Thermal prestressing of subsequently slotted reinforcement—Experimental and numerical development and optimization of an induction method

Abstract Many infrastructure buildings show deficits in their load‐bearing capacity and need to be strengthened in the short or medium term. However, the available methods are usually not effective for the self‐weight without temporary raising or prestressing the structure. To resolve this problem, thermal prestressing of slotted reinforcing bars is proposed here. The bars are embedded in a grouting material tempered from the outside and thermally expanded according to the temperature change. When the heat supply is stopped, the bond with the hardened grouting material restricts the re‐deformation of the bars during cooling. This creates prestress that counteracts the bending moment from the self‐weight and relieves the tensile zone of the cross‐section. Of course, it is important to limit the heat input to the slots when tempering. Heating the entire cross‐section would reduce the effectiveness of the strengthening. The paper shows the experimental and numerical development of an induction method to thermally prestress subsequently added slotted reinforcement. The strengthening method is first implemented experimentally in reference tests with varying slot shapes. The results are then used to calibrate a numerical computation that is used to investigate the influence of other boundary conditions on the effectiveness of the strengthening measure. The optimum result is derived therefrom and experimentally validated, taking into account practical construction needs. The results show that the numerical optimization of the induction method increases the effectiveness of the strengthening by up to 100%. Accordingly, the reinforcing bars must be installed in triangular, 10 cm wide slots filled with high‐strength concrete (HPC), which is tempered with the highest available thermal output and up to 95°C. In order to counteract the temperature increase in the concrete at the initial cross‐section, which reduces the effectiveness, it must be cooled in parallel. It is advisable to place dry ice at a distance of 6 cm from the edge of the slot over the largest possible width. For practical realization of the strengthening with usually several bars added in parallel, a minimum bar spacing of 40 cm must be maintained to ensure sufficient cooling between the slots. Alternatively, the bars should be attached and thermally prestressed in sequence.