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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
 

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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
Yes, install 6mil.

It's a moisture barrier.
 

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Not needed. Make sure they use welded wire fabric though. And depending on location, expansion joints are very important. after they pour spray it with water every day for at least a week.
 

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I have quite a few friends who are contractors and they have never heard of that. Maybe it is a regional thing. When my garage was built, they used fiber-filled concrete. 18x18" footings (code) and a 4" floor. Almost 10 years ago and not a single crack as compared to similar garages in the area who used wire mesh in the foundation. Not sure if it helps with the curing/drawing out of the water.
 

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You need plastic between the dirt and the rebar/ reinforcement, for a vapor barrier,,,TLW
 

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when you put down the plastic sheeting it helps keep the water from being sucked out of the concrete while curing.
 

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Use the plastic. If you ever decide to put some kind of coating on the floor it doesnt need moisture in it.
 

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Discussion Starter #11
I live in Kentucky. There is also about 3 feet of fill rock in the back to level it out. Its been setting for about 2 months to let it settle.
 

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Very few people use plastic anymore and cities that require it are just too lazy to correct the codes, but some areas do need it ^^like Florida because of all the building on swamp lands:p

You will get just as many "use it" "don't use it" replies.

I personally don't like the way the pad cures when plastic is used. All the moisture has to leave through the top when curing and it takes longer to dry out inside (I personally believe water pockets stay at the bottom and leave voids). Before a pour just make sure the dirt is wet so it won't draw all the water out too quickly while curing.
 

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Quick answer before reading my book LOL, use 6mil poly at the very very least.

what state are you in? we've always used 2" polystyrene since 99% of the garages we built were to be working/hobby garages that were heated in the winter time. At the very least 6mil poly/plastic to keep the moisture out in the winter time if you ever do decide to heat the garage, it helps cut down on how wet the floor gets. The barrier also, as mentioned above, slows the cure since the ground is'nt absorbing the mositure out of the concrete...most good guys will spritz grade with water before a pour...such as sidewalks/drives, so slow the grounds absorbtion of the water in the mud/concrete- just makes for a stronger product overall.

Anybody that buys into "no metal, just use fiber mesh concrete is a fool, and the person selling them on that is a crook/shister" Even out concrete rep/long time family friend says it is just a upcharge they can get but it does nothing to prevent movement....the only thing that will help hold concrete together when it does crack and ground forces it to heave is steel. Code here requires 2' grids on driveways but it's just second nature to do all our work following that grid pattern for typical slab work.

About the only places that "could be exempt" from using steel if your a cheap ass is a basement floor, or a garage floor that is inside footing and will be heated...because technically that slab is on a "condiditoned" area that will NOT be subjected to the heave cycle come spring after a long cold winter....but for us it was never an option, steel is cheap, the labor to layout and tie is fast...made me sleep better knowing down the road I would'nt have call backs for stupid shit like I hear all the hack guys whining about. Just like using sand under driveways/slabs...yep, it's a cheap and easy filler to get grade where it needs to be, but has poor drainage qualities..we've tore out ALOT of driveways/sidewalks that have hollow spots under them where the sand has washed out. We us 1" clean rock for our bases since it drains and cannot wash out.

As mentioned above, put a sprinkler on it that night after the pour/ and keep the concrete cool for a good week...slowing the cure process helps ensure minimal cracking if at all, and makes for concrete that is super super hard...just a "for the long term" approach to make it better.
 

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^^^ Guy knows what he is talking about in regards to the steel! The steel is really what makes it strong in the end...slow cure helps a lot too! We have 4" slabs in places that semi's drive across all the time, but we put rebar every 8"...no separated cracks! Look at the price of rebar...then buy as much as you can afford. Don't let the contractor tell you it's too much steel! Think about which is stronger--steel or concrete?

As far as the plastic goes, it's your choice. It has benefits and draw-backs. Try to figure out what your long term plans are and build accordingly.
 

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I am a superintendent for a large GC, and have probably put down close to a million square feet of concrete through the years, and every one of the pours had a vapor barrier of 6 mil visqueen placed with the seams taped. So, yes, I recommend putting it down. Use a 6x6 mesh, then use a soffcut saw right after finishing to put in control joints, usually in a 30'x30' pattern. The only time mesh wasn't used was on a Home Depot project, where the slab was 6", and they used a large aggregate in their mix. If you wish, you can use wet burlap covered with plastic to aid in the curing of the slab.
 

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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.
 

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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.

1st - good read. Well written.

2nd - do you know if the flooring is being placed after a 28 day cure time for elevated slabs.
 
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