Concrete formwork

[rick_rine]

I want to make a pergola. I want 8 concrete columns, Greek style. I live in Tassie so shipping of precast columns is out of the question. I have Googled and tried to find form work to make my own without any luck. Does anyone have any ideas, where to buy, how to make, web sites to visit?
Thanks
Richard

[KBs PensNmore]

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

[rick_rine]

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

[Bushmiller]

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

[ian]

mostly :whs:

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

[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! :wink:
Not saying it can't be done coz it can, just that it takes a well thought out & repeatable design is all.:2tsup:

[Bushmiller]

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

[Mobyturns]

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.

[rick_rine]

I have calculated (http://www.calculator.net/concrete-calculator.html?slablength=5&slablengthunit=foot&slabwidth=2.5&slabwidthunit=foot&slabthick=5&slabthickunit=inch&slabquantity=1&holediameter=2.5&holediameterunit=foot&holedepth=6&holedepthunit=foot&holequantity=1&tubediameter=15&tubediameterunit=centimeter&tubeindiameter=1&tubeindiameterunit=centimeter&tubedepth=240&tubedepthunit=centimeter&tubequantity=1&circcal=Calculate) 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.

[rick_rine]

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.

[rick_rine]

I found a brass fireplace surround and used it for for work on a retaining area last week.288729

[ian]

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! :wink:
Not saying it can't be done coz it can, just that it takes a well thought out & repeatable design is all.:2tsup: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


I have calculated (http://www.calculator.net/concrete-calculator.html?slablength=5&slablengthunit=foot&slabwidth=2.5&slabwidthunit=foot&slabthick=5&slabthickunit=inch&slabquantity=1&holediameter=2.5&holediameterunit=foot&holedepth=6&holedepthunit=foot&holequantity=1&tubediameter=15&tubediameterunit=centimeter&tubeindiameter=1&tubeindiameterunit=centimeter&tubedepth=240&tubedepthunit=centimeter&tubequantity=1&circcal=Calculate) 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


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

[rick_rine]

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)

[ian]

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

[DamianD]

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 :2tsup:. You could probably attach your reliefs after. Only thing is you can't reuse the tube.

Good Luck

Damo

[rustynail]

Split PVC pipe makes a good form, provided a good release agent is used. Whale oil is one of the best, but good luck with getting that. When making columns, we rip the pipe in half then use stainless steel split clamps to hold the two halves together. The closer the clamps are together the better the joint. The clamps with mounting feet are handy as they can be screwed to an available vertical surface, keeping the column upright during the pour.To overcome the problem of concrete separation a length of gal pipe down the centre of the form helps, provided the drop is not too great. The length you are talking would be fine. The steel pipe also gives some reinforcing to the finished column. It also gives a fixing point at both top and bottom of the finished column. Keeping agrigat size small also helps prevent separation, along with aiding a better surface finish. We have found external vibration to be more effective than internal on thin columns. It is most important to commence vibration from the start of the pour at the bottom of the column to help release trapped air and continue, up the column, right to the completion of the pour.By delaying vibration air pockets will develop and be unable to rise through the above layer of concrete.
Hope this helps and good luck.

[rick_rine]

289645 Thanks Damian and Rustynails, I greatly appreciate your advice.
I am pretty excited. So far I have about 8 bases made and that is a learning experience. I now have 4" tubes inside to hollow them out. This, along with spray and cook, means they fall out of the mould after 24 hours curing. I was concerned about the 6" columns but have seen some on the local pub veranda and they don't look too small.
I am going to practice by firstly making a base for a garden ornament ( see photo) and then making a small pergola of the same di mentions but just one stage, in another part of the garden. This will be 2x2x2.3 high. The final one wil be 5 stages of 2x2 to make it 10 metres long by 2 metres wide.
I am now using the vibrator on the moulds. It won't reach down into the columns though so I will just tap them. The height of tghe columns alone will be about 1.9 metres. This will make the pergola about 2.4 m high. I don't want to go throught the trouble and expence of gal pipes. I can see the advantage. My plan is to use 10mm deformed bar tied into the 3 bar trenchmesh and this will protrude up 2.2m. just below the top of the column pvc formwork. When pouring I will concrete in gal strapping to hold on the 195mm treated pine beams. Perhaps the deformed bar will minimise the cement separation?
Thanks again every one. This post had me thinking about the concreting I have done over the years. I estimate that I have poured 60 cubic metres by truck and hand mixed the same amount! I must have some Italian in my blood. smile.

