Criteria for identifying concrete homogeneous areas for the estimation of in-situ strength in RC buildings

•The estimation of in-situ concrete strength in existing RC structures is crucial.•The reliability of the most usual NDT and DT methods is discussed.•Suggestions for sampling and evaluation of test results are provided.•Possible improvements to current structural codes are proposed.•An application to a real RC building structure is reported. The investigation of the structural characteristics (geometry, structural details, materials quality, etc.) is a crucial step in the assessment of existing reinforced concrete (RC) constructions especially after earthquakes, fire or explosions, increase of live loads, etc. Specifically, the investigation of materials quality can also be required for new constructions in the case of non-conformity of the strength obtained from standard test specimens. Therefore the knowledge of the in-situ mechanical properties of constituent materials (concrete and steel) is a key issue in the evaluation of structural capacity. In order to achieve this goal, destructive (DT; e.g., cores) and non-destructive (NDT; e.g., ultrasonic, rebound) test methods are often adopted, either alone or combined. However, insufficient indications are provided in the current European and Italian codes to permit an effective use of these methods, individually or in combination, during in-situ investigation campaigns. To remedy this, this paper proposes some possible improvements to the current standards paying special attention to the identification of the concrete areas with homogeneous properties within the structure using non-destructive tests (rebound index and direct ultrasonic velocity tests). It also, using a duly selected number of destructive test values (i.e., cores), establishes a multivariate relation between NDT and DT data for each homogeneous area, thus also permitting the estimation of concrete strength in the other points where only NDT tests are provided. Suggestions for the application of the proposed procedure, are provided in the report of a real case.