why do people put plastic sheeting down before pouring concrete? I think i know but want to here from experts.?
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.
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.
It's cheating. If you did not put down enough gravel, then put down the plastic. Because when the dirt leaches water out of the concrete (or other reasons), then that part of the concrete is weak. (or in a wet area, it also creates problems). However, in all honesty, your result would be better WITHOUT the plastic underneath, and more gravel underneath (which costs more). Why? Because cured concrete is a bit of a sponge, and needs to also let the water it absorbed out. If someone later puts this inside a home by building over a patio, without enough gravel base, then this concrete would be constantly wet, and thus, would be an odd and difficult to figure out disaster of problem, perhaps even a mushroomy smell. So, in summary, for a foundation for a fountain, it's great...for sidewalks....ok as well. For a slab foundation, it's a disaster! For anything outside, where the winters are short....it's ok.
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.
