Evaluation of mechanical characteristics of steel bars by non-destructive Vickers micro-hardness tests

Materials’ properties evaluation represents a crucial aspect for the static and seismic assessment of existing Reinforced Concrete (RC) structures and infrastructures. While concrete's mechanical characteristics are classically investigated by both destructive tests (DTs) and non-destructive tests (NDTs), for steel reinforcing bars the use of NDTs is not consolidated: a large number of samplings are planned as part of the survey campaign, requiring strong effort to be both extracted and restored and being, moreover, not always sufficient to reliably determine the characteristics of each type of steel grade/rebar used in the structure. Within NTDs, the determination of the hardness value could be a viable way to estimate rebars’ tensile strength. Many experimental studies were proposed in the past and current scientific literature, most of them performed through laboratory tests and therefore requiring a strong time and economic effort: the adoption of portable instruments for the determination of the in-situ hardness of rebars can be a good possibility to reduce the impact on the structure/infrastructure and to optimize timing and restoration operations. Of course, the methodology adopted shall be opportunely calibrated and attention shall be paid to the interpretation of the achieved data. The present research work aims at testing the effectiveness of NDT Vickers micro-hardness tests in the estimation of the mechanical properties of steel reinforcing bars. A methodology is proposed to achieve reliable correlations between hardness values and tensile strength of rebars, accounting for parameters affecting in-situ measures and different typologies of steel grades/rebars.