[Bushmiller]

Thanks Damian and Rustynails, I greatly appreciate your advice.
I am pretty excited. So far I have about 8 bases made and that is a learning experience. I now have 4" tubes inside to hollow them out. This, along with spray and cook, means they fall out of the mould after 24 hours curing. I was concerned about the 6" columns but have seen some on the local pub veranda and they don't look too small.
I am going to practice by firstly making a base for a garden ornament ( see photo) and then making a small pergola of the same di mentions but just one stage, in another part of the garden. This will be 2x2x2.3 high. The final one wil be 5 stages of 2x2 to make it 10 metres long by 2 metres wide.
I am now using the vibrator on the moulds. It won't reach down into the columns though so I will just tap them.

Rick

A quite passable concrete vibrator to be used externally is the cheapest, nastyist orbital sand you can find. We used such as this very successfully on pebble mozaic pavers like these

289643

For those the sander was screwed to a base and turned on for three or four seconds. I think just holding it to the pipework mould would suffice. Cheapest sander you can find. I think we used an XU-1 brand. It's vibration you want :wink: .

Regards
Paul

[rick_rine]

289784I have 3 bar 11 mm trench mesh in my test project. I will use 12 mm deformed bar going up inside a 150 mm PVC tube. The starter bar will be tied into the trenchmesh. I would appreciate some ideas on how to keep the 12 mm bar in the centre of the PVC. The PVC tube will be about 1.9 m high.
the foundation formwork allows for footings 0.32 m wide and about 0.2m deep. Under the columns footings are about .3 x .3 m x .3m.

[rustynail]

Rick,
We found deformed bar in thin columns was a real problem. Smooth bar or pipe has far less "shattering" as the concrete is poured. The distance between the form and the bar is only 120mm or so, which means the bar has a pronounced separating effect on the mix as it goes down. Also, black steel protruding from the ends of the columns is a rust starter.
The method we use to keep the pipe in the centre of the column is a base board with a hole for the pipe and a routed groove circle for the form tube. Another options would be blocks holding the form tube central on the board.At the top, the pipe is held with a legged ring clamp which attaches to the same vertical surface as the ring clamps holding the form tube.

[rick_rine]

Thanks Rustynails. Today I poured the first column, I was happy with the way it went but I won't know how good it is until I pull off the PVC in a day or two. I had already bought deformed bar so tried to minimise its affect by inserting a 50 mm PVC conduit over it and tried to pour the concrete down that. It did not work that well. I forgot to get rid of the air bubbles often and had poured in one mix, about point eight metres height, before I remembered.
the reason for this post is my surprise at how the concrete shrunk quickly, I assume because of the water content. I had levelled it off at the top of the PVC tube but thirty minutes later it was down by thirty mm. I am glad I am making a practice project.

[ian]

Today I poured the first column, I was happy with the way it went but I won't know how good it is until I pull off the PVC in a day or two.
snip
the reason for this post is my surprise at how the concrete shrunk quickly, I assume because of the water content. I had levelled it off at the top of the PVC tube but thirty minutes later it was down by thirty mm. I am glad I am making a practice project.I think it more likely that what caused the settlement was either air working it's way out of the form, or free water being absorbed back into the mix.


out of interest how "sloppy" was your mix

[rick_rine]

Hi Ian. The gravel I am using is finer than standard and is very hard to get the mix right. It is as if the mix is a bit dry and you add one cup of water and it goes sloppy. The mixes have been wetter than I would prefer but I think it is going to work. I had a sneak-peek at yesterdays column, peeled it back a tad, and it looks okay so far. I did another column today and it also sunk by 30-40 mm. Not surprising when I think about it, it's only 2% of the length, but it was something I did not foresee. Using the 50 mm PVC has I think aided in the removal of air bubbles as when it is pulled out I jiggle it to settle the concrete. I have not been too aware of cement coming to the top excessively.

