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Small slab-on-grade question

McDesign | Posted in Construction Techniques on October 3, 2006 01:07am

I am placing a 5×9′ slab on grade for a big “step” before a larger concrete slab.  The big slab (10’x15′) goes down 22″; deep enough here for frost +12″.  It has structure on it, and will be tiled.

Adjoining this, and 6″ lower, is the small slab – it’s 5×9; it will be 2″ above grade, and 3-1/2″ below grade; total slab thickness about a 2×6. 

My plan is to use 3000 psi with fiber, put a good amount of #4 bar in it, on an 18″ grid, deepen the edges to 8″ thick total, tile it with 13×13 porcelain, and let the slab “float” with the soil.  The soil is well drained, well compacted sandy clay mixture.

Inspector says this is fine for what he considers a “sidewalk”.

Any comments yea or nay before I do this?  I guess the question is, should I dig a full perimeter 12×22″ foundation?

Forrest

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  1. User avater
    McDesign | Oct 03, 2006 01:51am | #1

    Bump - for me.

  2. dug | Oct 03, 2006 02:52am | #2

    Forrest,

             I would think this would be fine for our area. If the soil is well drained like you say I would prab'ly  forego the 12'' x 22'' peremeter and sleep well at night.

            Couple questions tho,  Are you going to isolate the 5 x 9 slab? Are you going to pour it with some slope for drainage? How are you going to handle this joint when you tile up to the step; caulk?

                    dug

    1. User avater
      McDesign | Oct 03, 2006 03:32am | #3

      The slab will be isolated; I'll handle the transition with color-matched sanded caulk.  There will be a step up at that point, with coping, so I don't think a gap would be noticeable if it occurs.  I'll probably put in a strip of vinyl flooring at the join in the concrete, and not peg the new part to the old part with rebar.

      I did form it up today with 1" slope along the 5' sides.  I'll post some pix tomorrow - it's that portico project.

      Forrest

      1. stevent1 | Oct 03, 2006 03:49am | #4

        Forrest,I like the turn down on the lower slab. Can you reach uninterupted soil?. By not drilling/attatching the lower slab to the porch slab, will you tile the risers on the upper slab be fore you pour the lower slab?Have you cosidered conecting the two slabs similar to pouring concrete steps. More framing but you can bend 1/2" rebar to match the profile.What ever you do, it will be beyond most of what is going on in the "No deposit, no return, got to have a gimmick" world of contracting I see going on every day.Keep up the good work and excellent threads.Chucklive, work, build, ...better with wood

    2. User avater
      McDesign | Oct 04, 2006 03:16am | #7

      Did it today -

      View Image

      Forrest

      1. dug | Oct 04, 2006 04:03am | #9

           Forrest,

                   Looks good. What color tile? What size?

            IIRC it was originally slate, why the change?

               dug

        1. User avater
          McDesign | Oct 04, 2006 04:57am | #10

          The feeling was the slate would not look "fine" enough, particularly at the edges, if the side faces just had slate as well  The decision was made to go with a heavy porcelain 13x13 tile that looks like travertine (heavily variegated beige/brown/yellow limestone/marble) but is freeze-thaw stable.

          The 1" X 2-1/4" nosing is actual travertine that matches perfectly, (about $1000 worth) and I plan to adhere it really well with modified thinset.

          Forrest

          1. stevent1 | Oct 04, 2006 05:42am | #11

            Forrest,

            Looks good. 80 bags? Hope you had a helper(mixer)Will you do the profile on the travertine?Chucklive, work, build, ...better with wood

  3. User avater
    SamT | Oct 03, 2006 04:49am | #5

    Forrest,

    That's not the way I would do it, but it is a great way to get 'er done.

    Wether it's your way or mine, you really should excavate all the topsoil from under the slab. Back fill to BOC with the cheapest mineral product you can find and compact it.

    If you want to stop frost heave under the slab, put 1/4" insulating foam under the slab, extending Frost Depth (10"?) out on all exposed sides.

