The evaluation of material properties in existing Reinforced Concrete (RC) structures represents a crucial aspect for the structural/infrastructural assessment, including static and seismic actions. Whereas the evaluation of concrete strength properties can be traditionally performed combining destructive (D) and non-destructive (ND) tests, with the aim of exploring almost uniformly the structure/infrastructure, for steel reinforcing bars the adoption of ND methods is still not a consolidated approach. It follows that a large number of bar samplings is planned as part of the survey plan, resulting burdensome from both an economic and a practical point of view and strongly impacting on the existing structure, if we consider repair operations (welding, etc.). Besides, stating the wide variability of rebars typologies and related properties, it’s not always easy to achieve reliable results covering the whole range of executive details. Recently, new approaches and methodologies have been studied aiming to find and validate reliable procedures to achieve information about the mechanical properties of rebars using ND tests, such as, for instance, hardness tests. The determination of the in situ hardness value could be a viable way to determine the ultimate tensile strength through non-invasive procedures. Several studies already exist in the current literature trying to correlate laboratory hardness tests to mechanical properties, but these procedures always require expensive and time-consuming operations (i.e. laboratory tests, extraction of rebars samples, etc.), partially limiting the utility of the method.

In the present paper, the preliminary results of a wide experimental test research aiming to determine reliable correlations between quick in situ hardness measures and the mechanical properties (i.e. tensile strength) of different steel grades are presented. Experimental hardness tests were performed on real case-study viaducts with RC structures using a Vickers micro-hardness tester, which is easier to be applied in situ respect to other instruments – such as, for instance, the Leeb tester, limited concerning the specimen constraint requirements (and therefore the issue of vibration). Tensile tests were of course preliminary performed on extracted rebar samples, in sight of above-mentioned correlations.

The aim of the research is providing to technicians and engineers, through a rapid in situ measurement of the rebars’ hardness – performed following an ad hoc protocol – the estimation of the ultimate tensile strength, useful for recognizing the steel grade and therefore the corresponding stress-strain performance, accounting for possible relevant parameters such as diameter, production process, year of production, etc.