Civil Engineering Reference
In-Depth Information
thickness of 50 μm, the limit state function for bond loss Z
b
in mole of acid
per square metre is:
Z
b
= 0.24 - 0.023 *
I(A) * t
(6.10)
with
I
(A) current density [mA/m
2
] and
t
time [years].
This expression applies for current densities between 1 and 20 mA/m
2
.
As an example, for a current density of 1 mA/m
2
it would take ca. 10 years
to reduce the bond strength to the criterion, from 1.5 to 0.5 MPa. This is
unrealistically short: many coating CP systems in the field still have sufficient
adhesion after (much) more than 10 years. This may be because the bond
plane thickness is taken too low, the current density is lower than 1 mA/m
2
,
the bond criterion and/or the NADF of 0.07 from the titanium study are too
pessimistic, or simply that the initial bond strength is higher. Other studies
suggest that the reduction factor is between 0.1 and 0.01. The present model
is apparently conservative, possibly by a factor of two. Further numerical
modelling work may help to resolve this.
The model applies in homogeneous situations, which may not always be
the case in practice. An inventory showed that local loss of bond occurred in
some CP systems in less than 10 years. In at least one case, it was reported
that the silicone joints between gallery slabs were leaking just above beams
with CP that showed loss of coating bond. Apparently, moisture ingress had
reduced electrical resistivity, increased current density and locally produced
excessive amounts of acid.
6.3 Field performance of CP systems in The Netherlands
An inventory was carried out among companies in The Netherlands that
installed CP to concrete structures. Object identification, year of installation,
type of structure and protected area and anode type were recorded. Also
they were asked about maintenance, repairs and failure of components. The
inventory produced the following observations.
Between 1987 and ultimo 2004, about 75 CP systems were installed on
various types of concrete structures like bridges, apartment and office blocks
and parking garages, with a total protected area of 65,000 m
2
of concrete
surface. This does not include about 40,000 m
2
of (small) precast ground
floor systems (Schuten et al., 2001). Some systems were installed in multiple
phases or had multiple anode systems. Most of the 75 cases are monitored
regularly and work well. No corrosion-related damage has appeared as far
as we know. Some systems have been neglected, however, and some cases
have suffered from vandalism. 52 cases were well documented and had an
age of at least 2 years. Their age distribution is shown in
Figure 6.4
and their
performance information is discussed in more detail.
Until 2004 there were 16 CP systems older than 10 years. Information on
four of them is absent. One has been neglected and does not work properly
