Actively controlled temperature induction to strengthen reinforced concrete structures

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Many existing reinforced concrete structures need to be strengthened. The typical motivation for this is an upgrade against increased loads such as those that occur on bridges due to traffic or on buildings due to additional service loads.The key issue and fundamental limitation of additional reinforcement is that it is not effective for self-weight without temporary lifting or pre-stressing. Thus, only service loads are proportionately distributed to the existing structure and the newly placed reinforcement. This is in particular detrimental with respect to reinforced concrete structures, in which the self-weight usually amounts up to around 70%.Here, a completely new method is proposed to solve this problem, namely installing of additional reinforcement during local temperature induction. This method works for both, beams and plates. It increases the bending load bearing capacity by additional reinforcement inserted into slots with fast hardening high-performance concrete (HPC) as a composite and thus supplements the existing reinforcement.The core idea is to induce the pre-strain of the existing to the additional reinforcement by systematic heating, and thus to equalize both strains. Then, the two reinforcements act together as a real unit regarding both, increased load-bearing capacity and limited crack widths. Locally, the heat input must be strictly limited to the slots where it is necessary for pre-stretching and rapid hardening of the HPC. By contrast, heating of the total cross-section would reduce the stretching of the additional reinforcement and is hence undesirable. Induction therefore requires precise temporal and spatial control. In the project, temperature induction is developed in a generic and interactive way between modelling (digital twin) and corresponding experiments. This is performed consecutively with respect to size scale (experiment) and methodological extension (model building). Modelling starts from the description of the mathematical system and the actuator as well as sensor placements and develops the control and optimization of the thermal curing and its thermo-mechanical coupling from this. Outcome is a model-based concept of sequential control with an observer that is able to realize transient temperature and strain profiles. On material level necessary thermal parameters are identified while rapid hardening of HPC and creeping of the composite is investigated experimentally. On component level, the net temperature input from thermal mats and infrared radiators is determined and the thermo-mechanical coupling is investigated to achieve the necessary pre-strain and to reduce undesired secondary cracks. Finally, model and experiment are combined to a fully instrumented large-scale demonstrator. For this purpose, a typical reinforced concrete slab shall be strengthened to carry twice its service load. On this demonstrator all developments are implemented and evaluated true to scale.