It then reacts with the cement and soaks back into the concrete within an hour or two helping to complete the curing process.
No plastic under concrete slab.
This water can take literally years to dry out through the top of the concrete slab.
The rate that moisture escapes from a slab depends on the concrete w c the density of the finish the ambient conditions above the slab and the moisture below the slab.
Why do concrete slabs need black plastic membranes under them.
The other contractor says no way no plastic holds the water in the concrete during the drying process and can leave webbing like visual affect in finish of concrete.
One contractor says to put plastic under the concrete it prevents moisture from coming up into the concrete.
The structural why fil.
Almost everywhere except california that slabs are placed on the ground a plastic sheet a vapor barrier is located directly under the concrete slab and on top of the ground photograph 1.
In fact most professional shed slab builders will install a polyethylene membrane under a shed concrete slab even if it is not specified.
So it is common to use crushed stones with no fines as a capillary break and then add a plastic sheet over the top of the stones in contact with the concrete as a vapor barrier.
A concrete vapor barrier is simply a sheet of polyethylene plastic visqueen placed directly on top of the sub grade before the concrete floor or slab is poured to help keep moisture from the soil from passing up through the concrete.
So why not just use plastic and not stones.
If the plastic is right under the concrete the bleed water is forced to rise to the surface.
Or just use the plastic sheet to do both be the vapor barrier and the capillary break.
Thick concrete samples having a w c 0 50 and sealed from below required 82 days for the vapor emission rate to reach 3 0.
The reason is that the vapour barrier stops moisture from the wet concrete escaping into the ground under the concrete.
What is a vapour barrier under a concrete slab and why do you need one.
Although it may seem water tight this grade of material allows a lot of water vapor to pass through.