concrete placement in long column
concrete placement in long column
(OP)
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.






RE: concrete placement in long column
BA
RE: concrete placement in long column
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: concrete placement in long column
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: concrete placement in long column
RE: concrete placement in long column
Dik
RE: concrete placement in long column
Design considerations in reinforced concrete include much more than just the loading. As hokie66 noted, constructability has to be considered as well (hokie66:"The key to placement in columns is to leave plenty of space for placement and consolidation."). 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.
RE: concrete placement in long column
RE: concrete placement in long column
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: concrete placement in long column
Congested should never be spelled as it was... don't know what happened...
Dik
RE: concrete placement in long column
RE: concrete placement in long column
http://fris2.nist.gov/NEHRPClearinghouse/NIST/PB20...
RE: concrete placement in long column
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: concrete placement in long column
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.
RE: concrete placement in long column
http://imageshack.us/photo/my-images/706/barsarran...
Here's the actual bars of the column in the other thread before concrete added
http://img16.imageshack.us/img16/5928/barsactual.j...
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)?
RE: concrete placement in long column
RE: concrete placement in long column
RE: concrete placement in long column
RE: concrete placement in long column
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.
RE: concrete placement in long column
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!
RE: concrete placement in long column
It's not the only column with the problem, the following is a second column beside it with lower segregation problem, now this is my main question, we chisel the soft part, what remains in the following are the hard part, you can see in the intact surface that there are holes, but chiselling it we found it is just the surface with voids, now my important question, how sure are you the inside of it also has voids? or are you saying it's not the voids that is the problem but just bad mix. Are you saying the bad mix can be solid too and yet not 100% 4000 psi? how to test it? pls. answer this because actually all the 10 columns lower part look like this so I need to be convinced first before stopping work in the building and doing very costly repair.
http://img23.imageshack.us/img23/271/honeycomb2.jp...
RE: concrete placement in long column
RE: concrete placement in long column
Yes. The concrete in the center can have no voids and still be bad concrete. YOU NEED TO TEST IT or better yet, just remove it. You could have already removed and replaced the concrete while you've been screwing around with trying to find answers in this forum that you seem to continue to ignore.
Are your managers just idiots or are they only concerned about costs? There are no specific references that I know of that will tell you that your specific conditions have resulted in bad concrete in the middle, yet there are plent of them that refer to the results of your concrete as problematic. I would point you specifically to the following references:
1. American Concrete Institute (ACI) Manual of Practice (There are similar British and Australian references, so take your pick)
2. Design and Control of Concrete Mixtures, Portland Cement Association
3. Concrete by A.M. Neville (in my opinion, the single best reference available for concrete)
We surmise that you have bad concrete from the poor placement techniques, lack of consolidation, difficult geometry and several other rather clear indicators.
Collectively, the responses you've been given in this thread have amounted to several HUNDRED years of experience, much of which has kept us from being in your position.
RE: concrete placement in long column
Ron, Hokie said "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."
We don't have the technology for superplasticize mix, can't do watertight formwork and metal. We just have wooden formworks and and we don't have pressure pumping. At most what we will do is to use wooden formworks and extend it to the top a bit and open the formworks at a side for the new concrete to come down.
http://img706.imageshack.us/img706/243/repairformw...
the dots are the concrete repair top and bottom, is this allowable? the space left after settlement above wil just be fill up with non-shrink epoxy grout... by keeping the formwork open at a side slant upwarded, the concrete would fill up the column bottom and fill up the side outwards. You think this is ok. This is what the managers suggested in case if it has to be removed because that's the only technology we have.
for the concrete replacement, we will use site mix for 1-2-3 concrete combo... do you think site mix can ever be 4000 psi? a concrete truck has 6 cubic of concrete and we only need little so manager told us to site mix the replacement.
What do you think of the above replacement procedure.
RE: concrete placement in long column
Did you bother to read Ron's posts? You are in a hopeless position and are apparently in denial. Did you read the reference I posted earlier? To build a special moment frame in reinforced concrete, you need a lot more know-how and resources than are available to you. Who did this design, anyway? If it was some engineering consultant from afar, they need to get to your site and sort this out.
