CFRP for over reinforced column

[PSR_1]

Hello! In what way could we utilize CFRP( carbon fiber) for columns that are provided with more rebar than the maximum allowed by the code? It is clear that CFRp can add the lack of ductility due to over reinforcement, but what design approach can be used? What design assumptions could lead to the solution?

 

[SWComposites]

CFRP wraps have been used for seismic strengthening of columns and for other RC repairs. There are several specialty companies that do this work. Have you researched online? Also, look for past articles at compositesworld.com

 

[PSR_1]

Hello! Yes I have checked, the case isn't covered by any research I have come across.

 

[SWComposites]

well, then give us a lot more details about your structure and what you are trying to do.
is this a repair? or seismic retrofit? or something else?

 

[KootK]

The usual column maximum rebar limit is about bar congestion. I'm not sure that matters in the context of a column that has already been successfully constructed.

Or are we speaking of a column with significant flexure that is over-reinforced for bending? If that is the case, then confinement of the concrete in the flexural compression block might be exploited to increase the allowable compression stress there which would improve matters, effectively, by increasing f'c. Stronger concrete requires more reinforcement to reach the undesirable point where the compression block would fail prior the tension resisting elements being mobilized.

 

[lexpatrie]

It MATTERS in the context of a column that has been "successfully constructed" i.e. Harbour Cay and Champlain Towers South had too much rebar for the size of the column. The limitations in the code are to prevent voids and allow reasonably unobstructed flow of wet concrete so you actually get what you're using in your calculations versus a network of rebar tied together with wires and no concrete besides the skin and toothpaste/mortar stuffed into it after the forms are removed to hide the fact and make it look like a real concrete column.

I have some fairly highly placed folks as former professors, when I asked how one would responsibly fix an overreinforced concrete column, I got a bit of a "we're going to research that shortly" answer. So you're on your own there, buddy.

If you can rule out there are voids in the concrete, provided it's a column, I suppose that's progress. What the CFRP is going to get you, I don't know. You're outside the research, as you clearly understand, so good luck. In-situ load testing?

This kind of flaw in a concrete building is extremely basic and VERY easy to diagnose, so everything, everything else in the design needs a very close look because it screams inexperienced designer, incompetence, bad supervision, etc. etc. etc. So there's the strength at the slab joint, as there are limits on the concrete strengths there, the depth of the slab, punching shear, actual depth of the rebar, stirrup size and spacing, lap lengths, etc.

(KootK - It's funny I found some odd thread recently from 2001 where you said you were relatively new. Here we are 23 years later). Same forum.

 

[KootK]

In my book, a "successfully constructed" column is one that doesn't have voids. In the big highrise markets people are commonly using very, very high reinforcement ratios in the zones by employing alternate bar splicing methods, self consolidating concrete etc.

At this point, I basically consider the 4% thing to be a soft limit mostly applicable to small time, back woods contractors. And even in that context, I'd not pitch a fit over an existing column with 5% reinforcement instead of 4% unless I had some, tangible reason to be suspicious of the quality of construction.

I've seen plenty of 1.5% columns that have turned out terribly with spalling and voids. In my book it's more about skill and craftsmanship than anything else.

 

[KootK]

(KootK - It's funny I found some odd thread recently from 2001 where you said you were relatively new. Here we are 23 years later).

Sounds about right. I started my first, non-pre-ENG truss gig in 2000 and came in with a lot of questions.

 

[MHobbs]

KootK I read the first three words and saw the parenthesis, I was excited thinking you had written a book on concrete reinforcement.

 

[KootK]

I wish. Sadly, my knowledge of concrete reinforcement could probably be summed up nicely in a modest pamphlet.