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Fwd: Corrosion and durability of reinforced concrete

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Just this morning I attended a CRSI seminar on corrosion of rebar in
concrete.  The speaker was Donald Pfeifer of Wiss Janney Elstner -- very
knowledgeable.  WJE is doing a comprehensive study of rebar corrosion
for FHWA, begun in 1993 and slated to conclude in 1998.

Since there has been some discussion of this topic here recently, I
thought I'd summarize the most interesting points for the benefit of the
listserver readers.

1.  The rapid chloride permeability test (AASHTO T-277) cannot be used
to compare the permeability of normal concrete to fly ash, silica fume
or slag cement concrete.  The differing chemistry of these pozzolans
skews the results and makes them appear less permeable than they are. 
There are currently quite a few lawsuits making their way through the
courts because of unrealisticly low (<1000 coulombs)test values
specified for concrete.

2.  High strength concrete is in some situations (ie, bridge decks)
worse than the old 6 bag / 3000 psi concrete.  Why?  Because it has
quicker strength gain, higher modulus and lower creep, so when
restrained (by bridge girders, for example) it cracks, where the old
concrete would creep.  One possible solution is post tensioning.

3.  The more cover the better.  Chloride content decreases geometricly
with cover.  Where there is a crack cover is reduced or eliminated, so
it is important to minimize cracking and to repair cracks.  A MinnDOT
study was cited of some bridge decks that received Iowa low slump
concrete overlays where were very effective in retarding chloride
penetration.

4.  Standard epoxy coating is better than black steel.  Of the thousands
of epoxy rebar structures built in the past 20 or so years, very very
few have had problems.

5.  For high value or difficult to repair structures in corrosive
environments, stainless steel rebar looks promising.  It adds 5 to 10%
to the initial cost, but predicted life is 75 to 100 years.  In England
they have had pretty good success with stainless clad rebar. In Europe
stainless mesh is reported now being used in tunnel linings.

6.  Epoxy coated rebar should be electrically isolated from black
rebar.  It is common to see an epoxy coated top mat and a black steel
bottom mat in bridge decks, or to have a stay in place galvanized form. 
If these are electrically connected, a galvanic cell is created.  [My
understanding on this point is not too clear.  Would the galvanized deck
cause corrosion of the epoxy rebar, or would it act as a sacrificial
anode?]

7.  Testing is not complete, but the corrosion inhibitor concrete
additives look promising.  A "belt and suspenders" approach is
recommended -- good cover, corrosion inhibitor, and epoxy rebar, for
example.

I hope you listreaders find this useful.  Additions and corrections are
encouraged.

-Jay B.


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