No Plastic Under Concrete Slab

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.

No plastic under concrete slab.

Why do concrete slabs need black plastic membranes under them. This plastic sheet is in direct contact with the concrete as it should be in a rational world. The structural why fil. Although it may seem water tight this grade of material allows a lot of water vapor to pass through.

Recently though research has shown that the old traditional layer of 6 mil visqueen polyethylene plastic under the slab is seldom effective for two main reasons. One laboratory drying study indicated that 4 in. Or just use the plastic sheet to do both be the vapor barrier and the capillary break. If the plastic is right under the concrete the bleed water is forced to rise to the surface.

This water can take literally years to dry out through the top of the concrete slab. What is a vapour barrier under a concrete slab and why do you need one. So why not just use plastic and not stones. 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.

The reason is that the vapour barrier stops moisture from the wet concrete escaping into the ground under the concrete. 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. 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. In fact most professional shed slab builders will install a polyethylene membrane under a shed concrete slab even if it is not specified.

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.