Thermally controlled prestressing of bonded steel lamellae for the strengthening of slab structures using the example of reinforced concrete bridges

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The transfer project is based on the research project "Actively controlled temperature induction to strengthen reinforced concrete structures" (project number 458161128), which has been running since the end of 2021. The transfer project deals with the subsequent strengthening of reinforced concrete slab structures with bonded steel lamellae, which are prestressed with controlled tempering. The aim of the transfer is to put the basic mthod of controlled temperature induction into practice. This will be carried out up to prototypical application on a real slab bridge in Schwerte (NRW, Germany). Researchers from the fields of concrete structures and digital process engineering are working together with three practical partners, namely a construction company for the development of the application technology for the steel lamellae, Straßen.NRW as the owner of the bridge and controlling expert from the point of view of a construction authority, and an adhesive manufacturer for the transfer of the hot bonding technology known from car body construction to use on aged concrete surfaces. In contrast to the basic project, the method of bending reinforcement is now transferred from internal bar reinforcement inserted into slots to externally applied steel lamellae. In addition, the method is expanded from beam to plate structures. The method is being extended from the laboratory to application under ambient conditions and scaled up to real building dimensions. The aims of the project are to identify suitable adhesive types and configurations, to develop and implement a robust temperature control concept, and to verify the method under real-world conditions. For this, experimental investigations and model-based analyses are to be combined with successive increases in size scale and complexity. They culminate in the joint prototypical implementation on the bridge on site. Special challenges arise in the application process. For this purpose, the interacting, partly competing, parameters of temperatures, adhesive layer thickness and duration of heat treatment have to be configured. At the same time, creep losses in the adhesive layer have to be quantified, compensated for in the temperature control and minimized. Real-world boundary conditions, such as degraded surfaces and ambient influences from changing ambient temperatures or humidity, must be considered for robust reinforcement. For the final anchoring of the steel lamellae, the idea of temperature grading along the lamella length is pursued. The temperature is thereby reduced as planned towards the end of the lamella, and with it the shear stresses to the concrete, so that no unwanted concrete spalling occurs at the external force application. At the same time, full prestressing is maintained in the center of the lamella. From the control point of view, suitable sensor placements and locations of heat induction or additive cooling for temperature control must be determined.