cold weather footers


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Old 12-23-05, 08:23 PM
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cold weather footers

Looking for someone with practical experience. I am having a home built and a few days ago the footers were poured. I was concerned as that evening the temp dropped to 18 degrees (about 40 thru the day). I mentioned to my contractor that I was worried about the freezing weather weakening the concrete. He assured me that there would be no problem as the concrete had calcium in it, and the dirt would provide some insulation. It was not covered.
A few days later, most of the footer looks fine, except there are areas where the footer still appears damp and there is a fine granular powder over the concrete.
I have read recommendations to keep the concrete above 40 degrees for the first few days and also found some data from canada suggesting that you can place concrete basically in any temp above 15.
Was wondering about any practical experience with this situation? thanks.
 
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Old 12-23-05, 08:47 PM
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cold weather footers

The temperature of the concrete should not drop below 40 or so for few days. The proper way and most common method is to cover the footings with insulating blankets. The Canadian note about concrete placement in the winter is prtially correct. You can place the concrete, but you must protect it from freezing.

Calcium chloride is used as an accelerator that promotes high early strengths. You must be careful when using it and there are is a maximum dosage rate. Calcium chloride can cause corrosion to reinforcing steel and any embedded steel.

your concrete will look different and could appear wet because it has had very little curing and will not finish curing until next summer because of the low winter temperatures. Normally the loads on a residential footing are quite low, so the early strength of the footing is not a great concern umless there is heaving.

I would be concrened about protecting the soil under the footing from freezing. Often the footings can freeze and can ahave differential frost heaving before the wall is built on them. If this is the case, the wall can crack in the spring when the soil under the footings thaws and returns to it normal position.

dick
 
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Old 12-26-05, 02:09 PM
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cold weather footers

i appreciate your reply. Its quite frustrating researching concrete information as there seems to be quite a variation of opinions. I've found sites that emphatically state not to pour below 50 and I've found those that state down to freezing and even below poses no problems. Also there seems to be varying data on final cure strength, etc... I wonder if anyone has ever had their footer fail?
Interesting thought.
Thanks for the information.
 
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Old 12-26-05, 02:59 PM
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cold weather footers

Concrete can be placed at almost any temperature provided you use the right mix design, heating of materials finshing and protection during curing. Forget about the air temperature. You are concerned with the temperature of the soil, the temperature of the concrete and the protection of the concrete after it is poured.

The temperature of the soil can be quite cold, but it cannot be frozen to any appreciable depth. The concrete will warm it somewhat.

In colder climates, the mixing water is usually heated and the concrete in the winter can be as warm or warmer than the concrete in the summer. Even in Minnesota, ice can be added to concrete in the summer. Warm concrete sets up faster than cold concrete.

As concrete cures it generates heat. Providing warm concrete causes the concrete to cure faster and generate more heat (heat of hydration) as it cures. There are limits how warm concrete can be. Special mix designs are used and additives are used to create the right mix for cold weather construction.

A good ready-mix supplier can produce good concrete in almost any weather. Central mix (mixed in the plant with the truck providing agitattion) usually provides better control than transit mixing (mixed AND agitated in the truck) because of the better control over the water.

In some situations the item controlling whether concrete is placed is the ability of men to work at those temperatures. I was on a site in northern Minnesota where we routinely had 100 to 300 yard pours at temperatures below -10 F. Colder than that manpower was difficult.

Concrete can usually be protected with insulating blankets. The soil if not frozen, it has latent heat in it and will not freeze the concrete. The first several days are the most critical. You want to acheive as much strength as possible before the concrete freezes. Freezing when not saturated can be tolerated and the concrete will begin curing again once it is warmed.

People refer to 28 day strengths. This is a term used to describe the design strength of the concrete after curing under laboratory conditions for 28 days. In all reality, the actual stregth may be above or below this strength when cured on a job site. Concrete can be built on before 28 days or can be loaded before it actually reaches the design strength since the load applied are usually much less than the design loads, which have a higher "factor of safety" than other building materials.

I have seen footings fail due to cold weather. Except in the case of slabs and localized surface feezing, the failure is due to soil conditions. Usually the failure is due to variable soil types with variable moisture contents freezing. If the concrete is poured on frozen ground, it settles unevenly when it warms. If the soil freezes after footing is poured, it may crack if the is not a load to overcome the soil distortion. In the case of uniform soil conditions (uniform expansion) or a large applied load (walls, rigid structure) the effects of the freezing are not seen if there is indeed enough freezing to cause heaves.

For a home, the footings are usually grossly over-designed. With good soils footings may not be needed (except to provide uniform bearing) since the area neede for bearing is so low.

Like all construction, you have to have qualified contractors (knowledge and equipment) and qualified suppliers (technical support and facilities) to build is less than desireable conditions. In cold climates, cold weather concrete is a fact of life and it is just done a little differently. A supplied with 10 plants may only run 3 or 4 large plants to concentrate the use of the cold weather facilities.

Dick
 
 

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