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casting concrete columns

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Hazelb

from United Kingdom

Joined: 21 Oct 2003
Posts: 133
Location: United Kingdom

PostPosted: Wed Aug 24, 2005 8:07 am Reply with quote

I've put a new patio in and instead of using a wooden pergola I would like to make one with cast 'concrete colums'.

This is a small scale DIY operation so I only have basic tools.

My plan so far:

concrete footings with re-bar ( to form support for column).

Use a plastic soil pipe as mould.

I'm not sure if I need some sort of grease/oil in mould? Sould I cast it in one piece or a section at a time?

What ratio of sharp sand/cement should I use? How long should I leave the mould on? Does it matter that I don't have anyting to vibrate the concrete to get rid of air bubbles.

Or should I just experiment?
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masona

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PostPosted: Wed Aug 24, 2005 9:56 am Reply with quote

Sharp sand will crumble and should use ballast mix which including difference size stones depending on the size of the column. It might be best to buy it as it will have to have metal rod frame inside for strength
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Thermo

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PostPosted: Wed Aug 24, 2005 12:30 pm Reply with quote

split the pipe in two and get some big jubilee clips to refasten it when you cast it. give it a coat of mold oil or failing that cheap cooking oil! As you pour the mix in get a long piece of batten to tamp it in with, filling a piece at a time. Once its all in position and filled, gently tap it with a stick for a while to get the air bubbles to the surface.
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Hazelb

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PostPosted: Tue Aug 30, 2005 7:23 am Reply with quote

I'm planning on casting the coloum in situ so how long should I leave the concrete to go off before removing the mould?
Will rapid setting cement powder be a good idea?

I'm casting the base and reinforcment today! Colums at the end of the week!
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Thermo

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PostPosted: Tue Aug 30, 2005 3:13 pm Reply with quote

the longer you can leave it the better, at least a few days. Not sure about quik setting stuff, not sure if it may be a bit weaker
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breezer

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PostPosted: Tue Aug 30, 2005 8:25 pm Reply with quote

it would be interesting to see progress pics, i want to see how they turn out too
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Hazelb

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PostPosted: Thu Sep 01, 2005 6:25 am Reply with quote

I'll post pics when it is done, but It will take me a few weeks. I'm still thinking of how to cast them ( in parts, or whole in situe ), and it may take a few attempts!
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TexMex

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PostPosted: Thu Sep 01, 2005 1:57 pm Reply with quote

I think your biggest problem is going to be removing the air. As MasonA has mentioned, you'll need re-inforcing rods. With these in place it'll be a b*gger to poke anything down the pipe, to tamp it as you go. Without something like a vibrating poker, I doubt you'll remove all the air. This will become apparent once you remove the plastic, you'll be presented with a finish, akin to a badly constructed sandcastle.

If you can find a method to introduce the concrete in sections, (of say 9" depth) and tamp well, before adding the next, it just may work. Seems like a lot of hard work though for just a concrete pole. Doubt I'd have the patience.

As a suggestion, How about making up a former, to render the columns afterwards in a fluted doric style. This would save you having to be so fussy about the finish of the casting.
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Hazelb

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PostPosted: Mon Sep 19, 2005 5:59 pm Reply with quote

Update 19/9/05

I cast the column 2 weeks ago.
I used a 110mm underground drainage pipe, lightly smeared with vasaline as a mould.
That bit worked fine. I used a fairly wet mix and left the mould on for 5 days.

The mould came off fine ( 1 week ago ) and the column has dried out.

Looks great but..........

It sways a bit when you gently push it! So where did I go wrong?

The column is about 2.3 m high. I used 1 piece of 10mm rebar ( should I have used 3 or 4 ?) . Was the concrete mix too wet ( did that make it weak?)

I'm planning to try again, with a larger diameter pipe. ( whats the next size up?)

It does look good though!
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Thermo

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PostPosted: Mon Sep 19, 2005 6:10 pm Reply with quote

is it set in the ground or does it just sway when its pushed while freestanding if you get my drift?
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TexMex

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PostPosted: Mon Sep 19, 2005 8:46 pm Reply with quote

For concrete to reach it's potential full strength it MUST have all the air removed. For this reason you must make it just bearly wet enough to cause the chemical reactions to start. Generally you can just about form a ball in your hand by squeezing quite hard. Any excess water will just be occupying space that should be filled with binder or agregate. Water itself, as you probably know, doesn't have any resistance to deformation.

This rather dry consistency is what leads to the problem that I described previously regarding the tamping. You may want to do some research on "super plasticisers". These allow the concrete to "flow" far easier, without introducing excess water.

I doubt that your single 10mm rebar would be enough reinforcement, but it really depends on your application.

Another idea that may work would be the use of a substantial disc of wood, with holes cut for the reinforcing rods (maybe half a dozen). and a handle attached. You could then introduce the concrete a little at a time, threadle this instrument over the rods, and poke it down into the pipe to ram the concrete down. Withdraw, add more concrete, and repeat until you have built up sufficient depth.
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Hazelb

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PostPosted: Tue Sep 20, 2005 8:00 am Reply with quote

The post seems firmly fixed at the base. It just starts to sway 1/2 way up.

The single piece of rebar I used was firmly fixed in a foundation.
As it was a single piece, it was not rigid and would sway around.

The concrete was poured quite wet, I hoped that would give me a better finish and avoid air pockets.

Has using wet concrete just made the concrete too weak? or sould the strenght come from the re-bar?
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TexMex

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PostPosted: Tue Sep 20, 2005 2:46 pm Reply with quote

Re-inforced concrete takes advantage of the two separate strengths of the materials. Concrete has a very hight compressive strength, whereas Steel has a very high Tensile strength.

When an item bends, the outside of the bend is being stretched, while the inside arc is being compressed. If you use the materials to resist these two stresses, you will reduce the bend. The reinforcing rods should, therefore, be placed along the edge that is most liable to be streched. This is why lintels have the reinforcing bars toward their bottom face.

In your application, the outside curve of the bend will depend on the direction of the force applied that produces it. Even so, the centre line of the column is midway between the streching and compression areas, and therefore, it will probably only have a minimal effect at best.

It sounds like you also have a problem with the strength of the concrete. I would expect an 8ft x 110mm concrete column to be pretty stiff, even without reinforcement. Your reinforcing rod is probably just stopping the concrete snapping altogether.

You really need to get the mix right (you are using agregate, not just sand and cement), and then remove all the air WITHOUT using excess water. I'd also suggest that you need to arrange your reinforcing bars towards the outside of the column, where they can have maximum effect.
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Hazelb

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PostPosted: Tue Sep 20, 2005 3:20 pm Reply with quote

What a good ratio of sand/cement/agg ?
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Thermo

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PostPosted: Tue Sep 20, 2005 5:09 pm Reply with quote

youd be better to make a small frame of the bars up, tied together with wire (about 4 should do it). That will tie it all together and give it the necessary strength
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