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Steel shed concrete base: looking for the easiest way

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  • 03-09-2018 2:30pm
    #1
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭


    In the attached picture, the timbers are 5" deep and the drop off in the slope from front to back leaves the back requiring 12" of fill to top of timber.
    The base only need to be 4" concrete so looking for ideas to simplify the work.
    Need to finish the shuttering first of course :)
    1. Hard core not really an option as the back 12" needs to be solid and vertical when finished.
    2. Ready mix not an option either as is only wheel barrow access and my guess is 1.2 m3 [3 by 2 by .2]
    3. What about using a dry mix of cement and gravel as the base layer, leaving 4" for concrete?
    4. If doing that, how long would the dry mix need to be left to "go off"?

      [*}Can borrow a mixer
    All suggestions welcomed.
    Thanks as always

    “I can’t pay my staff or mortgage with instagram likes”.



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    #2
    Ronney
    Registered Users Posts: 839 ✭✭✭Ronney


    Bring it up in lean mix to bottom of timbers or just pour it all in concrete


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    #3
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    Ronney wrote: »
    Bring it up in lean mix to bottom of timbers or just pour it all in concrete
    By lean do you mean dry or just a weaker wet mix?

    “I can’t pay my staff or mortgage with instagram likes”.



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    #4
    Ronney
    Registered Users Posts: 839 ✭✭✭Ronney


    Calahonda52 wrote: »
    By lean do you mean dry or just a weaker wet mix?

    A dry mix, would usually be about 20kN in strength


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    #5
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    Ronney wrote: »
    A dry mix, would usually be about 20kN in strength

    So in a mixer
    8 to one by weight gravel to cement?

    “I can’t pay my staff or mortgage with instagram likes”.



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    #6
    Lumen
    Registered Users Posts: 31,010 ✭✭✭✭Lumen


    Calahonda52 wrote: »
    Ready mix not an option either as is only wheel barrow access and my guess is 1.2 m3 [3 by 2 by .2]
    How far to wheel barrow?

    I did about 0.75m3 of ready mix for my oil tank base, it only took a few minutes to wheelbarrow (although the distance was only about 10ft).

    Could have easily have done double before the concrete man's patience would have run out.

    Whole job took about 15 minutes.


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    #7
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    100 feet, narrow passage.
    Also screeding such a large area, being in 12" of ready mix in wellies is not easy!

    “I can’t pay my staff or mortgage with instagram likes”.



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    #8
    Lumen
    Registered Users Posts: 31,010 ✭✭✭✭Lumen


    I guess the question is whether it's less hassle to build a foot high concrete wedge or level it.

    There's only a couple of m3 of soil to move around.


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    #9
    foxy farmer
    Registered Users Posts: 3,194 ✭✭✭foxy farmer


    Cut some plywood to close off your formwork. Screw or nail it to the 6x2. A few battens fixed to the edge running along the ground will stiffen the ply. Drive a few stakes or steel bars to keep the ply from bursting out when concrete is poured. If you can get some rubble or filling then build up a "platform" about 100mm + in from the shuttering. This will allow concrete down to the ground and support the floor along the perimeter. A base of 100 to 150mm thick will be quite adequate for your job. No need for huge concrete wedge.


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    #10
    Lumen
    Registered Users Posts: 31,010 ✭✭✭✭Lumen


    You could use 2 part PU foam for the infill, then you'd have an insulated floor (apart from the cold bridge under the walls).

    Like a ICF, except above ground.

    So maybe...dig a couple of narrow trenches levelled with aggregate, into those build two unmortared piers of 100mm precast lintels to rough level, bridge with two more lintels, line the inside with poly sheet and duck tape, pour the foam to top of piers, top the bridging lintels with spaced A142 mesh, build the formwork about 100mm outside the lintel frame (much easier as the pressure will be lower due to infill), pour the concrete.

    That should limit the concrete effort, effectively just locking the lintels into position and levelling the top.


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    #11
    hesker
    Registered Users Posts: 1,490 ✭✭✭hesker


    Buy 3 wheelbarrows and get 2 mates to help.

