## 3 Materials and Methods
### Preliminary Tests on Concrete Materials  
In order to validate the experimental setup, preliminary tests were carried out on two reinforced concrete walls. Being concrete material constituted by a microstructure with a characteristic length generally smaller than the structural dimensions, the impact tests are considered as suitable for its mechanical characterization. The tests were performed on restrained concrete walls sited in the Engineering Campus of the University of Perugia (Figure 3a,b). Given that the walls were only accessible from one side, the tests focused on the surface waves, i.e., R-waves. This preliminary study was carried out on two different walls (CLS1 and CLS2), characterized by different conditions of the superficial layer that was directly exposed to environmental actions: the first has a smoothed layer while the latter presents raw material also affected by different states of degradation. Figure 2: (**a**) Instrumented hammer PCB 086D20 with head bolt. (**b**) High-sensitivity piezoelectric uniaxial accelerometers PCB 393B12 used in the tests. (**c**) Data acquisition system. (**d**) General view of the experimental setup. In the absence of specific information regarding the physical and mechanical properties of concrete used and the thickness of the walls, any effect related to the geometry or to the presence of internal rebars is intended to be neglected in the expected results. Regarding CLS1 wall, the tests were carried out by measuring the local accelerations induced by the passage of elastic waves at a distance of 0.4 and 0.6 m of the two accelerometers from the impact given by the hammer, both in horizontal and vertical directions. For each placement of the accelerometers couple, five hits and related waves were recorded. As an example, Figure 4 shows the typical signals obtained by the instrumented hammer (green line) and the two accelerometers. The blue line refers to the accelerometer closest to the source (Acc 1) and the orange line to the farthest (Acc 2). Owing to the small distance between the source and the receivers, the S-waves could not be detected, while the R-waves were clearly identified as indicated by the red circle. The time interval between the two peaks allowed to estimate the propagation velocity of the elastic wave. Concerning the CLS2 wall, the presence of degraded areas on the surface required to carry out several measurements at different distances between both the source and the accelerometers and between the accelerometers, in order to achieve a more reliable estimation of the homogenized elastic parameters. In Figure 5 the layout of the ten measurement point positions is shown, while Figure 5b,c illustrates the details of the superficial irregularities characterizing the surface of the walls, in which the presence of holes and no smooth surface can be observed. Such a degradation state affected the estimation of the elastic parameters, as highlighted by the results. In particular, the maximum distance between the source and the accelerometers was 2.00 m, while the range of distances between the accelerometers was comprised between 0.20 and 1.60 m. As an example, the graph reported in Figure 6 is related to the data recorded by a couple of accelerometers 0.20 m apart and placed at a distance of 1.60 m between the source and the sensor Acc 1. The small distance between the measurement points determines a detection of a quite high propagation velocity of R-waves, which appears comparable with the measurements made on the CLS1 wall. On the other hand, the large distance between the  
Figure 3: Tested concrete walls: (**a**) CLS1 wall with a homogeneous superficial layer of the material; (**b**) CLS2 wall with raw concrete material characterized by an evident state of degradation. impact point and the receiver allowed to make evidence of the contribution of the S-wave in the measurement points, as indicated by the red circle. Figure 4: Typical values recorded by the accelerometers in the tests on the wall CLS 1.