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Corrosion
Products
As
corrosion continues, corrosion products build up around the reinforcing
steel. Corrosion products commonly observed when concrete cores
containing corroding rebar were broken open were ferrous hydroxide
(Fe(OH)2), hydrated ferrous chloride (FeC12H2O), and black ferrous
oxide (Fe304).The exact product formed depends on the availability
of oxygen, water, and chloride ion, but the result is essentially
the same. These corrosion products occupy from 2 to 14 times the
volume of the original steel, creating an expansive force that is
sufficient to cause the concrete to crack. Propagation of the cracks
leads to staining, spalling and delamination of concrete.
INNOVATIVE
CATHODIC PROTECTION SYSTEMS FOR CONCRETE REPAIR AND REHABILITATION
Introduction
Zinc,
as a sacrificial anode, has been used to cathodically protect ship’s
hulls for more than a century now. It has become a common practice
to use cathodic protection either alone or in combination with coatings
for buried pipelines, storage tanks and offshore structures. It
has been well established both in theory and in practice that the
process achieves an immediate reduction in corrosion rate by making
the reinforcing steel the cathode, inhibiting its tendency to oxidize.
In addition, the cathodic reactions at the steel/concrete interface
increase the alkalinity (raise the pH) by hydroxyl ion (OH-) generation
and drive chloride ions (Cl-) away from the steel as a result of
the negative charge on the ions being repelled by the negative polarity
of the reinforcement and attracted to the positive polarity of an
installed anode.
Thus
Cathodic Protection of steel in concrete has always incorporated
some degree of Realkalisation (increase in pH) and Chloride Extraction
(redistribution of chlorides). Both these processes reduce the risk
of corrosion of steel in the concrete and also achieve further protection.
Recent innovations in materials and design have made CP, - the old
remedy, ever more attracting and promising today.
Pressure-sensitive
Zinc-Hydrogel Anode
For
conventional CP systems (SA or ICCP) to function properly, it
requires the presence of a continuous conductive electrolyte between
the anode and the cathode. This is to form a closed circuit such
that the reinforcing steel is indeed made the cathode. In areas
of electrical discontinuity, reinforcing steel would not be cathodically
protected. One of the recent innovations is the self-adhesive
and conductive zinc-hydrogel anode that provides a continuous
electrolyte contact between the anode and the reinforcing steel
embedded in the concrete structure. The pressure-sensitive zinc-hydrogel
anode is essentially a sheet of zinc foil coated with an ionically
conductive hydrogel pressure-sensitive adhesive (Figure 1), which
serves as the electrolyte between the anode (zinc foil) and the
cathode (reinforcing steel in concrete). The hydrogel is covered
with a liner to help protect it from contamination. At time of
installation, the protective liner is removed from the hydrogel
by hand, and the zinc-hydrogel anode is adhered to the clean,
bare concrete surface.
This
zinc foil anode coated with hydrogel is provided in roll form
for coverage of the concrete surface to be protected. The ionically
conductive hydrogel performs two functions: (1) to enable the
anode to be securely adhered to the exterior surface of the concrete
structure, and (2) to serve as a continuous conductive electrolyte
between the anode and the concrete structure.
Fig.
1 Pressure-Sensitive Zinc-Hydrogel Anode (Illustration copyrighted
by 3M)
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