    3 of us moved 2m3 of concrete up a hill through a house and out to end of back garden a couple of years ago. Took about 20 minutes I think.

    Could really do with a 4th person to rake the concrete and level.


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    #12
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    hesker wrote: »
    Buy 3 wheelbarrows and get 2 mates to help.

    3 of us moved 2m3 of concrete up a hill through a house and out to end of back garden a couple of years ago. Took about 20 minutes I think.

    Could really do with a 4th person to rake the concrete and level.
    Thanks, easier said than done as I am only a few weeks in this new house in a new town so don't know anyone yet.

    Anyway, thanks for insights.
    Case closed

    “I can’t pay my staff or mortgage with instagram likes”.



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    #13
    my3cents
    Closed Accounts Posts: 9,764 ✭✭✭my3cents


    What I have done in similar situations (couldn't mix enough concrete on my own in one go) is to build a blockwork wall around the edge as part of the foundation then infill with hardcore before finishing off with about 4 inch of concrete.


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    #14
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    Ready mix plus a few lads with 3 wheel barrows coming this week, rather permitting to pour it:)

    “I can’t pay my staff or mortgage with instagram likes”.



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    #15
    my3cents
    Closed Accounts Posts: 9,764 ✭✭✭my3cents


    Well that isn't going to subside any time soon.

    Add some rebar and it will survive a nuclear attack :)


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    #16
    recedite
    Closed Accounts Posts: 13,993 ✭✭✭✭recedite


    The only problem with a wedge like that is that you have created a "slip plane" so there is a tendency for the whole thing to move backwards over time. That's why wedges are not used generally in foundations or slabs. A foundation on a slope would be stepped instead, so always a horizontal surface.
    Anyhow, it won't matter for a shed. Worst thing that can happen is the shed gradually moves back towards the patio by a few mm, but its too small for any cracks to appear, so it would all move at the same time.


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    #17
    Lumen
    Registered Users Posts: 31,010 ✭✭✭✭Lumen


    recedite wrote: »
    The only problem with a wedge like that is that you have created a "slip plane" so there is a tendency for the whole thing to move backwards over time. That's why wedges are not used generally in foundations or slabs. A foundation on a slope would be stepped instead, so always a horizontal surface.
    You could dig a rough channel across the slope in the middle before the pour to create a "rib" on the underside of the slab.


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    #18
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    recedite wrote: »
    The only problem with a wedge like that is that you have created a "slip plane" so there is a tendency for the whole thing to move backwards over time. That's why wedges are not used generally in foundations or slabs. A foundation on a slope would be stepped instead, so always a horizontal surface.
    Anyhow, it won't matter for a shed. Worst thing that can happen is the shed gradually moves back towards the patio by a few mm, but its too small for any cracks to appear, so it would all move at the same time.

    Thank you for that very insightful observation.

    The yellow clay is very very hard, so I will take Lumens suggestion and dig a key

    edit 18th

    What about driving in lengths of rebar down at the deep end?

    “I can’t pay my staff or mortgage with instagram likes”.



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    #19
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    Three lads, two barrows: was not even here :)

    updated picture added

    “I can’t pay my staff or mortgage with instagram likes”.



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    #20
    Four Phucs Ache
    Registered Users Posts: 1,017 ✭✭✭Four Phucs Ache


    Fair play, that looks like glass.


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    #21
    mengele
    Registered Users Posts: 868 ✭✭✭mengele


    Calahonda52 wrote: »
    Three lads, two barrows: was not even here :)

    updated picture added

    How much for the concrete?


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    #22
    recedite
    Closed Accounts Posts: 13,993 ✭✭✭✭recedite


    Calahonda52 wrote: »
    What about driving in lengths of rebar down at the deep end?
    Just saw this now, so obviously too late to reply. But anyway, you'll notice that people are using galvanised rebar and mesh now, whereas a few years ago they threw in rusty steel. Basically you have to careful not to damage the coating on the steel, otherwise you are potentially creating a weak point/ time bomb in your concrete. Its the reason a lot of motorway bridges around the world are collapsing, including that one recently in Genoa.
    The bridge was constructed using reinforced and pre-stressed concrete about 50 years ago. There are a large number of reinforced concrete bridges in Italy, Europe, USA, and Canada with the same age, which are suffering from corrosion of reinforcement and or pre-stressing tendon."
    Anyway, a shed base will be fine, it can't fall anywhere :)
    Looks like a nice job.


