Bridges at the breaking point

The collapse of the Carola Bridge in Dresden has demonstrated how important it is to identify infrastructure weaknesses at an early stage. Indeed, roads and bridges, railways and power lines in Germany are showing their age, and some of them have significant structural defects. According to the latest condition survey from 2022, commissioned by the German Federal Ministry for Digital and Transport (BMDV), 7,112 kilometers of highway, 8,000 highway bridges and 17,630 kilometers of rail are classified as in need of repair. Innovative Fraunhofer solutions can help simplify, expedite, and cut costs of maintenance, analysis and servicing work − an overview.

With half of all German autobahn bridges aging and overloaded, road closures and traffic jams are an inevitability. Fraunhofer IZFP is hoping to lighten the burden with a new system for continuous condition monitoring.

Since the start of December 2021, noisy, smelly heavy-goods vehicles have been clogging the streets of Lüdenscheid in western Germany. Every day, around 25,000 vehicles are diverted from the A45 and through the once tranquil town, because they can’t use the autobahn anymore – the nearby Rahmede viaduct had to be closed entirely due to serious damage. With renovation out of the question, the only option is to tear down and rebuild the 53-year-old bridge – and that’s no one-off.

Around half of the 28,000 bridges on the German autobahn were built in the period between the 60s and 80s. Since the beginning of the 80s alone, traffic volumes have almost doubled, with heavy-goods vehicles in particular showing a significant increase. But the problem is not just that there are more trucks on the roads, it’s that the vehicles themselves have grown to gigantic proportions. Trucks with a total weight of 40 tons and 11 tons of axial load are by no means rare. Dr. Jürgen Krieger, director and professor at the German Federal Highway Research Institute (BASt), lays out the facts clearly: “Our current traffic situation is incompatible with old bridges,” he says. The civil engineer heads the BASt’s Bridges and Structural Technology department. However, if traffic is to keep flowing, renovation and rebuilding work must be carried out in stages. “This means we have to ensure that these older bridges continue to function for as long as possible,” Dr. Krieger warns.

Continuous monitoring cuts costs

Prof. Hans-Georg Herrmann and his team at the Fraunhofer Institute for Nondestructive Testing IZFP in Saarbrücken hope to lend a hand here. The deputy institute director, engineer and sensor expert is working on a sensor system that will continuously monitor bridge conditions. “It is vital that we make the switch from reactive to predictive or proactive maintenance management, so that we can keep the costs and consequences for traffic as low as possible,” Prof. Herrmann affirms. Prof. Krieger agrees wholeheartedly. In the future, intelligent sensors will help detect problematic changes in the bridge structure at an early stage – before any visible damage occurs.

Previously, bridges were surveyed by specially trained civil engineers, with primary and secondary – almost exclusively visual – inspections taking place at three-year intervals. In a primary inspection in line with DIN 1076 requirements, the bridge is checked all over from touching distance. In other words, every square meter must be inspected with great precision, with the inspector close enough to reach out and touch the structure at any time – for example, to tap it. They must document and assess cracks and any other damage. If they are unsure, they must arrange for a more in-depth analysis. At that point, they can also call on established non-destructive testing methods, such as the ultrasound and georadar systems developed by Fraunhofer IZFP. “We use a multimodal approach to solve the problems that the civil engineers bring to us. This means we combine different, sometimes newly designed sensors and measurement and condition variables. The system will draw on a variety of information sources, just as a human being would with their own senses,” explains Prof. Herrmann.

This would offer many possibilities for inspections. For example, it could be used to measure the thickness of concrete structures or assess corrosion of embedded steel elements – a characteristic weakness of prestressed concrete bridges, which make up around 70 percent of the German highway bridges. Bridges from the 70s and 80s are a particular risk for stress corrosion cracking, which can cause cracks to form unexpectedly in prestressed steel and, in extreme cases, can cause bridges to collapse. “We have made enormous progress in terms of construction materials. Back then, concrete was far less durable and thick than it is today,” says Prof. Krieger. It all boils down to one thing: old concrete is delicate and prone to cracking. Water and road salt find their way into the cracks and start wearing down the aged structural steel. Prof. Krieger sighs. “There’s actually nothing more you can do at that point.”

To prevent bridges from reaching that stage, it’s important to continuously monitor – and if necessary, repair – the concrete road surfaces and the underlying steel mats that reinforce them. “We are working on miniaturizing our inspection systems so that they can be installed on site as permanent sensors,” says Prof. Herrmann. His plan is that the efficient, intelligent sensors will generate their own energy supply, ideally via solar cells and that the system will also be able to detect the bridge condition in real time. The data from the sensors can be read on site by means of wireless transmission to a cellphone or automatic transfer to a cloud. An initial subcomponent of the system has already been developed and is currently undergoing testing.

Having been responsible for bridge planning, construction and operation since the start of 2021, Carsten Chassard of Autobahn GmbH des Bundes would also welcome a sensor system that could serve as an additional tool for structural monitoring and sustainable maintenance. The civil engineer, who heads up the autobahn company’s Neunkirchen branch, looks after a 334 kilometer autobahn route network with 527 bridges. He also has a tale of woe to tell when it comes to corroded reinforcement steel and cracked concrete structures. However, his hope is that continuous monitoring will make it possible to take more targeted action at an earlier stage. “But the measurement program will always have to be adapted to each specific bridge and its unique static characteristics.”

And it’s important to take the findings of structural inspections, any damage that has already been identified and the characteristic weaknesses of various bridge types into account in the process. Crucial variables such as traffic impact and air and structure temperature must also be measured. Without them, the data cannot be classified and correctly interpreted. Above all, the sensors have to be installed at the points that experience the greatest level of stress. These must be calculated individually for each bridge. “We don’t have any standard ‘construction kit’ bridges in Germany. Placing a sensor at a point where nothing happens is a waste of time,” cautions Prof. Krieger. However, he adds that continuous monitoring of every single problematic element is not a realistic option. Instead, experts will have to choose representative objects and apply the results to similar bridges and structural elements – to avoid closures like Lüdenscheid and the accompanying financial losses, which reach well into the millions.

Fraunhofer Institute for Nondestructive Testing IZFP