    A frost heave occurs because an under surface ice puddle is sucking the water out of the surrounding soil. If under-slab is the last to freeze, it''ll already be dry.

    SamT
    1. DonNH | Oct 04, 2006 03:26am | #8

      >If you want to stop frost heave under the slab, put 1/4" insulating foam under the slab, extending Frost Depth (10"?) out on all exposed sides.

      A frost heave occurs because an under surface ice puddle is sucking the water out of the surrounding soil. If under-slab is the last to freeze, it''ll already be dry.

       

      So is that what you would recommend under the apron in front of a garage (I'm in NH - 4' frost depth).  When I pour my apron (looks like next year, the way things are going), I'd like to make sure it doesn't move drastically with respect to my garage floor, which is sitting on a 4' frost wall.

      I expect I'd want something thicker than 1/4" in this climate, and would have to go out a ways further for the same effect. Something like a mini-version of the insulating umbrella used by the PAHS crowd?

      Don

      1. User avater
        SamT | Oct 04, 2006 04:28pm | #12

        Don,

        What'cha gonna do with the driveway? 'Crete, gravel? Base?

         SamT

        1. DonNH | Oct 05, 2006 04:43am | #13

          Driveway is well-compacted, fairly well-drained gravel.

          Don

          1. User avater
            SamT | Oct 05, 2006 05:34pm | #14

            Don,

            Your considerations are

            Provide support for vehicles

            Keep as much water out as reasonable. Don't want any underground puddles.

            Insure  that underslab is the last to freeze.

            Allow the apron to float in relation to the garage slab.

            Water will flow through gravel to the lowest point in the soil underneath.

            Don't spend more on the apron than the garage cost.

            Driveways typically sit in a wide shallow ditch. They tend to hold whatever water penetrates the surface.

            I just 'fixed' the same issue here. In my case #6 above was #1 here. I installed a gravel drain bed apron with tile to open air, but I only had to trench about 18' to get there.

            I'm thinking (without engineering) that you will need about 18" of fill on the insulfoam under the driveway portion to prevent compacting ruts in it from the weight of cars. Unfortunately, if you excavate below the bottom of the driveway bed, any water penetrating the driveway gravel will want to flow down into the bed under the apron.

            If the bottom of the drive bed is between 12" -18", I would place a 1"-1 1/2" slab of hi-density, waterproof insulfoam with a 1/4" slope away from the garage, the bottom of the leading edge even with the bottom of the drive base, and proceed as in previous post. About a 2' "umbrella."

            Your driveway is probably less than 12" thick. Then I would excavate 18", place the umbrella, no slope, and pour short 6" stem walls, front and back only, to support the apron, infilling between them with well draining fill.

            If, perchance, the drive is thicker than 18", I would excavate the area under the insulfoam to the bottom of the drive bed, line the hole with 6mil, punch some drainage holes in the bottom of that, backfill to 18", then wrap the 6mil over the backfill to waterproof the whole thing. Set the insulfoam and proceed as previously discussed.

            It still feels like I'm fergittin' something. . . . .SamT

          2. DonNH | Oct 05, 2006 08:02pm | #15

            Thanks Sam.

            Gives me some stuff to mull over between now & next summer, when I'll hopefully get to that stage (current issue is to get the roof finished b4 snow flies.)

            When house was built over 20 years ago, they probably just pushed up a few inches of loam, then leveled with gravel.  Below that is coarse gravel mixed with rocks (NH potatoes) & boulders.  Right next to the garage (I'm looking at a 3-4' apron) was of course dug up & backfilled in the course of placing the frost wall, so it's not as well compacted.

            Don

             

          3. User avater
            SamT | Oct 06, 2006 06:22am | #16

             SamT

  4. davidmeiland | Oct 03, 2006 07:20am | #6

    Only thing I can add is that I would use a crack isolation membrane under the tile.

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