CPD Specifier May 2015 issue January 2014 | Page 29

Structural Repair

Anodes extend bridge life
Galvanic anodes prevent the corrosion of steel reinforcement
and can significantly extend the life of bridges,
says George Sergi, Technical Director
with Vector Corrosion Technologies Ltd
and a member of the
Structural Concrete Alliance

Figure 1: Anode installation in a crossbeam

The corrosion of steel reinforcement due to
chloride contamination of concrete structures is
a major worldwide problem. Where chlorides are
left in the surrounding sound concrete, traditional
patch repairs to the damaged concrete are rarely
successful, since the corrosion cell simply moves
into areas adjacent to the repair. But improved
performance can be achieved by embedding
sacrificial anodes around the perimeter of the
patch repair.
HOW DOES IT WORK?
The use of galvanic anodes prevents incipient
anode formation, more commonly known as
the ring effect. This is where the newly applied
concrete within a chloride-free environment drives
the corrosion of the steel immediately outside the
patch, which is still likely to be contaminated with
chlorides. Consequently, further repairs requiring
multiple interventions will be necessary within
relatively short timescales.
Sacrificial anodes installed around the perimeter of
the patch reinstate a form of intentional ‘cathodic’
prevention’ within the patch repair region so that
the adjacent areas remain cathodic and corrosion
initiation is prevented or mitigated. Furthermore,
anodes placed in a grid configuration when
chlorides have reached the steel and are about
to initiate or have just initiated corrosion, can
prevent corrosion for considerable periods, as
the essentially cathodic steel repels the chlorides
away from the steel concrete interface.
Other beneficial effects are the prolonged
realkalisation of the steel from the cathodic
reaction ½ O2 +H2O + 2e- _> 2OHThe migration of the alkali cations Na+, K+ & Ca++
to the steel, owing to the driving voltage between
the sacrificial anode and steel, enhance the alkali
concentration around the steel and both reinstate
and improve the protective passive film. There is
also some evidence that solid alkali rich phases
deposit within any defects and pores at the steel
concrete interface, improving the physical barrier
against chloride migration (Fig 2).

PROOF OF
PERFORMANCE
Specially designed
galvanic anodes were
first installed in patch
Figure 2: Mean current density by area of steel delivered
repairs in the late
by the galvanic rod anodes and point anodes
1990’s. One of the first
monitored i