[ian]

Rick

from this distance it's a little difficult to suggest much in the way of mix design

Given the small diameter of the columns, a 10mm coarse aggregate is probably where you should be looking -- and a rounded rather than angular stone.
Then for fine aggregate you probably need some 5mm and sub 2mm sand.
Cement content (for the fines) should probably be up around 20% of the total mix (including the water), unless you can get some graded fly ash in which case you could swap about 1/3 of the cement for fly ash.

I'd start with around no more than 450mm of water for every 1 litre of cement -- maybe start with as little as 400ml of water and add addditional water at the rate of 50mm per litre of cement -- I think you should be aiming for a water:cement ratio of about 50%. Maybe a bit more, maybe a bit less.

You will want a plastic rather than a sloppy mix.

Also, I suggest that rather than climbing a ladder to cast trial columns, you should cast some trial blocks. These could be stripped 36 hours after pouring to get a look at how they turned out.

Keep notes of the mixes you're using to find the one that will work best for your project


On reflection, the concrete dropping down from the top of the form is most likely due to settlement, which suggests uneven vibration -- some areas within the column are more dense than others

[rustynail]

Thanks Rustynails. Today I poured the first column, I was happy with the way it went but I won't know how good it is until I pull off the PVC in a day or two. I had already bought deformed bar so tried to minimise its affect by inserting a 50 mm PVC conduit over it and tried to pour the concrete down that. It did not work that well. I forgot to get rid of the air bubbles often and had poured in one mix, about point eight metres height, before I remembered.
the reason for this post is my surprise at how the concrete shrunk quickly, I assume because of the water content. I had levelled it off at the top of the PVC tube but thirty minutes later it was down by thirty mm. I am glad I am making a practice project.
Rick,
A 30mm slump does not surprise me at all, particularly if the mix is very wet. Slump is a percentage of height so you would be looking at only about one percent. Just top it up while the concrete is still wet.
It would seem you are working alone. Thats unfortunate as continuous vibration during the pour is critical to achieve consistent results.
I am a little concerned that the conduit over the reo bar may cause more problems than it solves. It will restrict the column diameter too much for easy concrete flow. Also, when the conduit is drawn out, it will allow air right to the bottom of the column and then all the way to the top as it is drawn out, with only fall-in of the concrete to fill the void. May be better to do away with the conduit and lube the bar instead.
When using fine agrigate, the amount of water in the mix requires far more care. For consistency, it would be better to use containers for each of your ingredients ie buckets or drums.
Dont leave the PVC formwork on too long as you may get sticking, particularly towards the top, due to the release agent being rubbed off during the pour.
Once stripped, column must be kept wet 7days to prevent surface cracking.
Your form work looks good. Show us a pic of the stripped column, no matter how it turns out, so we can make any recommendations if necessary.

[rick_rine]

Thanks Rustynail and Ian.I pulled the formwork off the first column today. I am quite happy with the result. The attached photos make it look much worse than it is. The mix was 10 sand/gravel to 2 cement. I did not then measure the water. The sand/gravel is quit fine with the gravel being no larger than 5 mm. It is not sharp but it is not round and smooth either. The bottom of the column, where I forgot to tap the formwork, is actually very good. The top of the column is not as smooth and the middle has the most air bubbles. I can see what you are saying rusty about the PVC. I agree with you but think next I will try much smaller PVC, 20 mm. I think it might help with the jiggling action. In the main project I will use smooth round bar as suggested and not use any PVC inside. Ian I don't really have much choice in aggregate mixes and to be honest, if this column is strong enough, I am quite happy with the result so far. My main regret is that I had the clamps 280mm apart. It is clear that at least down the bottom they need to be no more than 140 mm apart.( I ordered a dozen from blackwoods 10 days ago but they still have not arrived :~ ) This is shown in the 4th photo where there is the gap between the clamps. Where the clamps were there is no problem. I ran out of whale oil last week so until my next trip with the Japanese I am using vegetable oil. It worked well. It seems strange as it must get wiped off as the concrete is poured down the tube. The columns are 150 mm in diameter. Being curious I gave it a very, very gentle push. It did wobble a tad. I was hoping to build the pergola with a flat top. This however would give no diagonal bracing. I am a bit concerned that the structure, the main one, might not be strong enough.291343291344291345291346291347

