postcreted anchor base failure

[turnpike]

I set a stand-off post anchor base in postcrete yesterday and after a day came back and put a 100mm square 2m post in it. The whole thing sways. The anchor seems to be loose, with movement about 3-5mm backwards and forwards.

I thought postcrete set in 10 minutes. Have I tried to use it too soon? Is it no good for a really heavy 2m post?

Any ideas how I can fix it appreciated! I'm thinking I might have to saw the anchor away and use a resin bolted one instead?

My first thought after this was, I wish I'd just put the post direct into the postcrete, but I thought this would work well incase it rotted and needed replaced in future..

Cheers!

 

[xdave]

Are you trying to support a 2m 100mm square post above ground with nothing but the stand off bracket to support it? That will never work unless the post has cross bracing across two sides. You'd be able to push it over with one hand and it would wobble in the breeze.

If you will be putting any sort of force on this post it needs to be sunk in the ground. A good quality pressure treated post (not the DIY chain stuff) will last 25+ years with 1/3rd underground so long as you sit the base on gravel and full-fill concrete/postcrete around the sides.

Postcrete is for mass-filling holes in the ground to support fence posts not for structural strength. It has no real aggregate in it above 3mm. Even when fully set (the 5 mins is only until it goes green) you'd be able to break it if it was not supported on all sides by the edges of the hole.

 

[turnpike]

xdave, the post was going to be supported from above and to the side by beams which would be fixed to brick, but even so you're right, the stand off bracket isn't good enough. I've learnt that the hard way.

It will have a couple of horizontal beams going between it and a brick wall a meter away, so doesn't have to stand and support itself, but it's looking like the postcrete was a mistake, so I'm wondering how to fix the situation short of digging it all out again.

I think I might just have been expecting too much from the post before I'd attached the supports.

Presumably provided there are beams holding the post in place, then the bottom of the post could have been held in place with a thick vertical rod into concrete?

Thanks.

 

[xdave]

If the the post will be cross braced from suitable points and is designed to be free of the ground except to anchor it vertically then there is nothing wrong with concreting in stand off brackets if they can take the load, but use concrete not postcrete to give it the best chance of staying fixed. It would need to be cross braced though if it is to remain a single post or it will wobble.

A stainless rod sunk in concrete and a matching hole in the base of the post would help to spread the contact area (four set in from the corners would be better), but if you want the post itself to be rock solid except for the timber itself flexing then the only real way to do it is to sink at least 1/3rd of its length underground, or increase the surface area of the section in contact with the ground by fixing it to a plate and bolting that to the ground.

Your original approach may still be ok if the cross and top bracing will be sufficient. I would expect that the post you have it probably the minimum hieght you want it so sinking it underground may not be possible without replacing it, so you would have nothing but a few hours to loose if you fitted all the bracing to see how it fairs before replacing the postcrete. If despite that the bracket still wobbles where it contacts the postcrete then just dismantle the structure and set at it with an old chisel. It will be out within minutes if it was only set a few days ago.

 

[turnpike]

Thanks. If the only option was to sink the post into concrete I wouldn't mind about getting another post, but I don't fancy digging all the postcrete out again. I used hardcore, bricks and about 4 bags..

I think I'll drill in a stainless rod as a bit of extra stability now that the bracket is a little loose. Then as you say, build the rest of the frame and see how it feels.

 

[turnpike]

xdave, just a quick update fwiw, I still wasn't happy with the general ground fixing of the post, regardless of what support it will have.

I took it out and as you say, it was quite easy to smash the postcrete back out. Frighteningly easy in fact, half of it was still powder.

I think I'll never touch it again and stick to mixing my own concrete.