New reinforcement system for reinforced concrete bridges: Renovating bridges with «smart» steel
Many bridges in Switzerland were built before the 1980s - and are now approaching the end of their service life. Empa researchers are developing a new type of reinforcement system that will make ageing reinforced concrete bridges fit again. For the first time, they have combined ultra-high-strength fiber-reinforced concrete with shape memory steel, which contracts when heated and thus prestresses concrete structures.
Today, bridges are already reinforced with an additional layer of ultra-high performance fiber-reinforced concrete (UHPFRC). This high-performance concrete is applied directly to the roadway slabs and is particularly dense and resistant to water. Conventional reinforcing steel is embedded in it to increase the load-bearing capacity.
An Empa team led by researcher Angela Sequeira Lemos and Christoph Czaderski from the «Structural Engineering» department has now gone one step further: they are replacing the conventional steel reinforcement with iron-based shape memory steel (Fe-SMA) - an «intelligent» material that can remember its original shape. After installation, the bars are heated to around 200 degrees Celsius. As they want to contract but are held back by the concrete, internal tension is created. These internal forces can close cracks, lift deformed elements and extend the service life of a bridge - without the need for complex tensioning devices. «The beauty of this reinforcement system is its simplicity,» says Sequeira Lemos. «You anchor the rods, heat them up - and they do the rest by themselves.»
Large-scale tests in the Empa construction hall
First, the Empa team investigated the interaction between the shape memory steel and the ultra-high-strength fiber-reinforced concrete, which were combined for the first time. The researchers analyzed how well the two materials are bonded together even after the shape memory steel has been heated and what forces can be transmitted.
This was followed by large-scale tests in Empa's construction hall with five concrete slabs, each five meters long, which simulated self-supporting bridge decks. One slab remained unreinforced, while the others were provided with a layer of ultra-high-strength fiber-reinforced concrete - either with conventional reinforcement or with Fe-SMA bars. In order to simulate real-life conditions, the team first deliberately caused the slabs to crack before they were reinforced - just as would be the case in a real bridge renovation.
After installation, the researchers heated the Fe-SMA rods, causing them to attempt to contract to their original shape and prestress the reinforced concrete structure. Even during activation, existing cracks visibly closed and remaining deformations completely regressed.
Significantly stiffer and more durable
Using state-of-the-art measuring methods, the researchers continuously tracked deformations inside the slabs. Digital cameras monitored the cracks on the concrete surface, while tiny fiber optic sensors were embedded along the bars. «We use sensors that work in a similar way to fiber optic cables in telecommunications,» explains the Empa researcher. «However, instead of sending encrypted data through the fibers, we analyze the light that is scattered back. This allows us to see exactly how the rods deform.»
The tests showed that both the conventional reinforcement and the new system with shape memory steel at least doubled the load-bearing capacity of an unreinforced slab. However, under everyday conditions - such as those caused by normal road traffic - the combination of fiber-reinforced concrete and shape memory steel proved to be superior: it makes the bridge slab stiffer, delays permanent deformations and can close existing cracks or slightly lift sagging components. «We were able to show that our system not only works, but can actually revive existing bridges,» says Sequeira Lemos.
Perfect for damaged bridges
The materials used are still relatively expensive. The system is therefore particularly suitable for heavily deformed or already damaged bridges - in other words, where conventional reinforcement methods have reached their limits. According to Sequeira Lemos, it could also be used in building construction, for example for balconies or flat roofs, where compact solutions or good sealing properties are required.
The Innosuisse-funded project was developed in close collaboration with the OST University of Applied Sciences in Eastern Switzerland, the Empa spin-off re-fer and the Swiss cement industry association cemsuisse. Following the successful tests, the team is now looking for a suitable bridge for the first practical application. «If we can strengthen a real bridge with our system, interest from the industry is likely to grow quickly,» says Sequeira Lemos. «And as demand increases, material costs are also likely to fall - this technology could then have a lasting impact on bridge renovations.»
Source: empa.ch
