cold weather concrete question and another.

[sharptailhunter]

Hey there everyone!

I have been on this forum for a good while now and have been soaking it all in. Last fall I poured my base slab. I was planning on waiting till spring to begin construction of the rest of the oven. However, here in S.E. Idaho we have been experiencing a freaky warm spell. The past few nights haven't even dropped below freezing, that's crazy for February! I'd really like to take advantage of having my slab free from snow and begin construction of the hearth stand and slab.

So, is it ok to use regular bag concrete mix to fill the cores of the CMU blocks if there is a chance of freezing temps before it fully cures? Would an additive of some sort to the mix or a different kind of bagged concrete mix be better?

Next, I was planning on using Quickrete 5000 bagged mix to pour the hearth slab. I have worked with it quite a bit before and find it good to work with and easy to use. How would that work in these colder months?

Lastly, and this may be a little difficult to describe, are there any problems pouring a second "finishing" or "decorative" concrete slab on top of my hearth slab after it has already cured? That is, the oven and insulating fiber board will be sitting atop my 3.5" rebar reinforced concrete hearth slab. I'd then like to pour (after the oven is built) a 4.5" thick slab (that thickness accounts for the 2" thick fiber board and the 2.5" thick fire brick floor) all the way around the oven that would include the landing that would be flush with the level of the oven entry and floor. Hope that makes sense? I think I'm just worried that the two separate concrete layers or slabs wouldn't bond and create an area prone to cracking and separating??

Thanks in advance for any replies. I am always amazed at how kind people are on this forum and how they really just wanna help each other. It's very refreshing!

-Alex

[jeeppiper]

Re: cold weather concrete question and another.

1. First a warning: I am not a concrete expert so it is best not to listen to any advice that I give you.
2. Obviously, the issue with concrete and cold is the expansion of water during freezing. Personally, I think I would wait to pour any concrete until you get at least a week of solid temps above freezing. It takes a month for concrete to fully cure. There are additives you can put in to prevent freezing allowing you to pour in cold temps. Since you are just pouring the cores, I would think you would be okay if you use the additive. I dont think I would use additives and try to pour the slab though..Wait till warmer weather to pour the slab.

2. You will be fine using the Quicrete 5000. You just dont want to use the fast-curing stuff...that is for mailboxes.

3. I think there is an "etching solution" you can use to pour concrete on top of existing concrete.

These are just my thoughts....There are some concrete wizards on this forum; sit tight and I am sure one of them will chime in with some words of wisdom.

[NCMan]

Re: cold weather concrete question and another.

Hi sharptailhunter......You'll be fine w/your block fill as long as it stays above freezing for a few days afterwards. If you feel it may get close to those temps, you can always cover it up w/some blankets, etc. to help keep the heat in and the cold out.

[pemer]

Re: cold weather concrete question and another.

You can use regular bagged concrete mix and use hot water to help accelerate the set and then cover with old blankets or insulation .The chemical reaction will generate it own heat. If you are still concerned about freezing you can add Addition of 2% (by weight of cement) of calcium chloride is the traditional way to accelerate the hydration reaction-it is very effective and reasonably cheap. But-a big but-that much chloride can lead to corrosion of any steel embedded in the concrete (like rebar)

[dakzaag]

Re: cold weather concrete question and another.

As long as you can get it cured for a few days without freezing, it will be fine. Blankets and light bulbs can save you if the weather turns.

With a 4.5" capping slab, I would not be too concerned about bond. Gravity is a fairly reliable force...

Since this 4.5" will most likely be your finished surface, you will want to make sure the forecast is above freezing for a couple of days and that you cure it carefully to avoid permanent imperfections. I personally think some of those little dings are what gives the oven character, but apparently I am weird.

The best way to screw up an easy pour is to add something besides cement sand and water. You can add color without much problem.

I wish we were having wacky weather. 0 this morning.

[sharptailhunter]

Re: cold weather concrete question and another.

Hey guys! Thank you for the replies. I've hmm'd and haww'd about it and decided to wait until I'm certain I can keep the concrete above freezing without fear, too much to lose and not much to gain. Yes, the top -and separate- layer of concrete would be just a decorative finish table kind of thing. I have been working with the Quickrete 5000 for other landscaping projects. I personally like the little imperfections that the stain I put on it brings out, but I do want it to be structural. Guess I can wait a couple months longer and then really hit it. I have the brick saw and cement mixer and the CMU blocks, just need the warm weather.

[NCMan]

Re: cold weather concrete question and another.

Hi there, sharptailhunter. That's probably a good idea to wait. Best of luck to you when you get back at it. Keep us all posted!!

[the_dr_masuess]

Re: cold weather concrete question and another.

Hey Sharptail,

I'm in Salt Lake City and I poured my slab just the other week. Gotta take advantage of the good weather! My thoughts were... If it is good enough for new home builders to build house foundations (which they are) then its ok for me to pour something considerably smaller which will not hold as much weight. Anyway... As long as you have a few good days of some sun you should be fine.

[sharptailhunter]

Re: cold weather concrete question and another.

Well, looks like our beautiful weather has ended for awhile. I woke up to three inches of snow yesterday. Loren, glad to see that you were able to get started on your oven. I'll be following your build :-)

I do have another question for any of you who may be lurking about. How necessary or beneficial is it to drill holes into my base slab for the vertical pieces of rebar? If one were to do that, is it recommended to then epoxy the rebar into the slab?

[NCMan]

Re: cold weather concrete question and another.

No, that's not necessary.

[the_dr_masuess]

Re: cold weather concrete question and another.

I just did it in the corners so that I could hammer some rebar and tie the blocks to the slab. didn't use any epoxy though... just hammered them in to the slab.