On 14 Sep, 16:14, "george [dicegeorge]"
wrote:

I've been concreting in offcuts of round plastic drainpipe.
How would I seal these against smoke and fire?

You may not need to. Per the regulations, a conduit up to 32mm
diameter is acceptable without.

Above that, you need a barrier that achieves the same degree of fire
protection as the wall already achieved. This doesn't mean the same
construction, merely that if the wall was already required to have 1/2
hour resistance, the duct should do so too.

You probably need a draught barrier anyway.

Is there a better way to do it?

An intumescent barrier is one that's not necessarily a barrier when
cold, but will expand to form one during the heat of a fire.
Intumescent mastics (Screwfix etc., cheap) are convenient for small
ducts, especially if the cabling is permanent. For large ducts, pre-
made collares are usually used, but these can get expensive.

A cheap and simple barrier that's useful when cables might come and go
is just to use a lid of plasterboard or Viroc Versapanel, according to
the usual fire regs. This can be split around the cables.