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Harbor
Cathodic Protection System
NFESC conducted a series of electrical tests including continuity
and potential measurements in the spans between Bents 217 and 218
and between Bents 223 to 225 to determine the condition of existing
reinforcing and the feasibility of effectively mitigating corrosion
by cathodic protection (Reference 1). The reinforcing, steel was
found electrically continuous within each span. Continuity was not
good between spans across the transverse girders. Applying the criteria
of ASTM C876-91 to the electrical potential values, a 90 percent
probability of no corrosion existed in the tested areas. However,
there were visible rust stains around some of the cracks and potential
gradients between anodic to cathodic regions indicated moderate
corrosion activity near repair patches and cracks. Since the steel
reinforcing is electrically continuous, the alteration of natural
electrochemical processes that result in corrosion can be achieved
by driving electrical reactions in favorable directions with an
impressed current on the concrete surface.
Since cathodic protection is the only practical means of mitigating
corrosion of concrete reinforcing steel once it has started, NFESC
designed an impressed current cathodic protection (ICCP) system
for selected deck areas of the upgrade segments. These deck areas
are between the curb and the outboard pile girder as well as between
the two track slabs. The impressed current system is a low amperage
circuit consisting of embedded titanium ribbon as the anode, connections
to existing rebar that serves as the cathode, lead wires to a junction
box, and a rectifier to supply electrical power. The anode configuration
and rectifier sizing was established by the site survey of potential
polarization information. The design is detailed in Reference 2.
The design provides nominal current density requirements to mitigate
current corrosion rates and requires less than 30 volts with a maximum
current less than 0.5 mA/ft2 (5 mA/m2) Figure 30 is a graphic of
the ICCP design for a typical installation between two transverse
girders. It was determined that the rebar cover was sufficient for
placement of ½-inch wide(1.25 cm), titanium ribbon anodes
into slots cut into the underside of the deck without the anode
material making contact with the reinforcing steel. The cover is
also sufficient to achieve an adequate current distribution for
the embedded ICCP system. The anode layout was configured to provide
uniform current distribution over the surface of each area. To offset
a somewhat low reinforcing steel connectivity across transverse
girders (bents), six electrical connections to the reinforcing steel
(three for each anode grid) in each span was required.
Figure 30. Schematic of impressed current cathodic protection system
on each end of Bravo-25.
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