Thread: Concrete formwork

image.jpg 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.

Originally Posted by rick_rine

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

Mosaics photos @July07 022.jpg

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 .

Regards
Paul

image.jpgI 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.

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.

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.

Originally Posted by rick_rine

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

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.

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

Originally Posted by 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.

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.

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.image.jpgimage.jpgimage.jpgimage.jpgimage.jpg

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

Originally Posted by 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.

Thanks Paul. That makes sense.

Regards
Richard

Originally Posted by rick_rine

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.

[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. image.jpg
black woods have still not got the clamps so I modified the arrangement and I believe it will be better.image.jpgimage.jpg

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.image.jpg 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.