Civil Engineering Reference
In-Depth Information
the polymer cables, down to a maximum of about 2 mm depth. The total
carbon content (supposedly graphite plus polymer) was reduced from 90%
down to 74% in the degraded outer layers. These changes had occurred
over depths of typically 0.5 - 1.0 mm and the mean current density was
2 mA/m
2
(Mietz, personal communication, 2006). The amount of carbon
oxidised in 15 years estimated from these analyses is equal to or greater
than what corresponds to equation (6.5) with
F
(C) = 1. An explanation
may be that the investigated samples had undergone a higher local current
density than the average. Unfortunately, this renders these data unsuitable to
obtain a value for
F
(C). However, this case led us to the conceptual model
of a carbon oxidation front gradually moving into the carbon-filled anode
material, resulting in increased resistance.
Some other sources provide qualitative data. The performance of a Ferex
system at Rotterdam (Schuten et al., 2007), was more favourable than that
of the Berlin system. In Rotterdam, the anode system had not degraded
significantly over 12 years of operation, as observed by light microscopy.
Another Ferex system protecting 288 balconies at Capelle (NL) seemed to
operate satisfactorily after 17 years (Nuiten, private communication, 2005).
These cases may indicate that
F
(C) can be well below 1.
A laboratory study (Eastwood et al., 1999) on carbon-filled polymer
anodes (similar to Ferex) reported oxidation of carbon particles at high
current efficiency in acid and neutral solutions (
F
(C) = 0.8 to 1). At high
pH (> 11) and relatively high current densities, the carbon oxidation
efficiency is low at about 20% and the remaining 80% of the current
releases oxygen by reaction (6.1), so
F
(C) = 0.2. Most of the carbon seems
to be oxidised to humic acids. Carbonate ions promote oxygen release and
slow down carbon oxidation (at alkaline pH down to pH 9). Oxidation
of carbon was found to increase the electrical resistance, very strongly at
low pH and only moderately at high pH. Qualitatively, this corresponds
well with the Berlin CP case. Taking into account that practical conditions
may be more adverse than in the laboratory, we conclude that for the
investigated anode material under normal CP operating conditions (a high
pH, a relatively low anode potential and a low current density),
F
(C) may
be about 0.3. Of course this value may differ for different carbon-based
anode materials.
Summarising, a conductive coating CP system can fail due to progressive
oxidation of carbon particles from the anode/concrete interface. At some
point in time, the electrical resistance of the coating layer that has become
devoid of carbon particles is so high that sufficient current cannot be
transferred to the concrete. Consequently, the system fails and the end of its
working life has been reached. A similar type of failure has been reported for
early carbon-based systems (Brown & Tinnea, 1991), particularly with high
local current densities.
