Coating a specific length of a rebar with epoxy and leaving the remaining length black. 3

[mmmghorab]

I'm sorry for this stupid question.
Case: The column dowels must be epoxy-coated and extend above the ground level with 1m, then black steel can be used.
The common practice is to make the lap splice between the coated dowels and long. black steel above that limit, which for a reason it's not possible.

Q. Is it possible to coat a steel rebar to a specific length with epoxy and the remaining part kept black ? what about the bond at the section between coated and uncoated parts?
Please I need an quick answer through the specs.

 

[MiketheEngineer]

Sure it can be done - but why bother. Just coat the whole thing. While epoxy is not cheap - it really isn't that expensive and you will sleep better.

 

[dik]

... and it may corrode at the junction of the epoxy and black iron (steel?)

Dik

 

[Ron]

dik is right. A concentration corrosion cell will develop at the interface, causing an elevated rate of corrosion.

 

[dik]

Ron is right... Dik is right <G>... but, hardly worth a star...

Dik

 

[Ron]

Nay Nay, dik! It is THE cogent point to consider!

 

[mmmghorab]

MiketheEngineer. I'm the consultant and totally agree with you. But the contractor wants to paint the rebars partially as I said above. I searched all the specs and didn't find any text allows or prevent this action.
Is there any specs, research or text book ensure our opinion ?

dik & Ron. Do you have any specs, research or text book for that ?

 

[hokie66]

If the contractor intends to paint the bars on site, just tell him NO! Epoxy coated steel, done in the manufacturing plant, has problems with corrosion cells where damage occurs. Painting bars in the field is silly. Concrete protects steel, why isolate it?

 

[dik]

Hokie... I'm not a big fan of epoxy bars... but I'm aware of corrosion cells and the problems associated with partially repairing rebar in parkades... forcing greater corrosion away from the initial site... I'll see what I can dig up for Mike on the weekend...

Dik

 

[sybie99]

May I ask a question relating to this, why or when would one use an epoxy coating as opposed to galvanizing of reinforcement?

 

[hokie66]

Some like it, some don't. I consider galvanizing much superior to epoxy coating.

 

[Ron]

Agree again with dik and Hokie66. I am attaching a couple of articles in the next couple of posts for your info. One of them was done by a "Hokie"!

 

[Ron]

First article....

 

[Ron]

Second article...I've noted this one in the title as being "biased" since it is from a galvanizing group; however, it does contain some factual info and some points to consider....

 

[Ron]

...and finally, a PhD dissertation done by another "Hokie"...note that one Brown's PhD committee members is also a co-author of the first article (and others on epoxy coated rebar).....

(Note...Brown finds only a 5 yr. service life increase with using ECR...not much in the scheme of things, relative to cost and effort)

 

[dik]

Thanks, Ron... now that warrants a star!

Dik

 

[TXStructural]

First, do not use an epoxy "paint". There will be issues with bond between the steel and the coating. Epoxy-coated rebar is fusion bonded (think powder coated), which is applied within seconds of being sand blasted, and a quality assurance process still is required to be sure the bond is sufficient.

Second, connecting coated and uncoated bars results in a macrocell corrosion potential. This is where small holes in the coating result in significant local corrosion because the relatively large uncoated bar acts as an anode, sinking free electrons which would be "trapped" within the coating of an isolated, fully-coated bar. Corrosion is an electrical process, and the coating acts as a barrier to flow of electrons.

Do not partially galvanize anything. This will result in the partial zinc coating acting to protect both the coated and uncoated bar, leading to premature oxidation of the zinc, resulting is loss of protection.

Epoxy-coated bar is not really very expensive, particularly since you are not talking about a large quantity.

As to the epoxy bashing, there are 100,000 bridge decks in the US using epoxy-coated bar. Of these, fewer than 200 have experienced premature deterioration. The product is used every day in every state, because it represents the best value for tax dollars under most circumstances. Where the exposure is severe or construction is expected take its toll on the coating, stainless is frequently used. Galvanized bar is also very good in most applications requiring corrosion resistance, but provides limited protection in some conditions (the zinc coating is sacrificial so the life is limited by that.) The first line of defense against corrosion is proper cover and concrete quality. Make sure there is proper consolidation and shrinkage is minimized to avoid excess cracking.
The problems in Florida and Hawaii were related to handling problems, and macrocell corrosion where black steel was mixed with epoxy. Both of these are addressed in the relevant design and construction standards, some of which were changed when macrocell corrosion was identified after these events. Handling is addressed in industry publications and the ASTM A775 spec. The coating is tough, but if they tear up the coating during construction, it certainly can't be expected to work properly. FHWA has long term projects which are showing good performance of galvanized, epoxy, and stainless bars under very harsh conditions. We expect publication of these findings very soon.