Innovative solution for post-strengthening or prestressing reinforced concrete and steel structures

Due to increasingly stringent requirements for CO2 savings and recycling, the construction industry has for several years been focusing on renovation and repair of structures instead of immediate demolition and new construction. In this way, for example, the volume of cement required can be significantly reduced. Existing infrastructure structures, such as bridges, nevertheless show relevant damage in many places and need to be permanently reinforced. The revolutionary memory®-steel products open up new possibilities for builders and designers to reinforce or pre-stress existing steel structures and reinforced concrete elements.

The structural problems usually stem from aged concrete due to carbonation and corrosion and fatigue of internal steel reinforcement. Damage due to age or increased loads cause insufficient load carrying capacity, but increasingly, repurposing projects are also being planned that require modifications to structural components due to larger spans.

Representation of the action of memory®-steel when pre-tensioning and when activating in case of prevented return to original state (left), re-plate (center), re-bar (right).

An iron-based shape memory alloy is an innovative solution to reinforce existing concrete structures. The available products allow reinforced concrete and steel structures to be reinforced quickly and easily. This strengthens the structure in a solid manner, which also significantly extends its service life. Prestressing can prevent the occurrence and further propagation of cracks and reduce the stress level of the existing reinforcement. However, structures not only get a longer life, memory®-steel is also fully recyclable and can be reused as stainless steel after a structure is demolished.

Shape memory alloys have the property, among others, that after permanent deformation Ɛp,0 they return to their original state after a single heating (curve 1). If this is prevented (curve 2), a tensile stress Ɛp,0 is created in the material, which can be integrated into a structural element as a prestressing force. Memory®-steel is based on this property. In the workshop, this alloy is first attached to the structure in a prestressed state and then the action is activated by heating.

Tests with re-plate performed at the MFPA Leipzig - during application of the fire-resistant plaster (left) and heating of the underside of the plate (right).

The special iron-based shape memory alloy and its many possibilities within the construction industry are the result of many years of research by the Eidgenössischen Materialprüfungs- und Forschungsanstalt (Empa), the ETH Zurich and numerous international research institutions. The re-fer AG then further developed this development into a market-ready solution in cooperation with the company Sika. (1), (2), (3).

Memory®-steel is highly ductile under tensile stress with a fracture elongation of more than 20%. Due to the proportion of chromium of 10% in the alloy, the material can be compared to a class 1 steel in terms of corrosion resistance. Currently, this structural reinforcement is used in reinforced concrete in two variants: an externally applied band (re-plate), which is fastened on the end with nails, or a bar made of ribbed steel (re-bar), which is fastened to the concrete element with a repair mortar (cementitious). Quality control involves testing the thermo-mechanical properties of the production batches and checking the temperature when heated on site. Test equipment also exists to measure prestressing in elements that are integrated. 

For high-rise projects, re-plate is often used. This product, which is similar in dimensions to carbon fiber CFC laminates, has a cross-section of 120 mm x 1.5 mm and is delivered to the construction site pre-stretched and then fastened into the concrete foundation with nails made of stainless steel type X-CR 48 P8 S15. (4) For this purpose, holes in a standard pattern are made in the band made of memory®-steel at both ends at the factory. After correctly positioning the strap on the construction site, these also serve as a template for pre-drilling holes in the concrete structure. Then the nails are mechanically fastened. Finally, the entire length of the band is heated to a temperature between 100°C and 300° C with a gas burner or an infrared radiator.

By means of an infrared radiator one can control the temperature in a targeted manner and thus achieve a pre-tension depending on the temperature. With a gas burner one always works at the highest temperature, which ensures that the maximum pre-tension σmax of 380 MPa is reached. Damage due to overheating is not possible with normal tools on a construction site. This re-plate forms an external tension band without binding. (5) Protection against corrosion is possible with the SikaCor® EG-1 epoxy coating. In many cases re-plate is combined with CFC laminates in such high-rise projects. Here, re-plate is used to reduce deflection or the width of cracks, for example, while the carbon fiber lamellas provide additional tensile forces to optimize load-bearing capacity.

