Thingemabob

Here are the disadvantages to monolythic pour:

You need a concrete pumper or a shoot to keep the mix from separating as it falls down the column. The drop should not exceed three feet.

You will need a vibrator to consolidate the mix after entering the form. Do this in sucessive layers.

The form will have to be well braced for full height and thus more costly than partial height.

If you strip the form early, apply a sealer (oil based) to promote curing and retain water. Curing is very important and the main reason for leaving the forms on.

This should get you started.

1. Shortest time is about 24 hours. If you use a "high early strength" concrete mix, you may be able to remove the forms a little earlier, but the disadvantages are cost and rapid curing, which causes internal stresses in the column.

2. For the height of colunm you have, there is no disadvantage in pouring monolithically. Use a vibrator sparingly; too much vibrating will cause the aggregate to settle torward the bottom. The only reason to use a vibrator at all is to fluidize the concrete mix so that it flows well around reinforcing bars and into horizontal pockets (you probably won't have any in a simple column). There is no separation of aggregate due to pressure.

Spraying oil onto the forms helps to remove them. The oil itself does little to seal the concrete. You may seal the concrete with sealer when it's dry to the touch or later. Concrete continues to cure for years, but it is rated (3000 psi) and tested in the US at 28 days, hence loads should not be put on the columns until they have cured long enough to withstand the next floor load.

Form oil used to release the forms are not meant for curing the concrete. Use these products or similar instead for that purpose (seee link). Curing

Bill makes a good point, don't over-vibrate, but don't under vibrate. You will get honey-combs where coarse aggregate is exposed on the finish. Ugly and bad and to be avoided.

Thanks guys! 24 hrs minimum it will be then.

I have since found out that , due to square column heads that are going to be cast at the same time as the slab, my column heights have reduced to 1900mm.

that makes it a little easier to control, I suppose.

Thanks again.

Hello Thingemabob

You do not specify whether you are casting these columns in-situ, or elsewhere and intend to install them later into the final position.

Either way, if you steam-cure the concrete overnight (12+ hours) you will obtain very high initial strength, and will be able to remove the formwork the following morning.

Steam curing of concrete is quite cheap, and is commonly used for high-strength concrete pipes to 2 metres diameter, bridge beams to 600mm x 400mm, power poles and the like.

A properly designed steam-cured concrete article has some high initial strength, refer:http://cat.inist.fr/?aModele=afficheN&cpsidt=16778900

<"....Abstract

Heat treatment is widely used to accelerate the strength-gaining rate of concrete. In general, the ultimate strengths of the heated-treated concrete are lower than those of the standard cured specimens. When ultrafine fly ash (UFA) is included in concrete, the pozzolanic reaction is accelerated through the heat treatment. Sometimes, various chemical activators were used to activate the reactivity of fly ash. In the current study, UFA and slag were used as a replacement for cement, steam curing and chemical activators were used to accelerate hydration of cement and fly ash, and then compared with moist curing. This paper presents the influence of steam curing on the compressive strength of concrete containing UFA with or without slag. The experimental results indicated that the concrete containing UFA has low early strength after 13-h steam curing and that the difference between the 28-day compressive strength of concrete through 13-h steam curing and that of moist-cured concrete is large, but the concrete with UFA and CaSO₄ or Ca(OH)₂ has a high early strength, thus, the reactivity of fly ash must be accelerated. Concrete containing UFA and ground slag was prepared, whose compressive strengths were improved.....">

Wet curing which means after you remove the formwork, you wrap jute sacking around the columns and keep that wet, or drop a plastic bag over the column and keep the bag full, gives very high 7 day strength.

Some folks think concrete cures by 'drying out', when in fact it cures by crystallisation of a matrix of needle-like jagged crystals, growing throughout the mass, (similar to barbed wire) and interlocking, which maked the 'set and cured' concrete extremely resistant to crushing forces.

Fiberglass resins are a later example of the same sort of curing, where a matrix grows though the mixed liquids or pastes.

Kind Regards....

Thanks, a million fold.

You guys have been fabulous. By now my columns are already cast (in sito) and they seem in excellent condition. Everything went well and I'm keeping them wet. My main concern was to be able to remove the formwork as soon as possible and with your help I kept it to 24 hours.

In order to reduce air bubbles, I had to increase the water content somewhat as well as the vibrating.

Thanks again