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 (TheOld...@gmail.invalid) 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

--
Tim Watts

[Dave Liquorice]

On Tue, 08 Mar 2011 23:37:43 +0000, Tim Watts wrote:

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

> 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.

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.

--
Cheers
Dave.

[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