White Coating on HDG Rebar 2

[dik]

We have a project where the rebar was left to the weather and there is a white coating on the galvanised reinforcing steel. I suspect that this is a zinc oxide or hydroxide material.

As long as the coating is tight and one is not able to wipe it off, I've permitted this in past.

On a current project, the client is asking for 'back up' information on permitting this oxide coating to remain and that it poses no long term degradation of the rebar.

Can anyone cite a reference doc that states that this is normally harmless to the longevity of the concrete.

Dik

 

[hokie66]

Is this what you have?

http://www.ingal.com.au/FAQ/faq6.html

 

[dik]

I have that one already... also their manual...

thanks, Hokie

 

[TXStructural]

The applicable ASTM standard specification is A767, and the presence of oxidation is not addressed in this specification.
ACI 301-10 also does not address the issue.

ASTM A767 requires chromating of galvanized rebar, and this will reduce or eliminate oxidation of the zinc in storage and in the presence of fresh concrete. Check the supplier documents, or with the supplier, to verify that it was chromated. If it was, a lightly oxidized surface should be accepted.

The mechanism through which galvanization protects the underlying steel is through a oxidation of the zinc preferentially to rusting of the steel. If the material is allowed to sit out, unprotected for long periods, the thickness of the zinc coating can slowly decrease, but it does take quite a bit of time and weathering to result in a loss of durability.

Also, if the material is stored for any period where it is exposed to salts (road salt from storage under or near bridges in winter, within a mile or two of a coastline) the bars should be cleaned to remove chloride salts before placement of concrete.

 

[dik]

TXS:
"If the material is allowed to sit out, unprotected for long periods, the thickness of the zinc coating can slowly decrease, but it does take quite a bit of time and weathering to result in a loss of durability."

Can you provide a citation for this... I'm not disputing this; I agree with it, but cannot find any references. Prior to posting my query, I could find nothing in the ASTM and ACI references you quoted. I've added the reference to chromating to my General Notes.

Dik

 

[TXStructural]

The various conditions of galvanizing dictate the finished thickness of the galvanized product, but the minimum is specified in ASTM A767. The rate at which the zinc will weather is very slow, on the order of 1 micron per year. The best way to determine if there is a deficiency is to test a sample using methods in ASTM A767 (for weight of coating per unit area coated.)

Also, take a look at the materials on this web page:

http://www.galvanizeit.org/aga/know...g-steel-bars-for-concrete-reinforcement-rebar