle of the night before you can trowel it. Since you are using rebar make sure it’s a non corrosive accelerator like polarset.

 

[bp460]

Get it done this Fall... there is still time. A few years back, I replaced my front walkway the week before Thanksgiving. The evening temps were right at freezing and I didn't have any problems. Are you doing the concrete yourself?

-Brad

 

[Zmann]

I am finally building a shed

Build a shed over the slab before you pour


= problem solver ;-)

 

[copmagnet82]

I would pour as well. Have them do 4000 or 4500 psi concrete which has more cement=more heat and the rain is a plus.
Actually curing a lower temp *as long as it doesn’t freeze hard* is a plus. Elevated temps are bad. Really big pours they have to run chilled water though to keep the temps in check.


The kicker is are you just screening and floating or are you wanting a smooth troweled finish??? If you want it troweled you will need some accelerator at those temps or it will be in the middle of the night before you can trowel it. Since you are using rebar make sure it’s a non corrosive accelerator like polarset.

I wanted to have somewhat of a smooth finish. I was also thinking about broom finish, but I'm not sure if that's a good idea.

 

[copmagnet82]

Are you doing the concrete yourself?

-Brad

Initially, it was supposed to be my dad, brother and I, but since it's getting cold, I was thinking about hiring someone who does it for a living.

Why are you asking? Do you want to come help me out?

 

[ConCretin]

I concur with those who say you are fine to place. Temps in the 50's especially if you get some sun will produce a nice manageable set time and the concrete won't be adversely affected until low temps drop a bit below freezing. If you get a reasonably early start, you shouldn't have any problem getting a finish on the slab well before dark. Keep some poly or a tarp handy in case you get a shower and try to keep rain off the slab until the surface is hard.

A broom finish is good for exterior surfaces where smooth concrete can get slippery but not so much for interior surfaces you will likely want to seep.

 

[mitusa]

I concur with those who say you are fine to place. Temps in the 50's especially if you get some sun will produce a nice manageable set time and the concrete won't be adversely affected until low temps drop a bit below freezing. If you get a reasonably early start, you shouldn't have any problem getting a finish on the slab well before dark. Keep some poly or a tarp handy in case you get a shower and try to keep rain off the slab until the surface is hard.

A broom finish is good for exterior surfaces where smooth concrete can get slippery but not so much for interior surfaces you will likely want to seep.

This^^^. You want a smoother finish that is easy to sweep.

 

[copmagnet82]

This^^^. You want a smoother finish that is easy to sweep.

I know, I know, I know... I like the look of broom finish, but again, as you guys are saying, not a good idea for a shed. I was hoping some would tell me that it would be a good idea.

 

[CraigStu]

I would go ahead and poor now. Unless your shed will be used for some unusual activity, it's just a shed, so if concrete strength is slightly sub par it won't be a concern.

 

[bp460]

Why are you asking? Do you want to come help me out?

I am not a pro, but if you need an extra body, send me a PM. I'd be happy to lend a hand to a local GJ'er.

-Brad

 

[andyvh1959]

My 24x28 garage slab pour last year up here in Green Bay was Oct 23rd, and it finished out fine. And it hasn't shown a single crack yet this year. By Nov 8th framing was going up and it was a balmy 15 degrees within a week.

 

[jd_1138]

I would go ahead and poor now. Unless your shed will be used for some unusual activity, it's just a shed, so if concrete strength is slightly sub par it won't be a concern.

This

 

[copmagnet82]

Thank you all for the help. I will be pouring tomorrow. The temps went from 30's to 60's and it'll stay like that for a week. Two more questions:

1. I have #4 rebar every 18". Would you guys add mesh on top of that or will the rebar on its own be sufficient? The concrete will be about 5" thick.

2. The guy who will be helping me is saying to add some calcium chloride to the concrete to help it set quicker. Is that okay? I though that would corrode the rebar???

Thank you all.

 

[ConCretin]

You have more than enough reinforcing with the rebar. No need for mesh too. Is your rebar securely supported mid slab?

Calcium chloride isn't typically allowed on commercial jobs due to corrosion concerns although it is frequently used on residential slabs, which rarely self destruct. You could ask for a non-chloride accelerator if it gives you peace of mind. The concrete plant will have one readily available.

If the temps are in the 60's, I wouldn't go more than 1% with the accelerator especially if the slab is in the sun.

Good luck with your placement.

 

[copmagnet82]

You have more than enough reinforcing with the rebar. No need for mesh too. Is your rebar securely supported mid slab?

What do you mean by support mid slab?

Calcium chloride isn't typically allowed on commercial jobs due to corrosion concerns although it is frequently used on residential slabs, which rarely self destruct. You could ask for a non-chloride accelerator if it gives you peace of mind. The concrete plant will have one readily available.

