pouring cocnrete - long column

[vedat1]

Hello,we are constructing a warehouse with cocncrete columns and metal structure roof. my question is: Can we pour concrete into 15 meters long concrete column forms at once (pouring at 4-5 meter intervals trough windows on the column form). Or should we pour the first half first, then after form removal do the same for the seconf half. Is a cold joint allowed in such a situation, (especially where bending is most)? Thank you in advance

 

[Zambo]

What dimension are the columns in plan?

Both options are possible, but a lot depends on access for concrete pokers (vibrators) and a reliable method for placing the concrete.

Are you using steel forms or timber?

 

[vedat1]

Thank you fof quick reply,
Dimensions are 50 cm to 50 cm. We use plywood as form material.

 

[Zambo]

The pressure from 15m height of concrete is generally too high for timber/plywood forms. So plan on several casting lifts.

I would say try 4 castings, that gives 3.75m each lift. 2 x plywood sheet = 4.88m so you have a good overlap at the joint between lifts (in fact a bit too much, but 3 lifts is a bit high per lift)

Then at the top of each lift you can cast in something to support the formwork above. Perhaps just cast in some horizontal 25mm diameter pipes.

It is preferable if you can use a tremie pipe or the pipe from a concrete pump so you can get the concrete to the bottom of the column without it falling past all the rebar. But this depends on the column links - is there any space to get a pipe down?

The most interesting point is the scaffold tower for access and a method for adjusting the verticality of the formwork. The easiest way,but relatively expensive is to have a strong scaffold fully enclosing the formwork.

 

[dik]

Why concrete as opposed to steel? Firerating?

Dik

 

[vedat1]

Yes firerating, but mainly economy. As far as I know concrete is more economical then steel columns. And for 15 meters you just cant solve it with a profile section I suppose.

 

[Zambo]

veat1 - where are you constructing this building?

dik, I guess vedat1 is outside USA or Western Europe. If you take labour costs out of the equation concrete will be cheaper than steel.

I doubt you have to fire rate steel columns in a warehouse, if no occupants above. A smoke spill sure and possible a sprinkler system. Anyway the steel rafters aren't fire rated apparantly. But of course local regulations will be in place.

Precast concrete columns would be a good option. A large cranes would be needed to lift them into position, but the work would be quick and good quality. The interesting part would be the joint at the base.

 

[vedat1]

The site is in Russia . I ve tought about the precast column but considering the transport expenses it is expensive. Of course we may do it on the site, but the workmanship must be perfect and the joint detail is interesting as you say.

 

[dochardnose]

Look into Self Consolidating Concrete (SCC). The major admixture manufacturers, BASF, Grace, etc. Can provide information. A web search will overwelm you but utube has some interesting video of SCC in use.

 

[shin25]

I agree with zambo. For such a tall column the concrete head pressure will be too high to use timber form work. Some pressure in ths form work can be reduced by controlling the pour rate. But, ultimately the pressure is depended on many factors of the concrete mix. ACI published "form work for concrete" is a very good book for this kind of design. For your kind of situation, a steel plate girder form may be more appropriate.

 

[dik]

One of the factors influencing form pressure is the rate of pour and for a small x-section it may be large, but your forms are tied at each edge so they are relatively stiff... Have you considered round columns using fabric forming... alternatively, the columns can be site pre-cast, maybe

Dik

 

[civilperson]

Cold joints can be placed anywhere in the length of the column, (as long as reinforcement continues through the joint). If placed at a moderate pace, i.e. 2 feet vertical per hour or 0.6 m per hour, the forms as described will have sufficient strength since the plastic pressure of concrete will be limited by the set of earlier and lower concrete. This height will require many hours of placement or many cold joints. DO NOT USE RETARDERS! Perhaps accelerators, (non-chloride), could be incorporated to speed up the vertical rate of placement.

 

[Zambo]

civilperson,

the columns are 0.5 x 0.5m in plan. You recommend casting at 0.6m/hr. This is 0.15m3/hr - not a good idea.

Also, not a criticism, but an observation, you refer to cold joints. Cold joints are unintentional joints formed in concrete due to unforseen circumstances, the correct terminology for joints formed at planned intervals is construction joints.

The best method of casting the columns correctly is to select a casting height and then design the formwork to suit. If 15m is selected then the formwork will probably be uneconomic and a new casting height can be tried.

Precasting the columns is definitely an interesting idea.

 

[dik]

If you're stuck on concrete and workmanship is an issue, then site-precasting looks like your only option. You may have serious problems with workmanship for columns that long if you try casting them vertical. Precast them on the flat and use a strongback and a hoist.

Dik

 

[civilperson]

Zambo,
Why is casting at 0.15 m<3/hr a bad idea? This is 5.3 cubic feet per hour or two or three box mixes per hour, if 1/4 yd mechanical mixer is available then one batch /hr will suffice as production speed. Third world construction is labor intensive and still can be good quality. My assumption of hand mixed concrete comes from the limitation of only plywood for forms.

 

[Zambo]

civilperson,

I have spent most of my life working overseas in countries at various stages of economic development. Just believe me casting so slow is a bad idea and cold joints are the likely result.

Also where did you get the idea that plywood is some kind of inferior formwork material.