 Concrete
Clues
Ongoing research on the dielectric properties of concrete
by a Virginia Tech team of electrical and civil engineers may
help to increase the life and safety of the nation's concrete
structures.
"Concrete structures across America are aging and deteriorating,"
said Imad
Al-Qadi, a civil engineering professor on the team. "For
example, there are about 580,000 bridges, and it is reported
that 40 percent of them are functionally or structurally damaged.
Engineers are trying to prevent further deterioration and sudden
disastrous collapse."
Major efforts involve nondestructive testing to detect problems
before they become catastrophic. One common testing technique
uses ground penetrating radar (GPR) to determine patterns and
signature recognition schemes, which are then evaluated by an
expert. "This technique often raises more questions than
it answers," said Professor
Sedki Riad, the electrical engineering team leader. "It
relies on inconclusive data that can easily be misinterpreted."
Instead of relying on patterns, Riad and Al-Qadi are developing
tools and techniques to use GPR to identify the dielectric properties
of a concrete sample. The condition of the concrete can then
be determined by comparing the sample to a database they have
developed, which includes dielectric constants of concrete under
a variety of healthy and deteriorating conditions.
"We are trying to develop tools that can be used by the
maintenance engineers doing the inspection, without needing to
consult with GPR experts," Riad said. "We have analyzed
the properties of concrete over a wide electromagnetic range,
from .1 MHz to 10 GHz. This gives us the ability to build and
simulate different tools for different conditions. We can control
the depth and the resolution of the measurement. We can also
optimize the frequency for different applications. For example,
chloride can be detected at low radio frequencies and the thickness
of pavement can be measured at high frequencies," he said.
Measuring chloride content is a key advantage, according to
Al-Qadi. "Chloride is a key cause of concrete deterioration,"
he said, "particularly in the 25 snowbelt states. In New
England alone, they use 20 tons of road salt per lane per year.
The salt diffuses into the concrete and causes the steel reinforcement
to rust, which increases the volume of the reinforcement, and
causes the surrounding concrete to crack."
 The team is developing tools
including a system that rides on a truck and scans a continuous
surface, and a C-Probe, that can be used for smaller, more targeted
measurements. Conceptually, the C-probe resembles a simple two-plate
capacitor structure with both plates laid on the same side of
the surface of the structure under test. The plates are made
of a flexible metal sheet, such as copper or brass, to allow
for direct contact placement on the structural element. The probe
is flexible, to conform to different geometries, such as the
contour of a pile.
According to Riad, the Virginia Tech techniques provide several
advantages, and complement other nondestructive techniques. "There
are conditions where each technique has its use. Our C-Probe,
for example, can be used to measure places that would otherwise
be difficult to inspect, such as bridge columns in water, where
vehicles generally cannot go."
Riad and Al-Qadi attribute their success as a long-running
interdisciplinary team to communications. "We have to understand
what each other is saying," Al-Qadi said. "We haven't
become experts in each other's fields, but we understand the
overall concepts."
"It also helped me that I had a general engineering background
in my undergraduate education," Riad said. "Even after
all these years, I'm able to draw on my early understanding of
mechanics and vibrations of structures. It's very enlightening
when you work closely with another discipline to see just how
your knowledge fits in."
The team also makes sure that graduate students on the project
work together in the same laboratory, to exchange ideas. "This
way they understand better," Riad said. "They become
better engineers."
Virginia Tech electrical and civil engineers
are developing nondestructive testing tools for concrete structures
that can be used in difficult-to-measure places, such as bridge
columns in water. Above: Jason Yoho (EE) and Brian Diefenderfer
(CE) calibrate a mobile ground-penetrating radar system they
will use to obtain readings from concrete. They then compare
the readings to a large database developed at Tech, in order
to detect deteriorating conditions.
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