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Corrosion damage / Earthquakes

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Gail Kelley wrote:
"Corrosion of reinforcing steel is usually more of a
serviceability (cracking, 
rust-staining) issue than  a structural (safety)
issue.  

Corrosion damaged reinforcement could possibly be a
very significant issue 
during seismic (or wind) loading, however.  I'm not
sure how often loss of 
cross-section of the steel is an issue, given that
corrosion is usually pretty slow 
but loss of bond between the concrete and steel  could
be important - you 
might have essentially unreinforced members over a
great length.  In addition,  
loss of stiffness of the concrete member at critical
locations could be 
important."


Gail,
I think it might be more accurate to say that the
serviceability issues often show up before the
structural issues.  The usual order is:
1. Minor cracking and staining.
2. Cracking significant enough to disturb, then
destroy bond.
3. Section loss.

The big exception to this is in post-tensioned
construction, where the ducts may contain any staining
and perhaps leave room for expansion, delaying the
cracking that might warn of problems.  Section loss
adequate to allow the tendon to break may be the first
sign of trouble.

A lot of bridge decks, after about 15 years of
marinating in de-icing salts, will be found to be
completely delaminated.  The top mat is thus pretty
ineffective.  The decks still function pretty well;
maybe we don't need to pour all that rebar into our
decks.  The people in Ontario figured this out 20
years ago, and use a fraction of the deck rebar that
AASHTO mandates (maybe 25%).  As far as I know, their
bridge decks work as well or better than ours, because
they don't have all that expanding steel trying to
delaminate them.  AASHTO deck design is based on a
rational one-way continuous beam analysis with very
conservative effective (wheel distribution) widths. 
Ontario's code more realistically recognizes a two-way
arching action, restrained by the surrounding deck.

I'd be very interested in comments from someone with
better first-hand knowledge of the performance of
these decks.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota



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