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pouring concrete section by section
3

pouring concrete section by section

pouring concrete section by section

(OP)

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?

RE: pouring concrete section by section

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

RE: pouring concrete section by section

(OP)

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?

RE: pouring concrete section by section

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."

RE: pouring concrete section by section

(OP)
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."?

RE: pouring concrete section by section

(OP)
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?

RE: pouring concrete section by section

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.

RE: pouring concrete section by section

(OP)

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.

RE: pouring concrete section by section

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.

RE: pouring concrete section by section

(OP)


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?

RE: pouring concrete section by section

(OP)


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.



RE: pouring concrete section by section

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.

RE: pouring concrete section by section

(OP)

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)?

RE: pouring concrete section by section

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.

RE: pouring concrete section by section

(OP)

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)

RE: pouring concrete section by section

Looks like you have a problem then.

RE: pouring concrete section by section

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

RE: pouring concrete section by section

JAE,

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

RE: pouring concrete section by section

(OP)

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.

RE: pouring concrete section by section

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.

RE: pouring concrete section by section

Jumping back on the merry-go-round...JAE is right. You will need a combination of tools and things...rags, wire brushes, vacuums...having done it once, the workers will want to do it right the next time.

RE: pouring concrete section by section

Apparently I offended someone!!

RE: pouring concrete section by section

(OP)

hookie, after pouring the column with concrete and fixing the beam bars back to position, we plan to pour concrete to the beam after one hour same day because it would take so much time to clean the bars with rags, wire brush, vacuum if it has to wait another day.. Would this be enough to avoid plastic settlement cracks? What is the minimum hours before pouring the beam so the column top won't develope this plastic settlement cracks?

RE: pouring concrete section by section

Plastic cracks form while the concrete is plastic, and that depends on your concrete mix, atmospheric conditions, etc. When the concrete has hardened, any cracks which form are not plastic cracks.

RE: pouring concrete section by section

(OP)

in multiple ties in a column, there are crisscrossing tie bars as seens from above. Do you actually have to insert the vibrator each of say 9 holes or just 4 holes will do? I wonder if vibrating the 4 holes is enough to vibrate adjacent holes.. how wide do you think is the vibrating influence?

RE: pouring concrete section by section

For your 500 x 500 columns, one or two locations should do, as long as they are not right on the edge. Closer to the centre is better.

RE: pouring concrete section by section

(OP)

last question, you said if column below and beams/slabs above is poured together with concrete at same time, there is bound to be plastic settlement crack at or near top of column. Why, if column is only poured with concrete and the beams/slabs two days later, why won't plastic settlement crack be form near or at the top of the column??

RE: pouring concrete section by section

Because two days is plenty of time for the column concrete to harden, I hope. Hardened concrete is not plastic, so is not susceptible to plastic cracking.

RE: pouring concrete section by section

(OP)

but what I don't get is even it takes two or 10 days for concrete in the column to harden, what is the connection or problem of casting with concrete the beams/slabs above at the same time, how would it affect the condition of the column below?

RE: pouring concrete section by section

Round and round we go.

RE: pouring concrete section by section

(OP)
Jae, earlier Hookie said that "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 question is why?? Why would plastic settlement crack form if floor is casted together with column and not if column only? there seems to be no connections explained.

RE: pouring concrete section by section

You said before that you now understand the concept of plastic settlement cracking, but it is apparent that you don't. Try googling. Simply, deeper sections consolidate (or settle) more than thin sections. So if the column concrete settles after placement of the beam, there will form a gap at the top of column/bottom of beam juncture. You don't want that, I assume.

RE: pouring concrete section by section

(OP)

I know the concept of plastic settlement cracks. So you are saying that after the gap above the column forms, casting the beam next day would cover the gap, whereas if the beam and column is cast together, the gap can stay hidden, is this exactly what you mean??

The concrete is 4000 psi and the column is 500x500mm 3 meters high. I wonder how big is the upper gap form by this, any experience? Because if it is that serious, I have to call the concrete company to do the column first one day ahead of the beam as both column and beam poured are all scheduled same time on saturday. Thanks.

RE: pouring concrete section by section

ticas...you are getting a terrific lesson from hokie66...pay attention.

Plastic settlement is significantly influenced by the depth of the section. If you place concrete monolithicly as you are contemplating at the column, the section depth is significant, thus while in a plastic state, the potential for settlement and segregation is much greater. If you allow the column to set up before placing the slab, you only have settlement consideration in the slab thickness, not the slab + column.

RE: pouring concrete section by section

ticas,
Listen to Ron, as well. He knows concrete and construction. And yes, you have described how the gap forms. I have seen the results of this being done like you proposed, a couple of times, years ago. The gap was in the order of 5 mm. Just don't do it! Better to learn by reading than by experiencing it on site. Make the call.

RE: pouring concrete section by section

(OP)

Ron, why didn't you mention about beam. Beams are over columns and slabs on sides of beams. Why, do you design structure with slabs over columns without beams??

The column is 500x500mm 3 meter with 3 ties spaced every 100mm (4") and average of 16 20mm bars. Do you know how to calculate the settlement of this above the column for concrete of 4000 psi?

But then, if the beam is casted together with the column, then what will settle will be the beam and not just the column top, isn't it? Or maybe casting the column first ensure the beam-column joint won't settle or suffer plastic cracking? This is very important and the one thing that can make me decide to cast the column first.

RE: pouring concrete section by section

(OP)

I just debated with the project managers, contractor heads and company officials about why the column has to be casted separately from the beam/slabs. They tried to convince me that standard practice is to cast beam/slabs above and column below simultaneously especially in upper floors. This is because it's not practical to hire one pumpcrete just to fill up the columns.

I am able to convince them at the end why the column has to be casted first or else the bars in the column-beam joints can suffer plastic concrete cracks and endanger the building. So we will cast column first after I insist it.

Now my last question is, how long should I wait after casting the column before I cast the beams/slabs above it? Can one day be enough?

RE: pouring concrete section by section

Yes, one day is enough. And good for you, these contractor people were just trying to save money, and should know better, whether they do or not.

RE: pouring concrete section by section

(OP)
btw.. hokie, i want to buy dozens of masking tapes to wrap the beam bars on top of the columns so rags and steel brush won't be used maximumly. Any hidden undesirability of masking tapes or can you recommend other tape to temporary wrap the bars? We can't entirely remove the bars but only move them to insert the vibrators to the column, thanks.

RE: pouring concrete section by section

Tape won't hurt...as long as you peel it all off before placing the floor concrete.

RE: pouring concrete section by section

hokie66....this is as good an example of forum mentoring as I've ever seen in the forums! Nice job!

ticas....there is not always a need for beams in a column/slab system. It is common to have slabs supported by columns without beams. You obviously have to accommodate shear considerations (punching shear) at the column/slab interface, but slabs can often handle the imposed loads.

As for calculating settlement of the concrete around/under rebar...can't be done by any reliable method! This is a parameter that has multiple facets, mostly related to the mix, placement methods and......thickness of the section as we've discussed.

RE: pouring concrete section by section

Thanks, Ron. As JAE said, it was quite a merry-go-round, and as you well know, I'm not always that patient. I think ticas learned a few things. He certainly is persistent, which is a good thing.

RE: pouring concrete section by section

...and if I may add, ticas stated his piece and continued to put forth his position, but in the end didn't ignore the advice from the most knowledgeable and experienced participants. And, he faced down the contractor, forcing it to be done correctly, which is not a trivial thing to do. A good thread all around, in my opinion. I know that I learned a lot by following along.

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