Reinforcing the floor before stove installation - Basement floor with "built-in" Platon mat(?)

[mansjo]

Hello!

I'm renovating a basement and need to replace the stove and chimney pipe. Previously, there was a Hugo Miljö with a premodul-like chimney. This, along with the old pipe, has now been removed, and a new Contura 26T high with a matching premodul chimney is ordered. However, the installer questioned the floor's strength since the stove is placed in a recreation room with "matting" on the floor. So, some type of insulating mat lies on the concrete slab with a thin (I assume) concrete layer over it. The new stove weighs 590 kg + chimney, but I assume the chimney weighs about the same as the old one. I have no idea what a Hugo Miljö weighs.

My question is, do you think the floor will hold as it is?
If not, can you cut out a square meter and cast a solid floor?
If so, how should you build up this small floor section so that the entire floor can be tiled relatively soon?

Can you lay LECA blocks and then a thin plaster/concrete layer on top, or should you cast/reinforce all the way down to the underlying slab? How should the new slab meet the old one on the sides, and what type of cement/plaster should be used to avoid problems with tiling?

Thankful for tips!

/Best regards, mansjo

 

[KnockOnWood]

Here's how it's done at my place. It's a newly built house, so everything is much simpler

My stove weighs 550 kg. The chimney probably weighs less than 100 kilos.
Where the stove stands, a 1x1 meter piece is removed from the top 100 mm layer of foam insulation.
The concrete is therefore 200 mm thick there, compared to the slab's other 100 mm.
And of course, it is reinforced so that it "hangs together" with the rest of the slab.
It is otherwise done the same way, but narrower, under the "heart wall," which supports the trusses' lower frames.

Two thoughts:
a)
Chiseling up and casting a thicker concrete slab is quite simple.
But how to do with the reinforcement?

b)
It might be that the old stove stood in a place that was already reinforced,
e.g., right next to a load-bearing wall.

And a third thought:
A thick steel plate (10 mm?) can help distribute the load from the stove.
I have a neighbor who did so under a Leca brick + stone chimney stove.

Best to wait for someone who is knowledgeable in load-bearing!

 

[mansjo]

This is how it's done at my place. It's a newly built house, so everything is much easier

My stove weighs 550 kg. The chimney probably weighs less than 100 kilos.
Where the stove stands, a 1x1 meter piece is removed from the top 100 mm layer of cellular plastic insulation.
The concrete is therefore 200 mm thick there, compared to the rest of the slab’s 100 mm.
And of course, it's reinforced so that it "ties together" with the rest of the slab.
It’s done the same way, just narrower, under the "heart wall" that supports the roof trusses' bottom frames.

Two thoughts:
a)
To chisel up and pour a thicker concrete slab is quite simple.
But what about the reinforcement?

b)
It's possible that the old stove stood in a place that was already reinforced,
e.g., right next to a load-bearing wall.

And a third thought:
A thick steel plate (10 mm?) can help to distribute the load from the stove.
I have a neighbor who did so under a stove built in leca + stone chimney.

Best to wait for someone who is knowledgeable in structural strength!

Thanks for the reply!

I think I need to cut up the floor anyway since I have no idea what it looks like underneath, there could be a thin "crust" over the mat, and then it might not hold, or it might already be reinforced... but better safe than sorry

Edit... Found a drawing of how the floor should look at least... The floor is of the type "floor in potential residential space" (red underline in the picture).

 

[KnockOnWood]

I wonder if the floor will hold up well, if the stove is to be placed next to the load-bearing interior wall.
Just cut away the chipboard and insulation and fill it with concrete and some reinforcement, a square meter.

That was roughly your original thought too?

 

[Stuff]

I have similar thoughts about the renovation of a small den for the fall/winter.

Currently, there is an old larger model stove with a chimney pipe resting on top. The floor is made of chipboard with joists set directly against the concrete. I intend to tear this up and replace it with a floating floor on a Platon mat. I was thinking of arranging a more stable base under the stove and therefore am considering building up a pedestal about 12 cm high and approximately 1-1.5 m² in area. The top of the pedestal will then be level with the rest of the floor. The pedestal will be clad with tiles.

Will it be stable enough to build this pedestal with, for example, 9 cm leca blocks and mortar, or do I need to build a form and cast the entire pedestal out of concrete?

 

[Stuff]

No one had any comments on my thoughts?
Suppose I go with the track using lecablocks. I was thinking of using narrower lecablocks about 10 cm wide, which I would mortar together lying on their side, so they build up 10 cm + mortar vertically.
Is mortar alone sufficient as an anchor between the lecablocks and the concrete slab, or is additional anchoring required? It feels like all the load will come directly from above.

 

[KnockOnWood]

...Will it be stable enough to build this plinth with, for example, 9 cm lecablock and mortar, or do I have to build a mold and cast the entire plinth out of concrete?

Stacking some lecablocks on top of a concrete slab doesn't provide any reinforcement.

Set a form, reinforce properly, and cast it, so it becomes stronger. Then the weight will be distributed over the cast surface.

 

[vectrex]

But in all honesty, does a reinforced >10cm thick concrete slab really need to be reinforced for the weight of a measly stove, about 700 kg, that's almost nothing. My parents have a masonry tiled stove with a masonry chimney on top, this in a house with a modern slab on ground (built -92). The only difference underneath is that there is stronger reinforcement and cell plastic with higher compressive strength. That stove plus the chimney easily weighs > 3000 kg.

