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    • CommentAuthorNeilD
    • CommentTimeApr 3rd 2018
     
    Hi again,

    For my new-build I am having a 150 mm suspended concrete floor resting on the inner skin of thermal blocks (like the green building store Golcar house). Under the slab will be 300 mm EPS 70, 50 mm blinding sand and between 150 and 600 mm of compacted hardcore. Whilst I am calling the slab suspended it could also be seen as ground-bearing.

    Currently I have spec'd a single layer of A142 mesh based on the assumption that it will be ground-bearing but I am wondering whether I should reconsider using something thicker/going for a double layer or asking me SE to review it. My concern is that the ground under the slab will settle and that the slab will be left fully suspend and inadequately reinforced. The max 'span' is 5m.

    I would really appreciate any general thoughts and particularly input from someone who has used this set-up.

    Many thanks

    P.S the concrete for the foundations went in today - exciting times! :bigsmile:
    • CommentAuthorgyrogear
    • CommentTimeApr 3rd 2018
     
    Posted By: NeilDMy concern is that the ground under the slab will settle


    Was a soil test not done ?
    To me it sounds like you are setting off on a long journey, with "concerns" -- what were the "assumptions" based on?

    gg
    •  
      CommentAuthordjh
    • CommentTimeApr 3rd 2018
     
    Posted By: NeilDFor my new-build I am having a 150 mm suspended concrete floor resting on the inner skin of thermal blocks (like the green building store Golcar house). Under the slab will be 300 mm EPS 70, 50 mm blinding sand and between 150 and 600 mm of compacted hardcore. Whilst I am calling the slab suspended it could also be seen as ground-bearing.

    It's either one or the other, or possibly a raft. Given that you appear to essentially have a passive slab, I don't understand why you also have thermal blocks underneath it that it may or may not rest on?

    Currently I have spec'd a single layer of A142 mesh based on the assumption that it will be ground-bearing but I am wondering whether I should reconsider using something thicker/going for a double layer or asking me SE to review it. My concern is that the ground under the slab will settle and that the slab will be left fully suspend and inadequately reinforced. The max 'span' is 5m.

    I don't understand why you're specifying the reinforcement. Surely that is something your SE would/should do?

    Why does the depth of the hardcore vary? If the ground is likely to settle (and why do you think that is likely?), then you probably want a raft rather than a slab. In any case it seems to me that you should run, don't walk, to your SE with these questions.
    • CommentAuthorgoodevans
    • CommentTimeApr 4th 2018
     
    Neil - either the slab is suspended (i.e. supported by the walls under it) or it is ground bearing.

    A single layer of A142 mesh in a slab will not span far - It is sufficient to span any small, local soft spots under the slab. And allow for a little differential settlement between the wall/foundations and the slab.

    Now another question is why will the ground under the slab settle, but not under the foundations where presumably there is a higher load. In other words - the slab may float better than the foundations.

    A 5m suspended rc slab is no picnic - if you have the ground to support a slab use it - it's free load-bearing strata.

    However if the ground floor slab is to be suspended then the slab load will need to be allowed for in the foundation design (but the foundations are now built so this bit has been decided already). I suggest that you are probably committed to a ground bearing slab - the decision is whether to bond it to the walls (by casting it onto the inner skin). Or cast it inside the walls and float. My gut instinct would be cast it onto the walls as you have planned - and if your SE has passed that then keep with that.

    For my build I HAVE to have a suspended floor in my build (I wish it was otherwise) - I looked at all sorts of ways to use a cast instu suspended slab - in the end I ended up with creating sleeper walls every 3 meters under the ground floor and using beam and block with insulation above.
    • CommentAuthorgoodevans
    • CommentTimeApr 4th 2018
     
    And it looks like djh is burning the midnight oil - and our posts crossed - but I agree - check with your SE - if he has signed off on that design keep with it.
    • CommentAuthorNeilD
    • CommentTimeApr 4th 2018
     
    Thanks for replies.

    I'm afraid it is in my nature to question everything - professional opinion or not - not sure if this a good or bad traite for a self-builder!

    We have good ground; sandy loam and then bedrock (shale) which the foundation's sit on. The ground under the slab is sound so I am not unduly concerned about settlement.

    The reason for the varying levels of hardcore depth is because the ground floor has a step in it.

    My understanding is that the SE has assumed the slab is ground bearing despite it being built in. I am just checking to see if this is common assumption with a built in slab - I understand built in slabs are commonplace in radon areas so I'm wondering how it is approached here. My only concern is that if the ground below the slab settled even by a fraction the slab would no longer be ground bearing.

    My gut instinct is that in reality a gap between the slab and the EPS is unlikely to open up unless the ground settles independently of any loading from the slab but it would be quite a complex situation to model from an engineering perspective.

    For more info on the the design see here:

    https://www.greenbuildingstore.co.uk/golcar-passivhaus-ground-floor-foundations/

    Thanks
    • CommentAuthortony
    • CommentTimeApr 4th 2018
     
    It is not a suspended slab, it is ground bearing and the mesh is anti cracking. A lad I trained builds everything like that as it negates any cracking or settlement between wall and floor and makes doorways easy with no cracks there either.

    Build internal walls to underside of slab.

    Compact hardcore mechanically and it will be OK
    • CommentAuthorgoodevans
    • CommentTimeApr 4th 2018
     
    Yep - I agree with Tony - all is good.
    • CommentAuthorNeilD
    • CommentTimeApr 4th 2018
     
    Thanks for the reassurance guys. I'll try to stop the worrying!
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