These tests also have the advantage of allowing tests to be extended to a large number of elements, as they require limited preparatory work, reduced testing times and less expensive equipment [8; 9; 10; 11]. One of the most adopted procedures is the Rebound Hammer (RH) test which proceeds by impacting a spring driven hammer mass on the surface of the investigated r.c. element and measuring the rebound of the hammer mass. Indeed, the rebound depends on the energy absorbed by the impacted material which may be linked to its strength [12]. It is worth noting that to guarantee reliable measurements, the surface of the investigated element must be smooth. Therefore, to this aim, the surface is prepared by means of an abrasive stone in order to minimize the effects of smoothness and carbonation of the impacted surface on the measurements. Moreover, the technician must control that the mass impacts perpendicularly to the surface and, to improve the accuracy, several readings must be acquired by repeating the measure at each point of a grid fixed on the chosen element. However, as with any non-destructive test, even if the correct procedure is followed, there is still a number of factors affecting the rebound index (see [13; 14; 15]). The effects of variability of the concrete strength on non-destructive tests results have been pointed out for several structural typologies and environmental exposures: in [16] the authors analyzed and discussed the results of an extensive experimental campaign on a viaduct, while in [17] the authors analyzed the data acquired on a huge number of buildingsby means of the Sonreb method. In the latter work, the effects on the structural safety related to the assessment of concrete strength are investigated by considering different analytical formulations of the laws that correlated the concrete strength to the results derived from Sonreb tests and by proposing new empirical laws. In addition, the reliability of the estimated concrete compressive strength depends on the conversion model that correlates the measured rebound index with it. Consequently, the main challenge is the calibration of the conversion model to estimate the compressive strength of concrete. As previously cited, many factors influence concrete strength and are mostly related to the execution phase. This variability is well known and accepted in practical applications, as the influence of some factors may be canceled or significantly decreased only by adopting the precast process. Thus, usually, the aim of the tests for the assessment of concrete strength is not to acquire a local evaluation of this parameter, but to establish a value of the concrete strength that may lead to an accurate evaluation of the structural response of the investigated building to any kind of actions. For this purpose, the present study aims to assess this strength value. It should be noted that the aforementioned variability does not raise significant concerns when the sought mechanical property is directly measured, as it occurs when it is possible to drill cores and perform compression tests. However, when the choice is made to proceed by means of rebound hammer tests, this variability has an impact on the measurements, which cannot be precisely quantified. In fact, it has an effect on the variability of the coefficients of the conversion model that correlates the rebound index to the concrete strength. Therefore, a topic is related to the analysis of the variation of the coefficients of the conversion model and, thus, the accuracy of the assessed strength. Many Authors have underlined that in practical applications the concrete strength varies due to uneven maturation of concrete, effects of corrosion of the steel bars, and vibration puddling, which may be responsible for non-uniform distribution of the aggregates, and temperature during the curing phase, etc. To limit the influence of these factors in the following analysis, it has been chosen to perform the analysis on a single building.