Filling Cracked Window Ledge

[Chris Doran]

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.

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?

Previous attempts with outside Polyfilla haven't worked for any length of time.

Chris

[EricP]

It may be concrete cancer. If they have iron reinforcing inside them, it's rusting and causing the cracks. If so, replacement is the only solution as no matter what you fill with, the rust is unlikely to stop.

[Owain]

"EricP" wrote | It may be concrete cancer. If they have iron reinforcing inside | them, it's rusting and causing the cracks. If so, replacement | is the only solution as no matter what you fill with, the rust | is unlikely to stop.

In the meantime a temporary way of holding things together might be to drill holes either side of the crack and well clear of it, chisel out a recess/trough linking the two holes, and drop a U shaped steel staple into epoxy into the holes, to hold the two halves together. Then make good over to prevent water/frost ingress.

Owain

[Andrew Gabriel]

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.

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]

painted