Continuous Galvanized Rebar vs Hot Dip Galvanized Rebar 2

[STrctPono]

I am referring to ASTM A1094 vs. ASTM A767 rebar. A few years back I was getting solicited from a US based supplier of ASTM A1094 rebar (they shall remain unnamed). They have been in business for 3 years now and I believe this technology comes out of China. I did my research on them right away and concluded that although, yes, the continuously galvanized steel bar process has many benefits for the Contractor... the fact is that the amount of zinc that is going down per ft2 of surface area of reinforcing steel is less for ASTM A1094 vs ASTM A767.

ASTM A1094 - 2 mils zinc thickness specified.
ASTM A767 (HDG rebar) - Class 1 coating is 3.0 ounces/ft2 of zinc works out to 5.9 mil coating.

When I brought this up to the supplier, they didn't deny it and merely stated that in actuality they get closer to 3.0 mils zinc thickness. A non-acceptable answer IMO either way.

The reason that ASTM A1094 reinforcing is so appealing is because it can be bent after fabrication unlike HDG bar. But the reason that it can be bent afterwards is because it has a thinner coating of zinc. I have a current job where they are trying to substitute ASTM A1094 for the ASTM A767 rebar that I specified. I wanted to see if other people deal with these standards and if they have an opinion on the matter....

Thanks.

 

[canwesteng]

Sure, that supplier probably usually gets 3 mils, but the HDG guys probably get closer to 3.5 oz/ft^2. No one wants to throw out or recoat off spec material.

 

[hokie66]

The more zinc, the better protection. Simple as that. Not familiar with the continuously galvanized bar. Who does that? Does it come from China? As to the amount of zinc deposited in the HDG process, it depends on the bars size. The bigger the bar, the thicker the coating.

 

[dauwerda]

According to the paper found here (presented at the 2015 New York City Bridge Conference),

https://www.researchgate.net/publication/301609328_Continuously_Galvanized_Reinforcing_Steel,

the continuously galvanized reinforcing steel process offers just as much protection as traditional hot dip galvanizing does even though it's required thickness is less as noted above. The reason for this is that the continuous process inhibits the zinc-iron reaction (by introduction of aluminum to the zinc bath), so the overall galvanized thickness is almost all pure zinc rather than also having a large zinc-iron alloy layer that is created with the traditional process.

Here is an excerpt from the article:

"The new continuously galvanized rebar is produced by the same method used for the continuous galvanizing of steel sheet. These highly formable coatings can be bent, twisted, stretched and are limited only by the formability of the base steel. The formation of the zinc-iron alloy layers that occurs during the batch process is avoided in the new process by adding a small percentage (0.20%) of aluminum to the zinc bath and by having much shorter immersion times (1 to 10 seconds). Aluminum acts as an inhibitor to the zinc-iron reaction,
forming an extremely thin, iron-aluminum-zinc inhibition layer, and allows the production of an essentially pure
yet very formable zinc coating on the rebar. The continuously galvanized coating above the thin inhibition layer
will have the same essentially pure zinc composition regardless of the chemistry of the steel being coated.

Zinc coatings passivate very quickly when exposed to fresh concrete which enhances the long term corrosion
protection of the galvanized rebars during years of service. The initial passivation of a zinc coating when
embedded in concrete occurs within hours, and is affected by the chemistry of the surface layer. A coating with
a pure zinc layer is known to be more completely passivated than one that is an intermetallic zinc-iron phase
[1]. Galvanizing rebar in a continuous manner using a zinc-0.2% aluminum alloy will produce a galvanized
reinforcing product with 40-60 microns of pure zinc coating that can successfully withstand the subsequent
reinforcing bar forming operations, and has the potential to resist corrosion in concrete to an extent equal to
that of much thicker zinc-iron coatings."​

 

[STrctPono]

Dauwerda,

Great find! I always appreciate your responses.

The article is interesting. The explanation of the corrosion protection of the pure zinc vs. the zinc-iron alloy composition seems logical. I haven't enough chemistry background to critically question its validity. The article does state: "...will produce a galvanized reinforcing product with 40-60 microns of pure zinc coating that can successfully withstand the subsequent reinforcing bar forming operations, and has the potential to resist corrosion in concrete to an extent equal to that of much thicker zinc-iron coating."

If this is all true, it almost seems that this new technology would eventually make hot-dip galvanizing of reinforcing steel rebar obsolete. A method that is cheaper, easier to fabricate, and as corrosion resistant as the original technology...

I suppose I will reach out to the galvanizer and ask them for case studies that run side-by-side corrosion studies of the two different ASTM's. I imagine that someone has done a cracked beam corrosion test of the two. The problem is finding a non-biased study.

I would appreciate any and all additional input.

 

[STrctPono]

Just an update on this for anyone who cares... I got a response from a PhD over at the American Galvanizers Association. He did not address the difference in performance for the pure zinc vs. zinc iron alloy layer of the 2 methods as stated by the research report linked by Dauwerda. For anyone familiar with service life analysis of concrete structures there is the initiation period and propagation period. He stated that for the 2 galvanizing methods, the initiation period would not change but rather the rebar with the thicker zinc coating would have a longer propagation period. He also stated that there is no test data that compares the 2 methods side-by-side.

Since AGA represents the entire galvanizing industry, I would assume that need to be very tactful with their approach to a topic such as this for fear of favoring one galvanizing interest over another.

I ended up rejecting the substitution request. I think I will need to see some actual testing results for ASTM A1094 corrosion performance vs ASTM A767.