concrete placement in long column 4

[khinz]

For long column (3 meters), what is the proper way to place concrete at the bottom? It seems by pouring it from the hole above (long distance) and with many stirrups crisscrossing the path, there may be some slump separation as the concrete fall down the bars causing segregation at the bottom of the column especially the sides where it is more difficult to vibrate due to numerous hooks present.

 

[BAretired]

Pouring concrete through elephant trunks is recommended by some, but in practice, I have not seen contractors using them voluntarily.

BA

 

[msquared48]

10 feet is not a long column. If you have to, use pea gravel concretewith a smaller hose from the Pumper, retracting thehoe from the bottom as the concrete is poured.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

I hate trying to edit on I phones. It's impossible.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Mike, in my opinion that is not an acceptable solution. Using small aggregate is avoiding the issue, not solving it. Larger aggregate makes better concrete. The key to placement in columns is to leave plenty of space for placement and consolidation. I have seen some of these seismic concrete designs which show reinforcement arrangements which would preclude adequate placement, so I wonder about the geniuses who come up with those schemes.

 

[dik]

If these are the same columns from the other thread, the reinforcing is so conjected with cross ties that you have to place it with a teaspoon. It has to be done with a small elephant trunk and a pencil vibrator. Regular approach, based on those that didn't work, won't do.

Dik

 

[Ron]

Assuming that the column you are contemplating is the one shown in your last photo of the previous thread (as dik noted), you have to get back to basics.

Design considerations in reinforced concrete include much more than just the loading. As hokie66 noted, constructability has to be considered as well [highlight #FCE94F](hokie66:"The key to placement in columns is to leave plenty of space for placement and consolidation.").[/highlight] He is exactly right. This means that the size of the column has to accommodate placeability, the mix design must accommodate clearances and placement technique, the vibration scheme must accommodate the geometry of the column and the mix design, and the placement process, whether pumped, tremied or dropped must accommodate all of the above.

To give you an idea of clearance issues, the maximum size of the coarse aggregate in the mix ideally should not be more than 1/3 of the narrowest distance between the rebar. That means that if you are using typical 3/4" to 1" (19mm-25mm) coarse aggregate, you will have problems due to the congestion of rebar. Further, the minimum pump hose size will be on the order of 3" (75mm). It is not likely that you have those clearances based on the photo.

Have your concrete supplier design a mix for the clearances you have. Have your pumping subcontractor use a pump that will allow the hose to be deposited all the way to the bottom of the form, filling the form from the bottom without free-falling (dropping) the concrete (as Mike noted). Make sure that all the constituents of the concrete mix are properly done...saturated aggregates, proper compensation for water, proper admixtures and admixture dosing. Then make sure that you have adequate quantity and wand length for the vibrators to be able to properly consolidate the concrete. Don't vibrate the forms, but you can tap the forms with hammers to allow release of entrapped air.

Good luck.

 

[Archie264]

Wow, the wealth of information that I learn from this forum is excellent. It's a terrific resource, to say the least.

 

[msquared48]

I agree totally that if the column reinforcing pattern is too congested for the load seen, it should be made larger - that is the best alternative.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

Ron, there are other problems with the rebar placing... unless they can get in to place concrete, the aggregate will simply 'bounce' off the interior ties and segregate as noted in the actual construction photos. The interior ties prevent the ingress of a trunk or vibrator... maybe 'diamond' shaped ties can be used to provide a bit of clearance.

Congested should never be spelled as it was... don't know what happened...

Dik

 

[Ron]

dik...I agree. Bottom line is basically what you, hokie66 and Mike have said or indicated....column is too small!

 

[hokie66]

In khinz's other thread, he belatedly revealed that this is supposed to be a "special moment frame", in other words a concrete frame trying to mimic the ductility of a steel frame, so it is packed with reinforcing steel. For those of us who have no experience with this type design or construction, the attached paper is interesting. Look especially at Section 7 of the paper, where the authors talk about constructability. The congestion as shown in Photos 7-2 and 7-5 is impressive...for all the wrong reasons.

http://fris2.nist.gov/NEHRPClearinghouse/NIST/PB2009102037.pdf

 

[msquared48]

In looking at the plan detail on page 12, you'd think that someone would have thought about how to get the hose to the bottom of the pour. Seems like the horizontal steel should be spread at the center in each direction to allow a hose chase. Maybe bundling of four bars... As shown, this is just a recipie for rock pockets.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[PUEngineer]

Having worked on several concrete special moment frames; on the surface, I didn't see the column from the other thread as all that congested. That's all said without seeing a plan view of the column. But looking at the perimeter of the column I've seen similarly reinforced columns that were taller and they were cast without issues similar to what khinz has experienced. Those columns had mechanical splices rather than lap splices to help with placement and ease any possible congestion. A proper mix design, quality rebar installer and knowledgeable concrete installer with proper vibration techniques are additional keys to the success.

I had that NEHRP document during the design and detailing process, and found it to be very helpful. It was most helpful during the detailing process.

 

[khinz]

Here's a plan view of the column:

http://imageshack.us/photo/my-images/706/barsarrangementx.jpg/

Here's the actual bars of the column in the other thread before concrete added

http://img16.imageshack.us/img16/5928/barsactual.jpg

The concrete ready mix is 3/4 aggregates 4000 psi, it can pass thru the rebars but thrown at long distance (about 3.3 meters at the hole above). There is segregation as noted in the other thread at the bottom.
Now my problem is demolishing the lower 1 meter of it to replace the column, I can't insert any vibrator after trying it out at the floor above which has the same bars arrangement. So any suggestions how to reach the vibrator to the bottom inserting it from middle (1 meter from floor in the picture)?

 

[Ron]

khinz....if the concrete mix is designed properly FOR THE REPAIR, and placed appropriately, you might not need vibration for proper consolidation.

 

[hokie66]

Further to Ron's advice, filling that section will likely require a high slump, superplasticized mix. It is also going to require watertight formwork. I think the most appropriate means of placing is by pressure pumping. Based on what khinz has said previously, getting this all done where he is located is problematic, but that is what is required.

 

[Ron]

Agree hokie66...I see no other option.

 

[khinz]

Since we didn't have the specialized equipments. We may just downgrade the building to just 2 storey with roof slab instead of 3 storey with metal roof. I guess the column pictured is enough to support a roof slab, isn't it. I wonder how to tell or estimate the the compression strenght of a bad mix. It's in a new thread about bad mix.

 

[Ron]

khinz...STOP!! That column might not be capable of supporting the next floor. Just because the column remains standing after you place the concrete does not, by any stretch of your imagination, mean that it is adequate to support the loads necessary for the building; whether or not you add the third story. Apparently this is a seismic design, so that will impose loads on this building not seen in normal use.

I've read most of your posts in multiple threads. It is clear that you are in way over your head on this. You need some local, experienced structural engineering help. I'm not being critical, but apparently you do not have much experience in concrete technology, structural design or construction means and methods. Further, apparently your resources are limited to remove and replace the concrete. This should not be. You and the contractor must be prepared for such consequences and deal with them directly, not by trying to rationalize a half-ass solution that could get people killed.

Get the concrete out of the column if you have to chip it out with a hammer and chisel. Replace it properly.

Failing to immediately make a clear decision on this issue puts you, your company and the public at risk.

I've seen better looking columns than this fail during the construction process because no one had the balls or the foresight to make a tough decision, or they rationalized a bad condition to seemingly make it better. The last time I saw this happen, a worker was killed. Don't put yourself in that position.

DO IT NOW!