Plastic sheets under concrete

[ErrolFlynn]

Most of my concreting experience has been laying footpaths, and I’ve never used plastic under it. I’ve just whacked the concrete down. I’ve never seen the point in that sort of application.

However, I can see the point of using it for floors inside buildings. I presume it acts as a damp course. Stops moisture from seeping through. Though, I’m still confused.

I’ve never seen or heard of anyone laying the plastic and then carefully gluing the sheets together. Or maybe they’re supposed to but can’t be bothered. Or perhaps they do and I just haven’t been watching them closely enough.

It seems to me if there’s a risk of water ingress, like a concrete floor that will be close to the water table, then if you don’t glue the plastic sheets together, then water is going to get in and through the concrete and your floor is going to be damp.

On the other hand, I hear that a strong concrete mix is waterproof anyway.

The crux of the issue is, if you don’t glue the sheets together, is it pointless using plastic sheets?

[mic-d]

I’ve only done it once when I concreted a hay room. So no practical experience or theoretical experience either. But if you’re worried about joins then really the same applies for all boundaries. If it’s a problem you’d have to bring the plastic up around the perimeter too. Do you see this? I don’t know. I think if the plastic is laid on sand, ie compliant, the weight of the concrete effectively seals the plastic sheets and seal the plastic to the concrete. Also the fines of the concrete, ie cement, will physically seal anywhere it gets to. It’s a cheap extra barrier that adds to the array of construction tools so why not do it?
edit. Obviously you’d want to have an overlap of the plastic. Two sheets of plastic sealed together over 250mm I would think would seal quite well. Think of sandbagging which if done properly uses a sheet of plastic under the bags and up above the water height. It’s just the weight of the sand stopping/slowing capillary action.

[droog]

If it is laid correctly joins and penetrations are taped.
https://ncc.abcb.gov.au/editions/2019-a1/ncc-2019-volume-two-amendment-1/part-32-footings-and-slabs/part-322-preparation

Of coarse it all depends on the trades doing the work and the inspector checking it before the pour.

[ErrolFlynn]

If it is laid correctly joins and penetrations are taped.

That makes sense, but it would be difficult. If the two sheets don't lie flat and there's a crinkle between one or the other sheet, or both, the tape might not be able to make a good seal.

After posting, and pondering the issue, it occurred to me that an option might be to use some kind of paint as a sealer. Something flexible. Bitumous-based, possibly. Maybe the same material as used to waterproof bathrooms. The thing is, all you'd need to do would literally be to splash a heap between the sheets and around the joint. It wouldn't matter how it looked, just so long as you get it between all the air gaps, and as soon as the first bit of concrete rests on top of it, it'll be held in place.