Setting concrete Piers in cold weather
user-52983
| Posted in Construction Techniques on
I will begin a construction of a carport soon. I will be digging the piers and setting the cement. The weather here in St. Louis is quite cool and I anticipate that it will be at the freezing mark for a while. I am concerned about the cure time and what techniques there are to properly cure these piers? Should I cover once poured?
James Krak
Replies
Greetings James,
This post, in response to your question, will bump the thread through the 'recent discussion' listing again.
Perhaps it will catch someones attention that can help you with advice.
Cheers
40,000 Americans are injured by toilets each year.
I'm no pro but this is what I go by.
Concrete cures to most of it's strength in 28 days.
Also once poured and while curing creates it's own heat by chemical process. As a result my understanding is the first night or two it would have to get down to about 28° before affected by freezing. After it's firm, however, I have always tried to cover the pour with thick mulch hay and plastic or a tarp for at least a week.
There is ingredients you can put into it. Not sure if it's regular auto anti-freeze or probably a special masonry ingredient that will prevent freezing.
mostly I wait for a nice two days above freezing.
thank for the info. Looks like I have a good couple of days ahead here in my area.
JamesKrak
did u consider the fast set concrete, made for use in colder weather.should be able to find it in the big box stores, an extra $1 per bag. better than making a homemade brew. you can always do that with the other precautions: waiting for warmer weather, ucovering your pour with hay or an electric blanket :-)
do a search on the site for " calicum" it was about three weeks ago. A lively discussion of cold weather concrete.. 2+3=7
just my humble opinion but i would stay away from calcium as an additive to hard on rebar though i know your asking about piers
That was a good discussion. If the search engine doesn't work (I think it's junk about half the time) we could do a quick recap I suppose.
James, concrete cures by a process called "heat of hydration". When water is added to cement the chemical curing process is activated where, depending on the ratios of water, cement, and agregate, the binding, or matrixing action happens . This is when the pulling action of the sand, stone, and cement , or matrixing, happens. In summer, the heat of hydration can get too high, conversely in Winter it may not happen at all if the temperature gets too low. If you call for a ready mix truck to deliver the mix, you can ask if they will add the heat additive to be certain the "heat of hydration" process if off to the right start. Someone already told you here that the design strength, PSI of the mix, will be reached at full cure which is 28 days, 50% of the design strength is reached in 7 days.
It is a good idea, as someone said to cover the crete up to help hold the heat. I would also put a length of rebar in the footings to reinforce it too.
James,
To add to what's been said: If you dig one day and pour the next, try to cover your holes with plywood and maybe some foam board to keep the soil warmth in the hole before you pour. Afterwards, you cover the forms and concrete for a week or longer, if you can. Once you reach 500 psi, the concrete can survive freezing, but it will cure verrrry slowly if you don't give it that initial protection.
The electric blanket is good, too. I once did a small slab that I was planning to bury an electric blanket under it, just to make sure it cured okay. The weather got better, and I didn't have to do it.
Bill
We do a lot of cold weather concrete in Canada, its no problem if the right precautions are taken. Mix design plays an important part.
First, you can't pour on frozen ground as it will cause instability in the sub-grade similar to poor compaction. So cover the excavated areas to keep from freezing. Insulated tarps are available at a lot of rental centres - if the commercial guys haven't got 'em all already.
Ask the ready-mix plant for 2% calcium addition, but no more than that. Excess calcium may cause corrosion in the rebar over time. The calcium addition will cause a faster initial setup without further negative effect. At this time of year the batch plant should be making all its concrete with hot water, but check when you order. Between the hot water and the heat released by the hydration process, you can place and finish at several degrees below freezing without worry of surface freezing.
Start your pour as early in the day as possible to ensure that initial hardeneing takes place during the warmest part of the day.
Once the final finish is on the work, cover it with the insulated tarps. The internal heat generated will prevent freezing during the first night. The next morning, add a heat source if you expect freezing conditions through the day. For slabs, a couple of sections of scaffold will support the tarps enough to make a makeshift tent that can be heated with electric or propane heat sources. For smaller work like footings or piers, tent the tarps up over the concrete and put in a light bulb or halogen work light. It doesn't need to be toasty warm in there, you just have to avoid outright freezing. 2-3 days after the pour you will be fine.
If it hasn't already been done, I think this would be a good 'technique' article in the mag. Maybe around this time next year.
Scott.
As others have mentioned:
Keep your holes covered to not make them into cold traps.
Use a stronger mix (richer in cement) because 1) it will generate more heat and 2) you'll only get xx% of the concrete's potential if it freezes before fully cured so start with stronger stuff. Like a 6-sack or even 7-sack mix instead of a 5-sack mix.
If the plant is close, you have spec hot water in the mix.
How many piers? I'd seriously considered throwing a hair dryer down each one (you alrady have one or two and they are only $10 or so at Wall-mart). Set on low and left covered over night, you'd heat up the dirt around each hole 20-30 degrees and that would help A LOT.
Covering once poured won't make much difference in a pier. The 30 square feet in contact with soil will determine the cooling rate, not the 1 square foot on top in contact with the air.
<<I'd seriously considered throwing a hair dryer down each one (you alrady have one or two and they are only $10 or so at Wall-mart). Set on low and left cov>>I use a roofing torch (kinda like a weed-burner on steroids) it works great.
That (roofing torch) would add some BTUs right quick!How about lower tech: throw some alcohol and handful of FG down there and flick in a match. Lighter fluid would work even without the wick but I can't recommend it. I make too much money from client who (accidentially) spilled pteroleum fuels on their property.David Thomas Overlooking Cook Inlet in Kenai, Alaska
I don't know, I suppose it's worth a try.I've got a cheap Radio Shack indoor/outdoor digital thermometer I use for cold weather concrete. One part goes under the blankets, one part stays outside.After the alchohol goes out, we could cover the hole, wait an hour, and check the temp.