I have to pour some concrete footers in a basement in order to install a girder and posts. I was wondering if anyone might have some input as to mix type/proportions? Also, what are the implications for pouring when the weather is very cold, as this property is currently unheated during rehab. (Additives for working with concrete near or at freezing????)
Thanks,
Dan
Replies
How many yards are you mixing? Unless you have enough for a mini truck (4 yards) why don't you rent/buy a 2-bag mixer and use 5000# Sakrete? Follow the directions on the bag and avoid adding too much water. Remember it's going to be a lot of work. If buying plant concrete, consult with them on admixtures.
Strictly speaking, cold-weather concrete should comply with ACI 306 and you should maintain 50 degees for 3-7 days.
That being said, due to the heat of hydration released when concrete cures, "at or near freezing" isn't usually too difficult to deal with (ie - concrete makes it's own heat - for awhile) - you can cover the area with plastic and lay fiberglass batts over top, for instance. Typical ground temperature, if isolated from colder air, is always going to be warmer. If you can heat the subgrade (propane heater) or keep the immediate area from getting below 45 degrees for the first 24 hours (again, supervised propane heater) that helps a lot. Placement should not proceed if the subgrade is frozen so that's step 1.
The key period is that first 24 hours because after the concrete reaches a strength of around 500 PSI it is much less susceptible to damage from cold temperatures during curing, although it really should reach 3000 PSI to have resistance to multiple freeze-thaw cycles. The 5000# Sakrete, being a higher-strength mix, will tend to acquire this minimal level of strength earlier.
You can of course use a non-chloride set accelerator like - http://www.wrmeadows.com/wrm00018.htm or http://www.fritzpak.com/faq/faq_nca.shtml - do not use calcium chloride.
Oh .... don't forget the rebar ;o)
What do you mean by "very
What do you mean by "very cold"? Near freezing is easy to handle when placing footers, if the soil is reasonably warm to start with and you cover it somehow. And a basement should be 10 degrees warmer than outside, even if unheated.
Concrete Footers
Thanks very much Jeff and DanH,
This is very helpful.
I believe I'll be ordering a truck for this project, as I have 7 footers to install, each measuring 30" X 30" X 12" deep. I believe my architect told me a while back that the project specifies a 12" depth due to the fact that the basement is already below the frost line.
Thanks again,
-Dan
I would probably
bring in a Drizair dehumidifier and possibly a ~4000 watt electric heater. The dehu is going to help a lot with moisture if you pour that much concrete inside a house, and it generates some heat. I would add the heater if the house is large and uninsulated.
You're worried about too cold conditions at this time of year? With a basement, you'd have to be pretty dang cold for a long time to be an issue. I'd think the basement would stay 50 degF plus w/ little problem. Can't imagine super sub cold conditions, yet ... 'course I've no idea where you live, but I'm guessing most areas of the country are still in the 25degF + as a low temp (maybe 'ceptin' FrostBite Falls and Alaska??!!).