Fibre-optic sensors could help control the quality of roads

Dmitry Redka, Associate Professor of the Department of Photonics of ETU \"LETI\"

Researchers from Saint Petersburg Electrotechnical University ETU “LETI” and Riga Technical University tested new technology for monitoring the state of the roadway surface. Fibre-optic strain and temperature sensors collect data on changes in the roadway structure depending on the load. This information will help design durable roads and plan their maintenance. The study was published in the Journal of Sensors.

The pavement of any road deteriorates over time. It is impossible to stop this process altogether, but it is possible, on the one hand, to choose more durable materials and, on the other hand, to repair cracks and ruts in the roadbed structure in the early stages, until the damage requires replacing the entire surface. Therefore road construction industry is always looking for effective monitoring systems along with new materials. Roads should be equipped with sensors that allow not only to detect defects timely but also estimate the load on the road section. Using this information, a maintenance team could understand the levels of pressure and vibration created by traffic in that area and reinforce the roadway surface where needed.

Dmitry Redka, Associate Professor of the Department of Photonics of ETU “LETI,” used fibre-optic sensors for asphalt pavements in a joint project with Riga Technical University. TThese devices are known for their sensitivity and can be arranged in existing fibre optic networks to remotely collect data, so they do not require an electrical power supply. The sensors are based on the so-called fibre Bragg grating. It is a short segment in an optical fibre in which the refractive index is variated using UV light. As a result, this segment always reflects radiation only in a very small spectrum and transmits the rest of the light without loss.

FBG can be constructed so that the wavelength of the reflected light depends on changes in the ambient temperature, pressure on the fibre, or other parameters. Fibre optic sensors work thanks to this effect. For example, a temperature sensor will reflect a laser signal differently at +20°C and -15°C.

Dmitry Redka, Associate Professor of the Department of Photonics of ETU “LETI,” explains: “Our experiments show that fibre optic sensors can accurately measure roadway deformations. It is necessary to monitor the temperature because, in warm weather, asphalt is more pliable, and strain values increase. Using our constant monitoring approach, one could determine when deformations exceed the limit in a section and take it into account when designing new roads and repairing existing ones.”

Researchers embedded two types of fibre-optic sensors for measuring strain and temperature in a layer of asphalt on a Latvian road during its maintenance. The sensors were placed 25-30 mm deep at two points on one side of the roadway. Because unprotected fibre-optic sensors are fragile, they were encased in composite and ceramic tubes.

To test if the system is working, researchers used a falling weight deflectometer, a device measuring the surface deflection under load. The centre of the plate, on which the load falls, was placed at different distances from and directly above the sensors. This test showed that the most accurate measurements are possible when the load is directed right on the sensors. That is why in real-life monitoring, it is essential to consider the direction of traffic. Scientists also verified that temperature plays a major role in the deformation of asphalt: all measured values were lower in fall than in warm summer.

A key part of the experiment was monitoring actual traffic. About 3.15 million cars pass through the point where the measurements were taken in a year, and over 23% of them are heavy trucks. Physicists determined which types of trucks impact the roadway the most and calculated that in 33% of cases, a passing truck deforms the asphalt by 0.3 mm per meter.