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I'd like to understand how weather conditions affect pouring a concrete foundation.

Are there concerns about pouring a foundation during cold, snowy, rainy, or otherwise inclement weather? Is it safe to do so, and can it affect the longevity of the concrete? Are there other considerations? Extra cost?

What precautions should I request in a contract to ensure the foundation is poured effectively? Are there tests that can be performed prior to pouring and/or temperature limits that should be maintained?

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If it's too wet and water starts to accumulate in the foundation, you can end up with weakened concrete.

One of the key metrics for a concrete mix is the free water to cement ratio. If the water content is too low, you struggle to get it to compact properly , resulting in air voids, and weak concrete. If the water content is too high, the water can leave pores in the concrete, which also reduces the strength of the concrete.

If the temperature is low, the hydration reaction slows down. Further, if the temperature is too low, the water can freeze, which means that it is not available to hydrate the cement, again affecting the strength of the concrete.

  • Any idea if there's a way to as a homeowner verify water mixture during construction? Or do I have to trust contractor? – glenviewjeff Sep 20 '14 at 19:18
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    @glenviewjeff - look up "slump test", there's several ways to do it. – Fiasco Labs Sep 20 '14 at 19:20
  • Slump isn't the only measure though. You can add water to a 50mm slump mix to get say a 100mm slump to make it more workable and weaken the concrete or you can order a 100mm slump mix which will have additional cement (or additive) to keep the free water to cement ratio in the ideal range. The usual thing to look out for is that the mix is as specified and that there is no additional water added on site. – John Sep 20 '14 at 21:43
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    If you're concerned, have the contractor do some test cubes and have them crushed (usually at 7 and 28 days) to show that the concrete has reached the design strength. This would be unusual for work on a house though. – John Sep 20 '14 at 21:46
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Reportedly, it costs substantially more to pour a foundation because of additives and other precautions used to keep the concrete from freezing.

This page lists extensive guidelines and recommendations from the American Concrete Institute.

  • Never pour concrete on frozen ground, snow, or ice.
  • Concrete in cold weather is recommended to have low slump, and minimal water to cement ratio, to reduce bleeding and decreases setting time.
  • Use insulation blankets or heated enclosures to maintain concrete temperatures above 50° degrees Fahrenheit for three to seven days. Maintain the concrete temperature above 40° degrees Fahrenheit for at least four more days after the use of the insulation blankets or heated enclosures.
  • Fresh concrete frozen during the first 24 hours can lose 50% of its potential 28 day strength.
  • Keep concrete warm, over 5 degrees Celsius, for the first 48 hours, where concrete strength development is critical. When concrete is being placed below 5 degrees, but is not below freezing point, concrete will take longer to develop the required strength. Note that removing formwork when concrete is too cold or hasn’t reached desired strength, could damage concrete strength and surfaces and concrete might collapse.
  • Concrete in cold weather is recommended to have low slump, and minimal water to cement ratio, to reduce bleeding and decreases setting time.

This page also offers a number of other useful considerations, effects, and construction guidelines during cold weather including:

  • Chlorine-based additives can have a corrosive effect on metal re-bar and shouldn't be used.

This thread contains a lot of good information about pouring foundations in cold winter weather.

Other notable points from Glenn Good:

It is recommended to keep concrete above 40 degrees. With the proper insulation covering all areas exposed to the cold it will maintain this temperature in most cases. Keep in mind that this insulation must extend below the frost line as well. Rigid polystyrene foam insulation board can be used below the grade and left in place. The warmer the weather the less insulation required.

You may also want to try to keep track of is the amount of water in the concrete and the time it is poured. Concrete should be placed within 1 hour after it leaves the plant. The "slump" test is used to determine the amount of liquid in the concrete. Concrete should not be poured over a 4" slump. The lower the slump the less water in the mix and the stronger the concrete will be. Excess amounts of water cause the concrete to shrink more as it cures and as a result stress cracks will begin to form and weaken it.

Another item I should mention is the addition of calcium to the mix. This is often used in cold weather pours to accelerate the curing time and prevent freezing. The one major draw back with using calcium (and many other accelerators) is they have a tendency to deteriorate or oxidize (rust) the reinforcing steel that is used in the concrete. Chances are you will have steel rebar in your foundation and I would advise you do not permit the use of accelerators in the concrete mix. This will mean they will HAVE to use insulation to protect the concrete from freezing but you will get a stronger product that will last longer.

This site says:

Temperature extremes make it difficult to properly cure concrete. On hot days, newly placed concrete losses too much water through evaporation. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. In general, the temperature of new concrete should not be allowed to fall below 50°F (10°C) during the curing period.

Cold weather concreting is a common and necessary practice; every cold weather application must be considered carefully to accommodate its unique requirements. The current American Concrete Institute definition of cold-weather concreting, as stated in ACI 306 is, “a period when for more than 3 successive days the average daily air temperature drops below 5°C (40°F) and stays below 10°C (50°F) for more than one-half of any 24 hour period.”

Rule number ONE is that ALL concrete must be protected from freezing until it has reached a minimum strength of 3.5MPa (500psi), which typically happens within the first 24 hours. In addition, whenever air temperature at the time of concrete placement is below 5°C (40°F) and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered:

(1) Adjustment of construction schedule regarding loads imposed on the new concrete structure

(2) Placing and curing temperatures to produce quality concrete

The exposure of concrete to cold weather will extend the time required for it to gain strength. In structures that will carry large loads at an early age, concrete must be maintained at a minimum of 10°C (50°F) to accommodate stripping of forms and shoring and to permit loading of the structure. In many cases, achieving the required durability will require a protection period of more than 24 hours. This may not be an issue with residential applications where applied loads are typically small and may be applied in small increments over several days or weeks.

In no case should concrete be allowed to freeze during the first 24 hours after it has been placed. Since cement hydration is an exothermic reaction, the concrete mixture produces some heat on its own. Protecting that heat from escaping the system may be all that is required for good concrete quality, while more severe temperatures may require supplemental heat.

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In the Pacific Northwest, we have similar concerns about pouring concrete in the fall. I found a good article about the feasibility (and advantages) of pouring in Boise during the fall:

http://www.boiseconcreteco.com/blog

It seems that a milder place like Boise doesn't face the same hazards as those way north.

  • Links suffer from "bit decay" over time. It would be good to edit this to copy in the key points of what you learned from that site. – keshlam Sep 19 '16 at 2:21
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Concrete guys will always say you can pour. As mentioned soil should not be frozen where the footings are going in. There are heated blankets which run off of generators and bales of hay which can be used...in addition if it is allowable the concrete plant may add Calcium chloride to the mix which causes the concrete to harden faster but with less strength...however if you only need a 3000 psi mix or so then taking a 5000 psi mix and making it set hot with CaCl2 then this may be a workable solution. I would avoid pouring in the dead of winter/at the coldest point if possible...always better to try to pour in a warm week or a little bit before it gets super cold. Pouring in freezing temperatures also creates issues for backfill...backfilling with frozen earth is not advisable as you will have significant settling when it thaws...I have seen contractors bring in non-frozen fill but this is still suboptimal.

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