Hollow-sounding floor slab advice sought

22 Jul 2006
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United Kingdom
Hi all,

I'm in the process of converting a car port into a garage and have discovered the ground under the poured concrete floor slab may have dropped.

The garage is 48 years old and is in an area susceptible to subsidence (big pine trees surrounding the house).

One half of the slab sounds hollow when hit with a hammer. I cut-away a section and removed the concrete. The slab is about 4 inches thick and sits on a damp-proof membrane, which is over a bed of loose gravel. There is no evidence of the slab cracking anywhere. The concrete is still resilient (difficult cut through and a ballache to break with the sledgehammer).

The house is a keeper so I would like to do this right, but I would also like to avoid disturbing the floor if possible. I plan to pour a latex screed over the floor and use it as a workshop.

Can anyone with experience of concrete floors tell me whether I can leave it alone, or whether I should be re-pouring it before I lay the screed?

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It's really impossible to say. The floor has been there for all that time with no problem so might go on for many years yet with no problem. On the other hand if you load the floor with machines or whatever it might be just enough to cause a crack. If it did crack later what are the consequences of that - could you put up with it or would it absolutely have to be repaired? The only way to 100% avoid that is hack it up now and re-lay it with some reinforcement. But that's onviously going to cost you. So it's either cost now or cost and hassle later. Or of course it may never crack.
Cheers John. Kind of what I'm thinking too. The previous owner had a car parked on top of it, so it should hold a workbench. Maybe.

I don't really want to disturb it, so I'll fill the hole I made and leave it alone for now.

Always bet on black.

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