Civil Engineering Reference
In-Depth Information
few cells that did not function were simply ignored if the total number of
REs was still sufficient for a good impression of depolarisation. Although the
proper functioning of REs is critical from the point of view of depolarisation
testing, their failure is not a big problem in practice. Either there is
sufficient redundancy in the total number, or replacement is part of low-cost
maintenance. As a strategy, having more REs than strictly necessary is a good
investment. Tentatively, RE functioning may be checked by measuring their
electrical resistance with respect to the reinforcement. In general, a rather
narrow distribution of resistance values is found. A single value much higher
than the rest may suggest loss of contact, indicating the RE is not working
properly any more.
6.6 Summary and conclusions
This chapter describes time-dependent degradation processes and failure
mechanisms of components of concrete CP systems in the operation stage.
Most critical factors in time-dependent degradation of CP systems are
oxidation of the anode material (carbon), bond loss of the anode or the
cementitious overlay, corrosion of anode-copper connections and failure of
reference electrodes.
Oxidation at the anode/concrete interface is inevitable. It has two effects:
oxidation of the anode material itself, in particular carbon; and oxidation
of hydroxyl ions. Both are important for the service life of concrete CP
systems.
Oxidation of carbon particles in the anode material results in gradual
build-up of a high electrical resistance and eventually the CP system fails
due to insufficient current flow. The amount of carbon in a critical layer
thickness within the anode is considered as the 'capacity' of the system.
The 'load' is the amount of oxidation per unit of time and current density,
which is only a fraction of the total current density. A limit state function
was formulated with coefficients quantified from experimental data. No test
method is available yet for determining the 'oxidation capacity' of a carbon-
based anode material. Due to its chemical inertness, activated titanium does
not suffer from oxidation. The service life of titanium-based CP systems is
not determined by oxidation of the anode material itself.
Acid production in the anode/concrete interface results in dissolution of
the cement paste and eventually loss of bond, which will block current flow
and cause failure of the CP system. The processes involved are electrolysis,
migration and diffusion. A model was conceived for acid-induced adhesion
loss. The 'capacity' is a critical (bond) layer thickness of cement paste. The
'load' is the amount of acid formed per unit of electrical charge, which
is a fraction of the total current density multiplied by time. A limit state
function was formulated with coefficients quantified from experimental data
supported by tentative numerical modelling. No test method is available yet
for determining the 'acid capacity' of the anode/overlay/concrete in a CP
