## 3 Experimental Testing
### Metrological Characterization of the Contact Measurement Chain  
Although not used in the destructive compressive testing of masonry specimens, this study included the metrological characterization of contact dimensional measurement chains with LVDT \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Target Identification**} & \(X\) & \(Y\) & \(Z\) \\  & /mm & /mm & /mm \\ \hline
1 & 10.320 \(\pm\) 0.009 & 10.153 \(\pm\) 0.030 & 5.105 \(\pm\) 0.031 \\
2 & 10.344 \(\pm\) 0.010 & 100.137 \(\pm\) 0.033 & 5.023 \(\pm\) 0.011 \\
3 & 10.387 \(\pm\) 0.019 & 189.870 \(\pm\) 0.033 & 5.070 \(\pm\) 0.024 \\
4 & 100.241 \(\pm\) 0.018 & 189.803 \(\pm\) 0.026 & 5.152 \(\pm\) 0.017 \\
5 & 190.090 \(\pm\) 0.016 & 189.852 \(\pm\) 0.032 & 5.266 \(\pm\) 0.039 \\
6 & 189.827 \(\pm\) 0.009 & 99.927 \(\pm\) 0.031 & 4.939 \(\pm\) 0.012 \\
7 & 190.081 \(\pm\) 0.012 & 9.926 \(\pm\) 0.029 & 4.973 \(\pm\) 0.010 \\
8 & 100.318 \(\pm\) 0.012 & 10.047 \(\pm\) 0.030 & 4.934 \(\pm\) 0.006 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Three-dimensional spatial coordinates of the reference targets—referential A. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Target Identification**} & \(X\) & \(Y\) & \(Z\) \\  & /mm & /mm & /mm \\ \hline
1 & 10.197 \(\pm\) 0.018 & 10.733 \(\pm\) 0.034 & 5.077 \(\pm\) 0.028 \\
2 & 10.183 \(\pm\) 0.032 & 100.785 \(\pm\) 0.036 & 4.936 \(\pm\) 0.015 \\
3 & 10.213 \(\pm\) 0.057 & 190.752 \(\pm\) 0.032 & 4.959 \(\pm\) 0.011 \\
4 & 100.437 \(\pm\) 0.054 & 190.655 \(\pm\) 0.016 & 4.858 \(\pm\) 0.007 \\
5 & 190.448 \(\pm\) 0.053 & 190.558 \(\pm\) 0.025 & 4.811 \(\pm\) 0.012 \\
6 & 190.007 \(\pm\) 0.035 & 100.710 \(\pm\) 0.026 & 4.928 \(\pm\) 0.011 \\
7 & 190.112 \(\pm\) 0.022 & 10.751 \(\pm\) 0.025 & 5.032 \(\pm\) 0.037 \\
8 & 100.135 \(\pm\) 0.024 & 10.920 \(\pm\) 0.017 & 5.198 \(\pm\) 0.009 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Three-dimensional spatial coordinates of the reference targets—referential B.  
Figure 8: Dimensional measurement of the reference targets in the optical measuring machine. sensors in order to establish a comparative basis relative to the proposed alternative measurement approach described in Section 2.2. In a first stage, this laboratorial task included the horizontal calibration of the LVDT sensors (composed by its fixed coil housing and the moveable core stem) without assembly accessories (springs) and, in a second stage, the vertical calibration with and without the assembly accessories. This last stage aimed to determine the magnitude of the measurement uncertainty component related to the LVDT installation onto the masonry specimen to be tested, namely, the impact of the measurement position and the use of springs. The tested LVDT sensors (brand HBM, model W50K, Darmstadt, Germany), with a measurement interval between \(\pm\)50 mm, were calibrated in a controlled laboratorial environment (temperature between 19 and 21 \({}^{\circ}\)C, and relative humidity below 65%), based on the electrical tension readings obtained through the data acquisition system (brand HBM, model Spider 8, Darmstadt, Germany) to which each LVDT sensor was connected. In the horizontal calibration, displacement reference values were obtained from SI traceable measurement standard equipment, a unidimensional measuring machine (brand Trimos, model Tulm 0210-450, Renens, Switzerland), with a 200 mm range, resolution equal to 0.1 \(\upmu\)m and instrumental measurement accuracy of 1 \(\upmu\)m. From the sample of eight LVDT sensors calibrated in the horizontal position, the following maximum values of standard uncertainties were obtained: (i) calibration, 1.3 \(\upmu\)m; (ii) repeatability, 3.0 \(\upmu\)m; (iii) linearity, 36 \(\upmu\)m.  
In the second stage, the LVDT sensors were assembled in a vertical column having an SI traceable measurement standard displacement calibrator (brand Sylvac, model P50, Yverdon-les-Bains, Switzerland), with a 50 mm range, resolution equal to 0.1 \(\upmu\)m, and with an instrumental measurement accuracy of 1 \(\upmu\)m).