## 4 Experimental Outcomes
### Results: Post-Installation System P1  
The first post-installation method P1 highlighted some critical issues for the correct positioning of the sensors. Results from tests on the specimens (calcarenite and clay brick masonry) equipped with two ceramic sensors are shown in Figure 10. Sensors were unable to capture the trend and the peak load of the load cell reference curve. Ceramic sensors suffered from some installation defects. In detail, the mortar did not fully cover the sensing  
Figure 8: Test setup. Figure 9: Placement of digital absolute displacement indicators. area, hence an inadequate load transfer occurred due to the discontinuities (Figure 11a). The incompatibility between the ceramic material and the mortar caused detachment, sliding and rotation of the sensor (Figure 11b). Moreover, in one case (sample SP1_P1_L), the sensors were broken (Figure 11c) because stress concentration occurred. Results from the tests on specimens (calcarenite and clay brick masonry) equipped with two capacitive sensors are shown in Figure 12. In this case, the capacitance variation (\(\Delta\)C) recorded by sensors was able to reproduce the trend of the load cell reference curve. The copper faces of capacitive sensors established a better adhesion with the mortar layer. Figure 11: Installation defects: (**a**) discontinuity in the section; (**b**) sensor rotation; (**c**) sensor rupture. Figure 10: Comparison of outcomes from ceramic sensor and load cell for post-installation system P1: (**a**,**b**) calcarenite masonry; (**c**,**d**) clay bricks masonry. Installation defects occurred in the capacitive stress sensors in samples SP4_P1_C and SP4_P1_L: voids in the coverage mortar layer (Figure 13a) and sensor rotations (Figure 13b). When the contact mortar surface is not homogeneous, it involves a lower variation of the capacitance \(\Delta\)C, (Figure 12b). Initially, sensors nZ11 and nZ12 (in sample SP4_P1_L, Figure 12d) showed a low sensitivity to load variation due to voids in the mortar layer and rotation, and when the adherence was achieved near the maximum load, the sensor capacitance variations grew significantly. Figure 12: Comparison of outcomes from capacitive sensor and load cell, for post-installation system P1: (**a**,**b**) calcarenite masonry; (**c**,**d**) clay bricks masonry. Figure 13: Installation defects: (**a**) discontinuity in the section; (**b**) sensor rotation. Finally, Figure 14 shows the results of tests on calcarenite and clay brick masonry equipped with one ceramic stress sensor and one capacitive stress sensor. Comparing the capacitive and ceramic stress sensor curve, it is evident that the former one has a better sensitivity compared to the latter because the copper surfaces of the capacitive sensor established good interaction with the mortar filler layer. It is worth observing that in some cases, i.e., capacitive sensor nZ13 (Figure 14b), the interaction at the interface between the sensor surface and the mortar filler layer was lost, and a negative variation of the capacitance was observed. Table 3 summarizes the peak stress values recorded by the ceramic stress sensors (\(\sigma_{max,ceramic}\)) and the variations of capacitance at the peak (\(\Delta C_{max}\)) measured by the capacitive stress sensors. Additionally, the average values and the coefficient of variation (COV) are reported. The values of \(\sigma_{max,ceramic}\) are compared with the compressive strength provided by the load cell of the testing machine (\(\sigma_{max,loadedell}\)). Considering the average values of stress, it is worth noting that the ceramic stress sensors underestimate the peak load by about 83% and 68% with reference to calcarenite and clay brick masonry, respectively. The average peak stress measures by the sensors and by the load cell and the maximum variation of the capacitance recorded by the capacitive stress sensors are also shown in Figure 15, in addition to the standard deviation bars.