## 5 Conclusions  
This study aims at defining a new non-destructive approach for historical mortar strength evaluation, focusing on a quick, easy to use, in situ, and relatively economic device, named Equotip hardness tester, applicable to any ancient masonry mortars. The proposed non-destructive testing (NDT) is useful in acquiring strength information which is often absent for widespread structural materials in territories with old and high value built cultural heritage. Within this context, this work shows the first results of an ongoing experience on the mortars of walls from different historic buildings in the town of Camerino (Italy). The data resulting from the Equotip hardness tester were validated by the correlation with the data collected from the Uniaxial Compressive Strength test, allowing the following observations:  
* the Equotip device, due to its low impact energy, is able to evaluate the mechanical properties of ancient mortars, characterized by low strength. In particular, the low dimension of the impact tip of the Equotip device investigates the mechanical properties of the first millimeters of the mortars;
* the data collected by using both the'single impact test' and'repeated impact test' methods; results indicate that the first one is strongly affected by the device settling, underestimating the real mechanical properties of the mortar differently;
* the \"repeated impact test\" method used in this paper allows monitoring of material behavior following repeated impacts and thus discarding initial values affected by settling and final values affected by artificial micro-fracturing of the material because of the impacts. For this reason, the collecting methodology is fundamental to obtain reliable results;
* the comparison between EQ results and UCS laboratory test allowed the calibration of a relationship between the two parameters;
* the investigated mortars are characterized by low values of UCS, ranging from 0.17 MPa to 1.49 MPa, values largely lower than the strength currently required for new structural mortar; they were classified into three groups based on the UCS values and their different stress-strain response to the compression test; in particular, a brittle behavior has been observed for mortars with a higher UCS whereas a ductile behavior was noticed for mortars with lower UCS values; an evidently different mechanical behavior has also been observed between the mortar samples coming from the core of the masonry and the ones coming from the face. In particular, the mortar in the core of walls is generally weaker than the one coming from the wall face. Figure 11: Linear interpolation of \(Lmean_{situ}\) vs \(Lmean_{lab}\). [PERSON]’s coefficient of determination (R\({}^{2}\)) is reported in the graph. The reasons for the lower strength of the masonry core can be related to a number of aspects such as the different ages of the mortars, their mineralogical composition, the manufacturing, and application method. Such aspects, not so documented in the literature, are of relevant importance in quantifying more realistically the structural behavior of historical masonry structures;
* the EQ data collected in situ always have a lower mean value comparing to the ones measured on the specimens in the laboratory. This is an important observation to consider in order to give a correct estimation of the real UCS. This aspect has been previously evidenced by [15], pointing out that the NDT may allow a superficial evaluation of the mortar properties and, more so of other materials [21; 22; 23]. In fact, natural environmental processes, with time, cause decay mostly at the most superficial portion of the mortar, in this study evidenced by the reduction in strength derived from the \(\delta\) parameter. For the reasons just mentioned, the in situ data should be corrected in order to obtain a more reasonable evaluation of the mortar compressive strength.