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    • CommentAuthorgoodevans
    • CommentTimeJun 10th 2017
    does anyone have any experience using eps as side formwork for ground beams. Specifically, will I get a good bond between the concrete and eps after the concrete has cured. My concern is heave - I'm planning some compressible eps below the form work but should heave occur there will still be be some upward force on the eps form work which by then will be rendered.

    There are other options - I could rough up the eps for a better bond

    or partially screw in long eps 'rawl plugs' on the inside of the formwork so that the top of the plug is encased in concrete

    Or would I be better using standard formwork then using a cement based adhesive and / or mechanical fixings after the beam has cured to fix the eps.

    (I'm trying to find a way to avoid a ventilated suspended floor and use cast insitu rc using the Coredek Cellcore product and in addition L or U or flat eps forms for the external ground beams). If I can do this it will almost be a passive-slab (except for the penetrations from the piles).
    • CommentAuthorgoodevans
    • CommentTimeJun 11th 2017
    for a picture of what's intended ....
    • CommentAuthortony
    • CommentTimeJun 12th 2017
    I reckon the EPS will stick solidly to the concrete, I would mechanically fix outside only
    • CommentAuthorTimSmall
    • CommentTimeJun 12th 2017
    I don't think you'll need any mechanical fixings - feel free to do a test to convince yourself, but with ICF buildings, you have to destroy the EPS to get it off the concrete. If you're using the extra-low compressive strength EPS as beam formwork, you could always extend it up to 100mm below ground if you want to be extra-extra sure.

    Could you get some AAC blocks on-edge between the tops of the piles and the beam?

    Concrete circa 1.15 W/mK
    AAC - Celcon Super - (8.7N/mm²) 0.18 W/mK
    • CommentTimeJun 12th 2017
    If you want something to put on top of the piles under the slab, Compacfoam CF400 apparently has a design stress of 4.18 N/mm² *

    I wonder how big the tops of the piles would need to be? Hmm, 4.18 N/mm² is 426 tonne/m² so if there are 27 piles (IIRC?) the pile tops wouldn't need to be an unreasonable size, I think. Oh, and lambda = 0.065 W/m.K

    * http://www.compacfoam.com/26-compressive-strength.html

    Now you just need an engineer who's familiar enough with all this stuff to be willing to put his PI insurance on the line.
    • CommentAuthorgoodevans
    • CommentTimeJun 12th 2017 edited
    A 200mmx200mm precast driven pile will take a SWL of up to 350 kN (depending on the pile) giving 8.75N/mm2 - however for these piles the pile reinforcement is broken out and the top of the pile and it's reinforcement is embedded into the ring beam - no room for compacfoam.

    My feeling is that a .04m2 breach in the insulation every 4 to 5 meters of ring beam will probably not be worth plugging - especially if the pile has a deep eps collar around it under the beam.

    Thanks Tim/Tony for the reassurance - I wonder how the polypropylene boards that Cordek cover their Cellcore sheets with bonds to concrete.
    Posted By: goodevansMy feeling is that a .04m2 breach in the insulation every 4 to 5 meters of ring beam will probably not be worth plugging - especially if the pile has a deep eps collar around it under the beam.
    We built a few Passive Slabs with Piles but instead of putting the UF heating pipes into the slab we put down 100mm of insulation on top of the slab and put the UF heating into a screed. You're correct the area where the piles punch through the insulation is only about 1% but we felt heating the slab to 30 degrees would increase the heat-loss.
    • CommentTimeJun 19th 2017
    Are thermal bridge calculations not generally about the surface temperature reduction in the vicinity rather than energy loss considerations that might be measured by averages and percentages?
    • CommentAuthorgoodevans
    • CommentTimeJun 20th 2017
    VH, I'm not sure the slab will need heating to 30 degrees - a well insulated house should need nothing like that temperature. In addition, the natural temperature of the ground just below the insulating pile collar may be well above the average winter air temperature further reducing loss.

    I'm also going for medium density solid block work walls for additional thermal mass (and a solid feel) so for your solution I think I would need a second ring beam or, as a minimum, a thicker reinforced screed above the insulation to spread the wall load to keep the compressive strain of the insulation to reasonable levels.

    On balance - I think I'll live with the small thermal bridge at the piles safe in the knowledge that the walls and ground slab will be directly supported by the piles/ring beam.
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