## 4 Experimental Outcomes
### Results: Post-Installation System P2  
The experimental results seen above for the P1 installation system led us to improve the method for post-installation of sensors in existing masonry elements. Figure 16 shows the results of tests on calcarenite and clay brick masonry equipped with two ceramic sensors. In this case, sensors were fully embedded (Figure 17), but the mortar stiffness reduced the sensor sensitivity to the load variations. Ceramic sensors were unable to capture the load cycles; the sensing area records a load variation only in proximity to the peak load. The curves of ceramic sensors tagged 24, 28 and 26 assumed a sub-vertical behaviour almost to the peak load (Figure 16a,b). Moreover, ceramic sensors tagged 24 and 28 (Figure 16a), 26 (Figure 16b), 20 (Figure 16c) and 27 (Figure 16d), recorded an initial non-zero load value due to sensor calibration issues. While ceramic sensor tagged 30, (Figure 16b), assumed a trend of a noisy fluctuation because potential electromagnetic interference occurred in proximity of the peak load, it failed diverging to high values. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**ID Specimen** & \(\sigma_{max\_loadcell}\) [**MPa**] & \(\sigma_{max\_ceramic}\) [**MPa**] & & \(\frac{\sigma_{max\_loadcell}}{\sigma_{max\_loadcell}}\) & \(\Delta C_{max\_\text{[pF]}}\) \\ \hline SP1\_P1\_C & 7.36 & (C9) 0.18 & (C8) 1.02 & 0.02 & 0.14 & - & - \\ SP2\_P1\_C & 9.40 & (C6) 3.10 & (C7) 3.67 & 0.33 & 0.396 & - & - \\ SP3\_P1\_C & 7.72 & - & - & - & - & (nz17) 66.44 & (nz16) 71.51 \\ SP4\_P1\_C & 8.59 & - & - & - & - & (nz14) 272.32 & (nz15) 189.17 \\ SP5\_P1\_C & 8.71 & & (10) 0.52 & & 0.06 & & (nz18) 278.0 \\ \hline
**Average** & **8.63** & \(\mathbf{1.50}\) & - & - & - & \(\mathbf{192.57}\) \\
**COV** & \(\mathbf{8.7\%}\) & \(\mathbf{88.7\%}\) & - & - & - & \(\mathbf{46.5\%}\) \\ \hline SP1\_P1\_L & 17.72 & (C1) 4.93 & (C2) 8.01 & 0.28 & 0.45 & - & - \\ SP2\_P1\_L & 14.97 & (C4) 4.93 & (C3) 11.00 & 0.33 & 0.73 & - & - \\ SP3\_P1\_L & 13.80 & - & - & - & - & (nz10) 182.77 & (nz9) 233.79 \\ SP4\_P1\_L & 14.46 & - & - & - & - & (nz11) 10.75 & (nz12) 24.36 \\ SP5\_P1\_L & 18.31 & & (5) 1.00 & & 0.05 & & (nz13) 135.6 \\ \hline
**Average** & **15.85** & \(\mathbf{5.15}\) & - & - & - & \(\mathbf{120.49}\) \\
**COV** & \(\mathbf{11.4\%}\) & \(\mathbf{58.2\%}\) & - & - & - & \(\mathbf{65.2\%}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: 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 the capacitive stress sensors post-installed adopting method P1. Figure 15: Average and standard deviation values according to post-installation system P1: (**a**) calcarenite masonry; (**b**) clay bricks masonry. Figure 18 shows the results of tests on calcarenite and clay brick masonry equipped with two capacitive stress sensors. A general reduction in sensor sensitivity was observed in the capacitive stress sensors for lower load values due to the use of a rigid mortar for the sensor post-installation. This effect was less evident in the capacitive sensors tagged QW (Figure 18a) and Q5 (Figure 18b). Figure 16: Comparison of outcomes from ceramic sensor and load cell, for post-installation system, P2: (**a,b**) calcarenite masonry; (**c,d**) clay brick masonry. Figure 17: Sensor fully embedded after the test. Figure 19 shows the results of tests on calcarenite and clay brick masonry equipped with one ceramic stress sensor and one capacitive stress sensor. The trends of the ceramic sensor curves are different from the load cell reference curves, but peak load recorded are in good agreement with the reference peak load. Figure 19: Comparison between measurements provided by ceramic and capacitive sensors and the load cell data, for post-installation system P2: (**a**) calcarenite masonry; (**b**) clay brick masonry. Figure 18: Comparison of outcomes from capacitive sensor and load cell, for post-installation system P2: (**a,b**) calcarenite masonry; (**c,d**) clay brick masonry.