The article presents experimental research on the corrosion of prestressing steel bars with denotation CKT (fully threaded anchor bars), which are composed of high-quality prestressing steel of the grade Y 1050 (1050 MPa). The experiment was performed using an electrochemical accelerated test. The aspects of the electric current value influence, time dependence on the degree of corrosion, and especially the influence of the prestressing level in the prestressing steel bars on the degree of corrosion were observed and examined. The results of the experiment showed that if the sample was in a stressed state, its degree of corrosion increased. Specifically, for the maximal stress equal to 90% of the tensile strength, the corrosion degree was increased by approximately 7.3%, in comparison to the unstressed specimen. In this case, a 7.3% corrosion degree corresponds to a weight loss of 350 g. The theoretical degree of corrosion was calculated using Faraday’s Law, which allowed the prediction of a rough estimate of the corrosion degree obtained with known input data. The experimental results showed that there was no apparent difference in the corrosion morphology of the sample during the same time-dependent corrosion influence at the same prestressing level in the sample with the same electric current value.

Strengthening existing structures and bridges allows us to continue using them, increase their reliability, resistance, durability and extend their service life instead of demolishing them and replacing them with new ones. This helps to reduce CO₂ (decarbonization). The use of prestressed CFRP strips represents the use of new modern materials and new technology for strengthening existing bridges. The paper is focused on the use of prestressed CFRP strips for strengthening a concrete bridge made of precast prestressed box girders as the most suitable strengthening alternative in a given case. This is a technology that is more commonly used for strengthening structures, but it is not common to use this technology for strengthening bridges. There are relatively few examples of using this technology for strengthening bridges, also because these are dynamically loaded structures. The paper firstly presents the diagnostics and calculation of the load-carrying capacity of the railway bridge on a narrow-gauge railway line in Štrbské Pleso, Slovakia, and then the strengthening of the given bridge. The bridge is located in the mountains of the High Tatras in the northern part of Slovakia and bypasses two local roads. The bridge was made from the precast prestressed post-tensioned box girders of six single spans. The visual inspection, diagnostics, and verification of real dimensions and material characteristics were requested. The non-destructive and semi-destructive methods of testing were used to determine the geometrical and materials’ properties. After that, the calculation of the load-carrying capacity was done. For this purpose, a numerical 3D FEM model was created. For determining the load-carrying capacity, the standard approach, given in Eurocodes, was used according to provisions, which take into account the modified (lower) reliability levels and their adequate partial safety factors. From the calculation, it follows that the bridge should be strengthened. The strengthening of the superstructure was done using prestressed CFRP strips in the lower part of the box girders. This is one of the first applications of this modern method of strengthening, not only in Slovakia but in Central Europe as well.

Reinforcement corrosion is an increasingly serious issue worldwide. The structures gradually lose their load-bearing capacity, reliability and service life of the buildings decrease. Humankind should monitor and solve the problem of corrosion. From this reason, the research is focused on effect of corrosion on the structures not only in Slovak Republic. This study continues the previous work (Strieška et al., 2018), where were investigated corrosion maps of carbon steel (reinforcement in Slovakia between years 2004-2017. A dose-response function was used to evaluate the effect of corrosion on steel rebar. Input data for the corrosion models were obtained from the Slovak hydrometeorological institute database. Corrosion losses for individual monitoring stations across the Slovak Republic were determined by calculation. Corrosion maps were visualizing in programs Gimp and Qgis. The results were processed in the coordinate system WGS 84. The results are corrosion maps of corrosion losses per year between years 2017-2024 in Slovak Republic. Following, the effect of corrosion was applied on the structural beam subjected to bending and exposed in the aggressive corrosion environment. These examples were developed for the monitoring stations, where corrosion risk was the highest, and lowest in Slovak Republic and for the station Žilina. The graphs illustrate expected reduction in beam capacity subjected to bending caused by the corrosion of reinforcement over a 100-year period. The research demonstrates that the effect of corrosion significantly affects the structural reliability and durability. The analysed beam demonstrates reduced load-bearing capacity and does not achieve the required load-bearing capacity over a 100-year period.