Civil Engineering Reference
In-Depth Information
6
Service life aspects of cathodic
protection of concrete structures
6.1 Introduction
Cathodic protection (CP) of concrete structures has been successful in
stopping corrosion in reinforcement for over 20 years in Europe. Long-
term experience has been reported by various authors (Tinnea & Cryer,
2008; Nerland et al., 2007; Wenk & Oberhänsli, 2007; Polder, 1998).
Advocates usually state that the service life of CP is superior to that of
conventional repairs. Moreover, CP installations involve significant
investment and their service life is an important issue. Service life aspects
of CP have not received much attention, however. Analogous to modern
service life design methods for concrete structures (e.g. DuraCrete), a
performance based, limit state oriented and probabilistic approach should
be chosen. This will allow CP to be incorporated into modern maintenance
management systems, which surely will be performance oriented. This may
be an unreachable ideal at the moment; however, this chapter intends to
present some of the necessary elements. Such an approach is based on
identifying the most important failure mechanisms, modelling their time-
dependency, quantifying uncertainties and finally, calculating failure
probabilities as a function of time. This chapter describes time-dependent
degradation processes and failure mechanisms of essential components of
impressed current CP systems in the operation stage. The starting point
is that the CP system has been properly designed and executed and that
commissioning, testing and voltage/current adjustment in the early stages
have been carried out successfully.
Good performance is defined as the absence of corrosion-induced damage.
In practice, this is assumed to be the case if sufficient depolarisation is obtained
(BS EN 12696). Sufficient depolarisation can only be reached when current
flow is unhindered and control systems work properly. Normal maintenance
should be applied, that is, two to four electrical checks and one visual
inspection per year.
Degradation of the anode material itself, loss of concrete/overlay adhesion,
corrosion of anode-copper connections and failure of reference electrodes
are the most significant time-dependent failure phenomena. Quantitative
