On the early morning of September 11, 2024, part of Dresden's Carola Bridge collapsed into the Elbe – moisture transferred during construction led to corrosion inside the bridge, which over time caused numerous tendons in the prestressed concrete to snap. Temperature changes added to the problem. Unfortunately, the Carola Bridge is not an isolated case: In its 2022 condition assessment, the Federal Ministry for Digital and Transport classified 8,000 motorway bridges as needing renovation; the organization "Transport & Environment" arrives at double the number, with 16,000 bridges in need of repair. It is therefore high time to assess the condition of Germany’s bridges and continuously monitor them in order to close or renovate them in time when danger arises.

Acoustic Emission Analysis Means Listening

A bridge with finger-thick steel components is anything but easy to inspect – it does not reveal on its own how the prestressing steel is doing. In a project that started in October 2025, the focus is on the customer-specific development of monitoring systems for this special application. “Using acoustic emission sensors attached to the bridge, we can analyze the failure state and predict its development. We essentially map the interior of bridges and other structures and make hidden damage visible,” explains Dr. Lars Schubert, scientist at Fraunhofer IKTS. Distributed sensors across the bridge measure the sound generated within the structure – whether caused by a car driving over it or by a steel rod breaking inside the bridge. With algorithms being further developed at Fraunhofer IKTS, the condition of the bridge is recorded. Are there severely corroded areas? Have cracks already formed?

However, a single sensor does not get you far – sound attenuation in concrete is high due to the material properties, and many sensors are needed to extract the required information from the recorded acoustic signals. “For continuous monitoring, the sensors as well as the associated measurement technology must become significantly more cost-effective,” says Schubert. A novel sensor technology aims to achieve this: small MEMS sensors, short for “Micro-Electro-Mechanical Systems.” Since these automatically manufactured sensors are less sensitive than manually produced ones, the team is currently investigating their limits and working on adapting them for infrastructure monitoring. The researchers have also tailored the measurement technology that receives the signals for this application: away from an all-purpose measurement system toward a system specifically designed for bridges and other infrastructure. They developed their own electronics and are customizing them for the application.

Optical Fibers as Sensors

Another intriguing technology lies in optical fibers, mainly known from fiber-optic internet. “These fibers can also be used to monitor infrastructure,” says Bianca Weihnacht, researcher at Fraunhofer IKTS. Here, a fiber used in telecommunications serves as a sensor. What matters is not the light passing through the fiber, but the light interacting with strains in the fiber and being backscattered. If the cable remains unmoved, light reflected at a defect always returns after the same travel time – after all, the path to the defect and back remains constant. However, if the cable is stretched or shifted, the defect moves, and the travel time of the reflected light changes. In this way, local movements along the cable can be detected.

Since a fiber does not only have a single reflection point but rather a continuous sequence of reflection points due to production, it acts as a continuous sensor. “The technology of fiber-optic sensors is extremely attractive: it enables low-cost infrastructure monitoring – using just a single fiber without any wiring,” Weihnacht explains. For example, such a fiber can be attached to a bridge to monitor its strain. Furthermore, many fibers already lie on the seafloor and can be used as sensors. Even power cables contain optical fibers. An initial test conducted by the researchers in the basin of Hamburg Harbor was successful. Further investigations are underway at the Digital Ocean Lab, where industry leaders, researchers, and innovators work together to test and refine next-generation maritime technologies.

Optical fibers also stand out due to their properties. Electromagnetic interference, such as from passing trams, does not affect them. They are therefore ideal for urban infrastructure monitoring. Since they require no wiring, they can also be used in explosion-proof areas such as refineries, where flammable gases are present and even a small spark could ignite them. Another monitoring gap closed by fiber-optic sensors concerns structures on the seafloor: no measurement equipment must be placed underwater. Instead, the fiber sensor can extend out of the water and be connected to the measuring device in the air. “Fibers greatly simplify maritime monitoring – especially in extremely challenging environments such as the North Sea,” Weihnacht confirms.

Rapid Implementation and Introduction into Practice

Depending on classification, bridges fall under the responsibility of the federal government, states, or municipalities. Inspections are carried out by civil engineering offices; the further development of inspection technologies takes place in academia. For this reason, Fraunhofer IKTS works closely with engineering firms (e.g., MKP GmbH, FOLAB GmbH) in ongoing projects and receives significant support from experts such as Prof. Steffen Marx, Professor of Concrete Structures at TU Dresden. Close collaboration among all stakeholders enables rapid transfer of research results into practice. Due to Fraunhofer IKTS’s involvement in the German Society for Non-Destructive Testing (DGZfP), where Lars Schubert chairs the committee for condition monitoring and participates in the acoustic emission testing committee, standardization and incorporation into regulations are significantly simplified.

To further advance these topics, Fraunhofer IKTS is organizing a session on structural monitoring with acoustic methods at the “8th International Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures” from August 26 to 28, 2026 in Dresden. All interested parties are welcome!