Thickness for concrete slab for cast iron stove?

[Kristofer_Hlm]

Hello!

We are building an orangery and planning to have a cast iron stove for evenings and chilly days.

The idea is to cast a slab for it to stand on, raising it about 50 cm from the floor. The slab will rest on 19 cm wide lightweight concrete blocks that will be part of the planting beds.

The slab will be approximately 150 cm wide and 75 cm deep. So it will be about 110 cm between the supports. The back edge can be anchored to the existing concrete wall with one or more reinforcing bars. The stove weighs about 200 kg, and the slab will be reinforced with "regular" (6 mm?) reinforcing mesh. The plan is to use fine concrete.

Can anyone help with a suitably oversized thickness and if there's anything else to consider?

Thanks in advance & kind regards,
Kristofer

 

[Kristofer_Hlm]

No one has any thoughts on this?

 

[Snickarkirre]

Hello! I'll start by saying that I'm not a constructor, but I can offer an educated guess. Anchoring it to the underlying concrete wall sounds like a good idea. I think you should be fine with a 100mm thick slab. Using a mesh mat is smart, but I think you should reinforce the rebar with some 12mm rebar running across the slab from lightweight concrete block to lightweight concrete block. Place the 12mm bars at the bottom on spacers and then put the mesh on top. You can also use coarse concrete instead of fine concrete.

How much can the lightweight concrete blocks actually handle? There's quite a bit of weight in that construction. The slab weighs roughly 260kg plus the stove and chimney pipe, which is almost half a ton. You might be able to reduce the thickness of the slab, but then you would probably need to use thicker bars.

 

[Kristofer_Hlm]

Hi!

Thank you for your response, unfortunately, the slab was just poured a few hours ago.

Here's what we did:
- About 100 mm thick in fine concrete
- Edge beam at the front about 150 mm total height and 50 mm depth with two 8 mm reinforcing bars (one at the bottom, one level with the reinforcement mesh)
- 6 mm reinforcement mesh across the entire surface & attached at the back edge, with two extra 8 mm in the same plane (in addition to the two in the edge beam)

It's annoying that I had 12 mm reinforcing bars at home but thought it felt excessive. I hope it holds up anyway.

Thanks & have a nice evening,
Kristofer

PS. The blocks are 3 MPa, so 3 N/mm². Roughly calculated, they can then support 44 tons per side. DS

 

[Snickarkirre]

Hi!

Thanks for your reply, unfortunately, the slab was cast just a few hours ago.

It ended up being:
- Approximately 100 mm thick in fine concrete
- Edge beam at the front about 150 mm total height and 50 mm depth with two 8 mm reinforcement bars (one at the bottom, one at the height of the reinforcing mesh)
- 6 mm reinforcing mesh over the entire surface & attached at the back, with two extra 8 mm in the same plane (in addition to the two in the edge beam)

It’s annoying that I had 12 mm reinforcement bars at home, but thought it was overkill. Let's hope it holds anyway.

Thanks & have a nice evening,
Kristofer

PS. The blocks are 3 MPa, so 3 N/mm2. Quickly calculated, they can then withstand 44 tons per side. DS

Oh, it's a pity I wasn't quicker to reply. But it sounds like that should hold without problems. Smart solution with an "edge beam" at the front as reinforcement. It was just me thinking it might be wise to play it safe with 12 mm bars. But I do think your solution will work without issues, just let it cure for a week before loading it

Wow, then those will certainly hold without problems at least

 

[Kristofer_Hlm]

Yes, a bit unfortunate, but still nice to have a response and some discussion

Thanks, I thought the beam could be good for a bit of extra security. No risk of early load, it will probably take a few months before there is a stove on that slab. But I might as well leave the formwork and supports underneath for over a week, they don't bother anyone.

Thanks & take care,
Kristofer

 

[Snickarkirre]

Yes, a bit of a shame but still nice to have a response and some discussion

Thank you, I thought the beam could be good for a little extra safety. No risk of early load, it will probably be a few months before the stove comes on that slab. But I might as well leave the form and supports underneath for a good week, they won't bother anyone.

Thanks & take care,
Kristofer

No exactly, it's always nice!

Okay, that's good then the concrete has time to cure properly.

Thanks, same to you!