An experimental study on the within-member variability of in situ concrete strength in RC building structures

•Knowledge of in situ concrete strength is crucial in the structural assessment.•The reliability of the most usual NDT methods is discussed.•The results of an experimental program on as-built structural members are reported.•The role of the main factors influencing in situ strength estimation is examined.•Suggestions for sampling and evaluation of test results are provided. Reinforced Concrete (RC) buildings can require the determination of in situ concrete strength during the execution of new structures to investigate low-strength results from acceptance tests and, especially, in the capacity assessment of existing structures. Most structural codes and technical recommendations indicate that in situ concrete strength should be estimated by means of drilled cores (Destructive Test, DT), possibly supplemented by non-destructive tests (NDTs). Besides contributing to the identification of homogenous concrete areas and thus indicating locations where cores have to be extracted, NDTs can significantly reduce the total amount of cores needed to adequately estimate concrete strength in an entire structure. The paper firstly reports a brief review of the most usual NDT methods (rebound number and ultrasonic pulse) and DT methods (cores) and discusses the role of the main factors influencing in situ strength estimation. It then reports and analyses the results of a wide investigation carried out on an RC beam member extracted from an existing structure. The results show a low variability of rebound number and direct velocity values along the beam, and a high variability for surface velocity values and, especially, core strengths. The wide scatter in some test results has been seen in relationship to the micro-cracking condition arising from past applied loads. Suggestions for test location and interpretation of in situ and laboratory test results are provided.