i’m a small builder putting up a new home.my old foundation sub moved down south.i’musing a poured concrete 3500lb concrete with fibermesh for the slab “4” thick.the new sub won’t put a 6mil vapor barrier under the slab.he said it causes the surface to turn to dust.i think it will have the exact opposite effect.anyone out there with any advice?
matt
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Replies
Never heard that one before. Up here, the finishers don't like the poly as it does not let bleed/excess water drain out to the crushed stone. This slows the curing of the concrete and trowleling takes another 4-5 hours, most of it waiting. The longer the concrete is moist, the stronger it gets.
You're right, he's wrong. Around here the under-slab poly is an inspectable point. How would he explain that by leaving the poly off, you avoid the powdery dust? (efflorescence)
Scott.
Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”
The powdery dust on the floor at this time is not efflourescence.
What poster no. 2 said
it will also make the slab crack by leaving the poly
That would be only when they add too much water to make it easy flow in the fitst place.And the powder scale on top would be beause the water floats the portland to the top when they get in a hurry and float and polish it too soon, not because of the plastic
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Where does this idea come from?
Ask him how much slab work he does over rigid foamboard like Dow's isocyanurate boardstock.
Extruded closed cell foam, with smooth surfaces top and bottom, is about as effective a vapor barrier as PVC sheet, isn't it? I have seen a lot of slab work done over it, and never any powdering.
One reason for dusting (not effluorescence- this usually takes a few years to appear):
From the Portland Cement Assoc. text "Design and Control of Concrete Mixtures":
"Freshly cast concrete has an abundance of water but as drying progresses from the surface inward, strength gain will progress at each depth only as long as the relative humidity at that point remains above 80%. A common illustration of this is the surface of a concrete floor that has not been properly cured. The surface dries quickly so that it is weak and traffic on it creates dusting"