ᐅ Structural analysis for repurposing an existing concrete slab

Hello,

first of all: I understand that no one in this forum can assess structural engineering – and that is not what I am asking for. I am looking for general guidance for my situation.

Starting point: a "solid" concrete slab foundation, 10x10m (33x33 ft), built in 1980, currently supporting a simple timber-frame shed; no documentation available at the moment; request for the building records is still in process;

Project: residential construction on the existing slab in the form of a small, simple bungalow.

Question: How can an existing slab foundation be evaluated for load-bearing capacity without any documentation? How does this process usually work? How much does such a site inspection or survey generally cost – or is demolition and re-pouring usually more economical? Is it technically feasible to strengthen the existing slab by adding concrete on top?

As mentioned, I am not looking for a definitive statement for my building project yet – a structural engineer will have to be involved for the final assessment. I would just like to know beforehand if this is an (economic) dead end.

Thank you in advance for your constructive feedback!

thanatos

thanatos schrieb:
Starting point: a "solid" concrete slab foundation, 10 x 10 m (33 x 33 ft), from 1980,

thanatos schrieb:
No documents are currently available;

Well, that's the catch. Without the construction documents, no one knows the thickness of the slab, whether there are strip footings, or the amount/thickness of the reinforcing steel used.

If you have a lot of time to read, there was already a thread here from someone who really wanted to continue building on a slab foundation: https://www.hausbau-forum.de/threads/einfamilienhaus-meinungen-zu-unserem-entwurf.25622/ – in that case, however, it was already planned for a residential building (although he did not want to reuse those plans).

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Caspar2020 schrieb:
Well, that’s the crux of the problem. Without the construction documents, nobody knows the thickness of the slab; whether there are strip footings; or the amount/thickness of reinforcement steel used.

…and that’s exactly why I asked my question. It was about whether it’s economically feasible to determine this, or if it’s easier to just demolish everything. Or whether it’s possible to pour a new layer of concrete on top, since it should be at least as solid as compacted gravel. In that sense, the statement was of little help to me...

You can take core samples and have them tested in a laboratory for their strength. However, I only know this method in cases where planning documents are available (and an actual verification is required due to limit values).

Try calling one or two civil engineers or structural engineers in your area. Also, apply some pressure on the authorities to get access to the building file.

thanatos schrieb:
because it should be at least as firm as compacted gravel.

No; that is a bold assumption if the substructure is not known.

It will probably end up being a demolition. Breaking it up on site and reusing the material can save costs.

thanatos schrieb:
...The question was whether something like this can be determined in a cost-effective way...

Supply and drainage pipes would need to be installed within the existing foundation slab, which would significantly limit planning flexibility in some areas.

For demolition and removal, you can estimate one day with two to three people (one truck driver and up to two equipment operators).
If no materials need to be separated on site, the costs should not be too high.