In monitoring the conditions that lead to corrosion
of steel-reinforced concrete, the ECI addresses a multi-billion-dollar
world-wide problem – and one that exacts a grim
toll in human life.
The estimated cost to repair reinforced concrete structures
runs $200 per square meter of exposed surface. In the
United States, the annual direct cost of corrosion in
highway bridges alone is roughly $8.3 billion, including
maintenance, repair, replacement, and the cost of capital.
Indirect costs, including traffic delays and lost productivity,
may run 10 times that number.
But corrosion is more than an economic issue. In June
1983, a 100-foot section dropped out of the Mianus River
Bridge in Greenwich, Connecticut, U.S.A., killing three
motorists and critically injuring three others. The
steel pins that joined sections of the bridge had decayed.
In May 2000, in Concord, North Carolina, U.S.A., more
than 100 people were injured when steel strands corroded
in a pre-stressed concrete pedestrian bridge and the
structure collapsed onto the highway below.
When a structure is first built, the concrete itself
protects steel reinforcement by providing both a physical
shield and an alkaline environment. This causes a passive
film of iron oxide to form over the rebar, preventing
further corrosion. Over time, however, chlorides from
de-icing salts and sea water may permeate the concrete
and depassivate the steel. They penetrate the iron oxide
film, and set up corrosive electric circuits within
the structure. Carbon dioxide also poses a threat, reducing
concrete’s alkalinity and weakening its protection
for embedded rebar.
Once corrosion begins, it’s self-sustaining. As
steel rusts, its corrosion products occupy three to
six times the volume of the original rebar. This stresses
the concrete, generating cracks, delaminations, and
spalls. These, in turn, provide new means for water
and chlorides to reach the steel, which then corrodes
even faster.
None of this is visible until late in the process, however,
when cracks form. This is unfortunate, as visual inspection
has long been the mainstay of structural maintenance.
Supplementing the human eye, some maintenance engineers
have cut concrete samples for analysis. But this is
destructive and expensive, and it disrupts the use of
a structure.
Historically, engineers have also installed analog probes
into concrete. But these have generally measured only
one or two factors in corrosion. This has limited their
usefulness and reliability. Probes have also faced challenges
in delivering data. Many require inspectors to tour
a structure, plugging a reader into each probe by hand.
Others transmit their readings to a datalogger. Because
their signals are analog, however, they’re vulnerable
to electro-magnetic interference. This has made it difficult
for probes to transmit reliable data for more than 30
feet. Worse, it’s obliged structure owners to
install a multi-thousand-dollar datalogger to receive
the readings from each probe.
