Just as a doctor uses an ultrasound to examine the inside of the human body, a structural engineer needs tools to inspect the inside of concrete blocks. However, the options available so far have been quite limited in effectiveness.
Concrete, made up of a variety of materials such as stone, clay, chalk, slate, iron ore and sand, scatters normal sound waves, making it difficult to obtain clear images.
Recently, scientists from Japan and the United States have collaborated to develop a system that allows identifying internal defects in concrete buildings and bridges without having to destroy their structure.
The team members explained in a press release that their method involves sending sound waves into the material and capturing the waves that bounce back to create images of what’s inside, similar to an ultrasound.
“In our approach, the ultrasonic wave is broadband, using a wide range of ultrasonic frequencies rather than operating around a fixed frequency,” said Professor Yoshikazu Ohara of Tohoku University in Japan.
«The receiver is capable of accepting an even wider range of frequencies. By automatically adapting the frequency to the material, our system improves the contrast between defects and the background material in the concrete,” he added.
To carry out this development, Tohoku University collaborated with the Los Alamos National Laboratory in New Mexico and Texas A&M University.
One of the main challenges is that it is difficult to know what frequencies of sound waves will survive traveling through concrete, since different materials can interfere with different wavelengths.
To handle this uncertainty, the team used two devices: one to generate a wide range of frequencies to send into the material and another, called a vibrometer, to capture the waves that come out.
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The system, described in the journal Applied Physics Letters, can handle a wide range of frequencies, meaning that even if ultrasonic waves are scattered by materials in the concrete, those that make it through are still detected, no matter what frequency they are at.
‘As concrete filters out certain frequencies, the laser Doppler vibrometer simply captures the frequencies that remain,’ explained Professor Ohara. “Unlike conventional systems, we don’t have to change transducers or adjust the frequency beforehand. “The system adapts automatically.”
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The result is a high-resolution 3D image of the defect and its location in the concrete.
For a repair planner or field technician, this provides ‘concrete’ information: how deep the defect is from the surface, how large it is, and how it extends in three dimensions, making it easier to plan repairs more efficiently.
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