Aeneas Paul and Noah Sträter (right) are researching how to detect in a timely manner whether a bridge has internal damage.
Civil engineering
Transparent Bridges
Many bridges in Germany are at a critical age. If damage could be identified in a timely manner, a bridge’s service life could be safely optimized and reconstruction planned in time before the bridge has to be closed.
Despite appearing intact on its exterior, the Carola Bridge in Dresden collapsed in 2024. How could this happen? “If you take a closer look, no bridge collapses without warning,” says Dr. Mark Alexander Ahrens from the Institute of Concrete Structure at Ruhr University Bochum. “There are always symptoms leading up to it.” They just have to be noticed in time, and researchers have developed methods for doing so: Ultrasonic measurements help to detect the smallest of changes. Fiber-optic sensors detect strain on the surface that cannot be seen with the naked eye. Rubin, the science magazine of Ruhr University Bochum, reports on the topic in its current issue on “Bridges.”
Concrete as a sponge
The majority of long-distance road bridges in Germany are prestressed concrete bridges, which contain steel strands pulled through conduits. Once such a bridge has been fully constructed, we can no longer see what is happening inside. Ideally, nothing at all should be happening: The strands are supposed to be protected against air and water. “However, we do not view concrete as a rock, but as a sponge,” says Ahrens. “It breathes all throughout its life, reacts to weather, and undergoes normal aging processes.” In addition, air and water can remain on the strands if construction does not go perfectly. This leads to stress corrosion cracking, which is also the culprit behind the Carola Bridge collapse.
When faced with the known risks, what can be done to notice in time when a bridge is weakening? The researchers at the Institute of Concrete Structure have developed two methods to do just this. Noah Sträter employs ultrasound. “We equip the bridge with a network of ultrasound emitters and receivers, one about every five and a half yards,” he explains. The ultrasonic waves pass through the bridge and generate a characteristic pattern. By repeating the measurement later and comparing the patterns, researchers can tell whether and where a change has occurred inside the structure, as changes in the concrete affect the propagation of the ultrasonic waves.
Optical fiber detects strain
Aeneas Paul has developed another method: His approach is based on fiber-optic sensors made of glass fiber which are bonded in place across lengths of up to over 160 feet and can be distributed along the entire length of the bridge, even if it is miles long. The optical fiber is highly sensitive to strain.
Detailed article in the science magazine Rubin