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Special Report
Interdisciplinary Activities and Programs

 April 1998


bridge photoConcrete 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."

lab photoThe 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.

The Bradley Department
of Electrical and Computer Engineering
Virginia Tech

Last Updated, May 10, 1998
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