RE: concrete placement in long column
Your managers are nuts. They want you to take a bad situation and make it worse by site mixing a "1-2-3" concrete mix with a crew that doesn't even understand concrete placement, much less concrete mixing. ABSURD! A 1-2-3 mix works for mortar, but not higher strength concrete. Volumetric batching on site is difficult at best to achieve higher strength concrete with reasonable consistency.
If you must site mix, buy dry, pre-mix, bagged concrete from a reputable supplier. Make sure it is a higher strength mix and follow the directions exactly. Make sure it is capable of being made with a high slump yet still achieving high strength.
There is no need to leave a gap at top if you form as you propose. The open trough on one side will work, but carry the formwork above the joint and fill with concrete to above the joint. Once you are sure that adequate vibration has been done and the concrete is adequately consolidated, then slide a sheet metal "stop" down the side of the column all the way to the bottom of the trough. This will separate the concrete remaining in the trough from the concrete in the column. When the concrete has set and you remove the formwork, you will only have a small surface blemish to repair.
RE: concrete placement in long column
- Are you kidding me? Why are you asking this question? You said that this is a special moment frame - that term is meaningless with horrible materials, placement, let alone guessing at the compressive strength of concrete from a bag. This structure's usage better be relegated to housing farm animals.
Also, you have barely responded in kind to everyone's comments and questions - I don't know what to make of that, but I suggest you do a better job if you want future help of these experts - it is plain common courtesy.
In Russia building design you!
RE: concrete placement in long column
Ron, do you think it will work if I'll just order another concrete truck even if only less than a cubic needed? We will put them in basin and put the concrete thru the opening. But here's the problem, there is no way for the vibrator to reach the bottom (even half). If no vibrator are used, I wonder if it would recreate the original problem. Or I wonder if vibrator the concrete in the basin prior to throwing it at the hole can work. Or maybe we can do multiple lifts for one meter length.. for example.. every 1/3 meter so vibrator can be inserted at every lift?
By the way. I assume the bar rebars at bottom alone can support the weight of the finished columns above with some help from the shore supported the finished beams above.
Many thanks.
RE: concrete placement in long column
Vibrating outside the form does no good. Don't bother.
Insert the vibrator in the same opening as you place the concrete....should be adequate.
Good luck and let us know how it works out.
RE: concrete placement in long column
Nothing which you have proposed or suggested will solve your problems. Now that you have revealed that you have all ten columns in a storey which are like this, it may be necessary to demolish the entire structure that has been built above these columns, and start over.
RE: concrete placement in long column
Dik
RE: concrete placement in long column
Hokie. If I replace the concrete in the lower part (1 foot) of all columns. Won't the strength be the same as the original provided proper mix is used? I think it will. Why suggest demolishing as demolishing the finished beams and slabs above the columns would be very costly.
Also you said "Khinz....if the concrete mix is designed properly FOR THE REPAIR, and placed appropriately, you might not need vibration for proper consolidation." Can others share any experience with concrete mix that would no longer need vibration for proper consolidation? What must be the slump amount? This is my major issue now.
On monday. I'll look for all local concrete experts to examine the columns (if I can find them locally). But I need some knowledge now of concrete mix that won't need vibration to get some idea because in the column indicated above, the bad placement is 1 meter of the lower columns and I can't insert the vibrator in it because of the stirrup spacings of 80mm (and I can't split it into 3 lifts because I can't order 3 concrete trucks for different days just to get a few volumn concrete of the mininum 3 cubic truckload).
Many many thanks to all of you who made me contributed helpful suggestions and ideas.
RE: concrete placement in long column
On top of everything else, you need to hold the contractor to the specification of the stirrups.
Those overlapping corners need to be bent 'in' as shown on your drawing - it stops them popping open if the frame twists due to any reason (eg earthquake)
(also, comparing the drawing and the pick, you're missing 4 20mm bars in the corners, but I'm just going to hope you've given us the drawing for a slightly different col....)
Are you in Vietnam by the way? this feels very familiar.
RE: concrete placement in long column
RE: concrete placement in long column
You asked... "Why suggest demolishing as demolishing the finished beams and slabs above the columns would be very costly."
Answer: So is the litigation after the building collapses, let alone the damage to the reputation of the design engineer and contractor.
You have had a lot of very experienced and knowledgeable engineers giving some pretty sound advice here that you choose to do nothing with but question. I think there is a different agenda here.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com