## 3 Experimental Investigation
### Prestressing and Post-Installation of Sensors  
After a curing period of 28 days, specimens were pre-stressed by applying a load value equal to 20% of the average compressive strength, respectively, for clay bricks and calcarenite masonry.  
The load was applied through a steel contrast system consisting of two HEA200 profiles, one at the top and the other one at the bottom of the panels, connected to each other with four screw rods (two for each side) as shown in (Figure 3).  
Figure 1: Innovative stress sensors: (**a**) Ceramic. (**b**) Capacitive.  
Figure 2: Specim geometry (dimensions in mm).  
The aim is to evaluate the response of the post-installed sensors to the variation of load and therefore to local pressure inside the masonry, starting from an initial stress state, corresponding to what the masonry has applied for permanent loads, for example through the floor slabs.  
The experimental campaign investigates two operating methods for the post-installation of the stress sensors in masonry using the specimens previously pre-stressed.  
The first post-installation system (named P1) was performed through the following procedure. The first step is to drill two symmetrical holes with respect to the vertical axis of the specimen (Figure 4a), placed at one third of the length of the masonry. The holes were made by using a drill and a grinder with a diamond disk, diameter of 50 mm. Holes were then cleaned with compressed air and wetted with water. Afterwards, they were filled using a hydrated lime-based mortar having mechanical properties similar to the mortar used for the construction of the wall (Figure 4b). After partially filling holes, sensors were located inside the cavity (Figure 4c), making sure to arrange them in a perfectly horizontal position, reducing as much as possible any rotations and translations. After the positioning of the sensors, mortar was injected into the cavity with the aim of saturating it and trying to completely cover the surface of sensors (Figure 4d).  
The second post-installation system (named P2) is more complex and articulated. The execution and the cleaning of the holes is identical to the previous case (Figure 5a).  
Figure 4: Operating phases for the first post-installation system (P1) of the sensors: (**a**) hole drilling; (**b**) hole filling by using mortar; (**c**) sensor placement; (**d**) hole sealing.  
Figure 3: Steel contrast system.  
Subsequently, a plexiglass closure mask was fixed directly to the masonry using two metallic screws. This mask is equipped with two side holes necessary for fixing it to the support, while other two holes are used for the placement of injection tubes, and another central hole is used for positioning sensors inside the cavity. The end of the sensor and its power cable have been fixed to a plexiglass plate using a chemical adhesive (Figure 5b). This plate, as indicated in the following images, fits perfectly into the previously fixed mask. This system allows a perfect orthogonal placement of the sensor inside the masonry, avoiding any movements and rotations during the mortar injection (Figure 5c). The mortar used is a superfluid slurry with hydraulic, fillerized lime and eco-pozzolan-based binder. The slurry was injected using the lowest placed injector and proceeded to continuously inject the mortar until it began to leak from the highest placed injector. After these phases, the top injector was closed, and the slurry continued to be injected from the bottom injector to put it under pressure inside the hole (Figure 5d).  
For each post-installation system (i.e., P1 and P2), a total number of ten specimens, five in clay brick masonry and five in calcarenite masonry, were prepared. The specimens were labelled as \"SPn_Ps_m\", where 'n' is the progressive number assigned to each sample,'s' refers to the post-installation system (i.e., 1 = first post-installation system, 2 = second post-installation system) and'm' indicates the masonry type (i.e., C = calcarenite, L = clay bricks).