Forming and casting slabs yourself - experiences?

Hello everyone,
although it doesn’t directly relate to the topic of house construction, it has a similar structure.

I am building a pool in the garden and have two supply shafts here, which I would like to cover with a concrete slab, with an opening to climb in later using a ladder.
The walls are made from 24 cm concrete formwork blocks.

Dimensions of the 1st shaft (pool technology)
1.5m x 2m

Dimensions of the 2nd shaft (water tank)
2m x 2m

I thought of making the entrance the size of a manhole cover so that I can later have a suitable cover for it directly.

Now the question is how best to create a concrete slab on top. I have already googled, but I only find information on how slabs for houses have to be constructed, or mention precast elements.

A terrace made of wooden planks will be built on the slab of the 1st shaft, and soil will later be placed on the slab of the 2nd shaft to fully plant the area.

My specific questions are:

- how should I make the formwork for this?
- what should the reinforcement look like? (Is, for example, a simple welded wire mesh sufficient?)
- how thick does the slab need to be at a minimum?

I hope for your help and input.

Thank you very much.

Hello abc12345,

My questions are specific:

- how do I have to make the formwork for this?

The same as is required in a precast plant: a wooden floor and four side walls. The wooden floor will then remain underneath as lost formwork.

- what should the reinforcement look like for this? (Is, for example, a simple steel mesh sufficient?

With "just" a simple mesh, it definitely won't be enough here. You will probably need plenty of additional reinforcements as a ring around the outside. Your structural engineer will calculate that for you.

Keep in mind, you have to somehow hold the ring of the manhole cover in your ceiling. The access is probably planned in the middle?

- how thick does the ceiling have to be at minimum?

Your structural engineer will calculate that as well. I would never try to make a ceiling with a circular opening (ø60-80cm??) myself. Especially as you are talking about adding further additional loads on the ceiling. Especially in the area of the terrace, easily 100-150kg/m² (1 person + wooden construction, your structural engineer will probably calculate higher!) on top, that’s already a big deal!

I would have this ordered as precast parts. But keep in mind that you will then need a crane to unload from the truck and place onto your walls. Assuming a 15cm thick ceiling, you come to about 1.1 tons of weight for the small ceiling! I would definitely keep my hands far away from that.

Thank you very much for your response. I don't want to pay a structural engineer extra for such a little "ceiling". I had hoped that for such small spans, one could come up with a simple solution. When I consider what a structural engineer costs, in practice I can just massively overdimension and install steel beams like in our house's floor ceilings. But since I don't want to use a sledgehammer to crack a nut, as I said, I had hoped for some empirical values. A prefabricated construction is out of the question here. This is "only" the covering for a water tank and a technical area.

The dimensions 1.5x2 and 2x2 are not "just" a cover with a big hole in it.

If there will be soil and a terrace on top anyway, just go to the locksmith and have a suitable cover "plate" made for you. Handle on it to lift it up. Done. Otherwise, I agree with . Consult a structural engineer.

With a cover like that, I'm already in the four-figure range... I can definitely do it cheaper if I just insert 3 steel beams across the width, board underneath and support, and put concrete on top. That holds for houses, so it will surely hold as a ceiling too.

I definitely drive more cheaply if I simply insert 3 steel beams across the width,

Do you do that with the steel consumption? I estimate a reasonable double T beam HEA 120 at €150 per 2,000mm. That will also be four-digit without concrete. And your entry must also be properly processed (reinforcement), otherwise the edge will break off immediately if you put a proper load on it. Concrete is very brittle without reinforcement.