This master thesis focuses on validating the Graphical Concrete Strength Class (GCSC) method on a large scale. The GCSC method is an empirical approach for assessing the concrete strength class of existing reinforced concrete structures. It utilizes probabilistic and empirical models, considering factors such as the age of the concrete, environmental conditions, and cement type. The method employs two graphs to estimate compressive strength. The aim is to provide empirical evidence regarding the reliability and applicability of the GCSC method in real-world settings. The study involves inspecting 96 reinforced concrete bridges in Finland, with each bridge having 15-17 drilling cores. Different combinations of drilling cores are utilized to evaluate the method's effectiveness. The results demonstrate the effectiveness of the GCSC method even with fewer than three cores per test location and within the concrete strength class range of C16/20 to C50/60. The analysis of results using histograms indicates the level of approximation achieved by the GCSC method. The study highlights the importance of studying an adequate number of cores to achieve more reliable estimations. The GCSC method offers a practical and cost-effective solution for assessing concrete strength class, enhancing efficiency, and reducing costs associated with evaluating existing reinforced concrete structures. The findings contribute to validating the GCSC method and provide insights for future research to further enhance its accuracy and broaden its applicability.