Concrete is extremely strong at handling compressive loads, and if it's in an old house without insulation underneath, it primarily deals with compressive loads since there's no cell plastic to "spring" underneath.

 

[KnockOnWood]

But in all honesty, does a reinforced, >10cm thick concrete slab really need to be reinforced for the weight of a flimsy stove, like 700 kg, it's almost nothing. My parents have a masonry tiled stove with a masonry chimney on top, this in a house with a modern slab on the ground (built in -92). The only difference underneath is that it has heavier reinforcement and foam with higher compressive strength. That stove plus the chimney easily weighs > 3000 kg.

Concrete is extremely strong in handling compressive loads, and if it's in an old house without insulation underneath, it becomes essentially just a compressive load because there is no foam to "spring" under it.

Yes, reinforcement may very well be needed.

And your reasoning about "compressive load" is extremely amateurish, even downright incorrect.

On the underside of the concrete slab, there are tensile stresses, which concrete is quite poor at handling.
(That's why "prestressed" concrete is often used in beams, precisely to avoid tensile stresses.
However, this does not occur in house foundations.)
But it can vary greatly depending on where in the construction the reinforcement is placed,
At the top, middle, or bottom of the concrete.

Try, and hope for the best, and possibly make repairs afterward,
or let a structural engineer take a look at it.

 

[vectrex]

Yes, reinforcement may very well be needed.

And your reasoning about "compressive load" is extremely amateurish, even downright incorrect.

I reiterate, as I mentioned, that it essentially only results in compressive force. This is given that it is an old uninsulated slab (which it seemed to be here).

The tensile stresses you are referring to only occur if the slab does not have a substrate that is equally strong or stronger against compressive force. Compare with "ordinary" EPS S100 quality. It resists 100kPa short-term load without deforming, whereas simple concrete at least resists 20MPa. Thus, 200 times stronger.

In this case, we have a substrate of well-packed crushed stone, significantly stronger against compressive force than EPS.

Not sure what to say about the amateur label...

 

[KnockOnWood]

...Don't really know what to say about the amateur designation...

You can say whatever you want

But I agree that it was a stupid statement.

I hereby apologize!

But I still think you're wrong in your reasoning.
The concrete slab, and the foundation, are calculated for distributed loads, and not as in this case a load of 750 kg on a very small area, approximately 0.2 m2.

 

[vectrex]

You can say what you want

But I agree that it was a silly statement.

I hereby apologize!


But I still think you are wrong in your reasoning.
The concrete slab and the foundation are designed for distributed loads, not like in this case a load of 750 kg on a very small area, about 0.2 m2.

Apology accepted.

In principle, I agree with you, but not when it's only 750 kg. If it had been ten times that, it would be a different matter (even though it would probably have been fine, too).

20Mpa pressure corresponds to more than the weight of 400 tons on an area of 0.2 m2. And 20MPa is still a very moderate assumption on old concrete.

(However, I'm not saying it would work to place a 400-ton lump in the basement in question, it was more to illustrate how much "pure" compressive strength simple concrete can actually handle).

 

[Stuff]

Stacking some lecablock over a concrete slab does not provide any reinforcement.

Set a form, reinforce properly and cast so it becomes stronger.
Then the weight will be distributed over the cast surface.

Thanks for the good response. I hadn't really considered much that additional reinforcement beyond the slab was necessary. I had mostly been thinking about leveling with the remaining floor, getting a good surface for tiles, and ensuring the base itself holds up.
The slab is in the basement of a 1960s house, meaning there's no insulation underneath.

Can I cast a base reinforced with reinforcement mesh without anchoring further in the substrate?

I've read that some recommend drilling holes in the slab and anchoring reinforcement bars, for example with anchor mass, before casting. Personally, I am skeptical of this. If we reason from the perspective that the purpose of the base is to distribute the load of the stove and chimney pipe over the slab, any extra anchoring points would risk acting as point loads on the existing slab. It would be best to cast without extra anchoring and keep the interface between the base and slab as homogeneous as possible to achieve an even load distribution?
(I'm an educated civil engineer in vehicle technology, so I have some knowledge of strength of materials, but mostly applied to steel constructions. Concrete is a completely new area for me.)

 

[KnockOnWood]

I've read that some advocate drilling holes in the slab and anchoring rebar, e.g., with anchor grout before casting. I'm skeptical about this myself.

I agree.

But aren't there any real reinforced concrete experts here?

Stuffs' reasoning about the compressive strength of concrete seems quite irrelevant to me. If you calculate it that way, then no edge beams are needed either.

(Wonder how they calculated at the textile factory in Dhaka, Bangladesh )

 

[Stuff]

I agree.

But is there no real reinforced concrete expertise here?

Stuff's reasoning about the compressive strength of concrete seems quite irrelevant to me.
If we calculate like that, then there’s no need for edge beams either.

(Wonder how they calculated at the textile factory in Dhaka, Bangladesh )

You're not confusing me with Vectrex now? It's he who makes the reasoning about compressive strength.

(As usual) maybe we are overanalyzing it all but as all DIY enthusiasts want to, once it’s finished, you want to sit down with a little whisky by the fire and feel satisfied that any possible earthquake would shake the whole house apart but the stove would darn well stand firm.