DIYbanter - View Single Post

In article ,
(Chris Doran) writes:
I have an downstairs outside bay window ledge which is cracking up, as
they do. It's the common thick and deep concrete wrap-around thing
which is cast on site. The cracks range from hairline to a couple of
mm wide and seem to go right through, although the ledge doesn't seem
to be about to fall apart and I want to fix it before it does.

Googling this group suggests that PVA as a good filler/glue for cement
cracks, but the tin doesn't give any instructions for this
application. In fact it only talks about using it as a sealer, but
this (UniBond waterproof PVA) was the only "outside use" tin I could
find in Homebase.

Don't have a tin on me, but I think you'll find it's only regarded
as waterproof when mixed with mortar -- the raw PVA isn't.

What dilution should I use?
Should I add cement to the mix as the tin says? Or will this make it
flow less freely?
Do I pour it in a bit at a time allowing it to set between pours, or
just fill 'em up in one go?

Or should I use something else entirely?

Yes;-)

For narrow cracks, I used one of the oil based mastic tubes to seal
the crack. The important thing is to stop water penetration, as this
freezes inside and breaks away more of the rendering. In my case it
is all pointer over -- might need to do something different if the
fix will be visible.

Where there's a chunk of render missing, mix up some sand/cement 6:1.
I suggest adding waterproofer (combined with plasticiser will make
the mix easier to handle). PVA will help it stick too -- I use about
1 teaspoon of outdoor PVA per 2 shovel loads of sand.

Depending on the orientation and size of the missing area, you may
need to make up some shuttering to hold it all in place whilst the
render sets. I use bits of timber held in place with G-cramps against
the good areas of render, and wrap timber tightly with cling film for
easy release (which also gives a smooth surface, as you don't have
access to polish it with a trowel).

--
Andrew Gabriel