The structural capacity evaluation: the importance of Non Destructive Tests.

In the structural safety assessment process of existing structures, the knowledge of mechanical material properties is a key point. In fact, different experimental activities carried out on materials extracted from existing reinforced concrete buildings show a high strength variability, especially for concrete. In the past, the lack of standardized material controls and manual techniques of workmanship, determined not always uniform and homogeneous flows of concrete, with consequent very different values of strength within the same structure, but also in the same structural element. The most accurate and reliable experimental techniques are direct tests on the material, but these are very expensive and invasive. Alternatively, indirect methods estimate the material strength by correlating very different physical quantities; however, the non-destructive methods have a lower cost, they are easily executable and finally they cause no damage to the structural elements. In this work, a complete experimental activity, concerning destructive and non-destructive tests, was conducted on some elements (four portions of column and a beam portion) extracted from an Italian school building built in 1940. This building, demolished in 2003 due to a negative evaluation of the structural capacity, represents an interesting case study, especially for the important functionality. Destructive and non-destructive methods have been compared and appropriate correlation laws have been developed in order to predict the main mechanical properties of the material. Concrete compressive strength has been correlated to the ultrasonic pulse velocities and the pull-out test, moreover, the static and dynamic elastic modulus have been compared. Finally, a specific SonReb formulation has been performed on the base of available data