Concrete foundations, time of year pour
What time of the year is right for pouring concrete foundations and slab in residential construction? I am located in Fairfield County, Connecticut. This past winter I saw a house being built where it seemed to me they were doing work in the winter that I would have thought may compromise the construction. Possibly I have been reading up too much on building science. I can’t recall the temperature on the day of the concrete pour for foundations, or in the following days, so I thought I would ask this question in an open ended way.
It is probably the case that with the right expertise and variation in mix, a pour can be properly executed at other times, but I am assuming that one may not be sure of having people with the right skills. Thus I am looking for mainstream practice in this regard.
Replies
We've done pours in the middle of winter here in Alaska but you do need to tent or blanket them afterwards. Then depending on the weather it may have to stay like that for 3 or 4 days.
Dave
The first consideration is to never, ever place concrete on frozen ground.
Secondly, concrete needs to be above 40°F for several weeks. Concrete generates some heat so you can place it even if the temperature will drop below 40° for the first day or so if the concrete is in a mass (such as a wall) and not spread thinly in the weather (such as a sidewalk). Covering the concrete with insulating blankets is helpful.
If the temperature is going to drop lower than 40° for a couple of weeks than artificial heat is wise.
If the ground was properly compacted then frozen ground will not make any difference at all. Watch the steam coming off the concrete in cold weather, wet or setting up. Concrete generates its own heat up til the time it starts curing.Usually concrete in a formed wall will still be warm , you can see and feel the heat the next day,Depending on the mix, concrete in a wall form without covering or insulation can be poured in temperatures in the teens. The heat from the mix will prevent any problems except for the top 1/4".That portion might freeze and flake . If the top of the form was covered, it would prevent the flaking.The reason it usually is not covered is simple,frozen portion has no bearing on the strength of the wall.
Slabs can be poured over frozen ground too,as long as there is no ice on it.Thin spotty layers of ice or even a thin layer of snow are not a problem.The concrete will melt it as soon as it is poured.I've poured slabs on grade and high rise floors with temperatures in the mid twenties,we use blankets on grade after the slab is troweled. On high rise floors the floor below is tarped in and heated,no blankets .
mike
When the ground thaws it will subside and the concrete will be poorly supported and will move.
Makes no sense , whether the ground is frozen or not ,the soil is not moving before or after the foundation is poured. If what you surmise is true, every building in areas that get cold winters would be having settlement problems. No way do you have to wait for 40° temperatures to pour footings,walls, flat work etc.
Wayne, if you do not trust my explanations, try googling for info on pouring concrete in cold climates. I am sure you will find a dearth of info.
mike
OK - then why myst we always dig below the frost line for footers?
Very simple, frost heaves the ground. Has absolutely nothing to do with pouring concrete in a proper trenched footing.When the footing is poured, walls up and backfilled,frost can heave the area that is frozen but not the footing.
mike
Let me get this straight... you are saying that it's fine to pour concrete on frozen ground?
Actually I found a plethora of information rather than a dearth.
The ground expands when it freezes and shrinks when it thaws. If you place concrete on frozen ground, and the ground thaws, it will subside. When it subsides unevenly the strength of the structure will be compromised.
I agree that you never pour on frozen ground. But here in Canada, the rule is that that concrete cannot freeze in the first 48 hrs.
Using hot water in the mix, and ICF blocks, the local guys have proved you can pour a wall in -15C weather, and it won't freeze for several days, if you cover the top of the wall. Footings, they still cover with loose straw.
Many thanks for the information here. My original question has a context. I expect to start construction of my house early next year, with how early being open. While the property itself is dry, there are wet and low lying areas adjacent. So my concern is that the foundations and basement in a sense will need to be like the hull of a boat - they need to keep water out even more so than in any normal construction. If the concrete isn't just right, I could end up having perpetual water problems. While it seems that there are numerous technical solutions (heat, insulation, suitable mix etc.) to cold weather pours, they will only be as good as the integrity and competence of the concrete supplier or sub. I thought that since there is only one shot at getting concrete right, I could schedule this important step so as to maximize the probability of proper construction. If I was in the business of construction, and had a relationship with a foundations sub, my approach might be different. However, this is not so and I am thinking of taking an approach of caution.
we pour 365 days a year, 24 hours aday. now serious anytime its not freezing. and you can pour when it freezing, just not fun.
http://www.cement.org/tech/faq_cold-weather.asp
http://www.concretenetwork.com/jobsite/whentoprotectfresh.htm
http://www.miconcrete.org/page.cfm/89/
http://opkansas.org/Documents_and_Forms/cold_weather.pdf
When we pour slabs up here it's on 4' of compacted gravel at temperatures of 0 to 10 above. These are government or commercial jobs with federal and local inspectors looking over our shoulders. If it wasn't OK with them I guarantee they would stop the pour.
If we waited till it warmed up to 40 above we would only be able to pour for three months out of the year. I'm sure that you're not supposed to pour in anything under 40 but it still happens without the concrete failing or the ground giving way.
Are blankets, heaters, tents or other methods used to keep the concrete warm for a few weeks after a pour, or is it just left exposed to the 0-10 degree (Fahrenheit?) weather?
Yes, three or four days usually.