Concrete over a plastic 40mm waste pipe

[TheOldFellow]

I'm renovating a bathroom. Taking out some stuff I discover that the

40mm waste from the shower runs under the vanity and bath in a channel in the floor. There's about 75mm from the top of the pipe to surface level. The channel is about 100mm wide.

If I concrete the pipe in (I want to tile over that bit) will it be strong enough? If it's OK, what kind of mix should I use? Should I change the pipe to some other kind (currently it's solvent-weld and about 40 years old)

It's not going to be used for the shower in the new scheme, I'm putting the washing machine and condenser dryer there.

Thanks,

R.

[Tim Watts]

TheOldFellow ( [e-mail]) wibbled on Tuesday 08 March 2011

19:47:

Yes, plastic can be and often is set in screed and concrete.

Any joints in that section?

If you have any reason to doubt the pipe's integrity, you could replace that section with current 40mm and avoid any buried joints.

As for the strength - yes, 40mm strongish mix will be fine if that is from surface to the top of the pipe - and the mix packs well to the bottom of the trough. But I would consider a screed mix rather than concrete if the channel is not big - and by removing the larger aggregate from the mix, you will not be subjecting the pipe to any stress from pointy bits of rock.

You could do a number of things for belt and braces:

1) Run the 40mm inside a loose bit of 50mm provided you still get good depth on top.

2) Wrap a very thin layer of closed cell foam (2mm sheet as used in some packaging) around the pipe.

Both of those will mitigate the quite significant expansion of uPVC when it gets warm though I'm not sure if that is a real problem or not.

Cheers

Tim

[Dave Liquorice]

I was a bit reluctant to say bury the 40 year old plastic and was going to suggest constructing a bridge from 18mm ply supported both sides down to the bottom of the channel.

But replacing the old with new and padded inside a 50 mm pipe shoud be fine with no joints in the buried section. When you bury it ensure it has a correct and even fall, pack it underneath every foot to 18" to prevent it sagging and be aware of it wanting to float when you bury it.

Use compression fittings each end with as much free pipe as possible not solvent weld as sure as eggs is eggs one will need to change or adjust one connection or the other at some point.

[TheOldFellow]

Thanks, I got all this. New 50mm concreted in as a conduit for the new 40mm seems best for me. This will allow the 40mm to be withdrawn or rodded through the wall outside the house. Push-fit outside for neatness, and probably the same inside as it is just a riser for the washing machine hose and condensate drain for the dryer and dehumidifier. The pipe run is straight and only 2 metres so the fall will be fine. I'll make some sort of neoprene gasket to make sure there is no insect ingress or air movement via the inter-pipe gap.

R.

[Tabby]

direct burial's fine, it just makes it way easier to do plumbing maintenance later if you sleeve it with something. Pretty much anything would do as sleeving, its only to stop the concrete gripping the pipe. The only thing to avoid is burying fittings that arent solv weld.

NT