Yes, install 6mil.Building new garage and fixing to pour the concrete floor. Question is to put plastic under concrete or not? got bids from concrete guys and one said to put plastic under it and one said no plastic. Just need more opinions. What does the plastic help? Thanks
I don't do much residential engineering but on the comercial side of things we call out for 15 mill poly vapor barrier located directly under the slab, a minimum of 6" of 3/8" crushed stone the stone thickness will depend on what the geotechnical engineer suggests based upon test borings. Then the concrete mix design will have a low water to cement ratio of under .42 and to make the mix workable no field water only super plastiziers are added to give it a slump of 5 at the chute. Then an additive of Barrier One or equal is added to the concrete to prevent water vapors coming up through the slab. Saw cuts are then made depending on the project at around a 12'x12' pattern.
This may seem way over kill however there is a large problem in the last 7-8 years in commercal buildings with flooring adhesives. The adhesives are all water based, thus any moisture migration up throught the slab will debond the flooring from the concrete surface causing flooring failures. For those who think this is a crock of shit you can ask any large architectural firm because chances are they have gotten sued by the building owners on this matter and have lost. It is even a problem with elevated slabs on metal decks.
In 4" interior non-structural slabs on grade you can get away without steel and just by using poly fibers however the big item here is the dosage rate. (Nobody asks about this). You need a minumum of 1.5-1.75 lbs of poly fibers / cubic yd. Concrete finishers hate it because the slab is hairy due to the amount of fibers. Why use fibers 1 reason is project insurance rates on very large comercial projects. Welded wire fabric is a big tripping hazard and in the eyes of the insurance company is a liability. No WWF no tripping hazard in that part of the construction.
Curing is specified as a 7 day wet cure meaning it is wet all the time, typically with burlp and a plastic sheet and dripper hoses on the slab. It is critical to keep the slab hydrated especially in the first 24 hrs. The saw cuts need to be made no later than 12 hrs after you can get on the slab, after 12 hours you lost your window and the saw cuts will be much less effective.
All this for a concrete slab on grade. I am a structural engineer and have fits when a residential concrete guys doesn't do things properly. Sorry so long winded I had to list everything. And another thing, it's not cement, its concrete.