Another advantage of the re-plate ties is their behavior at elevated temperatures. In this respect, the shape memory alloy memory®-steel behaves like conventional reinforced concrete, which is noticeably less prone to breakage than CFC reinforcements or other bonded solutions, which require additional protection due to the low glass transition temperature of the epoxy resin in order to avoid problems in case of fire. Fire tests according to the ETK curve under load and investigations at the MFPA Leipzig have shown that with a simple fire-resistant plaster type SikaCem® Pyrocoat of only 22 mm, fire resistance class R90 (incl. safety factor 1.5) can be achieved. (6) Even after 120 minutes, no reinforcement problems could be identified. The combination of memory®-steel with CFC-laminates is therefore often an effective solution, in which only re-plate has to be taken into account for sizing in the event of fire and thus no expensive protection for the CFC-laminates is required.

For applications in bridge construction, reinforcement with re-bar, the bar made of ribbed steel, is very efficient. These reinforcing bars with a standard rib geometry can be integrated into pre-milled slots or embedded in the repair mortar. (7) Both ways were considered and optimized in detail during the investigations. The version in which the bar is laid in a groove is very effective for cantilevers with negative bending moments. The other method of reinforcement was already widely used in applications with positive bending moments in combination with a MonoTop® repair mortar from Sika, as well as in applications with negative moments embedded in a SikaGrout® casting mortar. If the bar is to be applied externally, first remove the aged concrete and then attach the bars in the desired position. With the two variants, first of all, only the ends of the bar are attached to the supporting structure at a predetermined length with cement mortar, after which that bar is heated with a gas burner and the prestressing forces are realized. With the repair or casting mortar, the re-bar and the supporting structure of the structure are then joined together along the entire length. In this way, the bar made of ribbed steel functions as a prestressed tensile element that is only subsequently attached. This re-bar has also been used successfully on several occasions in combination with bores and Sika's AnchorFix® epoxy resin adhesive.

Additional prestressing of the load-bearing structures of existing bridges can significantly reduce the width of cracks and also ensures that the cracks do not increase in size, improving the service life of the load-bearing structure. Pre-stressed reinforcement also ensures that fatigue loading at an existing reinforcement occurs at a lower level and reduced amplitude. Studies showed that re-bar as a U-shaped brace can also be used as reinforcement in applications with shear reinforcement (shear force problems).

Increased load-bearing capacity for a reinforced concrete slab with fire-resistant properties - Switzerland

In a parking garage, the load-bearing capacity of a reinforced concrete slab had to be increased. Due to the low height of the space and the fire safety requirements, a solution with re-plate ties was chosen. Initially, the re-plate was attached to the structure, after which the prestressing was realized by heating it with a gas burner and then allowing it to cool down. For fire resistance, a fire-resistant plaster type SikaCem® Pyrocoat was applied. Contractor: Stocker und Partner AG.

Since the reinforcement in some of the concrete supports was out of order, the action of the broken type 145/160 steel bars that normally absorb the tensile forces had to be taken over. Due to the high requirements regarding dust generation, odor nuisance and time limitation, as well as the low component geometry and low concrete coverage, a solution with laterally applied re-plate was chosen. Contractor: Laumer Bautechnik GmbH with in-house engineering office.

Between 1950 and 1970, a curved bridge made of natural stone was extended on one side with a reinforced concrete structure. However, in 2020, extensive carbonation of the concrete and corrosion of the flexural reinforcement was observed at both supports of the bridge. In addition, the load carrying capacity also had to be adjusted to meet current standards. In order to achieve a robust bridge construction while extending the service life of the structure, a solution with re-bar bars in Sika's MonoTop® 412 Eco repair mortar was used. To achieve prestressing forces of approx. 200 kN in the girders of the bridge, the final anchorage points were reinforced with steel brackets. Contractor: re-fer AG and marti arc Jura.

Due to an increase in load, the load-bearing capacity of a cantilevered concrete slab with respect to negative bending moments had to be increased. In addition, the existing deflection had to be reduced and a proper connection of the reinforcement to be applied with the building was necessary. The reinforcement was done using re-bar bars, which were inserted into the pre-milled slots with SikaGrout® 312 casting mortar. The connection to the existing ceiling was achieved using Sika's epoxy resin adhesive AnchorFix® 3001.