Civil Engineering Reference
In-Depth Information
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A detailed condition survey is very important for the correct configuration
of an ECE application. It helps to focus the treatment on the relevant
areas and to find criteria for planning treatment zones and application
strategies.
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ECE is not limited to the space between concrete surface and upper
reinforcement layer, but is effective as deep as reinforcement is present
as a cathode. In most cases, chloride has also accumulated behind the
upper reinforcement layer, so it can be removed by ECE. This requires
multi-stage treatments with some weeks' pause between the applications.
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It is even possible to pull chloride through a 20 to 30 cm thick slab if the
chloride containing concrete surface is not accessible. This requires at
least a three-stage treatment.
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The ECE efficiency can be raised by using a pulse width modulation
(PWM) mode which is more effective than permanent operation
(Schneck et al., 2001). The chloride transport is forced along the field
lines, but happens within the capillary pores that have other directions.
So chloride can get caught in sack pores and be released during a switch-
off period. The benefit of running ECE in a pulse mode has been verified
recently (Elsener and Angst, 2007).
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In combination with the alkalisation coming from the reduction of
oxygen and water on the reinforcement surface, some residual chloride
can be accepted without ongoing corrosion activity.
The durability of an ECE depends on the concrete quality and the
environmental influences on the structure after finishing the rehabilitation.
If applied on unprotected bridge decks, 15 to 25 years are expected until
new chloride-induced corrosion activity can be measured (Whitmore,
2001). If ongoing chloride ingress can be avoided or is not present (e.g. if
a broken drainage system in a hollow box girder was repaired or concrete
surfaces were coated), ECE will re-establish corrosion protection without
limitations.
8.15 Fields of application
Quite different to cathodic protection (CP), ECE will only be used for
rehabilitating corrosion cases. It is not feasible as a preventative action. The
main application background is the treatment of corrosion caused by de-
icing salts. Fire damage caused by PVC (which releases hydrochloric acid) is
another possible case. Marine applications are less frequent, but have been
done as well. Besides the technical feasibility, soft factors such as avoiding
traffic disruptions, dust and noise or working in areas with limited access are
important plus points.
In most cases, the treatment area of an ECE can be limited to 'hot spots'
of a few square metres - ranging from ca. 5 to 100 m² (to be identified by
the condition survey).
