Footings Freezing

[37ed]

I am an structural engineer in the Charlotte N.C area,for a concrete subcontractor.

The frost line here is about 12in or less in some areas.We have footings at 3ft 6in below subgrade ie. "BOF "is 3ft 6in below subgrade. The design engineer and soil engineer have ask our company to cover these footings with poly to keep them from freezing.the air temp at night is 17 degrees and ranges from 0 to 55 degrees in the day.

There reasoning for the poly over the ditch is to keep the bottom of the footing from freezing,thowing and creating mud.
However I feel that putting poly over the ditch is going to create the very problem that they are tryihg to avoid.

My reasoning is as follows,
1.The bottom of the ditch has a temp of 51 degrees constant.
2.The air temp runs 0 to 55 degrees.
3.I do not feel that frost will enter a ditch only 30 in wide.
4. I think that the poly will produce a greenhouse effect.


anyone have any feelings on this?

thanks in advance Ed,

 

[pmkPE]

Questions and Comments:
1) How do you know the bottom of the ditch temp is 51 degrees CONSTANT? Are you there betweem 5 and 7am when the temp drops down to its minimum? Cold settles as the air is denser i.e. heavier and therefore will flow into the low areas.
2) Frost does not enter any area, it forms where the cold air contacts with the moisture in the soil. The only time you will have a constant temperature in the soil is when it remains covered (at its original depth of cover). Remember, Frost is the freezing of the water that is in the soil. Naturally the deeper into the soil one measures the temp the less the drop in temperature. But that is a factor of not only the depth but also the ambient air temperature as well as the length of exposure (time) to the cold temperature. That is why the nominal frost depth up north can be several feet while in the south only inches.
3) A greenhouse effect in the sense of the term as I understand it requires solar energy for heat. Sunset to day is approximately 5:20 pm with sunrise at 7:40 am. (The weather channel shows that the temp this noon in Charlotte was 25 degrees while in Pittsburgh it was 26.) The poly will protect your subgrade from the extremes of cold but it will not be a greenhouse. Besides the sun is low in the sky at this time of year and you will mostly have shadows on your "poly"; not much opportunity for heat. Otherwise you would see a lot of homeless people cuddled underneath their "polys".
4) Also remember that the people responsible for designing and possibly overseeing the construction of your client's structure have stipulated that "X" actions be taken. These actions are typical for the conditions at this time of year and in general for protecting exposed subgrades and should not be disregarded. When competent firms bid on work they should have an understanding and knowledge of what is required during extreme weather conditions for the part of the country they serve.
5) When in doubt you did the right thing - you asked for questions and advice with your own reasoning based on what at first glance seemed reasonable considerations. I hope this helps in your decision making process.

 

[jheidt2543]

A few thoughts to consider:

1. Perhaps by "greenhouse effect" they meant the tendancy of poly sheeting to trap moisture vapor escaping from the subgrade and that freezing.

2. Prevention of frost penetration is the result of reducing heat transfer. Air is a good insulator (think of thermal pane glass) so, the poly sheeting will provide some protection. However, the amount of protection is a function of the temperature difference and the exposure time.

3. Here in the upper midwest, we use either loose straw or insulating tarps to cover footings over night. The straw material is relatively cheep, but clean up is a pain and when it is dry it blows around the constructoin site or the neighboring areas. I have seen some contractors just burn the straw in the ditch to "clean it up", but that only works if the straw is dry, otherwise you're sending smoke signals.

Tarps are initially expensive but easy to move around and reusable. You do have to hang them up to dry after use and roll them up and store them, but you can get 10-20 reuses.

Personally, I wouldn't use the poly for frost protection, but we get colder weather up here. Maybe, you could salvage the poly and use it as a vapor barrier under the floor slab?

Just a few thoughts.

 

[cvg]

thread158-112883

see the above thread for additional commentary on freeze/thaw, heave and insulation of foundations

 

[Ibeam]

1. You don't want the soil to lose compaction by freeze/thaw cycles. It's not frozen down there and you want to keep it that way.

2. You don't want your concrete to freeze while it's trying to cure.

3. When you say the poly will great a greenhouse effect, you just mean that during the day it traps in the solar effect and warms things up, which may get things a little steamy inside. But that's o.k., at least as I see it. If I'm wrong then maybe someone will point that out.

The Polyethylene is just one way of protection, just as others have said. If that's what is reccommended to you I'd just go with it. You obviously don't think it's called for, but it's a small price for a precautionary measure.

Insulated blankets are what we use whenever we're pouring concrete in winter/freezing climates. We were pouring valve vaults in Baltimore a year or so ago when they had 32 inches of snow fall.