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Reinforced
concrete is one of the most durable, versatile and widely
used construction materials. However, occasionally it
does not give the low maintenance life expected of it.
Sometimes this is due to incorrect expectations, sometimes
to inadequate specification or construction and sometimes
due to more adverse conditions than initially expected.
Consequently, there are many structures in the built
environment suffering from corrosion induced damage.
The
one estimate from the USA is that the cost of damage
due to de icing salts alone is between $325 and $1000
million per year to reinforced concrete bridges and
car parks. In the UK the Department of Transport estimates
a total repair cost of £616.5 million (approximately
one billion US dollars) due to corrosion damage to motorway
bridges. These bridges represent about 10% of the total
bridge inventory in the UK. The total problem may therefore
be ten times the DoT estimate. There are similar statistics
for Australia, Europe and particularly the Middle East
where the warm marine climate with saline ground conditions
increase all corrosion problems. Corrosion control is
made more difficult by the problems of curing concrete
in hot, drying environments and has led to very short
lifetimes for reinforced concrete structures. Deterioration
occurs on buildings and other structures as well as
bridges.
As
concrete is porous and both moisture and oxygen can
move through the pores and microcracks in concrete,
the basic requirements for corrosion of mild or high
strength ferritic reinforcing steels are present. The
reason that corrosion does not occur in most cases is
that the pores contain high levels of calcium, sodium
and potassium hydroxide, which maintain a pH of between
12 and 13. This high level of alkalinity passivates
the steel, forming a dense gamma ferric oxide that is
self maintaining and prevents rapid corrosion.
In
many cases any attack on reinforced concrete will be
on the concrete. However, there are two chemicals that
penetrate the concrete and attack the steel without
breaking down the concrete first. The culprits are chlorides
and carbon dioxide as these are the main common atmospherically
borne species that penetrate concrete without causing
significant damage and then promote the corrosion of
steel by removing the protective passive oxide layer
on the steel, created and sustained by the alkalinity
of the concrete pore water.
There
are many texts covering the mechanisms of corrosion
in concrete and assessment techniques as well as specifications
and recommended practice documents on how to select
and apply repair methods (e.g. NACE RP 0290 and RP 0390,
CEN EN 12696 and ENV 1504, ACI 222R etc.) |
