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Thread: Floating slab

  1. #1
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    Floating slab

    I am not a concrete guy but I am looking to pour a floating concrete slb for a shop 20 X 40. What should I puit down for a base?
    Thanks Jim

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    Clear away the topsoil and such,, get some nice sand. Your in frost country so put at least 6 inches of sand down
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    Super Moderator CM1995's Avatar
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    Like Grandpa said, use a good base material and prep the sub-base. We use crushed stone in my neck of the woods instead of sand since sand is very expensive due to the distance it has to be trucked in. We use a #57 stone underneath slabs. It's 1/2 -3/4 washed stone, good for drainage.

    Being in the "Great White North" you may want to consider some sort of insulation and don't forget a vapor barrier between your sand or stone and the concrete. Also what kind of reinforcing are you using?
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    I use FLBC (fractured limestone base course) as it has a lot of sand size particle along with a good mix of #57 size and down stone, and, it is about the cheapest thing here. Compact it very well.

    However, since the frost line here is about 1/8", I may be giving bad advice for your climate. (probably no more ignorant than CM1995 though, which makes me feel some better. )

    Like he said, use a vapor barrier, and, for a shop floor, use about twice the reinforcing you think you need. Don't forget, you may jack up a lot of weight on a little area.
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    i would say at least 35mpa concrete, and 10m rebar instead of a wire mesh if your going to have heavy equipment in there.....

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    A good bed (6") or crushed stone such as 3/4" minus or D-1 and a layer of 2" blue board (insulation)

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    Quote Originally Posted by akpolaris View Post
    A good bed (6") or crushed stone such as 3/4" minus or D-1 and a layer of 2" blue board (insulation)
    I would double that crushed stone, cheap to put it in, expensive if something goes wrong in the future. The 2" blue board is a good way to go. Tape the seems with TYVEK brand tape, this will turn that blue board into a sealed vapor barrier, two birds with one stone. The only thing that sucks with vapor barriers is all the moisture in the concrete when it is curing has to go through the top. It makes finishing a little more of a pain in the rear. If you are going to spend any time in this thing, and have the budget, put radiant heat tubing in the floor. One heating and cooling contractor we work with from time to time heats their shop with two hot water heaters and a circulating pump. It works out of be fairly cheap heat.

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    Super Moderator CM1995's Avatar
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    Quote Originally Posted by mitch504 View Post
    (probably no more ignorant than CM1995 though, which makes me feel some better. )
    Ignorance is bliss ... sometimes



    Quote Originally Posted by Turbo21835 View Post
    The 2" blue board is a good way to go. Tape the seems with TYVEK brand tape, this will turn that blue board into a sealed vapor barrier, two birds with one stone.
    That's a pretty slick idea with tape and blue board.

    Quote Originally Posted by Turbo21835 View Post
    The only thing that sucks with vapor barriers is all the moisture in the concrete when it is curing has to go through the top. It makes finishing a little more of a pain in the rear.
    The vapor barrier holding the water in during the finishing is a good thing in the summer down here and a bad thing in the winter. Although calcium chloride can be added in the winter to speed up the heat of hydration but I'm not a big fan of it. (That's southern winters, which is like spring time to you guys up north )

    Back to the slab topic -

    Just a suggestion - I would put #4 (1/2") rebar on a 4'x4' or smaller grid across the slab with #9 wire mesh sheets on top of the grid. (I prefer the wire mesh sheets instead of the rolls, easier to work with.) Placing the mesh sheets on top of the rebar grid keeps it up in the slab where it needs to be. Use some concrete bricks (not clay bricks they wick water) to hold the rebar off the bottom of the slab.

    Also need to think about expansion joints. I prefer cut expansion joints instead of metal or PEJ. You can cut the slab the following day 3/4" to 1" deep. Or if there is a green cut saw available you can cut the slab as soon as the trowels come off of it.
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    While my cocncrete isn't floating it does have heat in the floor. I would never be a a cold climate without it. I keep the shop 50 degrees, that is plenty if working and you are working on a nice warm floor.

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    Quote Originally Posted by CM1995 View Post
    Just a suggestion - I would put #4 (1/2") rebar on a 4'x4' or smaller grid across the slab with #9 wire mesh sheets on top of the grid. (I prefer the wire mesh sheets instead of the rolls, easier to work with.) Placing the mesh sheets on top of the rebar grid keeps it up in the slab where it needs to be. Use some concrete bricks (not clay bricks they wick water) to hold the rebar off the bottom of the slab.
    If you are going to that extreme, just do what one of the worlds largest engineering firms specs 12 inches of concrete for flat work. Inside that 12 inches of concrete is two layers of # 4 bar. 1 layer is 3 inches from the bottom, the other is 3 inches from the top. The rod is on 12inx12in spacing. Last building we did for them, 100x300, including foundation work and approaches, had over 60 tons of rebar in it.

  11. #11
    Senior Member stumpjumper83's Avatar
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    when your building a shop floor to have to remember as others have said that loads may be concentraded like 20 ton over 6 square inches (bottom of a 20 ton hyd jack). 4' on the rebar grid seems light, 2 layers on 12" seems overkill, but nothing is more of a pain than having to do the job a second time cause you were a cheap donkey, and now you must spend 3x what you did the first time to end up with what would have been another $200 in rebar if it had been applied the first time.

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    Super Moderator CM1995's Avatar
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    Quote Originally Posted by Turbo21835 View Post
    If you are going to that extreme,
    That's not extreme that's a typical floor slab spec. i used that on the monolithic slabs on the houses I built and didn't have any issues with cracks. #9 wire mesh sheets are our local name for the same wire mesh you buy in rolls at HD or Lowes, they are just easier to handle and place.

    The shop that I am using now is just an unreinforced 6" slab, 20 years old or so. No cracks or problems so far.
    Last edited by CM1995; 08-30-2011 at 06:45 AM.
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    Ground conditions are different everywhere.All good advice here but I would check with your local building dep't and/ or a local engineer first.Here in NY for instance,that floating slab on grade is dictated by a maximum footprint of something like 24'x24',with very strict [and a PIA] requirements.
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    Thanks for the advice guys. Pretty well everything I was wondering about was covered by one person or another. Much appreciated.
    Jim

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    I'm not a big believer in floating slabs, some make them work others not, I"d opt to pour some footings and walls and sit the building on that and then pour your floor inside, you could save some on fill and gravel to help offset the cost of walls, go at least frost depth or slightly more on the walls but thats just me and my two cents worth of input so take it for what it's worth.

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