Sound waves show state of roads

A Swedish scientist has developed a way of using sound waves to reveal the underlying condition of a road. The Swedish Road Administration says the method, which is expected to become the new standard, may lead to major quality enhancements and cost savings.

Since a road consists of many different materials it is difficult to predict how it will respond to future traffic and environmental loads. Because roads, unlike buildings, for instance, are 'built into' the ground, it is hard to inspect them visually. Inspectors typically have to drill cores and break up asphalt and concrete samples.

Nils Rydén, a researcher in engineering geology at the Faculty of Engineering, Lund University, developed the technology of using sound waves to obtain information about the composition and stiffness of the material on a computer screen.

Non-destructive testing with sound waves is based on measuring the dispersion of the waves in constructions in order to 'see' the stiffness and thickness of the materials involved as well as any cracks, etc. The velocity of the waves is directly related to the stiffness of the material, and differences in stiffness produce reflections that can be utilised for measuring the thickness of layers and for detecting hidden damage.

Sound waves with frequencies in the range 50-10,000Hz are used for this application. Higher-frequency ultrasound does not penetrate deeply enough in road materials, and x-ray inspection is too expensive and complicated.

Rydén was inspired to test the possibility of inspecting roads with sound when he wrote his master's thesis in 2000. At that time there were similar projects underway elsewhere, in the US, for instance, but Ryden says that today the Swedish method is the most advanced.

Damage to bridges, tunnels, dams, and nuclear power plants can also be uncovered using this technology.

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