Thickness for small suspended slab???

[Make it work]

Hi all,
I am setting up to pour a suspended slab for a landing at the back door. It will be about 1mtr square supported fully along one edge and on the corners on at least a full brick at the other end. I also intend using reo and at least 25mpa concrete.

So my question is, will a thickness of 75mm be OK or is this too thick or too thin?


Cheers
Alan M

[bricks]

75mm is way too thin.
Plus you need to have reo and stressing to hold the slab together after you take the form work away.
If I were you I'd just buy a concrete pre-fab slab/paver ( yes you can get them made and delivered), Place it on top.

Or fill the hole underneath with sand or rubble and pour a slab on top of that.

[pawnhead]

It's only a metre square and the clear span would be even less than that so 75mm would probably do if you're using a sheet mesh reo, but for an extra few bucks I'd make it 100mm.
To save buying a full sheet of reo @ 2.4m x 6m, I'd just buy a length of 12mm rebar and cut lengths @ 940mm. (you could get the reo place to cut them for you). This gives you 30mm of concrete cover at the edges so they don't get wet and rust, but make sure that they overlap your supporting base by at least 50mm. Lay them in a square grid pattern (both ways) 150mm apart(200mm would be OK, but play it safe for a few bucks more) , tie every second intersection with tie wire (or a bit of string) and support the grid on 4x25mm bar chairs, so that it's closer to the bottom where the tensile stress is located in a simply supported suspended slab.

Disclaimer: I'm a builder, not an engineer, but this is just how I'd do it, and from my experience, if anything it's overkill.

[ian]

You will need a piece of formwork, which wont be recoverable, plus supports under the slab to support the wet concrete.
In this situation I'd use 15 or 18mm fibre cement sheet for the formwork.

You haven't said what you are going to place on the slab, but if it's tiles I'd be inclined to forget the slab and use two or three thicknesses of 18mm FC sheet glued and screwed together.


ian
use be I'd be in

[Make it work]

Thanks for the replies,
The top of the landing and the top of the steps will be tiled but I do prefer concrete.
I may be able to make it thicker than 75mm but I don't have much to play with. I did think I may be able to put a lintal in accross the less supported edges and I suppose I can always drop the corry formwork down below the supporting brickwork.

OK, off to lay some more bricks. will post pics later.

Cheers

Alan M

[pawnhead]

Well I haven't really seen a slab that thin unless it was a garden path on ground, but then I haven't seen a slab that small designed before. But I did pour a slab that was engineer designed once that tapered down to 75mm. It was a cantilevered slab and it was 100mm thick at the supporting wall, then it overhung the wall by 600mm, and the bottom tapered up to 75mm for a sleek thin look at the edge. It had 12mm reo bars @ 200mm centres, top and bottom, that met at the end of the cantilever.
I'd say that 75mm would be OK for such a small span, but I'd go to whatever trouble was needed to make it 100mm since neither of us are qualified to make that sort of decision in the first place. Especially if there's a chance of anyone falling if it breaks.

[Make it work]

Good advice John, I think the steel is the answer, but I will bump it up as close to 100mm without going too high.

Cheers

Alan M

[bricks]

just thinking about your problem,

Why dont you build the supporting walls and the columns that hold the concrete up, then get a base made from bondeck or even a section of thick calv sheet or something.

You'll still have to put reo in but theres less chance of the slab dropping out from underneath?

[Terrian]

SL102 reo or better for a suspended slab, consult an engineer....

[pawnhead]

consult an engineer....

Start with an architect. He'll be able to help you get it through council with your development application, followed by your building application, advertisment fees, inspection fees, footpath damage levy, etc, etc.
With any luck, you may be able to pour as soon as next May/June if you've got enough money left.

[Terrian]

Start with an architect. He'll be able to help you get it through council with your development application, followed by your building application, advertisment fees, inspection fees, footpath damage levy, etc, etc.
With any luck, you may be able to pour as soon as next May/June if you've got enough money left.

that, my friend, is what I call going just a little overboard

I would hate to see the guy (?) pour this suspended slab, only to have it fail in a few months time

[DvdHntr]

that, my friend, is what I call going just a little overboard

I would hate to see the guy (?) pour this suspended slab, only to have it fail in a few months time

Minimum 100mm thick and the 12 rebars sound like the best option. I would use bondek/condek. Refer to their online guides.

[pawnhead]

Welcome to the forums DvdHntr. I see you're taking an interest in engineering matters with your posts so far. It's always good to have another well informed contributor on board.
What are your qualifications if you don't mind my asking?

[Make it work]

Here is the pic I promised. Now with the brickwork done it is easy to see where the slab must go.
I was going to put another course of bricks on but based on the advice I got from all you learned folks I will now make the slab thicker.
Before any one points it out, the sliding door is yet to be moved to its new position.

[pawnhead]

That's nothing. It's only spanning 700 at its widest. I'd just buy a sheet of MDF for formwork and set it up 100mm below your finished height, and let the concrete drop down to the brickwork just around the edges. Install it in two pieces so you can get it out when you strip. A few rubbish 4x2 joists under the perimeter and one in the guts, sitting on timber props, or brick stacks. MDF edgeboards, tied back the through bracing, with tie wire back to the deck.

You've got a bit of work going on there I see. If you've got enough of that trench mesh left over you could use that.