[Bushmiller]

RR

I thought your concrete mix might be a little light on with cement and Ian has mentioned that aspect is important. My take on concrete (but I am no expert) is that the finer your aggregate the higher content of cement you require. The reasoning behind this is that in an ideal world every particle of aggregate needs to be completely coated with cement. As fine aggregate occupying a given volume had a larger surface area than a coarse aggregate it becomes clear that you require more cement.

I would be reluctant to say exactly what the cement component should be and others may be able to advise. If not a call to the Cement and Concrete Association may be of help. However my memory is that they man the phones on a volunteer basis and may only be there during mornings for example.

The project looks as though it has potential. Good for you for trying.

Regards
Paul

[rick_rine]

RR

I thought your concrete mix might be a little light on with cement and Ian has mentioned that aspect is important. My take on concrete (but I am no expert) is that the finer your aggregate the higher content of cement you require. The reasoning behind this is that in an ideal world every particle of aggregate needs to be completely coated with cement. As fine aggregate occupying a given volume had a larger surface area than a coarse aggregate it becomes clear that you require more cement.

Thanks Paul. That makes sense.

Regards
Richard

[ian]

The mix was 10 sand/gravel to 2 cement. I did not then measure the water.
The sand/gravel is quit fine with the gravel being no larger than 5 mm. It is not sharp but it is not round and smooth either. The bottom of the column, where I forgot to tap the formwork, is actually very good. The top of the column is not as smooth and the middle has the most air bubbles. I can see what you are saying rusty about the PVC. I agree with you but think next I will try much smaller PVC, 20 mm. I think it might help with the jiggling action. In the main project I will use smooth round bar as suggested and not use any PVC inside. Ian I don't really have much choice in aggregate mixes and to be honest, if this column is strong enough, I am quite happy with the result so far. Hi Rick

as alluded to by Paul, you need enough fines in the mix to completely fill the voids between the aggregate particles.
In your case filling the voids is less about strength and mainly about the surface appearance -- the right mix design and enough vibration will give you a nice smooth column without any surface voids.

[rick_rine]

[QUOTE=
Cement content (for the fines) should probably be up around 20% of the total mix (including the water), unless you can get some graded fly ash in which case you could swap about 1/3 of the cement for fly ash.

I'd start with around no more than 450mm of water for every 1 litre of cement -- maybe start with as little as 400ml of water and add addditional water at the rate of 50mm per litre of cement -- I think you should be aiming for a water:cement ratio of about 50%. Maybe a bit more, maybe a bit less.

You will want a plastic rather than a sloppy mix.


Keep notes of the mixes you're using to find the one that will work best for your project


s[/QUOTE]
thanks Ian, today's column was measured accurately. I used 2 water, 3 cement and 12 aggregate. I could not use less water. So although it is what I would call a 4:1 mix in fact the cement is not 20% of the total mix. It is in fact only 17.6% if I understand you properly. If I make 2:3:10 that would satisfy the requirement and that is what I will do on the fourth and final trial post. The water to cement ratio is much lower than you suggest. Do you think this matters? I would think the less water the better. The posts do look a bit on the thin side. At least 200 mm would be better but I could not find any fancy formwork on line except the one I bought. Perhaps I should have tried harder but it will be okay. On the main project I am thinking of making the distance between the post 1.5 m x 2 m wide.
Rustynail, I used smaller PVC today but I will take your advice for the final post and lubricate the bar instead.
I think for the main project I will put in three bars of reinforcing, one 12 mm and two 6 mm. 291470
black woods have still not got the clamps so I modified the arrangement and I believe it will be better.291472291473

To to minimise the flex in the posts if I am too concerned I could go back to the idea of having the posts sit on a square pillar, say .4 x .4 x 1 metre high. Then the columns would only be 1 metre high.
just to clarify, although I am calling this a test project it will still be a pergola, just smaller and in a corner of the garden that cannot be seen from the house.291475 It is behind the trees. The main one 10 m x 2 m will be in front of the dam, easily seen from the house.

