Why do people put plastic sheeting down before pouring concrete? I think i know but want to here from experts.?

Putting plastic down before pouring concrete, is to prevent any water in the concrete from being absorbed by the underlying soil. If the water were to be absorbed by the soil, the process of the concrete 'curing' properly would be affected.Why do people put plastic sheeting down before pouring concrete? I think i know but want to here from experts.?
Putting the plastic down might help it cure faster, but that's not why it's necessary. Concrete behaves a lot like a sponge, and without a proper plastic vapor barrier, the moisture in the soil will continuously be drawn up through the slab. As the flooring contractor stated, this makes it almost impossible to put down any kind of flooring on the concrete. Any thing with glue will eventually come up, any kind of carpet will cause mold problems, and most kinds of wood will rot.Why do people put plastic sheeting down before pouring concrete? I think i know but want to here from experts.?
Plastic sheeting serves as a moisture barrier. Concrete is permeable and moisture from the ground can penetrate and dampen it, causing higher energy costs to heat and cool as well as possible damage to carpets and other things touching the concrete.One caveat: If you put a moisture barrier like plastic under the concrete in an open carport or other open structure such as driveways, it will make the concrete sweat and slick with moisture when the air temperature become warmer than the slab because the condensation will not be absorbed into the slab.
That prevents the salt and moisture in the earth from penetrating the concrete, intermingling with the lime in concrete. Some concrete contains ground ';Oyster Shell';, compounding the problem.


This occurs through hydrostatic pressure.


When that happens, ';Alkali'; forms and deposits on the surface of the slab.





When hard surface flooring of any kind is glued to the slab, Alkali destroys the adhesives.


Sometimes, before that happens Alkali will penetrate through the concrete creating deposits between grout lines of tiles of flooring. It appears as a white powder.





There are times when the plastic may be ruptured and sometimes it does not prevent some formation of Alkali.





Alkali travels, damn near like an animal and creates fits for flooring.





Much research and many products have been produced to combat Alkali.


None are 100% remedies. Sometimes Plywood sub flooring is the only answer.
it's been answered, I'm just ';voting'; It's to keep the moisture in the concrete from being sucked up by the dirt. If the water gets leeched out the concrete will be brittle and crumble.
Plastic sheeting creates a stoppage from the liquid concrete running into the dirt below. It needs this so the concrete can dry and not keep getting lower as it absorbs into the ground.
My uncle is works for a cement company and he says.... To layer it so it will be stronger. The more layers it has the more weight it can support. 1-2 layers 10 tons, 3-4 layers 15 tons, and so on.
So the ground won't absorb the water from the mix. With out plastic, your concrete will not cure to it's full potential.


Mass. builder for 40 years.
It's a damp proof course to prevent water penetration from below.
To keep moisture from leeching back up through the concrete
make sure they stick around.. else where damaging the construction..