The restoration of historic buildings is an important task in civil engineering. However, the assessment and rehabilitation of low-strength reinforced concrete elements remain challenging, as such cases are often not covered by existing standards or approvals. Reinforced concrete elements, particularly those built before 1942, are often identified as having very low strengths. To investigate the characteristics and correlations two historic low-strength reinforced concrete slabs in an existing building in Brieske (constructed in 1913) were examined with a dense measurement grid of surface tensile tests. The graphical evaluation clearly illustrates the distribution of highly variable local strengths. Additionally, core samples were taken and compressive and splitting tensile strengths were determined. The evaluation of the results show correlations that differ from those of normal-strength concretes. A derivation from compressive and split tensile strength significantly overestimates the tensile strength of the concrete compared to the surface tensile strength. It demonstrates that low-strength concrete components require more thorough investigation than normal-strength concrete. Recommendations for further investigations are derived from the findings and the identification of structurally relevant areas.