[rustynail]

If you are short of clamps, just use a doubled piece of fencing wire in the form of a bushmans twitch. If you put a clamp every 400mm and then a twitch in between each clamp that should be plenty, I trust you know what I mean by a bushmans twitch.
I dont think there will be much advantage in putting in extra reo. It will make it hard to get the concrete down. The rods would require ties to stop them bellying in or out during the pour. These ties would then load up with concrete in such a small area.
A bit of bicol in the water may help eliminate those small surface voids. Burt most importantly, plenty of external vibration,from the bottom up, during the pour.
I'd up the cement content too.

[ian]

Hi Rick

thinking back on the big pours we did in the past, the mix had around 330kg pf cement and 150kg of fly ash per cubic metre, and a water:cement ratio of under 0.45 = less than 150 litres per cubic metre. For some pours we dropped the water cement to 0.38, but the dry mixes were real bastards to place.
these were road pavement slabs and if the mix was too wet the concrete would run down the cross-fall. we needed the high fines content to get a durable and void free surface

for your mix you need plasticity so it stays together as it drops down the form,
lots of fines to avoid surface voids
and lots and lots of vibration to get the air out


if the form is a tending to wobble or bulge, stiffen it with a timber battern on one or two sides

[rustynail]

Rick, you may be making life hard for yourself with such thin columns. If you increased the size to 200 or 300mm it would give more pressure against the form, which would assist with air escape during vibration.

[rick_rine]

Hi Rustynail. The columns are looking pretty good. I would have liked to make them thicker but I could not find suitable moulds. To anyone reading this post, if you know of any please tell me. This is actually why I started this thread. I would prefer they were 200-250 mm .
Having said that, the project is looking as good as I could have hoped for. I am very pleased thanks everyone. The final column was poured today with the help of my eight year old son who tapped and tapped with me. I had been wary of tapping too much as I was concerned about the cement separating. I am not so concerned now as all the WWF advice has been to tap, tap and tap, also I have heaps of cement in the mix and I have not experienced the cement seperating in this job so far. The mix has been increased to 1:2:6 water, cement, aggregate. This, to me, is a 1:3 mix. Twice the strength of my normal, general purpose, concreting. The posts are very flexible and I am slightly concerned about this. I might need to put diagonal bracing on the structure, but it is just to hold up a vine.
I think the posts are too high and with the major project I think I will lower it by 150 mm.

[ian]

Hi Rick

I won't say "looking good" till I see the surface finish on your last post :)

re flexibility, I presume you mean the posts are prone to wobble on their bases.
This is to be expected until you tie the tops together to create a portal frame. For safety -- no one wants to be hit on the sconce by a concrete post -- you should continue to brace each post till the portal beams go in and are connected

[rick_rine]

Thanks Ian. No the posts are secured to the bases with the reinforcing rod coming through from the trench-mesh and there is no wobble on the base The concrete post actually bends slightly as it is pushed. I have seen this before on a 6" thick suspended slab. A builder jumped up and down on the fully cured slab that was cantilevered and unsupported where he stood and it bounced, or flexed, quite a lot.

[rustynail]

Rick,
Pipe rather than bar will give you more rigidity. If you are concerned about having a void down the middle of the post with pipe, just fill it with concrete at the end of the pour.

[Bushmiller]

Thanks Ian. No the posts are secured to the bases with the reinforcing rod coming through from the trench-mesh and there is no wobble on the base The concrete post actually bends slightly as it is pushed. I have seen this before on a 6" thick suspended slab. A builder jumped up and down on the fully cured slab that was cantilevered and unsupported where he stood and it bounced, or flexed, quite a lot.

