Page 168 - Corrosion Engineering Principles and Practice

142 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 143

Outer blanket with electrolyte Inner blanket with
electrolyte
Anode mesh

Rectifier +

–

Rebar
(cathode)

Concrete
FIGURE 5.45 Schematic of electrochemical chloride extraction and
re-alkalization treatments.

Chloride extraction has been recommended for structures that do
not contain pre- or post-tensioned steel and have little damage to the
concrete itself. The current densities and applied voltages involved
are significantly higher than those used in cathodic protection,
therefore increasing the risk of hydrogen evolution and subsequent
hydrogen embrittlement (Chap. 6). The unsuitability of the technique
to prestressed concrete is thus not surprising. Further requirements
are a high degree of rebar electrical continuity and preferably low
concrete resistance. Since several days or even weeks are involved in
the extraction processes using suitable current densities, the technique
is more realistically applicable to highway substructures rather than
bridge decks, where it could cause serious traffic disruptions as
illustrated in Fig. 5.46.
In practice, the chloride extraction process does not remove the
chloride ions from the concrete completely. Rather, a certain percentage
is removed and the balance is redistributed away from the reinforcing
bars. There is also increasing evidence that the protective layer around
the reinforcing steel is considerably enhanced, elevating the threshold
for corrosion.
As with ICCP, the applied current density has to be controlled.
Under excessive current magnitude several problems can arise, such
as reduction in bond strength between rebar and concrete, softening
of the cement paste around the rebar steel, and cracking of the
concrete. Concrete containing alkali-reactive aggregates is not
considered a suitable candidate for the process as the expansive
reactions leading to cracking and spalling associated with these
aggregates tend to be aggravated [29].