## 5 Comparisons and Discussion  
The results of the the experiments showed a significant influence of the post-installation methodology (system P1 and system P2) and the stiffness of the post-injected mortar and  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**ID Specimen** & \(\sigma_{max\_loadcell}\) & \(\sigma_{max\_ceramic}\) [MPa] & & \(\sigma_{max\_loadcell}\) & & \(\Delta\mathbf{C}_{max}\) [pF] \\ \hline SP1\_P2\_C & 8.12 & (28) 7.46 & (24) 5.55 & 0.92 & 0.68 & - & - \\ SP2\_P2\_C & 7.48 & (26) 7.19 & (30) * & 0.96 & * & - & - \\ SP3\_P2\_C & 7.67 & - & - & - & - & (Q9) 248.12 & (QW) 226.01 \\ SP4\_P2\_C & 7.35 & - & - & - & - & (Q5) 187.32 & (QM) 278.49 \\ SP5\_P2\_C & 6.67 & & (25) 7.42 & & 1.11 & & (Q8) 93.36 \\ \hline
**Average** & **7.46** & & **7.04** & - & - & **187.78** \\
**COV** & **6.3**\% & & **5.5**\% & - & - & **35.6**\% \\ \hline SP1\_P2\_L & 11.91 & (29) 7.14 & (20) 6.07 & 0.60 & 0.85 & - & - \\ SP2\_P2\_L & 11.63 & (27) 5.29 & (21) 8.48 & 0.45 & 0.73 & - & - \\ SP3\_P2\_L & 12.22 & - & - & - & - & (QC) 305.05 & (QO) 271.63 \\ SP4\_P2\_L & 11.47 & - & - & - & - & (QG) 251.26 & (QZ) 250.07 \\ SP5\_P2\_L & 9.88 & & (23) 9.81 & & 0.99 & & (QS) 228.18 \\ \hline
**Average** & **11.42** & & **7.77** & - & - & - & **255.73** \\
**COV** & **7.11**\% & & **18.7**\% & - & - & **9.71**\% \\ \hline \end{tabular}  
* unreliable value.  
\end{table}
Table 4: Peak stress values recorded by the load cell of the testing machine and ceramic stress sensors and capacitance variation values at the peak recorded by capacitive stress sensors post-installed adopting the method P2.  
Figure 20: Average and standard deviation values, according to post-installation system P2: (**a**) calcarenite masonry; (**b**) clay bricks masonry.  
of the sensor with respect to the masonry materials, in particular for the joint between the stone units. The post-installation method P1 has the advantage of being simple but showed many drawbacks because it was unable to ensure adequate filling of the hole and therefore a homogeneous distribution of the pressures on the sensor which could therefore be partially ineffective. This is particularly true for ceramic sensors, while the performance of capacitive sensors that are able to capture the stress state variations seems to be good for measurements. The post-installation method P2, on the other hand, has the disadvantage of being more complex, but it guarantees a more precise positioning and effective filling of the hole. In this case, however, the variations in stiffness due to the injected mortar and the post-installation system showed a better response of the ceramic sensors but a worse performance of the capacitive sensors, which lose sensitivity and are unable to perceive pressure variations in small ranges. The conditions at the peak stress during the test were generally well recorded by both types of sensors even if a precision calibration of the sensors to estimate an alert range for the recorded pressure variations is not yet possible. It is in fact necessary to investigate materials to use as the injection medium of the post-installed sensor to improve compatibility with the existing mortar and optimize redistribution of the pressures on the joint with the sensor installed. This is the fundamental point that guarantees the best performance of the sensors. The greater sensitivity of the capacitive sensors to the pressure variations recorded with the first post-installation system indicates that the capacitive sensor is quite sensitive as long as there is no sudden variation in stiffness between the grout of the joint and that of the injection.  
Subsequent studies and a new experimental campaign are planned, with the aim of finding the optimal post-injection system and calibrating the capacitive sensors to pressure variations, since the pressure-capacity link is non-linear.