Das Verbundverhalten von Betonschrauben mit großem Durchmesser wird mittels Auszugsversuchen mit enger Stützung untersucht. Durch das Eindrehen von Betonschrauben in das erstellte Bohrloch wird eine kraftschlüssige Verbindung hergestellt, über deren Verhalten wenig bekannt ist. Zusätzlich zur konventionellen Installation auf Basis des Hinterschnitts wurde eine Variante mit zusätzlichem Verbundmörtel untersucht. In ersten Versuchen wird der Einfluss der Verbundlänge der Schrauben auf das Tragverhalten durch Variation der Anzahl an Gewindegängen für verschiedene Betondruckfestigkeiten betrachtet. Um genauere Auskunft über die Spannungsverteilung im Kraftübertragungsbereich zu erhalten, wurden anschließend zwei Arten von Messschrauben mit Dehnmessstreifen im Verbundbereich entwickelt. Aus den Erkenntnissen, die mit diesen Messschrauben gewonnen wurden, wird eine Ansatzfunktion abgeleitet, mit welcher die Verteilung der Längsspannung beschrieben werden kann. Aus dieser Verteilung kann anschließend analytisch auf die Verbundspannungsverteilung rückgerechnet werden. Abschließend wird ein Vergleich mit dem gängigen Ansatz einer konstanten Verbundspannung untersucht.

Due to growing traffic volume and more restrictive design standards, structural engineers require new strengthening systems for concrete structures. Especially the level of calculated shear reinforcement has increased with the introduction of the Eurocode standards. Therefore, new methods to improve the shear strength of existing structures are needed. Concrete screws are used as a fastening system in concrete. Due to the mechanical connection, a very robust bond between the reinforcement element and the concrete is given. The new idea is to use these anchoring elements as postinstalled shear reinforcement. To show the usability and enhancements of the new method, several test series with concrete girders have been performed at the University of Innsbruck. Using concrete screws in several different configurations all test beams showed a remarkable increase in shear-load capacity. With a new design approach derived from the test results it was possible to apply the new system to several pilot-projects. One slab bridge built in the 1970s was strengthened to the load level according to Eurocode. Another pilot-project was realized at a railway bridge with a two span concrete box girder. This bridge was built with tendons vulnerable to stress corrosion cracking. To ensure a ductile failure, strengthening was necessary which was realized with concrete screws under ongoing traffic on and underneath the bridge. Currently a shear strengthening of a city tunnel in the central area of Munich is planned. In the roof slab about 7000 concrete screws are projected to be installed as shear and bending strengthening system. K E Y W O R D Sbridge strengthening, concrete screws, postinstalled reinforcement

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<p>The Altstadtring-Tunnel is one of the essential east-west traffic routes in the city center of Munich and was constructed in the late 1960s. Segment 34 of the tunnel was built directly underneath the existing Prince-Carl- Palais, a historic building from 1804. Therefore 15 pre-stressed concrete girders with an effective depth of</p><p>3.5 m and a maximum span of up to 30 m were built which now form the tunnel roof slab. These girders were pre-stressed with steel nowadays well known for stress corrosion cracking. A recalculation of the slab showed that no ductile failure can be guaranteed in case of a progressive rupture of the tendons. Therefore, a concept for strengthening the slab was developed using concrete screw anchors as post installed bending and shear reinforcement. The concrete screw anchors are normally installed as anchoring elements in cracked and non- cracked concrete and are available with diameters up to 22 mm. Developing this concept further, it is straight forward to use these anchoring elements as post-installed reinforcement in existing concrete structures. This new strengthening system was developed at the University of Innsbruck in the last few years and can fulfill the special requirements of this project, such as installation of the strengthening system from underneath the tunnel slab during ongoing use of the structure. High strength steel with diameters of up to 63.5 mm will be used as post-installed bending reinforcement covered with a new shotcrete layer on the underside of the tunnel slab. In total 59.3 tons of new flexural reinforcement and 7199 concrete screws for strengthening the shear capacity of the girders will be used to ensure a ductile failure of the tunnel slab. The on-site work started in March 2019 and is expected to take two years to complete.</p>