Thickness for small suspended slab???

[DvdHntr]

What are your qualifications if you don't mind my asking?

Structural Engineer, MIEAust

Was looking for something else and came across the board. When I saw some of the posts I thought I may be able to help. Also, I am doing a renovation and when I come to do some of the trickier parts I thought I could get some advice.

[Make it work]

Welcome to the forums DvdHntr.

Welcome from me too, I am newish here too and have found it very helpful and enjoyable.

Make sure to explore other parts of the site, the list is long and varied but all good.

I'm sure you will have good input and advice.

[Pulpo]

Alan

Nice work on the brickwork.

I would also agree that 100mm would be my choice.

This looks rather very straight forward.

I usually use f72 or f82 mesh meaning 7mm/ 8mm thick and 200mm square spacing, but an entire sheet would be a waste, and some 12mm reo would be a good choice.

As for the formwork, ideally form ply is the best option for the base and would have some temporary support in the middle.

Some timber or form ply for the sides which also needs to be supported.

It becomes a bloody mess when part of the form falls apart at the pouring stage.

Personally I would not use compressed fibro cement sheeting for form work [too expensive] nor would I use mdf, maybe in a pinch.

Good luck

Pulpo

[pawnhead]

nor would I use mdf, maybe in a pinch.

For a single use it's fine and it could even be reused a few times depending on how wet it's been. It's less than half the price and if you used a sheet of formply it would only be good for edge boards after cutting it up for that slab. Probably put a couple of joists through the guts, but that's no big deal.

[Make it work]

I have some 19mm yellow tongue flooring offcuts that will be big enough and I have enough reo. All that is stopping me is that I am waiting till the frame of the house is done, so the new slab won't be in the way of anything.

I will post more pics once I have made the formwork.

How long should I allow for the new slab to cure before I remove the temporary supports?

Thanks to all so far.

[bricks]

How long should I allow for the new slab to cure before I remove the temporary supports?

Thanks to all so far.

Just asking why you absolutely need to have a void under that slab?

Surely the best option would be to put plastic water barrier ( forte-con) around the inside of the brickwork and back fill with sand?

As for cureing time, the number that usually gets thrown around is 28 days, you could cover the top of the slab with plastic after you pour, This helps to stop the top drying out faster than the bottom and prevents cracking in the surface, ( it cures evenly).

And make sure you insulate the concrete slab from the side of your house.

Nice brickwork by the way.

[Make it work]

Just asking why you absolutely need to have a void under that slab?

Surely the best option would be to put plastic water barrier ( forte-con) around the inside of the brickwork and back fill with sand?

Before I demolished them, the old back steps were filled with sand and bricks and ANTS. Little buggers, there were millions of them and we could never kill the nest, also with what I now know about protecting against termites, it was a good point of undetected entry for them.
I left a void that will have an opening door to be able to inspect for nasties and spray with the annual pest control.

As for cureing time, the number that usually gets thrown around is 28 days, you could cover the top of the slab with plastic after you pour, This helps to stop the top drying out faster than the bottom and prevents cracking in the surface, ( it cures evenly).

And make sure you insulate the concrete slab from the side of your house.

I put 10mm expansion joint between the brickwork of the step and the house and plan to do the same or leave a gap between the slab and the framework, is this what you mean?

Nice brickwork by the way.

Thanks I'm slow but getting good at it now.

Thanks for the info, I appreciate all the advice I can get.

[brynk]

greetings - here is something else for you to consider

in situtations i have encountered where an un-reinforced load-bearing masonry wall is supporting a slab, if the poured slab is not prevented from keying in to the top course of the bricks by a sheet of gal / plastic or similar then cracks will appear in the brickwork from the side-wise forces applied by the concrete.

the best method i have seen is a sheet of plastic laid up the entire outer face of the wall, over the top and a little to the inside - this will protect your very smartly done face-brickwork from getting concrete splatter all over it, as well as stopping the conc from tieing into the top of the wall itself.

another option, if you were to use a pre-cast concrete panel then the problem may be avoided - then all you would need to do is lift it in place once it has cured for a suitable period of time, say, 28 days. another would be to put mortar along the top course & let it cure before forming & pouring your slab.

r's brynk

[thebuildingsurv]

You would have to be pretty tough to lift a concrete panel ?

[brynk]

as they say, the devil's in the details
(but then, someone once said god is in the details as well? does that mean god and the devil are both in the details? or can only one exist there at any given time? could it mean that god is the devil, taking on the suitable form depending on what side of the bed he got up on this morning? maybe it depends on the detail itself? if it's a mongrel detail then it could be the devil, and if it was a mongrel detail that worked out it could be god? what about the boring old run-of-the-mill details? do they also get graced by god &/ the devil? what if it was one of many of the same details? the first one works out but it was hard, then the rest of them come in just fine? and the biggest question of all, why don't they answer all of this in one of the aussie standards?)

maybe pour it on a platform right next to the brick then slide it across on rails, jack it up, take the rails out then lower it? or cast in a lifting lug then hoist it into place with a clever arrangement of pulleys and string? archimedes would be happy to help!

[pawnhead]

the best method i have seen is a sheet of plastic laid up the entire outer face of the wall, over the top and a little to the inside - this will protect your very smartly done face-brickwork from getting concrete splatter all over it, as well as stopping the conc from tieing into the top of the wall itself.

You can get a specially made slip joint for that. Two strips of galv sheet metal with grease between them.

as they say, the devil's in the details (but then, someone once said god is in the details as well? does that mean god and the devil are both in the details? or can only one exist there at any given time? could it mean that god is the devil, taking on the suitable form depending on what side of the bed he got up on this morning? maybe it depends on the detail itself? if it's a mongrel detail then it could be the devil, and if it was a mongrel detail that worked out it could be god? what about the boring old run-of-the-mill details? do they also get graced by god &/ the devil? what if it was one of many of the same details? the first one works out but it was hard, then the rest of them come in just fine? and the biggest question of all, why don't they answer all of this in one of the aussie standards?)

I see that you're cogitating about God there brynk. My mate Socrates asked this question;
"Is the pious loved by the gods because it is pious, or is it pious because it is loved by the gods?"
Euthyphro dilemma

And my old mate Al said this;
"I cannot imagine a God who rewards and punishes the objects of his creation and is but a reflection of human frailty." Albert Einstein

[Make it work]

if the poured slab is not prevented from keying in to the top course of the bricks by a sheet of gal...

I have put ant capping on the top course of bricks because it is up against the timber frame on 2 sides, so that serves this purpose too.

The new door was delivered late last week and will be fitted this week.

Then it's slab makin time...

I will post more pics when I get to that stage.

Thanks ALL...

[Make it work]

We have a FRAME, we have a DOOR, we have a SLAB and we have much RAIN.

Why does it always bloody rain when I pour concrete?

I ended up making it 170mm thick, the same height as the steps and I used 65 X 65 X 5 gal RHS on three sides to support the slab. I used 12mm rebar @ about 125 centres, welded in a grid and made a removable timber formwork which will come off early in march.

Pics will follow when I remove the plastic covers.

[Make it work]

Pics as promised

1. The formwork supporting the slab
2. The finished slab, waiting to be tiled (eventually)

[sports fan]

thats a solid set of stairs/ landing ! you could use that void as a bomb shelter