## 2 Materials and Methods  
The materials used in the present experimental study were obtained from several Greek sites, namely quartz from Assiros, near Thessaloniki; marble from west Crete, Chania region; and quartzite and metasandstone from west Crete, Kissamos area. More than 100 kg of each material was sampled from several locations of the same site in order to obtain representative samples. Mineralogical analyses show that marble consists mainly of calcite, while quartzite and metasandstone of quartz and some mica. The quartz samples used in this study were of high purity white quartz. The main difference between quartzite and metasandstone is the degree of transformation. Quartzite, due to dynamic re-crystallization, consists of fine crystalline and elongated quartz grains, which are well oriented; due to the mechanical strain imposed, individual quartz crystals are present. However, metasandstone consists of a fine crystalline matrix with individual quartz crystals, which exhibit lack of orientation due to the lower degree of transformation compared to quartzite. In addition, large gaps are observed between its grains due to the high porosity. Table 1 presents the porosity and density of each raw material, as determined with the use of Archimedes method [47]; all measurements were carried out in triplicate and average values are shown. The standard deviation of porosity and density was 0.002 and 0.005 respectively, for all materials tested. Cylindrical cores of representative samples with NX size (diameter 54.7 mm) were used to determine the physico-mechanical properties of each material. Both ends of the core specimens were cut flat and polished using a cut-off and a special polishing machine, respectively. The height to diameter ratio of all specimens was kept equal to 2 in order to minimize the size effect. From each material type 6-8 specimens were cored (Figure 1a,b). Specimens exhibiting cracks, discontinuities, or alteration phenomena as a result of coring were not used in tests and replaced with new specimens. In this study, the non-destructive P-wave velocity and Schmidt hammer rebound tests were carried out. The P-wave velocity test was performed using the PUNDIT 6 (Portable Ultrasonic Non-destructive Digital Indicating Tester) device (CNS Farnell, London, UK) at a frequency of 1 MHz. According to the procedure, two transducers were placed opposite the two parallel surfaces of the core samples (Figure 2a,b) and the travel time of the ultrasonic waves, from one transducer to the other, was recorded. By taking into account that the distance between the two surfaces of each specimen is known, the velocity of the ultrasonic waves can be calculated [48]. It is known that the ultrasonic velocity depends on density and the elastic properties of each material and is higher when the sample has adequate density, uniformity and homogeneity [49]. \begin{table}
\begin{tabular}{c c c} \hline
**Material** & **Porosity \%** & **Density g/cm\({}^{3}\)** \\ \hline quartz & 0.02 & 2.65 \\ marble & 0.3 & 2.72 \\ quartzite & 0.9 & 2.59 \\ metasandstone & 8.0 & 2.11 \\ \hline \end{tabular}
\end{table} 
Table 1: Porosity and density of the raw materials used. The Schmidt hammer rebound test was performed using an L-type hammer with impact energy 0.735 Nm. Core specimens were tested in a V-shaped cradle block with an approximate weight of 23 kg. Prior to test execution the circular ends of each specimen were divided into eight equal arcs, from the ends of which eight straight parallel lines along their longitudinal axis were drawn (Figure 3). The test was performed by placing the core specimens in the cradle and impacting the hammer vertically along the drawn lines, starting from line No1 (Figure 4). In this manner, five individual impacts were conducted at equal spacing and consequently five rebound values \(R_{L}\) were recorded along the first line drawn. Then, the same procedure was repeated by rotating the specimen 45\({}^{\circ}\).