Does concrete cast furniture need to be monolithic

[Adsibob]

I had a concrete base for a low table cast in concrete. From a bird’s eye view it is  basically a U shape, with 90 degree corners, or a square with one side missing, except the back side projects out. Each side is about 8.5cm to 10cm thick and it is 40cm high. Before they poured the concrete into the plywood frame that had been built for it, we put some rebar in to strengthen the 90 degree corners. 
It’s possible that the height is a bit short, about 1cm to 2cm too short. On top of this U shape we are putting a steel plate, and on top of the steel plate, a heavy wood burning stove, although there is support for the stove from the fact that one of the “sides” of this U shape is almost directly underneath the stove, slightly off Center to allow space for the incoming air supply pipe.

 

My question is whether I can add another 1cm to 2cm of concrete to the  top of this U-shaped structure, to correct the potentially incorrect height, or will that mean that the structure is weakened by the fact that it was not made by one monolithic pour?

[Bonner]

It is a thin layer of concrete which will probably crack but not really a problem. Just make sure the new concrete is keyed into the existing and the steel plate is free to expand and contract. Suggest you sit the plate on a thin bed of mortar as well to ensure it is fully supported.

[SteamyTea]

Post up a picture, I am struggling to imagine what you are describing.

 

Concrete is fine, strength wise, when in compression.

Edited January 1, 2022 by SteamyTea

[Adsibob]

On 01/01/2022 at 08:48, SteamyTea said:

Post up a picture, I am struggling to imagine what you are describing.

 

Concrete is fine, strength wise, when in compression.

Here is a picture showing the concrete plinth on the bottom and then the steel rods and steel plate we are planning to put on top (the pictures are showing it floating in mid air, but it will be screwed down onto the plinth). To give a bit more explanation, the overall height of the top surface needs to be 430mm from FFL.

 This was going to be achieved by 410mm cast concrete PLUS 18mm of cuboidal hollow steel rods arranged in a sort of ladder formation (though my diagram omits the “rungs”) and then 2mm thick steel sheet, bent over the side of the  front-most rod to give the appearance of a 20mm steel plate at the top, but at a much reduced cost.

wood burning stove going on top of the steel plate (either side of where i have drawn the hole for the air supply pipe)

My concern is that actually, the top of the concrete is not level. At places it is 410mm as intended, but at other places it is 10mm to 15mm too short. 
My builder’s plan is to level it with self levelling compound. Is this the best way to fix this issue - won’t it just crack?

 

Edited January 4, 2022 by Adsibob

[markc]

Self levelling will be fine, just roughen the concrete surface to let it get hold (assuming it has been floated)

[SteamyTea]

You could put in a pinch of glass fibre choppings in the concrete mix, though not sure how much it will affect the surface finish.

 

Edited January 4, 2022 by SteamyTea

[Adsibob]

2 minutes ago, SteamyTea said:

You could put in a pinch of glass fibre choppings in the concrete mix, though not sure how much it will affect the surface finish.

 

What would this achieve?

The surface finish isn't that important, as it will be covered. But it does need to be level/flat.

[SteamyTea]

4 minutes ago, Adsibob said:

What would this achieve?

Will improve the tensile strength, which will help with reducing the possibility of cracking due to bending. Bending and buckling is really just the compressive and tensile strength counteracting each other.

Edited January 4, 2022 by SteamyTea

[Adsibob]

14 minutes ago, SteamyTea said:

Will improve the tensile strength, which will help with reducing the possibility of cracking due to bending. Bending and buckling is really just the compressive and tensile strength counteracting each other.

Interesting idea, though potentially not without risk. This item, for example, states "do not apply to [sic] thick as exotherm will generate making the mix reach high temperatures" not sure what that actually means though.

[SteamyTea]

3 minutes ago, Adsibob said:

"do not apply to [sic] thick as exotherm will generate making the mix reach high temperatures"

That is when used with a polyester resin, concrete is different.

[Adsibob]

6 hours ago, SteamyTea said:

You could put in a pinch of glass fibre choppings in the concrete mix, though not sure how much it will affect the surface finish.

 

Is this what you had in mind:

 

https://www.ecfibreglasssupplies.co.uk/product/chopped-strands-6-mm

[SteamyTea]

1 minute ago, Adsibob said:

Is this what you had in mind:

 

https://www.ecfibreglasssupplies.co.uk/product/chopped-strands-6-mm

Yes.  Don't need much as it disperses quite a but when mixing.

Pretty sure it is already used on thin floors.

https://concretecountertopinstitute.com/free-training/introduction-to-gfrc-glass-fiber-reinforced-concrete/