Wrapping Existing Column With New Concrete - Low-Shrinkage Concrete Mix 4

[KootK]

I'm reviewing a design where an existing round concrete column is being encased with 6" more concrete all around. Roughening for composite behavior, new cage, and all that jazz.

I have this picture in my head where the new concrete tries to shrink but is restrained from doing so by it's bond with the existing concrete. That, in turn, result ins distributed cracking and puts the new longitudinal rebar in tension and compression alternately from one crack to the next, in effect, prestresing the original column.

So my questions are these:

1) Am I considering this correctly?

2) With a prestressed column that one might get from a precaster, the prestress does not add to the axial load on the column for stability purposes. The same would be true here, yes?

3) Would you be specifying a low shrinkage mix for this application? For what it's worth, the contractor would like to use SCC for obvious reasons.

4) If you were to specify a low shrinkage mix, how would you determine the performance characteristics to specify here?

5) The new concrete will be rather heavily reinforced. Should that effectively eliminate my shrinkage concern on it's own?

This is a stocky parkade column that is in need of additional compression capacity. Moments and slenderness will be ancillary concerns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

4. Your drawing depicting radial shrinkage is not correct in that radial shrinkage will cause the concrete surrounding the column to pull tighter to the column.

After thinking about it I agree, that makes more sense that you're taking the circumference dimension and making it smaller. I'm sure there's some thickness loss as well but I imagine it's insignificant compared to the change in circumference. To satisfy my curiosity I'll have to take a close look at one of our manholes coming out of it's form to see if I can tell which way it shrinks.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Guastavino]

Kootk,

1. Get an apple, cut a slice of it (enjoy the rest) to fit perfectly around a wood dowel.
2. Wait a few days and observe how it shrinks.
3. Bill your client for the apple.
4. Fill in the gaps with engineering judgment.


But seriously, I would guess concrete shrinks somewhat similarly