Concrete hinges can withstand extremely high loads and rotations, while requiring only minimal maintenance. Their use is widespread, mainly in bridge construction, but they also find applications in the prefabrication of tunnel segments. With the right design and implementation, they can meet the highest requirements for the durability and resistance of a structure. However, the existing models and design procedures are relatively outdated. The models are based solely on empirical assumptions, whereas the shear resistance of the joint itself plays only a marginal role. The following paper aims to compare existing design models against experimental results in order to find the most suitable design approach that reliably captures the performance of a hinge under a shear load. An experimental program was developed in which 9 samples of concrete hinges were tested for different levels of axial loads and degrees of reinforcement. The results of the experiments were then compared with the selected design models, and a numerical nonlinear analysis was conducted.

In-situ research and laboratory examination provided on concrete core samples from old bridge structures confirmed that a thin layer of ordinary cement-based render coat protects the underlying concrete from carbonation. The average measured value by phenolphthalein test was less than 2 mm after more than 100 years of service life in exposure class XC3 of EN 206. Low carbonation depth is explained by the presence of a thin (2-4 mm) layer of the protective render coat (PRC) situated on the bridge concrete. The place, where the PRC was of good quality was almost impermeable and the carbonation of the concrete underneath was even 0 mm. If the PRC locally spalled, the carbonation depth of the same concrete, at the same structure and environment reached at this place even more than 80 mm. It was observed that low carbonation of concrete significantly depends on the non-permeability of the PRC. A narrow free space filled with the carbonates causes increased non-permeability of the PRC creating thus the built-in limestone-based (anti-carbonation) barrier with the ability to dramatically reduce CO2 penetration into the beneath concrete over time. Values from in-situ and laboratory research are presented in the article with the explanation of these phenomena.

In-situ research and laboratory study of the concrete of old bridges shows that despite the low strength classes of concrete and the long time of exposure to CO2, it is possible to moderate the depth of their carbonation. Many old bridges were found during the in-situ survey in Slovakia, which showed negligible carbonation under an old cement render (PRC) even after more than 100 years of direct exposure to CO2. At the same time, it was found that if this protective layer was significantly damaged or missing in some places, the depth of carbonation of the same concrete reached considerable depths, locally 70-80 mm. The article presents and summarizes the findings from in-situ and laboratory research with a possible explanation of this phenomenon.