R-Value of Air Entrained Concrete

[MJB315]

All,

I'm in the building design world trying to make some sense of energy / insulation codes. I am seeing that air entrained concrete has a higher R-Value than standard poured concrete (see below). Whereas it makes sense, the difference surprises me. Is air-entrained concrete really this good at insulating?

Standard Concrete: R = 0.8 (F*h*sqft/BTU)/in
Air Entrained Concrete: R = 3.9 (F*h*sqft/BTU)/in

If so, how much air needs to be entrained? The standard 6% +/- per volume?

MJB

 

[Montemayor]

MJB:

I can’t tell you how much air should be entrained in your concrete, but I can assure you that the more air you have entrained in your concrete, the higher the R value (the better the insulating quality). Of course, there has to be a limit.

This is so, because air (as all gases) has a TERRIBLE thermal conductivity value. Just take a look as its value as compared to any liquid or solid. While this is a terrible value, it is a GREAT value when viewed as an insulating agent. And that is exactly how industry makes good use of this natural resource. In fact, nature has started the whole application. The fact that Polar Bears can withstand Artic conditions like nothing at all is simply because nature has given them a fur coat that traps air pockets within the fur and it is the air pockets – not the fur – that form the basic insulating of the bear. The same effect is mimicked by Pittsburgh-Corning in fabricating FoamGlas insulation – perhaps the most efficient (& expensive) form of industrial insulation sold. FoamGlas is nothing more than fused glass bubbles that are filled with air or an inert gas – like nitrogen. Foamglas is so effective it is often used in cryogenic insulation. And because the glass cellular composition seems to give the product a great compressive strength, slabs of Foamglas are routinely used under the flat bottom plate of cryogenic or cold liquid storage takes to lend structural support as well as excellent insulating value at the floor plate. You may have heard of it or already used it.

I hope I have convinced you that air-entrained concrete should have a much better insulating property than common concrete. When I lived in the Chicagoland area in the 1960’s, I had my new home’s slab poured in air-entrained concrete because of that very fact. I picked up the idea from friends in the Chicago area who knew of this and the contractor who did my pouring also recommended it. However, I never got any information as to whether air-entrained concrete has a trade off in structural strength when poured in this type of application. I suspected that it did, so I increased the amount of rebars by 30%. I never had any troubles with my house foundation and we stayed warm with the hot-water radiant heat I installed.

I am interested if others have had the same experience.

 

[ione]

As pointed out by Mr. Montemayor what makes air, such as other gases, a good insulator is its poor thermal conductivity. Air has a thermal conductivity of 0.0256 W/(m*K) for air against 0.6048 W/(m*K) for water at 20 °C. This is basically due to its structure, low density and poor molecules interaction.
Having identified the poor air thermal conductivity as the “culprit” of the low thermal conductivity of air-entrained concrete, I would like to underline that the percentage of air entrained (generally from 3% to 6% by volume) is not the only parameter to look at. The distribution of air bubbles and their size is very important as well. Chemicals are added to the admixtures in order to lower the surface tension of the water to enhance bubble formation and stabilize the layer between water and air thus preventing bubbles coalescence.
Anyway, as pointed out once again by Mr. Montemayor, strength of air entrained concrete is reduced because of voids presence and typically this reduction is in the range of 2% to 6 % for every 1% of entrained air. The above reduction in strength performance can be compensated by the reduction in the water-to-cement ratio.

 

[moltenmetal]

OK, I'd personally be surprised if a 6% air addition resulted in an order of magnitude reduction in thermal conductivity, unless that 6% you're talking about was 6 MASS percent air and what you ended up with was basically a concrete foam which contained a very significant volume of air and had a greatly lower density than that of ordinary concrete. The 6% air entrained concrete I've seen is far from what I'd call a concrete foam.

 

[IRstuff]

This article seems to discuss appropriate proportions of air in concrete:

http://www.essroc.com/assets/essroc/corporate/c11_airentrainment.pdf

TTFN

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