RR

I think you have just discovered the reason for reinforcing in concrete.

Concrete has little tensile strength (but extremely good compressive strength). The reinforcing is there to make up the deficiency in tensile strength.

In a column there is also the problem of resistence to sideways force. Fortunately the main force will be compression and probably not very much of that either; Just the roof.

I agree with Rustynail that you would make life easier for yourself by increasing the diameter of the column.

Regards
Paul

[ian]

The final column was poured today with the help of my eight year old son who tapped and tapped with me. I had been wary of tapping too much as I was concerned about the cement separating. I am not so concerned now as all the WWF advice has been to tap, tap and tap, also I have heaps of cement in the mix and I have not experienced the cement seperating in this job so far. The mix has been increased to 1:2:6 water, cement, aggregate. This, to me, is a 1:3 mix. Twice the strength of my normal, general purpose, concreting. The posts are very flexible and I am slightly concerned about this. I might need to put diagonal bracing on the structure, but it is just to hold up a vine.
I think the posts are too high and with the major project I think I will lower it by 150 mm.


re flexibility, I presume you mean the posts are prone to wobble on their bases.
This is to be expected until you tie the tops together to create a portal frame. For safety -- no one wants to be hit on the sconce by a concrete post -- you should continue to brace each post till the portal beams go in and are connected


Thanks Ian. No the posts are secured to the bases with the reinforcing rod coming through from the trench-mesh and there is no wobble on the base The concrete post actually bends slightly as it is pushed. I have seen this before on a 6" thick suspended slab. A builder jumped up and down on the fully cured slab that was cantilevered and unsupported where he stood and it bounced, or flexed, quite a lot.
Hi Rick

from this distance, I think the wobble has two sources
1) the concrete is still very green -- it will probably take another 2 weeks to develop near full strength
2) the mix used for the first few columns is too weak. From recollection the tensile strength of concrete is less than 1/10th the compressive strength. I think you said that your first few mixes were 1 cement to 5 gravel/sand and pretty wet. At a guess this would give a compressive strength in the range of 10-15 MPa, with a corresponding tensile strength possibly <1 MPa. The mix with the much higher cement content probably has a tensile strength around 3.5 MPa. I can't tell from nearly 1000km away, but I fear that your first few columns may not be durable.

I think the higher cement content mix will fix the wobble -- you shouldn't need additional reinforcing.
However, I'd keep at least one decent size bar down the centre to act as a tie down rod for the column and eventual pediment

[rick_rine]

I have finished. The columns do look pretty good. I am very happy and thanks for all the advice. I will make some changes when I make the big project.

[ian]

hi Rick

you can't leave it like that

you have to tell us how the last column compars to the first -- surface voids, wobble, etc

[rick_rine]

Hi Ian. The last post was tapped a great deal, almost constantly. I did not see much difference. It still had many air bubbles. It was the strongest and did not wobble as it was a 1:3 mix. The one with the least air bubbles was the first post which was made of wet mixes, ones that I was unhappy with as I thought the mixes too sloppy.
If I build the big project I will 1) make mixes 1:3 2) make the mixes a bit wet. 3) mount the posts on square pillars about 900 mm high to reduce wobble. 4) make the posts 1500 mm apart. 5) tap constantly.

[ian]

Hi Ian. The last post was tapped a great deal, almost constantly. I did not see much difference. It still had many air bubbles. It was the strongest and did not wobble as it was a 1:3 mix. The one with the least air bubbles was the first post which was made of wet mixes, ones that I was unhappy with as I thought the mixes too sloppy.
If I build the big project I will 1) make mixes 1:3 2) make the mixes a bit wet. 3) mount the posts on square pillars about 900 mm high to reduce wobble. 4) make the posts 1500 mm apart. 5) tap constantly.
Hi Rick

diagnosing from a distance is always tricky ...

