Pouring concrete when it's cold

[Erin]

We need to get our footers in so we can start stacking walls!

Yeah, we should have done this back in November during Indian summer, but we weren't ready then.  Consequently for the last couple of months we've been looking at our hole in the ground, ready to pour our footers, but not sure the weather is going to cooperate.

I know concrete needs to be 50-80 degrees or thereabouts for best curing.  But I also know people are pouring concrete all over the place this winter.  Here in NW Kans/SW Neb we tend to have fairly mild winters.
Last week, for example, we had several days of low 60s with night time lows of mid to upper 20s.

What're your thoughts on this?

Should we just hold out a few more months?  (And put ourselves a little further behind) 
Should we look at something like choloride?  (I don't think you're supposed to add that to reinforced concrete, though, right?) 
Concrete blankets?

I'm going to call my concrete company in the morning and get their thoughts, but I wanted you guys' as well.

[glenn kangiser]

They add 1 to 2% calcium chloride to to concrete to make it set in the winter.  Here they are using hot water in Yosemite Park to mix with the concrete to speed up the set -- It's just done by the one local company that I maintain trucks on and use for my concrete pours that I know of. 

I have seen where a few engineers don't want calcium or ask for their approval before adding, but it has been done for years with no problem on residential and farm stuff around here.

[n74tg]

Just because others are pouring concrete "all over the place this winter" doesn't mean it's a good thing to do.  They sure won't be happy down the road if they have footer problems.  

Do you want the "best" footers you can get?...then

1. insure the temp around the footer doesn't go below freezing at night.  If nighttime lows will get down to freezing then cover the footer and put straw on top of it.  Do something to keep it warm.
2. WAIT at least a WEEK after pouring before you do ANY building on the footer (two weeks would be better).  Don't even remove the forms for a week.
3. keep the top of the footers wet and under visqueen for that whole WEEK after pouring or spray some curing compound that helps the slab/footer retain water.  
4. line your footer trenches with visqueen before the pour, so that the footer is encased as much as possible in plastic.  Curing concrete is unfortunately drained of water by things that absorb water, ie soil and the air.  The longer the water can stay in/with the concrete the stronger and more durable the footer will be.  
5. try to avoid thinking like the contractors who push to get to work on construction early, ie before the concrete is ready to be built on.  Like I said above, just because you see them do it doesn't mean it's a good thing.  They unfortunately have to work fast to make a profit on the job.  On your house, you don't have to do that.  You can take the time to do it right.

Good luck on it, keep us informed of what you decide.

[Beavers]

If you have had the footings dug all winter you don't want to pour on them now.  Pouring concrete on frozen ground is not a good idea, it's almost as bad on pouring on top of mud...it's not stable ground.

Also calcium cloride isn't a very good idea.  It is corosive to reinforcing steel, causes increased shrinkage (cracks), and increases the potential for scaling.  The scaling might not be a big deal for footings, but the other problems would be.

I have to be certified by the American Concrete Institute as a concrete inspector as part of my job.  I'll double check my spec books at work tomorrow, and post the specs for cold weather concrete pouring if you want. 

[glenn kangiser]

Good info, Beavers.  We don't have any freezing here to deal with - or barely at all so never see the problems.  The park is 45 miles where there is freezing but I haven't worked there.

[Beavers]

I forgot that our spec book is online, here are some of the sections dealing with cold weather concrete.

I know that a lot of home builders pour concrete all year long, but they don't have to deal with a spec book or inspectors.
These are the NE Dept. of Roads specs for construction.


h. Operations will not be started until the ascending ambient air
temperature reaches 40°F (5°C). Mixing and placing operations shall be
discontinued when the descending ambient air temperature reaches 40°F
(5°C).
i. In no case shall concrete be placed upon an unstable or frozen
subgrade or foundation course.


