pouring concrete section by section 3

[ticas]

when you are pouring concrete section by section in a column, which section is allowed partial pour in your practice? Can you do it at middle of column? I know the best is to pour the entire columns at once but the problem is you can't insert the vibrators thru the upper floor column-beam joints in the lower column because they are blocked by the beam longitudinal bars. What do you usually do?

 

[hokie66]

You place the column concrete to the beam soffit before the beam reinforcement goes in.

 

[ticas]

the third floor slabs and second floor columns will be poured together at the same time in one day. but the beam top bars are already installed in place in the third floor and it's blocking the access to the columns of the second floor with already formworks in place. Are you saying the top bars need to be removed temporarily so the vibrators can get in the center? But the vibrator can get in the sides of the columns. This is insufficient to vibrate the bottom of a 3 meter column and sized 500x500mm?

 

[hokie66]

It is poor/unacceptable construction practice to cast a column and the floor above together. That is a sure way to get a plastic settlement crack at or near the top of the column.

The vibrator needs to be in the space between the ties, as you say "in the center". Trying to vibrate in the cover concrete is also unacceptable practice.

The best analogy I can think of is that "You are putting the cart before the horse."

 

[ticas]

why would this cause a "plastic settlement crack at or near the top of the column."?

I think the plausible solution is to remove the top bars blocking the vibrators access to the column inner ties below. But what has this got to do with "plastic settlement crack at or near the top of the column."?

 

[ticas]

ok, i remember what are plastic settlement cracks... so i'll let them put concrete separately... perhaps the columns first by 80% concrete filled, then next day the rest of the column below and beams above and slabs would be poured. Is this the best course of action? Would this avoid plastic settlement cracks?

 

[hokie66]

You are on the right track, but the normal practice is to pour the column to the bottom of the beam or just below. An inch or two below the soffit is enough. The only reason you leave it slightly low is to avoid overfilling the column form and having to remove that before casting the floor.

 

[ticas]

What are the allowable column sections of which you can pour partial concrete section by section and which part can't be poured in section but in whole. Of course I know column-beam joints have to be pour monolithic, how about the column sections below it.

 

[hokie66]

You can pour columns in lifts, but it is not usual practice. The joint should then be clean and slightly roughened before the subsequent lift, so with formwork standing, that usually precludes double lift columns. Tall columns are often poured in multiple lifts.

I don't know what you mean by "column-beam joints have to be pour monolithic", as the sequence is column, beam, column, beam, etc.

 

[ticas]

Do you think the column can be concrete poured 80% or 2.4meters of 3 meters first and next day the upper 0.6 meters poured together with the beams and slabs above?

 

[ticas]

if columns are not poured monolithic with beams (poured together).. during seismic activity, the only thing holding them are the rebars, don't you see a problem with this?

this is the reason why we tried to pour column and beam together so the concrete can act as one unit but the problem is the beam bars block the vibrator from reaching the column below so we may pour the columns first after totally removing the beam bars again.

 

[hokie66]

The reinforcing is the only thing holding it together. Concrete has very low tensile strength on its own. I see no advantage in holding the column pour down as you suggested.

 

[ticas]

the column has 16 rebars and 3 sets of ties, one tie holds the entire perimeter bars of the 500x500mm columns, the two other ties hold the inside bars in between the corner bars in perpendicular. Now if the vibrator would be accessed from above, it can only reach 4 holes because the center hole is blocked by thick beam rebars above which can't be removed without too much work. I think vibrating the 4 holes at the sides can handle the concrete at the center from forming honeycomb? Do you actually have to insert the vibrator to each hole as seen from above the column (with ties crisscrossing the column)?

 

[JAE]

hokie66, I didn't know you were such a fan of merry-go-rounds.

ticas - refer to hokie's first post above. The beam rebar won't be installed during column concrete placement (not "pours").
Therefore, the beam reinforcement will not be in the way of vibration efforts in the column.

 

[ticas]

JAE, all the beams and columns formworks are already installed in place and concrete ready to be poured on friday but realized the vibrators can't be inserted into the beam holes.. and not all beam bars can be removed just the sides so only the side holes can be inserted with vibrators to pour the columns first (with the loosen bars to be put in place next day)

 

[JAE]

Looks like you have a problem then.

 

[JacksPanic]

Please explain to me how seismic resistance is increased by casting a slab/beam monolithically with the column below.

 

[hokie66]

JAE,

I'm just a kid at heart. But thanks for reminding me...I'll jump off now.

 

[ticas]

we have a problem, the beam bars can't all be removed but can only be moved around while the vibrator is inserted. The plan is to cast concrete to the column below first and the beam next day but won't concrete touching the beam bars and drying diminished the bonding when fresh concrete is poured next day? Has anyone encountered this? Can you just use cloth to clean the beam bars, would this work? It would take us a week to remove all the beam bars and this is not an option.

 

[JAE]

I would specify that any and all concrete film or material be removed from the beam bars prior to placing the beam concrete.
They can use a power tool with a wire brush to remove the fresh concrete film.

Then be sure to have them remove any concrete debris from the bottom of the beam form or the top of the new column concrete below.

I concur with hokie that the column concrete should be at or just below the bottom of the beam.