Civil Engineering Reference
In-Depth Information
on its performance in the field. Monitoring systems can be installed
if required, but at the expense of the simplicity of installation and
operation.
3
Anodes have a limited life and need replacement at intervals that can
be difficult to define. Most galvanic systems aim to provide at least ten
years life before the anode needs replacing. However, systems have been
designed to last up to 40 years.
4
If the concrete is too dry, the system will not work although under such
conditions the risk of corrosion is likely to be reduced as long as the steel
remains dry as well as the anodes.
5
Although well behind impressed current cathodic protection which has
protected over 2 million m
2
of concrete structures, galvanic systems are
progressing rapidly with at least 200,000 m
2
in less than five years of
serious commercial exploitation.
6
There is now a range of anodes. These include:
• Thermal sprayed metals (usually zinc or alloys)
• Encasement systems
•
Adhesive zinc sheet systems
•
Embedded anodes for extending the lives of patch repairs
•
Embedded anodes as discrete anodes.
7 The production of small galvanic anodes for patch repair systems is
a useful adjunct to conventional patch repairs. It is not designed to
provide full cathodic protection and would require a proper cathodic
protection design to ensure adequate protection and a reasonable life of
the patches.
8 The driving voltage of galvanic systems is low and may be inadequate to
provide full cathodic protection in very high chloride conditions.
9 The limited driving voltage may make it attractive for protecting
prestressed structures liable to hydrogen embrittlement and for epoxy
coated reinforcement where electrical continuity cannot be guaranteed
and the consequences of discontinuity are small.
4.3 Application of cathodic protection to highway and
marine structures
The full range of anodes can be applied to highway bridge substructures. In
the example shown in
Figure 4.11,
conductive organic paint coatings were
considered to be too short-lived, and thermal sprayed zinc was offered as an
alternative to conductive mesh plus overlay. It did not change the dead load
or the profile and was a lower-cost option. However, the cantilevered ends
were protected by probe anodes as they were more exposed to water run-
down, as well the less accessible diaphragm walls between the longitudinal
steel beams.