1) It looks like a 1:3 mix is the choice in respect to eliminating "wobble"

2) given your vibration technique -- external taping of the form -- a wet mix is preferred to a dry one

3) there's no reason to have short columns -- I rather like your slender trial columns -- the wobble is mostly due to a mix with too little strength

4) I can't comment on the column spacing -- with the right strength concrete this will mostly be an issue of aesthetics

[rick_rine]

293893 Put points on the beams. I had half a dozen cans of interior paints in the shed with a bit left in them. Mixed them all together to make four litres and sprayed it today. Looks great. Yes I know it is interior and inferior paint. I'm not worried. The vine will cover it.
I am very proud and thanks for your help and support people.

[rick_rine]

319216319217

[rick_rine]

Having trouble uploading photos but I thought I would share my new test pergola. This one has an 800 mm high square base which will reduce the sway in the posts. Also the posts are closer at 1.5 m centres. I reverted to standard gravel and sand mix 3:1 gravel:cement. I used a mechanical vibrator in the posts and in the base. One thing I overlooked was to tie down the base. What happened to the first one is that the concrete lifted the formwork up off the foundation. This oversight was easily fixed with the remaining posts. There is a photo of the first pergola as it is now and the work in progress on the new one. There are more air pockets in these posts, probably because, as I said, I reverted back to standard gravel instead of the fine stuff but it could also be because I used the mechanical vibrator and did not tap the concrete as often when I placed it in the PVC pipe.
319216319217

[rwbuild]

I know we come from the land down under but can us locals get a photo that we can relate to?

[rick_rine]

I am enjoying building the third and main pergola. I estimate that I have laid 100 cubic metres of concrete in my handyman life but this has been the ultimate in learning about this fantastic, malleable material. The mould I bought from America was stated to last 16 pours. I have made over 30 and have about 16 more to go so hope it lasts the distance.
Beside the practice/test pergolas I have made lots of tests columns and moulds.
There will be just under 9 ton of concrete in this pergola if I build the cross bars from concrete which I am seriously considering. I will then be able to curve the arches.
I have been very worried about lining up all the columns but am very pleased so far.
It is a great project both physically and mentally.
Mini-orb has been used form the columns.
I can't post the pictures properly. Sorry.

[DropSaw]

Hi rick_rine,
i spent 30 odd years making molds
statues column's retaining walls fountains pots
all out of concrete i used fibreglass to make the outer casting.
iv worked for BGC concrete in
perth for 12 months before getting sick of that.
ideally you want the thickest concrete possible
and use a plasticizer, this is an additive to make
your concrete go from 30 slump to 100 slump the additive last for about 15/20 min so you need to work fast
before the concrete returns to 30 slump or what ever it was before adding plasticizer, we used it all the time for tilt up walls were high strength was needed. The less water you add to concrete the less shrinkage you'll get.

[rick_rine]

351882
The columns have been made with mini orb and look great. In the columns I used 10 mm reinforcmnt and a 4-1 gravel cement mix. In the beams I have used nylon fibres and two six mm reinforcment bars. It has been over two years in planning but looks amazing. Mostly mixed by hand. Poured in-situ. Almost perfectly straight and level surprise. I am using 4 concreting gravel, 3 bickies sand, and two cement for the crossbeams. They are 3 metres long with a span between the posts of two metres, they are .1 wide and 0.14 high. The supporting beams are 0.1 x 0.18. The posts are on a 0.3 x 0.3 base with 0.15 diameter columns moulded with mini orb. No scaffolding was needed as they sat up right and I poured the concrete in with a garden pot.

[rick_rine]

351883351884351885351885

The he pergola is twelve metres long, two metres wide, has 18 columns and has taken six months about so far to build. I am thinking of a clematis or wisteria. I got the idea when I was in Italy a few years ago. One of a kind! No one else would have one of these in their backyard, and probably no one else would want one either 😃

[Bushmiller]

Rick

Seeing those columns immediately made me think Appian Way and Aqueducts. It looks extremely impressive!

An amazing project.

Regards
Paul