Thread: Concrete formwork

I saw where someone had made their columns from corrugated iron. A box was made for the base and top, shaped to suit. If you could make a sample of what was required, then make a split mold from it out of plaster of paris.
Kryn

Thanks Kryn but I would not try to use a plaster cast eight times on a 2.4 metre column as I doubt it would work. I appreciate the comment and had already thought along those lines. I think it , the mould, would have to be made of fiberglass or something similar. I am thinking of mini orb to curl around in an 180 mm column.
i will make the base and top separate from the column. I wonder if a plaster cast would last 16 times and how to make a nice pattern. I do not want a simple pattern. Ideally there would be leaves and more.
i am also keen to explore ideas for the roof beams, grape vines and such.
appreciate all ideas.
thanks Kryn.
Richard

Richard

If you grab hold of an old carpenters book such as "The Australian Carpenter and Joiner" you may get some ideas there. In my copy of that publication there is a section on Boxing for a square column and I think you would have to make a modified version of that.

Pouring concrete into a vertical form is problematical.

1. There are tremendous pressures at the base if the column is a significant height and the structure has to be very strong, but with threaded rod it can be used many times.

2. You will need to prevent the concrete separating if it is poured from a height (2.4m ?). Baffles

3. You will need to be able to vibrate the mould to achieve a smooth finish on the column. The mould has to also be able to withstand that.

4. I would suggest a square form with packing to take something like a mini orb profile. The mini orb can be treated with a bond breaker to enable re-using and easy dismantling. It would have to be in two halves, but the corrugation lends easy mating of the halves.

5. Use a fine aggregate.

6. Reinforced cage (deformed bar) will be essential.

7. Cast the top and bottom separately.

8. It is not going to be an easy process.

Good luck

Regards
Paul

mostly

what you want to do is doable, but problematic for the unwary/inexperienced

getting a good smooth finish (no holes, no exposed agregate) off the form is dependent upon your concrete mix, placement method, vibration, curing
you are probably looking at readymix, a concrete pump and 8 moulds -- not a cheap exercise
mixing yourself? well, you'll need around 4 portable mixer loads (2 cu.ft per load) per column and getting an invisible join between the mixer or barrow loads could be challenging

the finished colums will weight in the order of 200+ kg -- which suggests a crane to move them into position


maybe, shipping them in from a precaster will not be that expensive in comparision

If you do make them, they would be far and away stronger load bearing, if they were "pre stressed", which means the reo bar reinforcement is put under longitudinal tension (like wire strainers) before the concrete is poured and tension not released until after the concrete is fully cured.

So the mould in addition to everything already described - should be devised such that its possible to add substantial tension to the reo bar by use of a straining mechanism that's secured to something immovable (buried in the ground) and capable of considerable tension (hydraulic jacking system or block and tackle for eg).

The weight bearing capacity of the column pre-stressed is far higher than un-stressed if that makes any sense.
Just to add to your woes!
Not saying it can't be done coz it can, just that it takes a well thought out & repeatable design is all.

Richard

Another thought for a re-useable mould is to obtain a large diameter steel pipe (about 200mm) that the mini orb can be placed inside. The pipe would need to be cut in half lengthways and then provision made to bolt it together. You might be able to pick something up from a scrap metal merchant if you decided to go down that track.

Regards
Paul

Check out building demolition material recyclers, as they may have some of the old concrete moulds lying around the place. In FNQ we used to hand mix and hand pour concrete columns in situ for highset houses using both timber and steel moulds into the 1970's. Most of those mould were nothing more than a square cross section with arised corners formed with a quad, tri-quad or ovolo section. Italian builders were quite adventurous with their sections.

I have calculated the weight of a column 150 mm x 2.4 m as being 90 kg and using .04m3 of concrete. This does seem low. I have a concrete vibrator but will just tap the formwork often. I will not worry about prestressing the reo as it is a garden ornament/feature so the structural integrety is not vital, if it cracks the reo will hold it together. I have bought a plastic mould from USA for $25 + $50 shipping. This features leaves and will be used for the top and bottom pads. I hope it lasts 12-16 times. maybe I could make a mold of a finished form...
The columns I think I will use 150 mm PVC sliced up one side. I don't think miniorb will work. Perhaps I will just have them plain.
I might make concrete beams to go from post to post to hold the wisteria but timber treated would be lighter and easier.
Thanks everyone and please post any more suggestions/ideas. I will not be doing the job until summer really sets in.

