Fiber-Optic Sensors Will Help Control the Road Quality
Dmitry Redka, Associate Professor of the Department of Photonics of ETU "LETI," together with his colleagues from the Riga Technical University, has developed a technology for using fiber-optic sensors to monitor the condition of the road surface.
Researchers from ETU "LETI" and Riga Technical University tested new technology to monitor the integrity of the road surface. Fiber-optic strain and temperature sensors collect data on how the roadway structure changes depending on the load. This information will help design reliable roads and plan their repairs. 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 select more durable materials and, on the other hand, to repair cracks and ruts in the structure of the roadbed in the early stages, until the damage requires replacing the entire surface. Therefore road construction needs 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. Then, during the next road repair, it will be possible to take into account the pressure and vibration created by traffic and strengthen the pavement in the right places.
Dmitry Redka, Associate Professor of the Department of Photonics of ETU "LETI," used fiber-optic sensors for asphalt pavements in a joint project with Riga Technical University. That is not only high sensitivity that makes them appealing but also the fact that they do not require a power supply: they can be installed in an existing fiber-optic network and receive data remotely. The basis of such a sensor is a so-called fiber Bragg grating. It is a short segment in an optical fiber in which the refractive index is variated using ultraviolet light. As a result, this segment always reflects radiation only in a very small spectrum and passes 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 fiber, or other physical measures. Fiber optic sensors work thanks to this effect. For example, a temperature sensor will reflect a laser signal differently at +20°C and -15°C.
"Our experiments show that fiber 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, it is possible to 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 fiber-optic sensors for measuring strain and temperature in a layer of asphalt on a Latvian road during its repair. The sensors were placed 25-30 mm deep at two points on one side of the roadway. Because unprotected fiber-optic sensors are fragile, they were integrated into 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 center 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 located right on the sensors. That is why in real-life monitoring, it is essential to consider the direction of car movement. 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 for every meter.