## 3 Experimental Testing
### Experimental Apparatus  
Solid and hollow ceramic brick masonry specimens were retrieved from the walls of one building built in the beginning of the 20 th century in the city of Lisbon (Portugal), which was undergoing rehabilitation. These specimens were prepared and instrumented for testing at LNEC, with the classical apparatus described in Section 2.1. In particular, eight LDVT sensors were installed in the central region of the specimen frontal and rear surfaces (see Figure 6b), four of them in a vertical position (aiming the determination of the modulus of elasticity based on the measured vertical force and corresponding deformation), and the remaining four in a horizontal position, in order to determine the horizontal strain, and consequently, the Poisson ratio. All the LVDT sensors were fixed directly onto the specimen's ceramic bricks, avoiding bedding joints. The proposed optical approach was also implemented by fixing monitoring targets in the specimen's ceramic bricks and placing the measurement referential with the reference targets close to the observation surfaces (see Figure 6b). The observation distance of the camera relative to the specimen observation surface was optimized in order to establish a suitable field-of-view (with all the monitoring and reference targets visible on the image) and the maximum spatial resolution on the recorded images. Readings from the LVDT sensors and camera images were properly synchronized using a dedicated signal generator, as schematically shown in Figure 9.  
The specimens were subjected to axial compression in a universal testing machine (brand SERVOSIS MFL, model UPS 5000 V, Madrid, Spain) NP 4015-1:1990 force class 0.5 in the measurement interval comprised between 300 and 3000 kN, as shown in Figure 6a, based on an adaptation of the standardized method described in EN 1052-1:1998 [7]. Since the specimens' fracture stress is unknown, several increasing loading and unloading cycles were carried out for both solid ceramic brick masonry specimens (increments of 100 kN, starting from 100 kN) and hollow ceramic brick masonry specimens (increments of 50 kN, starting at 50 kN, due to the known reduced axial compression resistance when compared with solid brick masonry). A total of three cycles were performed for each load level, considering loading speeds between 0.5 and 1.5 kN\(\cdot\)s\({}^{-1}\) (as the maximum applied compression force increased throughout the performed cycles, the loading speed was also increased [8; 9; 10]). For safeguard purposes, LVDT sensors were removed after the appearance of cracks in the bricks, since the collapse of masonry specimens is quite fragile and destructive, as shown in Figure 10.  
Figure 10: Collapse mechanism resulting from compression tests: (**a**) ceramic hollow brick specimen; (**b**) ceramic solid brick specimen. Figure 9: Synchronization of image acquisition and contact measurements. In addition to the axial compression force reading obtained from the used universal testing machine, vertical and horizontal dimensional measurements were performed in the frontal and rear surfaces of the specimen, as shown in Figure 12, noticing that the contact and the optical measurement points are not spatially coincident. Based on the specimen's length and width measurements (exemplified in Appendix C, Table 11), stress versus strain curves, regarding the loading and unloading cycle corresponding to 1/3 of the fracture stress, were obtained as shown in Figure 13 for the case of a solid brick specimen (ID M0M10). Figure 12(a) presents the values based in the contact dimensional measurement chain and Figure 12(b) presents the values based in the optical measuring system, referring to the same observation surface and illustrating the determination of the modulus of elasticity. Figure 12(b) shows the presence of noise in the extension measurements obtained by the optical dimensional measurements, when compared with the strain measurements obtained by the contact measurement chain (Figure 12(a)).