Originally Posted by Bushmiller

Richard



2. You will need to prevent the concrete separating if it is poured from a height (2.4m ?). Baffles

l

Hi Paul, I imagine you are referring to the cement separating, That is why I am concerned about using the mechanical vibrator. It did not occur to me that dropping the concrete in from a height would have the same effect but I think you might be right. Maybe do it on an angle and pouring the concrete down the sides would alleviate it.

I found a brass fireplace surround and used it for for work on a retaining area last week.image.jpg

Originally Posted by Timless Timber

If you do make them, they would be far and away stronger load bearing, if they were "pre stressed", which means the reo bar reinforcement is put under longitudinal tension (like wire strainers) before the concrete is poured and tension not released until after the concrete is fully cured.

So the mould in addition to everything already described - should be devised such that its possible to add substantial tension to the reo bar by use of a straining mechanism that's secured to something immovable (buried in the ground) and capable of considerable tension (hydraulic jacking system or block and tackle for eg).

The weight bearing capacity of the column pre-stressed is far higher than un-stressed if that makes any sense.
Just to add to your woes!
Not saying it can't be done coz it can, just that it takes a well thought out & repeatable design is all.

this is not one for the faint hearted or the "she'll be right" brigade.
things to keep in mind if you go down that path
Pre-stressing needs a very strong mould (usually one made of steel) to push back against the force pulling the stressing wire
pre-stressing can easily induce a bow in the column
you don't want to be anywhere near a stressing wire if one lets go
Concrete properties for pre-stressed product tend to be rather tight in terms of mix ratios
pre-stressing can be an "inexact" science

Originally Posted by rick_rine

I have calculated the weight of a column 150 mm x 2.4 m as being 90 kg and using .04m3 of concrete. This does seem low. I have a concrete vibrator but will just tap the formwork often. I will not worry about prestressing the reo as it is a garden ornament/feature so the structural integrety is not vital, if it cracks the reo will hold it together. I have bought a plastic mould from USA for $25 + $50 shipping. This features leaves and will be used for the top and bottom pads. I hope it lasts 12-16 times. maybe I could make a mold of a finished form...
The columns I think I will use 150 mm PVC sliced up one side. I don't think miniorb will work.

I'm calculating just over 100kg per column.
the column will need to be well braced so it doesn't fall over
you will need a platform of sorts to stand on while you fill each column with concrete
also I think 150mm might look too slender for a "grecian" style column
to avoid spalling you want to make sure you have about 50mm cover to the reo -- which in a 150mm column translates to a something like a single bar up the middle

IMO round steel or timber posts are starting to look like a very viable option

Originally Posted by rick_rine

Hi Paul, I imagine you are referring to the cement separating, That is why I am concerned about using the mechanical vibrator. It did not occur to me that dropping the concrete in from a height would have the same effect but I think you might be right. Maybe do it on an angle and pouring the concrete down the sides would alleviate it.

Yes concrete will separate if just dropped into a mould.
tipping the mould to 45° doesn't aid much to avoid separation and your bracing now needs to support the entire length of the post.

to avoid unsightly voids, air bubbles and the like vibration should be vigorous and in the concrete itself

Thanks Ian for your comprehensive posts. It is clear you have a great deal of knowledge on the subject. I am now thinking of a 6 stage column, and yes treated timber would be easier and cheaper, but that is not what I want. So I will have a foundation, probably 4 bar trench mesh, 8-9 metres long. A square column base easily formed by myself with reliefs made on a scroll saw. This would be made do that black ply for form work. This would be 1.2 metres high. Then there might be the relief form from American 200 mm high and a custom orb column 1-1.2m high then another American bought ($50 for postage) form on top. Then I the treated pine or concrete frame for the top. It will all be tied together. These columns will weigh about 160 kg.
I appreciate your comment about the look of a thin column. It is something for me to consider but if it is only 1 or 1.2 m over a square pillar it might be okay. I must draw it up but I am only getting ideas together. My original post was really hoping to find retailers who sell for work but I am over that now. I think the ideas are coming together and I thank you all at WWF.
ps. Where in Sydney are you. I grew up in Hornsby (Mt Colah)

Originally Posted by rick_rine

ps. Where in Sydney are you. I grew up in Hornsby (Mt Colah)

almost in the Harbour, I once joked that I was eying off the Opera House as a shed

Hi Richard,

I used to do a lot of commercial formwork and often used farmatube for round columns. It comes in diameter 150mm+. Very easy and fast to use, all you have to do is secure the bottom and top making sure its plumb and pour the wet stuff in. Give it a tickle with the vibe or formies vibrator (slap it with your hammer a few times) and strip the tube off the next day . You could probably attach your reliefs after. Only thing is you can't reuse the tube.

Good Luck

Damo