Moulding Concrete in Plastic Tubes

[Randy Winkman]

I'm making a new edge for a flower bed to hold back the soil. It needs to be strong and be set into the ground, not just sit on top of it. Previously I used sections from big, round wooden stakes (80mm diameter). A sort of heavy duty version of the edging rolls you see at DIY stores. But even they don't last long so I decided to try to replicate them by pouring concrete into plastic tubes made from two 76mm gutter pipes taped together. They need to be 300mm long. Even though I'm mixing the concrete as I have done before with 1 part cement to 5 parts sharp sand (tamped down into the mould) I cant actually get anything that cures properly, even after a week. When I open the mould the concrete never actually seems to have dried/cured fully and invariably breaks into 2 or 3 parts. It has the appearance of something that's never going to work, even if I left it 6 months. Those 2 or 3 parts aren't crumbly though and do seem quite strong. Might it be the case that concrete wrapped in plastic is never going to cure well?

[dublove]

You sure you're mixing up your 'concrete' right?

1part cement & 5parts sharp sand is just a weak mortar.
You you need to get some ballast in there to make it concrete. Just pic a 25kg bag up at a diy or building outlet.
Try 1part cement, 2parts sand & 3parts ballast.

[Randy Winkman]

dublove said:

You sure you're mixing up your 'concrete' right?

1part cement & 5parts sharp sand is just a weak mortar.
You you need to get some ballast in there to make it concrete. Just pic a 25kg bag up at a diy or building outlet.
Try 1part cement, 2parts sand & 3parts ballast.

Thanks. I assumed that because I was making in in a relatively small tube that it would be better without stones. I assume the point you are making is that the stones give it the strength?

[Equus]

dublove said:

1part cement & 5parts sharp sand is just a weak mortar.

Yup, this.

You're creating long columns of mortar. At the very least, it will crack into the sections you're experiencing due to shrinkage as it sets.

Yes, it's the stones (aggregate) that give it strength.

[victoriansmallholding]

The stones do indeed give it the strength but I'm surprised yours did not set. Perhaps your cement was out of date, there was insufficient cement or too much water. For small items cast in concrete you could just use finer pea gravel to reduce the negative appearance of the stones.

[m3jappa]

use something like 3 parts sand 2 parts 10mm shingle 1 part cement.

do it quite wet and really pack it into the tube, you could also put a bit of threaded rod or re bar in there to hold it all together.

however would take ages, must be something else you could try?

[sparkythecat]

Why not fill lengths of 75mm plastic downspout with concrete instead ?
You could leave the concrete inside the lengths of pipe and use them like that.

[Equus]

victoriansmallholding said:

...but I'm surprised yours did not set.

From the description ("those 2 or 3 parts aren't crumbly though and do seem quite strong") I suspect that the mortar is setting, but any residual water in the voids of the matrix will never find its way out if you've got a long, waterproof tube enclosing it, so it will still look damp when the mould is removed.

The other problem you will have is that it will be difficulty to consolidate any voids in the concrete. When you're casting 'proper' concrete of any depth, you use a poker vibrator (no sniggering at the back!), to settle out the voids, but this will be difficult to do, in a long thin tube. Suppose you could try pouring gradually, while shaking/tapping the tube as you go.

Even with aggregate, concrete is relatively weak in bending and tension, though, so it'd never going to be that strong unless you put rebar in it (which is why concrete fence posts have steel reinforcement in them, of course - the steel takes the tension).

[Randy Winkman]

Thanks everyone. I think that the next step is to mix in some stones to give it lateral (?) strength so (hopefully) it doesn't just break into sections. The reason I didn't do this at the start is because a lot of the stones in ballast can be quite big and seemed out of proportion to the tube diameter. I think I'll just chuck out the bigger stones. Cheers again.

[paulrockliffe]

The other way to do it is to make a 'float' to the profile you want and then use that to cast the concrete to shape insitu, rather than using a mould. See Pinterest for details.

[Aluminati]

Buy bag of post mix, tip in tube, add water, tamp with stick. Like really tamp.

Works a treat.

[magooagain]

The mix you are using is too weak. Make a 3 part ballast to 1 part cement mix. Also put steel rod in.

[scottydoesntknow]

magooagain said:

The mix you are using is too weak. Make a 3 part ballast to 1 part cement mix. Also put steel rod in.

5 and 1 is plenty strong enough for concrete. As above, use ballast not sharp sand and vibrate. Tap the side of the tube with a rubber mallet maybe, poke with a stick, run an orbital sander over it... A length of rebar will help also. You're just making round fence posts after all and they hold up just fine.

[Randy Winkman]

paulrockliffe said:

The other way to do it is to make a 'float' to the profile you want and then use that to cast the concrete to shape insitu, rather than using a mould. See Pinterest for details.

That's an interesting idea. I'll check that out.

But my next move will probably be to use ballast and if that doesn't work put a steel rod in each one. As I said, they don't crumble, only break into 2 or 3 sections and they clearly wont be able to do that with a rod in them.

[magooagain]

scottydoesntknow said:

5 and 1 is plenty strong enough for concrete. As above, use ballast not sharp sand and vibrate. Tap the side of the tube with a rubber mallet maybe, poke with a stick, run an orbital sander over it... A length of rebar will help also. You're just making round fence posts after all and they hold up just fine.

You will never see any sort of cast concrete work at 5 to 1 mix.

[DozyGit]

Correct concrete mix, add rebar or fibres to give strength in tension. Alternatively get a concrete lintel and use.

[Randy Winkman]

magooagain said:

scottydoesntknow said:

5 and 1 is plenty strong enough for concrete. As above, use ballast not sharp sand and vibrate. Tap the side of the tube with a rubber mallet maybe, poke with a stick, run an orbital sander over it... A length of rebar will help also. You're just making round fence posts after all and they hold up just fine.

You will never see any sort of cast concrete work at 5 to 1 mix.

What should it be?

[R_von_S]

Equus said:

The other problem you will have is that it will be difficulty to consolidate any voids in the concrete. When you're casting 'proper' concrete of any depth, you use a poker vibrator (no sniggering at the back!), to settle out the voids, but this will be difficult to do, in a long thin tube. Suppose you could try pouring gradually, while shaking/tapping the tube as you go.

You can apply the vibrations from the outside on something of that size, it'll be just as effective. fnarr...

More seriously, just tap repeatedly on the outside of the form, at this scale you're just trying to encourage some of the (relatively) free fluid to flow into all of the gaps to improve the surface finish.

[Equus]

R_von_S said:

Equus said:

The other problem you will have is that it will be difficulty to consolidate any voids in the concrete. When you're casting 'proper' concrete of any depth, you use a poker vibrator (no sniggering at the back!), to settle out the voids, but this will be difficult to do, in a long thin tube. Suppose you could try pouring gradually, while shaking/tapping the tube as you go.

You can apply the vibrations from the outside on something of that size, it'll be just as effective. fnarr...

More seriously, just tap repeatedly on the outside of the form, at this scale you're just trying to encourage some of the (relatively) free fluid to flow into all of the gaps to improve the surface finish.

:cough:

[shtu]

I'd be going to the local merchant and seeing what paving edging they have. Probably less than a fiver each and no troubles with finish, strength, appearance, etc.

eg https://www.jewson.co.uk/landscaping/garden-paving...