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concrete slab for garage

cpcf250 | Posted in Construction Techniques on September 5, 2007 04:44am

OK – I have done a lot of reading, but still have some questions.

What I know – Rebar & wire mesh are not the same as fiber mesh

What I have –  A 40’x 60′ garage area under an existing barn.  Perimeter drains installed on outside of foundation.  Additional drainage installed under what will be the floor area, this ties to the outside perimeter drain.  Had a small amount of water in one area (1/8″ – 1/4″ so I’d rather add some additional drainage to make sure any water that can find it’s way in can find it’s way out out).

Approximately 9″ – 10″ deep of 3″minus gravel level and compacted with a small bulldozer.

Now the questions:

–  Do I need to put down a vapor barrier ? (locatation is Western Maine)

–  Do I need to put a final layer of sand (or something else) on top the gravel? or on top of a VB if used?

–  I’m thinking 5″ slab with fibermesh and some left over #4 rebar (only enough to space 4′ to 5′ on center)

The only parking/driving on this of any real wieght will be by one John Deere 4320 tractor with frontend loader and backhoe (~ 6,000 lbs) and a jeep. 

Thoughts?

The budget is getting tight at this point, just trying to wrap up with spending more than necessary.

Thanks !

 

 

 

 

 

 

 

 

 

 

 

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Replies

  1. DavidxDoud | Sep 05, 2007 04:55am | #1

    I kinda doubt the compaction is uniform, so that doesn't give you much leeway going lite on anything else -

    vapor barrier is cheap and helps with the cure of the concrete as much as retarding moisture migration up thru the slab (so I'm told)

    sand is not necessary -

    5" is pretty light to park/drive your tractor on - jeep, no problem - if the tractor is gonna be in a dedicated spot, increase the slab depth and put your rebar there -

    I'd bite the bullet and install wire mesh throughout - it only hurts once -

     

     

    "there's enough for everyone"
  2. User avater
    shelternerd | Sep 05, 2007 05:13am | #2

    The vapor barrier is necessary because it will help the slab hold moisture and complete its hydration (curing) the sand is not necessary and you don't get a much from the fiber as you would from tightening the re-bar up to 30" OC or less (on chairs, AKA dobies) and skipping both the fibermesh and the 6x6 wire mesh. Covering the concrete with plastic or old tarps after the first day and out to as long as a month will also help it to cure harder.

    In western Maine you need to think seriously about frost heaving, probably more than about compaction. Sounds like you have a good perimeter drain to keep water out from under the slab which is nine tenths of the game if you can't heat the barn to keep it from freezing.

    Where are you from in Western Maine? I used to live near Bethel and still have land there. (now living in North Carolina.)

    ------------------

    "You cannot work hard enough to make up for a sloppy estimate."

  3. User avater
    SamT | Sep 05, 2007 06:21am | #3

    compacted with a small bulldozer.

    That is NOT compacting. Think about it. What tractor do you use when the ground is kinda soft and you don't want to leave ruts. If your tracks are only 10" wide and 8' long, and a small bulldozer weighed a whopping 10,000#, that's still only a load of 5 pounds per square inch. I'll bet your little Cat has half that loading. A Crawler tractor is made to MINIMIZE compaction.

    Try this; drive your JD in and back out one time and look at the depth it dents your base.

    You better get back in there with a large jumping jack compactor and beat the livin' tar outa that 10" deep base and pray that you are actually getting some compaction down to the bottom. I assume you're not going to pull the top 5" out and compact it in two lifts like you really should. Or hire a professional compactor with modulating impacts to do it for you.

    Lets see, 6000# JD, call it 1500# on each front tire. You'll need to spread the 1500# over 300 sq inches to get 5 psi loading; 17^2 is 300, You better plan on a 17" thick slab and use two layers of #5 rebar 12" OC both ways.

    SamT

    1. Scott | Sep 05, 2007 07:16am | #5

      What SamT said.If you're going to drive a tractor in there you need to think about slab/soil loading.Scott.Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”

    2. davidmeiland | Sep 05, 2007 07:28am | #6

      Once again Sam is right. The bulldozer will provide minimal compaction, and the loads on your mimimally reinforced slab will cause it to crack. I would use the bulldozer to scrape out at least half of the gravel, then bring in a plate compactor and drive it around until you're dizzy. Then add the rest of the gravel back and do it again. Then lay down 6 mil poly (poor), Moistop Ultra 6 (better), or Stego Wrap (best) over the gravel. Then install a mat of #4 rebar 12" OCEW on 2" dobies. Then pour 6" of six-sack mud with angular concrete rock and fiber in it. After finishing keep it covered for a week.

      FWIW, the fiber is mostly for aesthetics. It will help prevent hairline cracking by diffusing the stress into microcracks instead. It will help a bit with point loads such as forklift tires to prevent spalling. The rebar will provide some structural strength, but the bulk of that really comes from the compaction of the subbase.

      1. cpcf250 | Sep 05, 2007 04:35pm | #7

        Thanks for the responses.  I am near the Bethel area, nice part of the state if you aren't on the coast.

        I didn't think pouring concrete directly on 6mil plastic VB was good because it reacts with plastic and degrades it rapidly?

        I don't mind spending more time compacting better, but I won't be pulling out 5" at this point. 

        As for not wanting to spend extra money at this time - I think a lot of folks tend to overbuild for the intended purpose and that what I'm trying to avoid.

        Finally - 17" slab for a 6,000 lb tractor? you can't be serious.  My Superduty weighs in @ 7,300 lbs and I can't imagine going more than a 6" thick pad for that or a tractor.  What am I missing here?

