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How about heated slabs?

I posted this on LinkedIn and figured I should raise the question here, where all the brainiacs hang out...Big Grin

Here's something to think about: Concrete slabs will trend to acclimate with the surrounding atmospheric conditions. Very old and dry slabs (55% or so) will absorb moisture when the ambient relative humidity is higher than the slab, and they will emit moisture when the ambient RH drops.

In the tropics a slab may be dry when it reaches 80% RH if that's what the ambient RH is. Remember that 'dry' means it emits no moisture, not that it has zero moisture in it. There is always free water in concrete.

So here's the conundrum, and I have not done any testing regarding this, but if we follow the standard psychrometric chart: A slab held at a constant 85 degrees with a 75% RH reading indicates there is nearly twice as much moisture in the slab compared to a slab at 65 degrees and 75% RH. By raising the slab temperature, you lower the RH.

If your concrete slab will always remain warm, could it then be suitable for flooring with a higher RH than commonly accepted?
JD Grafton
Concrete Answers for Flooring Problems

Look up my hot / bikram yoga post.

This is kinda similar to the EMC concept for timber flooring installers.

EMC = equilibrium moisture content. It's taken us years to get the guys to even test timber flooring, now it's gonna take years to teach them what to do with the figures once they have them. "I moisture tested the timber, and it's 8-9%, which is within the manufacturer's stated tolerances for that species" they would say triumphantly, then slap the timber down.

Then you have to explain that "tolerances" have nothing to do with it. It's about what the timber is likely to do, once installed in the premises in which they will spend the rest of their wooden lives. What's the climate like year round? What about the heating/cooling system? Is it evap?

So if the premises is going to maintain an average of around 55% RH and 26deg (that's celsius for you guys), then the ideal moisture content for a given timber might be, say, 10%. From that content, it will expand and contract a very small amount. However, if the premises is going to be 65-75% RH and 20deg for most of its life, it might be ideal for the timber to have a higher moisture content to maintain its shape more consistently.

Is this the same kinda thing for a slab in a moist area? Or does it all become academic once you slap a non-permeable surface on top of that slab in any conditions?

These are all huge questions. Carl Sagan couldn't pose any better.
The problem with socialism is that you soon run out of other people's money.
- Margaret Thatcher


can you post a link to the linkedin conversation?

Here you go Evan:
JD Grafton
Concrete Answers for Flooring Problems

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