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?

[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?

[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.

[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.

[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?

[Randy Winkman]

shtu said:

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...

Thanks. Trouble is that it has to follow an s-shaped curve.

[Randy Winkman]

sospan said:

Have a look at this site. May give practical tips.....
https://castawaymoulds.com.au/how-to-guides/guide-...

Cheers.

[Randy Winkman]

T1547 said:

So as it's only 300mm long section, you might be able to just use a relatively normal concrete mix and fill the pipe (vertically) in 4-5 layers tamping each layer very well as you go to ensure compaction. It might well work, particularly if you use a piece of rebar for reinforcement and some fibres for crack control too.

But for a more interesting/geeky method...

You're essentially trying to create a highly fluid, self-compacting concrete to fill the pipe form. In commercial construction applications this is used where vibrator access is not achievable or if there is a complex form/reinforcement congestion to fill.

In precast concrete manufacturing i.e. lintels, fence posts (pouring concrete into moulds) this would be achieved through carefully controlled concrete mix design, vibrating tables/moulds and the use of superplasticisers - chemicals that make the concrete much more fluid for a given water content.

As best you can try to recreate this, I'd suggest:

10mm max aggregate would be better for the dimensions you are filling.

2:2:1 or 2:3:1 (4-10mm coarse aggregate : 0-4mm sand : cement) mix would be best. Self-compacting concretes make use of much higher fine(s) aggregate and cement content than traditional mixes.

You will want to add water to make your mix more fluid. Keep in mind more water in concrete = less strength and more shrinkage (cracking).

Aim for maximum 0.5 water : cement ratio i.e. if you're using 400kg/m3 cement, use 200kg/m3 of water.

If you can find an off the shelf concrete plasticiser from DIY store you'll need to use this to obtain the additional fluidity in the concrete mix (up to max recommended dosage on the bottle) you need rather than adding extra water. This keeps the w/c ratio low but consistency high.

Add a reinforcement bar. Glass or polypropylene fibres will help with shrinkage control too (but offer little benefit in tensile strength).

The gentle vibrating of the pipe with power tools that someone else suggested is not a bad idea and should help with getting rid of air pockets and voids in the pipe.


Of course it would be easier to just buy a precast concrete lintel etc (but not in the spirit of the challenge!) but if you really want to cast it yourself to your required dimensions and don't want to try and create the mix, use this pre-bagged highly fluid concrete https://www.uk.weber/files/gb/2018-03/04.020_Five_... - Weber Five Star Repair Concrete, it will do a far better job at filling the form than anything that could be made diy.

Probably much more information than you wanted.

Good luck!

Edited by T1547 on Monday 19th August 21:22

Good stuff. Cheers.