Formwork for a suspended slab

[Brayden]

Hi There, 

Love the expertise and forum, super useful and greatly appreciated

I'm looking to pour a suspended slab.

100-120mm thick and 5m x 2.7m, suspended ~300mm off the ground with a double brick perimeter on all 4 sides bar a 400mm opening. I've poured a number of slabs and am confident in the process when pouring against a compacted base, however, the question I cant seem to solve;

Setting up the formwork will require a base. What is best to use for the base material, ideally that you can leave in place post curing?

 

Can I use formply and leave it after the concrete is set? (removing would be super hard) 

Or 

Am I better to use some corrugated sheeting to better stand the test of time and remain as a premanent base?

I have seen Deckform as product however seems overkill/commercial for my purpose

Drawing attach of the bricked area (and no, i didnt have a ruler or eraser handy, soz)

Thanks

Brayden

[Jewelleryrescue]

Hi @Brayden 

Great you done some concreting you will catch on quick.

So you're thinking is to use a permanent base form underneath the slab sitting on the base and forming sides for the slab and then the concern is the test of time for the material used under the slab.  EG Ply form work will rot sitting between the base and cement slab.!

Well consider having formwork sitting inside the base wall dimensions flush with the top of the base wall is open to direct slab contact. 

So when you pour the slab it will be poured directly on to the base wall and into the middle formwork (just need to temporary seal the edge gaps for the fresh poured cement not leaking.)

I would drill in a 4 reo spikes into the top of the wall (50mm) and bend them over to support steel reo mesh so the slab will never move, and it is all tied together.  OR bend the reo  spikes down and placed into drilled holes in the wall.

So the inner formwork ply is good 20mm thick or two sheets of 10mm just need to support that ply Maybe (a 90mm x 45mm timber frame) so the poured weight dosnt warp it so use as many supports under it as you can every 50cms as a guide so your slab isnt formed drooping.

Life span

Concrete is strong but if hit in the middle can fail if hit hard or stood on.

First the span 5M by 2.7 I think the slab will need steel support across the middle sections with angle iron 40mm x 40mm x 5mm wall as a minimum length ways or crossways.  Pre painted for longer life Zinc paint.

Sit the steel on the wall edges on top of the form work so as the cement is pour, they will be built in the slab spanning the base walls. This extra strong steel can be suspended into the slab too. You can use square tube too just make sure side wall 5mm as a guide.

Hope this helps.

[JacobZ]

Hi @Brayden,

A warm welcome to the Bunnings Workshop community and thank you for your question.

It certainly sounds like a fantastic project, but I would be very cautious as you are looking at roughly 1.6 cubic metres of concrete. One cubic metre of concrete typically weighs between 2200-2400kg, meaning your slab will weigh over 3000kg, which is obviously a lot of weight

Unfortunately, due to the sheer size and weight and the risks involved with the slab you are looking to pour, I think this is a question that should be directed to a suitably licenced person. Someone like an engineer or a concreter specialising in suspended slabs is likely the best person to speak to.

As @Jewelleryrescue has mentioned, the spans involved are very large and you will almost certainly need to add heavy gauge steel angles such as this Galintel 150 x 100 x 6 x 2700mm Solid Base Angle to support the underside of the formwork with heavy gauge reinforcing bar inside the suspended slab. The size and calibre of these materials should be discussed with a suitably licenced professional to ensure they are capable of supporting this weight.

While formply is often used for suspended slab pours, I wouldn't suggest using it as a permanent fixture. If over time water gets in and rots the formply, you have a lot of weight sitting on top of rotting material. For obvious reasons, this is not ideal. You would be better off using a fibre cement product such as James Hardie 2700 x 600 x 19mm Secura Exterior Flooring as there is no potential for rot like with timber products.

Ultimately, I'd love to see this project come to life, but I think it's important that you speak to suitably licenced people to ensure it is built safely and to a high standard. It is quite an investment in terms of time and money, so it would be awful to see something go wrong.

Allow me to tag @Nailbag, @Dave-1 and @TedBear for their thoughts.

Let me know what you think and if you have any further questions, please don't hesitate to ask.

Jacob

[Jewelleryrescue]

Hi @Brayden 

Another way is to think why you are making a concrete slab.

You can create a very strong timber or steel frame and place a concrete sheeting as the top layer if the concrete finish is what you desire. This will remove the super weight issues but will still be heavy.

Like a bathroom floor build.

[Brayden]

Great feedback and views, thanks @Jewelleryrescue and @JacobZ 

Really appreciated 👍 

I have an engineer on hand from a previous project so will give them a buzz to spec it up.

[Dave-1]

Good Evening @Brayden 

Wooooo that is some weight to have suspended and wel out of my scope of knowledge. I would recommend a builder who has experience in laying suspended slabs and would probarly ask to see some examples where you can have a chat to the people who have had one installed. It may feel a bit reare but that is a lot of concrete and you want it done right.

Are you intending on putting a structure on top of it as that could also change a bunch of calculations?

Dave

[Nailbag]

Hi @Brayden 

This is a project is outside my area of expertise, but sounds like well within @Jewelleryrescue wheelhouse.

Nailbag