Steel reinforcements in concrete structures are mostly prestressed hydraulically. This requires ducts for guiding the tension cables, anchors for force transfer and oil-filled hydraulic jacks. Because of the space requirements of all the apparatuses required, strengthening of older structures sometimes fails due to large space requirements of this method.
Experts from Empa and re-fer AG have introduced an alternative method to strengthen buildings: shape memory alloys based on iron, which contract during heating and permanently prestress the concrete structure. As a result, hydraulic prestressing can be avoided. The new building material will be marketed immediately under the name “memory-steel”. Several pilot projects, such as the reinforcement of various reinforced concrete slabs, have already been successful.
Development of memory-steel
The development of memory-steel began in the early 2000s. Decades prior, Empa had already pioneered the strengthening of concrete with carbon fiber reinforced polymers (CFRP). This led to the idea of using shape memory alloys for prestressing concrete. Initial tests with nickel-titanium alloys were positive. However, the material is far too expensive for use in the construction sector. In 2009, Empa researchers succeeded in developing an iron-based shape memory alloy, which they also patented.
New opportunities for old buildings
Memory-steel should prove useful for strengthening existing buildings. As soon as new windows, doors or lift shafts are installed in the concrete structure of an old building, a new reinforcement of the load-bearing structure is often unavoidable. In industrial buildings, the load-bearing capacity of an old suspended slab sometimes has to be increased. Such tasks can now also be easily solved in confined spaces: Either a strip of special steel is fastened under the ceiling using dowels and then heated with electricity or an infrared radiator. Alternatively, the reinforcement can also be set in concrete – a groove is milled into the surface of the concrete slab, then a ribbed reinforcement bar made of memory-steel is inserted into the groove and filled with special mortar. Finally, the profile is heated with the aid of direct current and thus prestressed. Another variant is to embed the reinforcement bar in an additional shotcrete layer.
Precast concrete elements with special geometry
In the future, memory-steel could also be a proven method for manufacturing precast concrete parts with a previously unknown geometry. The hydraulic prestressing currently used creates friction in curved structures, which greatly limits the use of this method. With a memory-steel profile embedded in concrete, highly curved constructions are now also possible. When heated, the profile contracts uniformly over its entire length without friction losses and transfers the stress to the concrete.
The ready-to-install memory-steel profiles are manufactured by Voestalpine Böhler Edelstahl GmbH & Co KG in Austria. The company is also working with re-fer and Empa to further develop the composition of the alloy.
Source: Empa, Swiss Federal Laboratories for Materials Science and Technology
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