September 14, 2007

TISEC INC.

Infrastructure testing methods have come to the forefront, especially since recent bridge collapse disasters. Concrete and steel, as shown here, can use similar acoustic emission monitoring to identify potential concerns.

Engineering

Montreal infrastructure evaluation company develops method of testing concrete bridges

Firm investigates method designed to test concrete

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The bridge catastrophe in Minneapolis scared the wits out of structural engineers because North America is rife with aging concrete superstructures.

The lessons learned remain to be seen but one Montreal-based infrastructure evaluation company has developed a method of testing concrete bridges that could prove to be a lifesaver.

The technology, known as acoustic emission monitoring, has a proven track record with steel bridges. Officials at Tisec Inc. see it as the perfect early warning system for potential catastrophic cracking in concrete crossings.

Acoustic emission testing differs from other crack detection methods, such as ultrasonic or impact echo and x-rays. Those methods, which require probing for cracks, are more labor-intensive and can be pricey.

". . .probability of early detection of small defects is quite high with acoustic emission"

Robert Hay

Tisec president

Acoustic emission monitoring works through the use of advanced acoustic sensors installed on bridges. The sensors pick up stress waves or sound waves created when cracks or deformations grow, says Robert Hay, a materials engineer and president of Tisec. The intelligent systems project technology then produce reports on the nature and severity of the defect producing the emission.  

“We just listen for crack propagation. The probability of early detection of small defects is quite high with acoustic emission.”

The period of monitoring varies. Depending on loading (traffic weight and volume), it could take up to a month of continuous monitoring to get conclusive results, he says.

Concrete cracks grow in many small steps before they become catastrophic. “We want to detect any of those precursor steps long before the cracks become critical and start to propagate.”

Tisec is not new to the crack detection game. The company has tested more than 300 steel bridges in North America and India over the past 15 years.

Monitoring concrete bridges, however, has its challenges. Steel is homogeneous and it is forgiving to cracks. That is not the case with concrete – a complex mixture of materials, including rebar – that has more potential failure modes.

While visual inspection of bridges is the frontline method of locating a potential problem in a concrete superstructure (fine cracks, for example), Hay suggests acoustic emission monitoring could become a viable second step in determining the severity of cracks so an appropriate maintenance program can be implemented.

In Ontario, bridges and overpasses are tested every two years. When cracks are discovered, follow-up inspection methods, such as ground-penetrating radar, are used but that can be costly.

Tisec’s testing system for concrete superstructures is the result of two years of research and development by Tisec Inc. and Université du Québec à Trois-Rivières (UQTR), in collaboration with Edmonton-based Canadian National Railway Company (CN). The team is largely funded by Ottawa-based Precarn Incorporated, a not-for-profit company representing a national network of corporations, universities, colleges, research institutes and government partners engaged in the development of intelligent systems-enabled solutions.

Hay says concrete crossings started springing up in North America about 50 years ago. Many of them could be nearing the end of their lifespan but replacing them is an “overwhelmingly expensive” option.

“Maybe inspection-based maintenance is the solution,” he says, noting that if visual inspections show minor flaws, acoustic emission monitoring can determine the extent of the problem, allowing engineers time to implement maintenance programs before structural problems worsen.

Acoustic emission technology was discovered in the 1950s and is widely used in chemical industry applications, particularly to monitor high pressure and storage vessels.

As yet, Hay hasn’t met with engineers to discuss the use of acoustic emission monitoring simply because Tisec is a small company with its hands full testing steel crossings.

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