Can my house collapse from adding a concrete slab?

Question

I'm at the final steps of the job right before the pour but it only recently hit me that I might be on to collapsing my own house...

I bought an old house (europe) built in the 1960s using stone as foundation (no concrete) and solid brick. The first floor is half underground and the walls are lined with stone on the outside and brick on the inside. The wall thickness is about 50cm on the first floor and 25-ish cm on the second. I'm saying all this so you can get a basic picture that it is a pretty sturdy house.

One of the few places where there is concrete used is on the porch (not sure if it's the correct term) where the entrances for each floor are. Here's a very bad drawing (added colors for easier differentiation), the grey parts are the stairs and the porch slab which are made from concrete:

So getting to the problem, the slab has visible cracks on it (which I suppose is normal) but there are patches on the underside where the cement plastering (they did as a finish) has fallen off and the rebar is visible and is in quite bad shape - rusted and easily breakable. The slab has visibly sagged in the middle by a good 3-4cm compared to the corners where it is supported but I cannot tell for sure if it wasn't poured this way or has sagged as it has deteriorated. It's dimensions are 2.7m by 2.8m by 5cm.

In order to strengthen it I decided to pour another slab on top of it and add metal bracing that is supported on the columns, here's a top view of what I have done:

In red are the brick columns, the one next to the stairs extends past them and the one on the bottom left is part of the wall of the first floor. The bottom right is the one that extends vertically to support the roof. In blue I have used 60x40x2 steel profile that is supported on all 3 columns and in purple I have 50x30x5 steel profile that is supported on the blue profile pretty much in the ends only because as I said the slab has sagged. In orange I have put 8mm rebar and in green 6mm rebar.

I thought it would turn out very strong, but in reality, the diagonal purple bar that goes through the center is 3.8m long and if I step on it it bends visibly and I'm only about 80kg. If I get other people to come step on it as well it doesn't bend very much more than with me on it only, which means it can take the load but it needs to bend first. Which in turn means, that if I pour concrete there it will fill the gap between the steel profile and the old slab (the sag space) thus any load that would otherwise cause the profile to bend will be taken by the old slab anyway. Which kind of negates the entire operation...

What I'm most concerned about is, as I calculate the required concrete to be about 0.5m3 which would result in roughly 1200 to 1500kg, what are the odds of the old slab breaking, taking along that outermost column and half of the roof along with it?

Answer 1

That's a well-put question, I think, but I wish you had asked before placing all the steel.

I hope you can find a structural engineer who understands concrete well, who can examine the pillars and existing slab, and give you more specific advice. However, the following are my first observations (I am not a licensed engineer, but have worked with one for years and have poured quite a lot of concrete).

I have poured some un-supported slab floors like the ones pictured (the design was given) and the steel was always all in round reinforcing bar format (none in tubes, rectangular or otherwise), and it was a bit more than what you have pictured. I have never seen tube used for reinforcing; I do not believe it is used commonly anywhere and it is probably not well-understood, unlike ribbed reinforcing bar which has been studied inside and out and has well-established practices around its use.

Also, the reinforcing bar in new slabs is usually all orthogonal to the sides. This is (partly) because, if you analyze the section of poured concrete that contains the steel bar as a simple beam, the shortest span is the most efficient for a given depth (thickness) of the slab; and the bar that crosses diagonally is longer rather than shorter, and so can carry less weight (as you have already found by your experiment when you stepped on it).

Since the old slab has sagged, you should consider that it has failed; you cannot properly consider it to have any strength at all in the new sandwich that you are proposing, even though it still holds itself up. This means that it is, at best, dead weight in the final product, and at worst an important safety risk, since chunks of it could (eventually) drop out onto the floor below (or cause complete catastrophic structural failure, although at a glance this seems unlikely because the new slab could hold the pillars in position).

Answer 2

A structural engineer is what you need to hire.

The slab you are proposing is capable of collapsing suddenly.

The incorrect mix of loads will cause your suggested slab to randomly collapse explosively.

Both the newly suggested slab and the original are excessively thin. Generally speaking, you need at least 12 cm of thickness for a 3-meter span, resulting in a three-ton increase in weight. An engineer would examine the bearing walls and foundation to see if they can take extra load.

You would need a bond beam consisting of 4 bars caged by tie bars and 12 mm deformed bars running perpendicular to the edges of the slab 20 cm apart.

The bond beam needs to adequately anchor to the walls. All these are very heavy!

I would build the new slab out of 4x10 cm wood rafters at 30 cm spacing and cover it with plywood. Then cover the deck with thin tiles. This alternative is going to be much lighter.

Again, ask an engineer and consider removing the defective old slab.

Answer 3

The purpose of reinforcing metal (called rebar in the US) inside concrete is to keep it together. It isn't there to provide structural strength iself, it is to allow the concrete to remain solid.

I submit that you need a structural engineer to tell you if adding several thousand kilograms of concrete to your upper floor is possible without risk. Homes aren't built like you describe whereI live, but adding a huge load on your own (and at a span you've proven doesn't last) would not be a good idea.