[Greenbuilding] re: concrete & frost

[Robert Tom]

On Sun, 04 Nov 2007 15:20:55 -0500, Joyce Coppinger <jc10508 at alltel.net>  
wrote:


> Ask your concrete contractor what they mixed in to make the concrete
> workable at temperatures at or below frost. If they mix in antifreeze,  
> ask him how this is a viable green approach.

The admixture that is typically used to make concrete workable at  
below-freezing temperatures is calcium chloride -- a salt.

The salt, as in the oceans, simply lowers the freezing point  
slightly( only a couple or few  degrees F) or , keeping the water in the  
concrete mix in liquid state so that it is available to the cement for  
hydration, a process that you want to facilitate for at least 21 days --  
longer would be even better.

Too much calcium chloride in the mix ultimately has the effect of  
weakening the concrete.

If frost is visible on the still green concrete, then obviously some of  
the water in the mix froze and as such is no longer available to the  
cement for hydration and if something is not done, the result will be a  
weak concrete that dusts/erodes easily most likely becoming butt-ugly crap  
in a few short years.

However, the good news is that the concrete may still be salvageable by  
putting moisture back into the concrete, covering it up with poly sheet to  
keep that water in the uncured concrete and insulating over top to prevent  
freezing from occurring again.  That insulation may be straw bales (with a  
covering over the straw to keep it dry in order to retain its insulative  
property) or rigid foamed plast-ecch! or even bags of old clothing.

The process of hydration will generate some heat initially but the idea is  
to trap some geothermal heat under the umbrella of insulation to prevent  
freezing of the uncured concrete.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at chaffY a h o o  dot  c a >
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