Thread: Concrete formwork

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

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.

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.

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

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.

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.

Originally Posted by 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.

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

Originally Posted by rick_rine

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.

Originally Posted by ian

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

Originally Posted by 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.

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

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.

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

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.

Originally Posted by 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.

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

image.jpg 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.

image.jpgimage.jpg