New concrete slab on top of existing

[greznik91]

Im dealing with insufficient existing RC slab above a residental building (first floor). Existing slab is aprox. 120 mm thick but reinforcement is unknown. the problem is that a slab cant be removed (and replaced with new one).

The slab is vibrating if you jump. There was a screed (100 mm) above this slab that is now removed.

I was thinking about making a new RC slab above existing (spans are small) so I was thinking about making it 100 - 120 mm thick. I would design a new slab so it would be sufficient alone (im not considering existing slab below). There would be no connection (no anchors for a horizontal shear between existing and a new slab). I know it would be easier to just remove existing slab and make a new one but thats not a possibility.

my question is how much load is to be expected on existing slab bellow since a new one is made on top of it?
Do loads distribute based on stiffness since there is no horizontal connection between both slabs?
In worst case scenario, the load of a new slab is approx. the same as 100 mm of screed that has been removed.

When pouring a new slab the existing slab would be used as a formwork (wtih some supports bellow it) - as I said spans are small.

Your thoughts?

apologies for my poor english

 

[greznik91]

Id really appreciate some thoughts about this.

 

[hokie66]

I would want to make the slabs work compositely. This can be done merely by ensuring the existing slab is thoroughly clean and slightly roughened by shotblasting.

 

[RickyTickyTavi]

Is this an elevated slab?

Have you explored FRP strengthening?

 

[BAretired]

If you pour a new slab on top of the existing, make sure that the foundations are adequate to support the additional load; also, you might want to add a few temporary shores under the existing slab until the new concrete has cured.

The existing slab and the proposed slab both seem rather shallow for the 5 m span, so composite behavior as suggested by hokie66 might be a prudent plan.

BA