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    #23
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    Thanks: its 2mm off square in 3.385m :D

    “I can’t pay my staff or mortgage with instagram likes”.



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    #24
    Calahonda52
    Registered Users Posts: 12,195 ✭✭✭✭Calahonda52


    Am going to put a timber floor in it, have access to 40mm MDF [it's FOC:D ]

    So, will have to line it with plastic.
    What I was thinking was of "sticking" say 6" or 9" wide dpc onto the concrete before they come fit the shed, and then seal the bottom of the shed against the dpc with say tec 7 or similar.
    Then cut the plastic to suit and glue it to the dpc.
    then put pressure treated timber lathes on plastic and lay the mdf which should be heavy enough to not need fixing to concrete..
    Any thoughts?

    Shed not due till end Nov

    [How would I stick DPC to the concrete? ]

    “I can’t pay my staff or mortgage with instagram likes”.



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    #25
    recedite
    Closed Accounts Posts: 13,993 ✭✭✭✭recedite


    No need to stick it. Try to get your hands on some radon barrier, its extremely tough and it works as a DPC too.
    The shed will presumably be fixed to the slab with rawlbolts or similar, so let these pierce down through the DPC. A few holes in it should not matter, just blow out any concrete dust and squirt in some silicone or Tec 7 so no moisture wicking can occur from the holes later. Then you can seal around all the edges afterwards, as per the plan. Your battens won't need to be pressure treated because they will be dry.


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    #26
    Lumen
    Registered Users Posts: 31,010 ✭✭✭✭Lumen


    Don't see the point in using a strip of DPC since it's only €65 for a 12.5x4m sheet of 2000G DPM sheeting which will give you two layers with plenty to spare.

    So maybe one sheet flat over the concrete (under the shed walls), and then another inside the shed, lapping up the walls, maybe use a perimeter batten to tidy it up allowing expansion gap with the MDF.

    I'm not sure it's worth sealing anything since water from the outside is going to have to climb a couple of inches of battened DPM to get anywhere near the MDF.


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    #27
    recedite
    Closed Accounts Posts: 13,993 ✭✭✭✭recedite


    If using a radon barrier, it would be one big sheet, the size of the slab. So fairly easy to hold in place without any "sticking" while the shed is being fixed. Its easy to tear polythene while walking around on it in heavy boots.
    IMO you'd be better of squirting the sealant in below the shed frame, (and not trying to have the DPC rise up the inside wall) because it will stop water and dirt pooling under the steel frame. Also because mice and insects might try to get in. OP claims 2mm accuracy of course, which is smaller than a mouses skull, but we'll see what gaps appear under the steel when its laid out on top of the concrete slab/DPC :)


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    #28
    Lumen
    Registered Users Posts: 31,010 ✭✭✭✭Lumen


    recedite wrote: »
    IMO you'd be better of squirting the sealant in below the shed frame, (and not trying to have the DPC rise up the inside wall)
    Yeah, that's putting a lot of faith in sealant though.

    What about detailing at the doorway? Is door opening in or out? Can water get under it to the flooring?


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    #29
    Roen
    Closed Accounts Posts: 4,620 ✭✭✭Roen


    I know you got the MDF free but any reason for not just leaving it bare concrete?


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    #30
    recedite
    Closed Accounts Posts: 13,993 ✭✭✭✭recedite


    It starts with the mdf flooring.
    Before long, its oak paneling and wallpaper.


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    #31
    Idbatterim
    Registered Users Posts: 17,840 ✭✭✭✭Idbatterim


    I’m thinking of using ground screws for my next project. Way quicker, no mess. The catch is cost obviously ...


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