[Ferro List] Re.:More On Tom's Crud Testing (Keith)

[Keith B]

Hi Chris

I suspect that I've found more than you on galvanizing problems, but 
you're experience is the same as mine regarding Tom's reports - I found 
absolutely nothing similar anywhere.  Re. asphalt: as I came back on 
line, I was working simultaneously on an offline question from Uwe, on 
Tom's problem(s), and brooding on a bucket of asphalt emulsion which had 
become accessible.  All prompted an excursion to briefly study asphalt 
emulsion technology tying back to old patents in my files, and I found 
myself warming to it as a much undervalued resource.  Yes, probably both 
in principle and in practice you'd likely develop a yielding film on 
your wire, but maybe not as bad as you think, such may actually be an 
advantage, and durability of the reinforcement might improve by decades 
to centuries.  More on that for another post or back channel discussion; 
Tom's experience is a more current and more important issue.

There's nowhere near enough published (with easy free access) on the 
behavior of galvanizing in cement.  Those who sell galvanized 
reinforcement are positive on its benefits, but lack depth (or mention) 
of its problems and disasters.  From users and potential users, there's 
more balanced treatment, generally positive, but very positive economic 
assessments sum to, 'it's a cheap way of substantially increasing 
structural durability - certainly cost effective - if we could trust it."

Galvanizing has been observed to work extremely well, but also to fail 
with little benefit.  Where it works, 5-25% of the galvanizing is 
sacrificed during emplacement and early cure due to chemical attack and 
formation of the calcium zincate long term barrier, after which 
corrosion proceeds at microns per year to extend life 40-50 years.  
Premature failure seems to reflect a compromised coating or excess 
chloride in the original mix (rather than later intrusion).  Don't goose 
cure with calcium chloride...

 From Galvanize vendors, I particularly note a suggested contractual 
requirement to limit reinforcement bends to no more than 10 degrees, 
rebar OR WIRE (my emphasis).  Along with that goes a specification for 
treating damage with zinc loaded epoxy paint and a limit on the number 
of sites allowable per bar.   That suggests fragility of the coating 
(and/or its passification) is a big issue impacting handling, 
emplacement and original design.  It also appears to rule out 
galvanizing for most FC; unrolling material violates the bending limit 
and typical ties are too frequent and damaging.

Most of the good information on passification comes from makers of 
galvanized steel cooling towers, where the life of the galvanizing is 
the life of their products, such are man rated and the mechanisms are 
obvious to the user/buyer.  Nothing like incentive to promote 
activity...   From both research papers and manufacturers info, 
galvanizing differs by method but we probably only need to consider hot 
dip.  That does not leave just leave a simple jacket of zinc.  What 
results is a series of zinc/iron alloys with the iron content decreasing 
outwardly to a final notionally pure layer of zinc forming 5-20% of the 
coating thickness.  Notionally pure because the zinc bath may actually 
be itself an alloy or variously contaminated.  That's a trap for buyers 
of cheap galvanizing because the vendors won't tell you or likely don't 
know.  Chromating is known to be beneficial on galvanized stock to 
resist "white rusting", but it's actually used routinely by the better 
galvanizers as the lst step in manufacture.  Customers who buy bright 
galvanizing get little or no chromating; dull galvanizing is either old 
bright stock or well and carefully made stuff with good chromating.  
Note that the manufacturer's chromating is not intended as a service 
coating, just to preserve the product in good condition until the buyer 
is ready to use it. 

Galvanizing in storage or for exposed duty relies on "natural" 
passification by a zinc carbonate layer.  Bright zinc first oxidizes, 
then the oxide hydrolyzes to the hydroxide, picks up carbon dioxide from 
the air or carbonic acid and forms a complex carbonate hydrated 
hydroxide resistant to further action and covering the zinc below.  
That's long been referred to as "natural passification" and is all a 
cooling tower manufacturer can use because chromating is streng verboten 
for anything to do with water supplies.  Accordingly, manufacturers have 
put much work into studying it and defining stringent initial operating 
conditions for new water towers to form and later maintain a sound 
coating.  pH is critical.  Never below 6.5 and not much over 9.0. 

It follows that an FC worker can have his galvanizing thoroughly messed 
up by acid rain - oldsters: see more rusty corrugated iron than you used 
to? - and also follows that "natural passification" is rapidly attacked 
at the pH of fresh OPC mortar.  That's a race which pretty much stops 
with early cure, perhaps first cure (no data) and a carbonate layer 
handicaps the cement. Action stops, as noted above, with the formation 
of a calcium zincate replacement for passification.   Obviously, speed 
of cure is a factor.  Noted:  Keep w/c ratio below 0.6 and chloride 
below 1%.  Here chromating is the gold standard.  Chromate in the OPC 
will do it or help, but a chromate dip before emplacement is better.  
It's unclear whether it protects directly or promotes favorable crystal 
form, or both, but the effect is to minimize loss of zinc prior to 
formation of an effective calcium zincate layer and perhaps to increase 
bonding to the forming cement matrix.

Nasty notes:  From several sources, it seems that there are two 
alternative crystal structures which can form in natural passification, 
one good and the other useless crud or worse.  No details though.  
Putting bright zinc into fresh mortar was said to cause action rapid 
enough to evolve bubbles of hydrogen.  In what I can find now, that 
action was reviewed from the perspective of potential hydrogen 
embrittlement of the steel and not seen as a major issue.  I have, 
though, memory of having something in my files from about 10 years back 
saying that such hydrogen evolution could harm the properties of mortar 
adjacent the reinforcement, but that it became  non-issue with brief 
weathering of bright galvanizing. 

It's the reported denaturing of the matrix which is the root of the 
importance of  Tom's unfortunate experience.  Again, I can't find 
anything like that now, but does galvanizing weathered in an acid rain 
environment, with the bad/wrong crystallization, or flexed, with perhaps 
a brittle or undesirable alloy set, expose much bright zinc?  Perhaps in 
cracks which expose steel to start electrolysis?  Tom has offered to 
send me samples of his mesh and locally bought OPC, so I'm thinking hard 
about test design.  Comments and thoughts welcome and references for the 
above to anyone who wants them.

kb

Christopher Glasspool wrote:
> Hello Keith,
> I'm glad your back at posting again. Your writing or voice often reminds me of my old discussions with my Dad.
> What you say on this makes perfect sense except that I'm not seeing any problem anywhere on the Internet on this. See My part 1,2,3 posts to Tom.
> What I don't like about an asphalt emulsion barrier is that it is so closely resembles the release form materials that I've used. It might lubricate the reinforcement! Should I find with my testings that a intimate bond problem occurred, I would be more likely to try an acrylic bonding agent like Acryl 60 sprayed on the mesh, but at this juncture I don't expect to find a problem, from what I'm reading and investigating.
> One more test than what Tom as tried would be some sort of empirical pull out test, maybe using a block of mortar with plain steel and galvanized and treated nails halfway embedded, that gets pulled after curing. - chris
>
>