14. Concreting in Cold Weather:
a. The following requirements shall govern the placement of concrete
when the ambient temperature is less than 40°F (5°C):
(1) The temperature of the concrete shall not be less than 50°F
(10°C) immediately after being placed.
704.03 Concrete Construction
- 464 -
(2) The Contractor shall furnish heating equipment and/or enclose
and protect the structure in such a way that the concrete and air surrounding it
shall be maintained at a temperature between 50°F (10°C) and 100°F (38°C)
for the first 72 hours after the concrete has been placed, and at a temperature
between 40°F (5°C) and 100°F (38°C) for the next 48 hours. The temperature
of the air surrounding the concrete shall be gradually reduced to the outside air
temperature at a rate not faster than 5°F/h (2.8°C/h).
(3) Curing moisture must be maintained.
(4) The use of "salamanders" or other open flame heating units is
prohibited. A shield shall be provided on heating equipment involving
combustion so that no exposed metal will be in contact with the source of heat.
b. (1) The Contractor may use form insulation to maintain concrete
at the temperatures indicated in Paragraph 14.a.(2) of this Subsection.
(2) The temperature of the concrete at the time of mixing may be
adjusted to assure that the temperature of the concrete will not exceed 100°F
(38°C) due to the heat of hydration.
(3) The insulated forms shall remain in place for 5 days.
(4) The forms may be loosened slightly, if necessary, to control
the temperature of the concrete below the maximum value specified. If
loosening of the forms is necessary, prior approval of the Engineer must be
obtained before the forms can be loosened.
(5) When forms are to be removed after the specified 5-day
period, the fall of the concrete temperature shall not be at a rate faster than
5°F (2.8°C) in any 1 hour.
(6) The Contractor shall provide 4 galvanized steel conduit tubes
1 inch (25 mm) in diameter and 12 inches (300 mm) long for each application
of insulated forms. These tubes shall be provided with satisfactory rubber
stoppers. The tubes shall be placed in the concrete as directed by the
Engineer and shall be used to take the concrete's temperature. After curing is
completed, the tubes shall be removed and the remaining holes shall be
grouted.
c. The Contractor shall assume all risk connected with the placing of
concrete during freezing weather, and permission given by the Engineer to
place concrete during such time will in no way relieve the Contractor of the
responsibility for satisfactory results. Any concrete showing damage from
freezing shall be rejected.

[glenn kangiser]

That answers a lot about best practices in freezing areas.  Thanks

[rwanders]

What is described above is an excellent example of a "two belts and several suspenders" methodology. Can't really say it's wrong---just overdone unless you are building another world trade center in an earthquake zone. I did civil inspection and concrete testing for many years and modern concrete mixes are extremely forgiving----most so-called 3000 psi mixes actually cure out to 5000+psi since it is much cheaper to send hot mixes than to demolition subpar pours. Concrete is commonly poured in subfreezing weather in the north with fairly simple and commonsense protective measures----usually insulation blankets or if very cold, visqueen tents and heaters for 3-4 days. Do thaw ground before pour though with preheating. Average home doesn't stress average concrete footer anywhere near its' design strength. Small footers need heat more than large due to less heat from concrete setting processes.

[glenn kangiser]

That sounds reasonable.  Most small houses would be happy with anything resembling a rock under them it seems -

I would not let it stop me from my build if I thought it would not cause me a problem if there was a bit of spalling - etc.

[bobtheengineer]

From a Northerner's (North Dakota, yea it gets cold up here) perspective:

Use warm water, high early concrete.  Keep insulation blankets on for a week.  You are good as long as the temperature doesn't get below 0 F.  Below 0 F it can still be done, but you'll have to do all the above, plus add an external heat source.   That is pretty temperature sensative for the first 3-4 days, after that it doesn't matter too much.  Curing concrete puts out enough heat on its own, you don't have to worry too much.  For a footing, I don't think I'd care too much.  Usually you'll just get surface spider webs when it freezes, you don't care how your footing looks, just that it stays put.

[Erin]

Having spent several years just south of Bismarck, I have to say, northern Kansas is the tropics. 

We've had rain falling since sometime yesterday evening. 
Yes, gentlemen. 
Rain. 
As in, too warm to snow!   


So far as pouring: 

I just got off the phone with the local Redi-Mix and asked their thoughts on pouring this time of year.  "Well ma'am, I'm not sure you'd really want pour in the rain..."   
But otherwise, he said it's not really a big deal.  Keep it covered with several layers of straw and all should be good.   They can also add calcium if we decide to go that route...

[glenn kangiser]

Rain is not a problem for footings if the truck doesn't get stuck.  Plastic will keep the surface cement from washing away.  Not pleasant to say the least though.