Civil Engineering Reference
In-Depth Information
that had undergone CP for up to 9 years at about 1 mA/m
2
were studied
by light microscopy (PFM) and scanning electron microscopy (SEM). The
conductive coating had a thickness of about 150 μm, an estimated carbon
content of 40% by volume (density 2250 kg/m
3
), totalling 135 g of carbon
per m
2
of concrete surface. If it had been oxidised at 1 mA/m
2
for 9 years
with
F
(C) = 1, about 9 g/m
2
of carbon would have been oxidised, so ca.
7% of all carbon in the coating. Oxidation would have occurred near the
coating/concrete interface first, then progressively deeper into the coating,
as illustrated in
Figure 6.1.
After 9 years, a zone of at least 10 μm thick
near the interface would have become relatively devoid of carbon particles.
SEM would probably have shown this, which did not seem to be the case.
This would also have increased the electrical resistance of the CP system,
which was not reported. The evidence suggests that not all of the current
is involved in oxidising carbon particles, so
F
(C) most likely had been
significantly lower than 1.
Another source does not relate to a conductive coating CP system, but
may provide useful information. Mietz and colleagues have reported data
on samples taken from a CP system in Berlin after 15 years (Mietz et al.,
2001). This system used carbon filled polymer cables as anode (FEREX from
Raychem) of 8 mm diameter with a 2 mm copper core. A length of 10.7
running metres of cable was used per m
2
of concrete, so the anode/concrete
surface ratio was about 0.25. During the first 7 years the system performed
well, but it did not perform well during the remaining period. Increased
system resistance required progressively increasing the driving voltage for
sufficient protection (tested by depolarisation) in the later part of its life; at
some point, sufficient depolarisation could not be reached any more. Autopsy
of the anode cable was carried out using SEM and microprobe analysis after
15 years. It showed that carbon had disappeared from the outer layers of
New
Aged
+
+
+
+
Conductive
coating
Conductive
coating
Concrete
Concrete
Carbon oxidation
front moving in
Carbon oxidation
front moving in
C + 2 H
2
O → CO
2
+ 4 H
+
+ 4 e
C + 2 H
2
O → CO
2
+ 4 H
+
+ 4 e
Figure 6.1
Model of oxidation of carbon particles in conductive coating anode; left:
particles.
