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Old 11-17-2008, 12:44 PM   #1
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Cold weather concrete - Curing questions


Hello,

I just poured a 32' x 45' slab 4" thick on Saturday afternoon up in mid-Minnesota. The temps from Saturday morning to Sunday morning were 37 high, 25 low. Sunday to Monday the temps were 31 high, 25 low. We put concrete blankets on the slab as soon as practical, around 3 hours post pour, likely sooner. Its hard to tell as there was a flurry of activity during that time. At any rate we put them on immediately after power troweling.

The forecast for the week is as follows (as far as they can tell):

11-17: 29/18*
11-18: 33/27*
11-19: 37/27*
11-20: 32/20*
11-21: 30/24*
11-22: 33/28*
11-23: 35/25*
11-24: 36/24*
11-25: 30/25*
11-26: 32/26*

How long should I leave the blankets on there? The average temps for November as 41/25, but we have been below average all month. The average temps for December are 27/10. Should I leave them on there for the full 28 days? How long should I wait to drill my wall anchor holes in?

Thanks in advance!

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Last edited by DigitalFusion; 11-17-2008 at 12:56 PM.
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Old 11-17-2008, 02:41 PM   #2
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Cold weather concrete - Curing questions


It would be best to leave the blankets on there for three or four days. You can drill holes for anchors anytime, but if they're expansion anchors you would be wise to wait to put them in until the concrete has cured for a week or two.

The blankets are a good idea as long as the hydration process is generating heat. Their added benefit is that they keep the wind off the surface of the slab, which is every bit as damaging to concrete as cold weather can be.

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Old 11-17-2008, 02:53 PM   #3
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Cold weather concrete - Curing questions


There is also one thing to remember. - People get air temperatures that do not necessarily agree with concrete and soil temperatures early in a cooling season.

Your soil was not frozen and brobably was much warmer than the soil. The blankets are doing a good job of retaing the latent heat of the soil and the heat of hydration from the curing of the concrete. Keep then on as long as practical until you gat a chance to take them off just before a layer of insulating snow.

If you used a higher strength concrete (more cement) or high early cement, that reduces any concerns.
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Old 11-17-2008, 03:30 PM   #4
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I know I should keep them on as long as practical, but I dont know how long that really is. Concrete gives off heat only for as long as hydration continues, correct? So at a certain point it stops giving off heat and the blankets are not doing all that much?

The mud setup waaaay faster than we expected. Turns out the concrete came from over an hour away as all the other plants were closed down for the season, plus I added a 1/2% calcium. We didnt have time to get the anchors in, so now I have to use wedge anchors. I would like to start framing up the garage as soon as possible, but I first need to make sure the concrete is strong enough to take the blankets off, and drill into and place the wedge anchors.
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Old 11-18-2008, 02:26 AM   #5
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Cold weather concrete - Curing questions


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Originally Posted by DigitalFusion View Post
We didnt have time to get the anchors in, so now I have to use wedge anchors. I would like to start framing up the garage as soon as possible, but I first need to make sure the concrete is strong enough to take the blankets off, and drill into and place the wedge anchors.
Are you sure it's ok to use wedge anchors, per code - not epoxied?
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Old 11-18-2008, 08:32 AM   #6
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Cold weather concrete - Curing questions


agree w/dick (concmasonry) & 'mite,,, btw, you're not alone - no one knows when conc cures unless you took sample beams for testing & they've been cured as the site conc.

once hydration slows measureably ( thermometer/your hand under the blankets ), set your anchors - if the 1st spalls the conc, go to the epoxy units,,, however, you may have to warm the immediate conc w/elec lamps which isn't a big deal.

ca cl's designed to shorten curing time - it also shortens working time as you noticed.
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Old 11-18-2008, 08:46 AM   #7
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Are you sure it's ok to use wedge anchors, per code - not epoxied?
Wedge anchors are usually allowed, provided they're set deep enough, which is uncommon! My suggestion would be epoxied rods for sure.
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Old 11-18-2008, 10:24 AM   #8
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So, if I leave them on for a week I should be fine?

Tell me more about these epoxy anchors. I dont have any experience with them. Where does one get them? How much are they? Pros/Cons of different brands?
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Old 11-18-2008, 10:49 AM   #9
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any const supply house'll have 'em or call your hilti guy,,, an alternative we often use is drilling the hole, cleaning well, dip threaded rod into the epoxy, & insert into the conc,,, 1 thing you didn't mention's how thick the avail conc measures 4" floor over what ? ? ?

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Old 11-18-2008, 11:06 AM   #10
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Floor is 4" thick 4000 PSI, edges where the anchors will be is 12" thick and 8" wide, going up at a 45* angle to the 4" floor. There are 4 runs of #5 rebar in the perimeter, with #3 in the main floor. Its poured over 20 mil poly, with 2" R10 high density insulation as there is 5/8" PEX in the floor. All this is over 12" of compacted wash sand, 8" of compacted wash sand in the perimeter. Below that is about 2-10" of clay, with a bed of sand mix that seems to be about 4' thick in spots judging from where we dug out a stump outside of the garage area.

My buddy is bringing a laser temp finder home from work today for me. I asked him about it last night as I want to make sure the concrete is maintaining a 55* + temp.
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Old 11-18-2008, 03:03 PM   #11
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all sounds good to me so maybe you're not the avg h/o you've got a ( unipour - monopour - thicken'd edge ) slab.

once hydration slows of its own accord, you can't afford to keep it warm,,, believe it or not, it will continue to strengthen even til next summer & beyond !
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Old 11-18-2008, 03:18 PM   #12
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I used to work doing concrete in the summers years and years ago. I never had any experience with cold weather pouring, save one slab they called me for help on, which was poured in 0-10 degree temps over heated sand with heated blankets over the top. I didnt do any of the post-pour care tho, so I was not sure how to care for this pour.

I have been out there daily, checking on the slab, taking a bucket of warm water with a sponge out there several times a day squeezing out water onto the slab in any areas that look dry. Its been ~96 hours (4 days) since it was poured and its looking pretty good. I think a few spots may have dried out too much, but I guess we shall see.

So there is no way to answer how long I should leave these blankets on then? Is there any way to tell when hydration has stopped? I know concrete never really truly stops curing, but doesnt it reach like 60-75% strength within the first 72 hours at 55 degrees? Would it cold shock the concrete to pull all the blankets off tomorrow afternoon (forecasted 36* high, but 22 mph winds) and check the entire slab for any dry spots and then cover it back up?
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Old 11-18-2008, 03:52 PM   #13
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when did you saw the joints ? ? ? that's critical to prevent random cracking,,, we saw them when the conc's cured sufficiently so a green conc blade & cooling/clearing water doesn't ' ravel ' the conc,,, 'nother way's to saw them dry w/asphalt blade,,, either way works.

far's the blankets, i'd leave them on for another week.
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Old 11-18-2008, 04:00 PM   #14
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havent cut the slab yet. I have always done it dry after curing.
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Old 11-18-2008, 04:33 PM   #15
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ABSOLUTELY F'N WRONG !!!!!! get on it asap & hope you're not too late !!!!!!

if we hadn't cut yet, we'd be cutting w/cured conc blade & 2" deep,,, usually we only cut 1" on ' green ' ( not cured but not plastic anymore ) conc,,, only clowns come back & saw jnts the next day you might notice some micro-cracking already,,, this time cold weather's an ally, not the enemy !

ps - those clowns explain random cracking by saying ' there's 2 types of conc - that which's cracked & that which will ! '

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