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.

 

[khinz]

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.jpg

 

[khinz]

pls. show any references proving that inside the segragation sections with soft parts removed (picture above) the the mix is bad and even though there is no voids, the compression stengh is merely a fraction.. say 500 psi. I need this material because management told me when the slumps thrown from upwards, they all splash to the sides and it is the sides which has the problem and not middle or inside the column. I want references to refute this.

 

[Ron]

You have to be convinced to stop work on this? I'd have to be convinced to let it keep going...and that's not likely.

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.

 

[khinz]

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/repairformworks.jpg

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.

 

[hokie66]

khinz,
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.

 

[Ron]

khinz...this just keeps getting worse!

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.

 

[a7x1984]

khinz: I am troubled by the content in your posts and responses. You have been urging (and nearly pleading) Ron et al to give you a golden answer that will solve your concrete problems in their entirety seemingly in lieu of a competent project manger.

we will use site mix for 1-2-3 concrete combo... do you think site mix can ever be 4000 psi?

- 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!

 

[khinz]

Ron and others. First many thanks for all the assistance.

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.

 

[Ron]

Yes. Order the truck. Get as many columns prepared as you can, then order the truck and place the concrete in all of them from the same truck.

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.

 

[hokie66]

khinz,
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.

 

[dik]

I stopped replying because it was starting to make my head hurt!

Dik

 

[khinz]

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.

 

[patbrry]

Khinz,

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.

 

[Ron]

Agree with hokie66 and dik...I'm done.

 

[msquared48]

khinz:

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