If the temps are in the 60's, I wouldn't go more than 1% with the accelerator especially if the slab is in the sun.

He told me to order a mix with 1% NCa. We'll be pouring at 1530, so we will have about 2 hours before the sun sets.

Good luck with your placement.

Thank you.

 

[ConCretin]

What do you mean by support mid slab?

Ideally your reinforcing will be pre-positioned a couple inches above the ground on chairs of some kind. While it's possible for the finishers to pull it up as they go, it's often neglected or tromped back down in the heat of battle. It won't do you much good laying on the bottom for your slab.

 

[copmagnet82]

Ideally your reinforcing will be pre-positioned a couple inches above the ground on chairs of some kind. While it's possible for the finishers to pull it up as they go, it's often neglected or tromped back down in the heat of battle. It won't do you much good laying on the bottom for your slab.

Oh, okay. All rebars are sitting on rebar chairs.

 

[firebirdparts]

You shouldn't worry about the cold. It's not cold enough to worry. You do need to worry about the rain. Maybe that's gone away too, I don't know.

Anyway, "the low 30s" is not freezing, and it's not that easy to freeze concrete in the ground.

 

[pcmeiners]

Agree with Firebird, the ground under the concrete will remain warm for days of a just below freezing air temps. Have access to blankets just in case a deep freeze is called for in the near future (unlikely).

 

[NUTTSGT]

A 10'x18' shed with 5" of concrete and 1/2" rebar. Holy overkill Batman.


I like it

 

[kaymccampbell]

Good luck on your pour. I'll be pouring a couple slabs over the next 7 days, while the weather holds up.

 

[Ozz316]

If the temperatures are dipping into the 40s at night blanket the concrete. It will hold in the heat of hydration. That is probably the most important thing. FYI rebar and mesh serve two completely different purposes in concrete. It’s your pocket book, but NCA usually runs about 5 times more than calcium. Yes it can confide rebar, but your not building a bridge deck here. I PROMISE YOU if you have a basement and it was poured in the fall/winter the mix had Calcium it, and that thing has way more rebar in it than your pad.

 

[TractorJeff]

If I was the person doing the pour with a buddy, NO WAY would I ask for accelerator to get it to set up faster! I could see myself in a World of DooDoo!
The Ground will keep it warm, if the weather changes, blankets or straw to cover it.

 

[skulldrinker]

Ive seen them build little plastic greenhouses above the slab and use a torpedo heater to keep it from freezing. But that's how they do it in Chicago.

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[copmagnet82]

Just wanted to say thank you to everyone that helped me out with the decision to move forward with the pour. I was supposed to pour on Wednesday, then it got rescheduled to Friday, and then it got rescheduled again to Monday, but it's finally done. Here are some pictures. I hope the slab will stay solid for years to come. Again, thank you for the help.

 

[The Cobbler]

good stuff! great that you got it done...

 

[joes169]

Ive seen them build little plastic greenhouses above the slab and use a torpedo heater to keep it from freezing. But that's how they do it in Chicago.

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Just a little PSA, non-vented heat and fresh/curing concrete can easily lead to carbonation, which has similar side effects as just letting the slab freeze in the first place. Portland cement is exothermic, you just need to capture the heat for a few days..........

 

[copmagnet82]

Just a little PSA, non-vented heat and fresh/curing concrete can easily lead to carbonation, which has similar side effects as just letting the slab freeze in the first place. Portland cement is exothermic, you just need to capture the heat for a few days..........

Hmmm... since it's in the high 40's/low 50's during the day for the next 10 days, would it be a good idea to cover it with blankets or at least plastic sheeting before it gets to very low 30's/high 20's starting tomorrow night? The concrete will be 48 hours old tomorrow night.

 

[joes169]

Hmmm... since it's in the high 40's/low 50's during the day for the next 10 days, would it be a good idea to cover it with blankets or at least plastic sheeting before it gets to very low 30's/high 20's starting tomorrow night? The concrete will be 48 hours old tomorrow night.

Technically, concrete can freeze once it reaches 1500 psi without issue (some say it can freeze as low as 500 psi, but that's far too risky IMO) which you've probably far exceeded by now. It's a small area, if you can collect enough bed sheets or old blankets to cover it all tomorrow late afternoon, it probably wouldn't be a bad idea.....

 

[copmagnet82]

Technically, concrete can freeze once it reaches 1500 psi without issue (some say it can freeze as low as 500 psi, but that's far too risky IMO) which you've probably far exceeded by now. It's a small area, if you can collect enough bed sheets or old blankets to cover it all tomorrow late afternoon, it probably wouldn't be a bad idea.....

My friend offered me some concrete blankets, so maybe I'll do that just to be safe.

 

[kaymccampbell]

That's a heavy duty slab. Nice work. My little 4" slabs can't hold a candle to it.