Civil Engineering Reference
In-Depth Information
Equation 8.1
Possible reduction reactions on the reinforcement surface forced by
ECE.
1
−
−
O
+
H
O
+
2
e
→
2
OH
(
a
)
2
2
2
−
−
e
.
g
.
Fe
O
+
3
H
O
+
2
e
→
2
Fe
(
OH
)
+
2
OH
(
b
)
2
3
2
2
2
H
O
+
2
e
−
→
H
+
2
OH
−
(
c
)
2
2
Both processes - chloride migration and reduction of oxides, oxygen and
water - run at the same time, but do not depend on each other in predictable
terms. Whereas chloride migration depends on the applied voltage, the cover
thickness and permeability of concrete and the water content, the current is
controlled by the voltage, the temperature, the resistance of concrete (as a sum
parameter of concrete cover, permeability, soluble ions and water content)
and the charge transfer resistances on anode and reinforcement. This does
not correlate with some other publications, e.g. (Polder et al., 1993), but has
a practical backup when high amounts of chloride can be removed at quite
low current densities and charges as well as only slow desalination progress
being observed at high current densities and total charges.
So the ECE not only reduces the chloride content of the concrete but also
raises its alkalinity as a result of the reduction reactions. This improves the
corrosion protection additionally, since with a high OH- content also an
increased chloride content can be present in the concrete without triggering
reinforcement corrosion activity.
On the external anode, which is usually of a dimensionally stable material,
we find other electrochemical reactions that lead to very acidic conditions:
the oxidation of water (8.2a), hydroxyl ions (8.2b) and chloride as well as
the formation of chlorine (8.2c). According to (Elsener et al., 1993) the
reaction of water and chlorine can also cause an acidic environment (8.2d).
Equation 8.2
Possible oxidation reactions on the anode.
+
−
2
H
O
→
O
+
4
H
+
4
e
(
2
a
)
2
2
1
2
OH
−
→
O
+
H
O
+
2
e
−
(
2
b
)
2
2
2
−
−
2
Cl
→
Cl
+
2
e
(
2
c
)
2
H
O
+
Cl
→
HCl
+
HClO
(
2
d
)
2
2
8.2 Development, components and application systems
The idea of moving dissolved ions in solid materials with the help of an
electrical field comes from geotechnology, where it is been used to clean
soils. Between 1973 and 1975, ECE was tried first on a laboratory scale and
then on a removed bridge deck in Ohio, USA (Slater et al., 1976, Morrison
et al., 1976). With high voltages - up to 220 V - and developing high current
