On Thu, 16 Jul 2009 15:23:26 +0100
John Rumm wrote:

TheOldFellow wrote:

I'll need about 90mm below the tray for a trap, this will just leave
enough to get through the wall to a gully. It's all horribly tight.

Now. How do I get the concrete out? I've thought:

It looks rather like the edge was bricked round and then someone just
dumped a barrow full of weakish mortar in the middle. So it will be sat
on the screed, rather than cast as a part of it. The result is that it
will lift quite easily once you can start it moving.

1) Hire a breaker - but I don't want to damage the underfloor, or the
drain the runs right under.

Any breaks you make it in should just split it, and not the screed. I
expect even a SDS chisel used downwards about three or four inches from
the edge would first crack it, and then split off the side of it. The
bit split off would have then been slid across the screed a little, and
hence be well and truly detached from it.

Yes, that's what I thought. Well after a half day's work I have
chipped just over half of it out. It certainly didn't cleave where I
expected. But, as expected it cleans off the screed OK. Should be
fine as a base for the new shower tray.

If you work in until you have a section of of the middle of it, you
could probably pry or lever the edge bits up in one lump.

2) Drill many holes with SDS+ and then take it out in chunks.

You could do, but it probably won't be necessary. Once you have broken
the bricks off from around the side, and started taking the edge off
with the SDS you should be able to get it into manageable lumps that
will lift off. If you make a notch with the SDS and stick a pry bar
under it (on a wooden block if you are keeping the tiles on the floor!)
you may even find you can get it out in reasonable lumps.

The occasional blow to the side of the lump with a club hammer will help
loosen it up as well.

I would expect you will get it out in under 20 mins with a small SDS.

3) Drill and cut at the parting line and then drive a chisel in with a
sledge. I'm not sure if it will part with the floor though.

One layer of concrete laid on another has little strength under either
tension or shear loads (although its good under compressive ones) unless
they have tried hard to achieve a good bond.

I think they tried!

Never mind though I will get the b...r out. Thanks for the
encouragement though.

R.