Application of Embedded Distributed Fiber Optic Sensors on a Highway Bridge as a Support for Bridge Inspections

In recent years distributed fiber optic sensors (DFOS) have found their way into the structural health monitoring (SHM) of bridges and other civil engineering structures. In contrast to traditional strain gauges and temperature sensors, which enable measurements at predefined points, DFOS allow the continuous monitoring of each point along the optical fiber. This technology therefore offers exceptional benefits for observing the development of cracks in reinforced concrete structures, since their exact location and width is normally not known beforehand.
SHM is mostly considered an expensive method reserved for ageing structures already showing signs of deterioration. The retroactive installation of DFOS on bridges is indeed cumbersome, as the access to the structure requires special equipment and the cables must be installed using adhesives, often in a groove cut in advance. If the cables are embedded in the structure before the pouring of the concrete however, the labor costs drop to a bare minimum. Furthermore, in contrast to the subsequent installation of the DFOS, the measurements can be compared to an initial state during or immediately after construction.
In a research project funded by the Austrian motorway operator ASFiNAG this concept was tested on a 420 m long prestressed concrete bridge. Fiber optic cables were installed for measuring strain and temperature. The cables were scanned at several stages of the construction process using different types of interrogators. Additionally static and dynamic tests with predefined loads have been carried out, supplemented also by other monitoring techniques such as high precision ESR strain sensors, laser scanning, GNSS sensors, robotic total stations, laser vibrometers or a camera with motion amplification® video processing algorithm. The aim is to validate the concept of supporting regular bridge inspections by data obtained from DFOS measurements. This can provide valuable additional information on the location of cracks and their development over time from the initial state.