Joining a concrete slab

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  1. #1
    Senior Member HandyDaddy's Avatar
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    Jun 2009
    Langwarrin, Melbourne

    Default Joining a concrete slab

    Hi all

    I've already searced the forum archives and found a few threads on joining concrete slabs, but I wonder could anyone help out with some additional info here.

    My house is timber / brick veneer on a slab, and the edges of the slab are stepped down. The timber frame sits on top of the slab and the brick walls sit on the step at the slab edge.

    Now I plan to extend the existing slab when I build an extension. I'm going to sub the construction of the new slab out to the pro's, but as I am planning the construction stages of the extension it has crossed my mind - will the brick wall sitting on the slab edge have to be taken down before the new slab is layed?

    It would seem to me that the new slab would sit over the ledge on the existing slab, in which case my wall will have to come down first. I was hoping to keep this wall up until later in the project when the timber frame of the extension was connected into the main house.

    Does anyone know if the slabs will overlap?
    I havn't appointed a company to lay the slab yet so cant ask them.


  2. #2
    Senior Member Batpig's Avatar
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    Sep 2007
    Northern Brisbania...


    Dear Handy,

    Generally, it depends quite a bit on how reactive your foundation soils are to seasonal changes in moisture-content. For example, if a Soil Test defines the site to be of the Class "A" or "S" variety, you could probably get away with just butting the new Slab up against the outside of the brick wall, but on the proviso that you thickened it down to the Footing on which the bricks sit, and tied it down to that footing with some bent Reinforcement Bars that have been epoxied at least a couple of hundred millimetres into the top of the Footing. Depending on the nature of the earthworks involved, it may also be necessary to articulate the brickwork joint where the extension meets the existing building. Having said this, it should also be borne in mind that earthworks for a proposed extension can sometimes tip a Site Class into the next bracket up, because the Consulting Engineer often looks at the "footprint" of the original house only (and no larger...), to give him an idea of the amounts of cutting and filling that will occur when he is classifying a house site and/or designing the footings. An extension beyond this original footprint could easily involve greater depths of cutting that brings "reactive" clay closer to the surface - hence "upping" the Site Class...

    At any rate, if for any reason you're staring down the barrel at either a Class "M", "H", "E", or "P" site, you're going to need to do a bit more than just butting the new slab up against the existing brickwork. For one thing, the new Slab is going to have its own internal Thickenings and/or Beams, and these are going to make the Slab sufficiently stiff to cause substantial cracking at the junction of the new and the old, if they cannot somehow tie in to the end of Thickenings or Beams in the existing Slab. One particular trick that I've seen that has stuck in my mind is the selective removal of bricks to enable Reinforcement Bars to be drilled into the old Slab edge where needed. If common sense is adhered to, this can be done quite safely, because the remaining courses of brickwork above will simply "arch" over the removed bricks. You remove a couple side by side where you need to concentrate a few bars (such as at the end of a Beam or Footing), or just every second brick where you're just trying to anchor the new Slab's reinforcement. You will obviously also have to dig right down beside the existing Footings and do the same thing with epoxied Starter Bars down there too.

    Sorry I can't elaborate beyond this, but it's a tricky little subject in its own right. You definitely need to use "Hanky-Heads" (that is - Concretors...), who have done this sort of thing before. Even then, it would be very touch-and-go if there wasn't either an Engineer or a Builder calling the shots. The thing is - if Footings and Slab are done wrong, you can never really make up for this down the track (that would hurt if you were bought the extended house of somebody else, and it started cracking on you down the track...) But anyway, that's why you generally need an Engineer's Soil Test and Design when it comes to Footings and Slab (although I'm not sure what the Council Regulations are down there...)

    Hope this gives somewhere to start with your thinking.

    Best Wishes,

  3. #3
    1K Club Member
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    Nov 2008


    You have to also make sure you don't compromise termite barrier if butting up to existing brickwork

  4. #4
    Senior Member HandyDaddy's Avatar
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    Jun 2009
    Langwarrin, Melbourne


    Thanks for your answers especially your very informative reply, Batpig

    I'm not intending on doing any of the concrete slab myself. It will be designed by an engineer after having soil test done, and the pinning of the slabs together will be done by the concreters. I expect the ground to be an 'M' classification, well at least thats what it was 20 yrs ago when the original slab was put down.

    My question is a little simpler than how do I connect the slabs, its simply, will the new slab overlap the shelf on the edge of the existing slab? If this will be the case I will have to plan to take down the brick wall prior to the slab being layed. The wall will have to be taken down completely at some stage anyway, its just a question of when.

  5. #5
    The Master's Apprentice Bedford's Avatar
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    May 2009
    Yarra Valley Vic oz


    It probably still comes down to what the engineer says, but I would over lap the slab as otherwise you end up with two joins when you remove the bricks, and you have to fill it in later anyway.

  6. #6


    Sitting thinking about it I have come to the conclusion if it were my house the bricks would come down first. Id get some of those cheap blue tarps to protect the wall from rain while you get the job done. These Id nail to the studs with sarking nails. You are going to be guessing as to what you will find when the wall comes down and you dont want any guesswork when laying a slab.

  7. #7
    Building Designer ausdesign's Avatar
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    Sep 2004
    Strzelecki Ranges Victoria


    Generally you would provide N12 dowells from the existing edge beam to the new.
    The new slab would be able to be founded as per the existing - i.e. same beam depth - assuming the existing slab performed correctly structurally and the bearing capacity, soil type was the same.
    The brickwork from the old to the new MUST have an articulation joint (no toothing of brickwork)
    You can either remove or retain the existing brick veneer wall.
    Peter Clarkson


    This information is intended to provide general information only.
    It does not purport to be a comprehensive advice.

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