        BTW - meant to mention 4000 PSI

        Thanks again,

        Chris 

        1. VaTom | Sep 05, 2007 05:38pm | #8

          Tough crowd, eh?

          I've got a large quantity of 6 mil poly in concrete contact.  I'm unaware of any degradation problems.  Nothing leaking so far, underground houses. 

          Routinely I run a 4500 lb tractor with 450 lbs in the bucket over an exposed aggregate slab (no large stone), welded wire imbedded, underlaid by 4" of xps, underlaid by washed stone, underlaid by fill that was compacted no more than yours (8000 lb crawler).  No problems.

          Which is not to say that your compaction amounts to much.  It doesn't.  Rebar is for tensioned concrete.  I didn't use any in my shop, which occasionally has had traffic as heavy as your tractor on my 4" (welded wire) slab.  No appreciable cracking, even though there was some fill involved.

          My other tractors and truck are substantially heavier than yours, they stay off my slabs.

          My house slab, OTOH, has major cracking.  Hired a guy who had a great reputation who insisted on watering the mix immensely, screeded by himself (mostly self-leveled).  Lots of calcium only helped him.  He was the expert, best in the area.  Boy was he wrong.  The other slabs I poured, 4" slump.  They aren't very flat, but they're almost crack-free.  All 3500 lb.

          Good luck with your project.PAHS Designer/Builder- Bury it!

        2. User avater
          SamT | Sep 06, 2007 12:47am | #9

          Yeah, I did kinda exagerate for effect. Those numbers assume that your 'crete has no more shear strength than sand.But, it illustrates the fact that a slab gets its load bearing abilty from the compaction of the base. A slabs' purpose is to provide some little bridging strength, a lot of lot of resistance to abrasion, and a nice surface.A slabs' load bearing capacity is more directly related to the bearing capacity of the base than it is to its thickness or even light reinforcing.I'm glad to hear that you're willing to compact it. It is possible to insure proper compaction with a jumping jack on ten inches of fill, but it's a lot of work.SamT

  4. User avater
    PeteDraganic | Sep 05, 2007 06:56am | #4

    Allow me to just interject that when a person makes a statement like...

    The budget is getting tight at this point, just trying to wrap up with spending more than necessary.

    I worry that this is not the time for this job to be done.  If it can't be done properly, then it shouldn't be done.

     

     

    http://www.petedraganic.com/

  5. [email protected] | Sep 06, 2007 02:00am | #10

    As said before your base needs compaction.  Depending on how much is "gravel",  how much is fines, and how much clay there is in it, you might be able to compact it with a hand held compactor.  But, I seriously doubt you can. 

    I would recommend calling a good equipment rental place and see if you can rent the sheeps foot attachment for your backhoe.  It can tighten up your base far faster, than you can by any other method available at this point.  The boom on the hoe can push down far harder than even a four hundred pound gorilla on a jumping jack compactor.  Be sure to mark the drain line locations, you could crush them if you aren't careful.  I have seen the sheeps foot get good compaction in 2-ft loose lifts, if you roll it back and forth until it walks out of the fill. 

    Rebar also helps hold things together when they do crack.  You need to put #4s @ 12-in on center both ways, or pour in small panels.  The 5-inch of 4,000-psi mud gives a panel size, of about 8-ft or 10-ft square.  If you really don't want to use much bar, keep the panel size down, to minimize cracking, and use the bars between panels to keep things close to the same level.  I personally don't like welded wire mesh, because I have never demoed a slab with it any where near the middle of the slab, 90% of the time it isn't in the slab, and is laying under it, on the gravel. 

    Won't be as smooth, but you won't have shrinkage cracks that will get larger over time. 

    1. cpcf250 | Sep 06, 2007 06:53pm | #11

      Thanks for the follow up replies.

      I will buy more rebar (kinda thought I was going to have to) and get the grid size down to 12" o.c.

      I will also look into the sheep's foot attachment for the John Deere and see if I can compact that way.  

      I appreciate the ideas and insight.

      Chris 

       

       

  6. woodturner9 | Sep 06, 2007 11:21pm | #12

    Perimeter drains installed on outside of foundation.  Additional drainage installed under what will be the floor area, this ties to the outside perimeter drain.  Had a small amount of water in one area (1/8" - 1/4" so I'd rather add some additional drainage to make sure any water that can find it's way in can find it's way out out).

    Since no one else has commented on this, I'll add my two cents.

    Plumbing code generally does not allow floor drains to be tied into the perimeter drain - they have to go to the sanitary sewer.  Not sure if you meant floor drains or tile in the slab when you say "additional drainage", but if they are floor drains, they need to go to the sanitary sewer.

    1. cpcf250 | Sep 06, 2007 11:39pm | #13

      Thanks for the heads up that drainage under the slab shouldn't be tied to a floor drain which goes to a perimeter drain.  Not an issue though, because no floor drains to be installed.

      Chris

  7. fingersandtoes | Sep 07, 2007 07:31am | #14

    Choose whatever combination of compacting and reinforcement you are comfortable with from the other replies, but for my money I'd saw cut a slab that size into smaller areas which will eliminate most of the potential cracking.

    1. davidmeiland | Sep 07, 2007 04:13pm | #15

      How will saw cutting eliminate cracking if the cracking is due to poor compaction? Sawcuts will minimize shrinkage cracks or at least control their locations.

      1. fingersandtoes | Sep 07, 2007 05:10pm | #16

        Don't get me wrong, I am not arguing for poor compaction. But uneven settling of a slab will cause it to crack. and saw cutting controls the location of these cracks.

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