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Re: Carbonation Induced Corrosion

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Carbonation is a very slow process, and dealing with it in existing
buildings is a relatively new issue.  I expect that the decision to use a
coating, or other procedure may need to be based on theory rather than
experience or observations of effectiveness.

Sika has a couple of products that are intended to deal with corrosion due
to carbonation.  One is a coating that is designed to block the migration of
Co2 into the concrete without blocking water vapor transmission.  Another is
a treatment by impregnation of an aqueous solution that is supposed to
migrate to the steel and form a protective barrier on it.  I've also heard
of a treatment that is intended to reverse the degradation of alkalinity in
the concrete by long-term application of a dissolved chemical from a
saturated blanket, sometimes assisted by an induced electric field -- I
don't know of a product for this.

You can test for carbonation using a sprayed-on phenothalene (the litmus
paper chemical) solution; low alkalinity (pH less than 7) is indicative of
carbonation, though rebar corrosion begins at a pH below about 9.  Your
local testing lab is probably able to provide the service; include
carbonation testing in the lab work.

You may have damage that can't be repaired by protecting against or
reversing the carbonation process.  Cracks of up to 1/2" are likely to have
resulted in separation of some reinforcing from the adjacent concrete.

Carbonation is a BIG problem that we are just beginning to get a handle on.
My rule of thumb is that reinforced concrete has a serviceability period of
100 years plus or minus a few decades because of the slow relentless
migration of Co2 into concrete over time.   If carbonation is the problem
your building is a little ahead of schedule at a little over 30 years, but
the rock finish and freeze-thaw may be contributing culprits.  Uncracked
concrete is less susceptible to carbonation -- cracks provide entryways into
the concrete for Co2-carrying air.  A stucco application onto the surface
(under the resistant coating) may be a way of closing cracks and helping
minimize air migration into the walls.

I've been trying to follow some of the dialog among concrete professionals
on durable concrete that has been underway for a few years.  Some propose
high-performance concrete [hpc] (high cement, low w/c, silica fume or other
pozzolonic admixture, good curing procedures).  Other propose lower
cementitious content on the theory that the high early strength of hpc
results in early cracking with accompanying entryways for harmful
intrusions.  I'm having trouble finding hard data from the low cementitious
folks.  If anyone has any information on their work, I'd appreciate hearing
about it.

Nick, I don't think your previous post got to my mailbox.

Nels Roselund
Structural Engineer
South San Gabriel, CA

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