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]

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.

 

[Ron]

Apparently I offended someone!!

 

[ticas]

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?

 

[hokie66]

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.

 

[ticas]

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?

 

[hokie66]

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.

 

[ticas]

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??

 

[hokie66]

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.

 

[ticas]

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?

 

[JAE]

Round and round we go.

 

[ticas]

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.

 

[hokie66]

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.

 

[ticas]

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.

 

[Ron]

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.

 

[hokie66]

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.

 

[ticas]

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.

 

[ticas]

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?

 

[hokie66]

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.

 

[ticas]

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.

 

[hokie66]

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