Concrete made too weak - help!

[MarmiteMayhem]

Hello Just put up some fencing. What a weekend. Dug tight hole, put post in, filled with concrete, tampered down, and next day put in panels. Great - job looks pretty good. However, only after we finished, I noticed the cement hadn't gone down as much as I expected. Turns out, its unlikley the requested 6:1 mix was made - more likely

10:1. (Don't ask - but put it down to a novice mistake)

Now - fence is perfectly straight, but concrete isn't yet set (48 hours) - its very loose (as one may expect!!!!!). This MAY be down to the ground being wet and/or the amount of rain we had.

Anyway - you see where I'm coming from......

No way are we going to dig it all up again to redo - its not worth it unless the fence falls down. No way can we dig around and change the concrete.

Bit of a f**k up really.

I was wondering, is it worth making a cement/water mix, and pouring this around the base of each post? If the "concrete" is simpy badly stuck balast, surely this may help it bond? Saying this as I remeber the solid lump I ended up with after another DIY task. If so, how much cement per how much water?

As you can imagine, I don't want to tell the wife the entire weekend with mates helping was a waste due to crap concrete - I'm pleased I have a wife that "helps" :)

So - would my suggestion help? Would it be better than nothing and praying? Or is everything okay; 10:1 mix is okay; and its just due to the weather? I know my suggestion may not be normal practice - but pretty worried!

Much appreciated.

MM

[Phil L]

Just leave it as it is, even ballast is better than soil for infill, and I suspect there's /some/ cement in the mix, IME even a 20:1 mix sets, eventually, although not as fast or hard as a proper mix....that said, your

10:1 mix won't be far off normal strength if you've used ballast, ballast is around 2:1 stone to sand, meaning your mix is around 6 stone, 3 sand and one cement....maybe you've come across a patch at the top where the rain has washed the cement out of the mix before it's had chance to set.

[Guy King]

The message from [e-mail] contains these words:

It'll probably go solid eventually. More likely if it was rammed well down. Left loose and raggedy it won't be very strong no matter what you do/did with it.

As Phil L says - protect it from the rain.

[The Natural Philosopher]

If its set crunchy, the rain won't hurt..only in the first 24 hours is rain a problem as it may leach cement from the surface, leaving it powdery..

Now this is how I think cement works.

You have stones. In order to stop them moving you add sand, This should be enough to just fill the gaps between the stones. Now you add cement to fill the gaps between the sand...if you fully fill them that cement is as strong and as impervious to water as it ever can be.

Too much cement, and the cement becomes the weak point. Too little and the sand has voids in it..so the mixture gets porous friable and WEAK.

Now, in all cases of concrete posts I have gug up or pushed over, it was not the concrete that gave way. It was the soil around the concrete. Ergo my take is that the concrete itself was overly strong, and more of it and weaker, would have done just as well.

Conclusion? Just leave it to set solid. It will be more than good enough.

Ive worked with 10:1 when I ran out of cement..bricklaying..yes, its a bit crunchy and you can scrape it out with a chisel or a trowel, but it still set hard enough to do the job.

[dom]

The advice I got for setting fence posts was to get a lump of concrete on the end of a post and set as deep in the ground as I was willing to achieve. I agree that the strength of the mix matters little. At my old house (which was very wind exposed in the middle of the fens), I simply bored deep, positioned the posts, poured in dry postfix mixture, poured a bucket of water on top. 7 years later those fences are still perfectly upright. Depth and mass.

[Staffbull]

just leave em they'll be fine, I've done some in the past by digging a hole and filling round the post with dry mix (but at 6:1) and leaving it to the elements, set solid in a few weeks :-). easier than getting amicer to some places !

[Weatherlawyer]

I don't know if I'd take them up but I'd be prepared to if they do fail. What a plonker to mess that up and save how much money in the process?

But even had he put the right mix in, he shouldn't have put the panels in for a week or so.

[Phil L]

The panels go in at the same time as the posts, I've only ever known two people to do it that way, IE to get all the posts in first then try and fit the mis-shapen and downright 'miles out' panels in later, both of them only ever did one fence[1]....the procedure is dig two holes at 'about' the right spacings, get one post in, then all the panels, then insert the next post up to it/them and concrete both posts in, then dig the third hole and repeat.

[1] both took 3 times longer than budgeted for, both were an absolute mess when finished and also used up dozens of the old style carborundum disks (before D-disks)...the reason is that concrete panels are rarely the same length, and the timber ones are a law unto themselves

[Christian McArdle]

Exactly. That's how I did mine. Otherwise you're never going to get it mm perfect when you fit the panels.

Christian.