Does spray painting rebar with Construction Marking Paint effect bond between rebar and concrete?

[wijgeng]

I am curious if anyone has any experience or knowledge regarding the effect that construction marking paint applied to non-epoxy coated rebar might have on bond strength (i.e., development length). Specifically, we are constructing large, circular foundations that require personnel to walk on top of the top mat of rebar. In order to "improve safety", it has been decided that they will spray paint a 4' wide "walk-way" all the way up the foundation. This results in a 4' wide by 30' long strip of rebar that is covered in paint (at least on the top half of the bars).

My concern is that this might act to debond the rebar from the concrete which would result in an increase in development length (similar to classifying these bars as epoxy coated rebar). However, I think that this case might even be worse than epoxy bars because the epoxy is fused to the bars whereas this spray paint is simply applied to the surface of the bar.

Any thoughts or concerns on this practice?

Thank you!

 

[TehMightyEngineer]

Does the development length work if it was epoxy coated?

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

https://www.facebook.com/AmericanConcrete/

 

[dik]

Probably very little unless a lot of area and a thick coat.

Dik

 

[KootK]

Everything affects bond to some degree so the all the way conservative approach would be to not allow it. Here, personally, I'd be okay with it given that;

1) I've seen a little research indicating that it's not big deal.
2) The mechanical action of the knurls is still in play and counts for a lot.
3) You're only painting one side of the bars.
4) In one direction, it's just some bars of many.
5) In the other directions, it's just a part of the span in the field.

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.

 

[Jerehmy]

Since light mill scale, rust, or a combination of both need not be removed from reinforcement, I think a very thin layer of paint would also be OK.

 

[dik]

light mill scale and rust actually improves the bond...

Dik

 

[DETstru]

Most substances don't impact bond at all.

Here is an article from the American Society of Concrete Contractors:

https://www.ascconline.org/Portals/...3-coatings-that-affect-bond-reinforcement.pdf

Linked here is one of the studies they reference (click download to see the pdf):

http://www.concreteconstruction.net/how-to/materials/how-clean-must-rebar-be_o

Linked here is the other paper they reference:

https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&ID=12535

 

[wijgeng]

Thank you everyone for the responses.

MightyEngineer - I am not sure if the bars work as epoxy coated. We are not the engineer of record.

KootK - I agree with your points. All of these concepts "help us out" in this situation.

DETstru - Interesting articles. I am very surprised that bond breakers do not affect bond as is stated in the ascconlne.org article..

I've attached a photo showing the current practice.

On a similar note, if none of these substances (e.g., form release agents, bond breakers, cement splatter, and potentially spray paint) effect bond, why does epoxy coating effect bond? Is it more related to the thickness of the epoxy coating that reduces the effectiveness of the knurls?

 

[Guest262653]

Could you paint only the border lines of your "walkway"?

On a side note, if such a footing had any problem due to this spray painting, the problem wouldn't be the spray painting...

 

[wijgeng]

avscorreia - They potentially could paint the border lines, they actually have some pictures of that as well. However, I don't think spray painting a walkway helps anything. If it were up to me, I would probably just place a plank on top of the rebar with lugs on the bottom that "lock" into the rebar so it doesn't shift. Then they can paint the ramp any color they want to and it has no impact on the foundation.

I am curious, what does your statement about the footing having problems that are not due to the spray painting imply?

 

[Guest262653]

In the year I spent on a bridge construction site, we also used planks for rebar "walkways". This was an advantage also to keep the rebars clean of dirt. Safer and cleaner.

My statement was just a smart ass comment. Sorry for not being clearer. What I meant was that the impact of the painting on a foundation of that size was certainly negligible and that if it had any problems it wouldn't be because of the reduced bond on such a reduced length.

 

[DETstru]

wijgeng, that looks like a really interesting foundation! Do you have any more photos?

 

[wijgeng]

avscorreia - Haha, thanks. I just wanted to make sure you didn't spot an error with your eagle eyes!

DETstru - Yes it is an interesting foundation! It is a wind turbine foundation. See the attachment for a photo of a different foundation.