Re: Fence Replacement

[Cursitor Doom]

Gentlemen,

So I've ripped out the old rotten fence posts. Some came out with concrete 'root bowls' attached, others left the bases behind. On the ones which left the concrete behind, there are 4" square holes where the original posts sat and they're stable and substantial so I'd like to re-used them if poss. Trouble is, the new posts are slightly smaller so there's a gap of about a quarter of an inch all the way round the new posts which needs filling with something, but the gap is too small for the shingle in concrete to slip down into. Any suggestions as to how to proceed?

CD

[The Natural Philosopher]

Expanding foam

Waterproof and will gap fill all day. I saw a video in which someone used it instead of postcrete, and IIRC the verdict was 'perfect, but expensive'

[alan_m]

Postcrete - put it in dry and use a watering can for the water afterwards. Sets in 10 minutes.

[alan_m]

Or mix your own kiln dried sand and cement. That's all those tubs of repair concrete are. Again fill the gap with the dry mix and just damp it down from the top.

[The Natural Philosopher]

quarter inch gaps will not respond well to any sort of concrete. sand and cement might work, but its hard to ensure that there won't be voids in such a small space. Hence expanding foam.

[Cursitor Doom]

Yeah, I 'm kind of struggling to see how expanding foam will cope with the vagaries of outside exposure to the elements plus the inevitable stresses imposed by gales, hurricanes and whatnot. It's going to get squished to nothing pretty quickly I'd imagine.

[Chris Hogg]

Get some Postcrete or Postfix or similar and sieve out the gravel? Then the resulting gritty powder should fill the gap with no problem. Making sure the post is hard up against two sides of the hole will give you a wider gap on the other two sides, rather that trying to fill equally all around.

[The Natural Philosopher]

Rubbish. Its used to hold windows in isn't it?

[The Natural Philosopher]

FFS if that's what you want buy a bag of cement and a bag of sharp sand.

[Cursitor Doom]

Is it?? Anyway, I don't think it would hold up given the much higher leverage it would experience at the foot of a fence post in a gale.

[alan_m]

[The Natural Philosopher]

Well it would, and it does. The compressive strength of foam is greater than the compressive strength of wet soil.

The fence posts in the local supermarket that blew over either snapped due to the concrete rotting the bases, or the concrete itself tore out of the ground.

Concrete does tow things - ensures a wet base to a wooden post as concrete is hygroscopic, and is only as strong as the soil that surrounds it.

Foam may not be ultimately as strong, but as long as its stronger than the soil its OK

[The Natural Philosopher]

Didn't do the job properly. Didn't use enough foam

[Tim Lamb]

Yebbut. The surface area of the soil/concrete interface is much greater than that of post/foam/concrete. All the plastic windows in this house are secured by screws with the gaps filled with foam.

I would mix a slurry of sand/cement in a bucket to gap fill.

[Jeff Layman]

And some accelerant because that's what Postcrete contains.

[#Paul]

This isn't really a disagreement with your general point, but wet soil placed under a varying load (e.g. due to wind gusts) remains wet soil, even if slightly displaced. However, expanding foam has the potential to be crushed or damaged, and will never recover; thus there is the possibility of an accumulation of minor damages over some time period that add up to something more serious (e.g. a loss of volume, and hence a loose fence post).

#Paul

[Murmansk]

I had success replacing a rusted metal fence post by getting a new post made that was "the next size down" and inserting that into the remains below ground of the previous post, secured with the kind of resin you use for resin fixings - worked very well and a lot easier than expanding foam I'd have thought.

[Cursitor Doom]

Not when you're placing the posts into concrete as I am with this project (see above in the thread).

[Cursitor Doom]

Exactly.

[The Natural Philosopher]

Why bother? I can wait 24 hours for posts to set.

My most sturdy posts that hold a 5 bar gate were put in with.....

..nothing at all. I made the holes very close fitting and packed clay around the pressure treated timber

The secret is going a long way down and making the holes fit well