## 4 Discussion  
Tables 2-7 show that the minimum strength of the tested samples were 14.12 kN, 12.60 kN and 29.50 kN for the group of samples with G = 0.4 MPa, G = 0.8 MPa and G = 1.4 MPa, respectively. Since UNI EN 15129 [15] limits the value of tensile stresses acting on elastomeric isolators to 2G, by multiplying 2G by the sample base area (0.1 \(\times\) 0.1 m\({}^{2}\)), the isolators should have strengths at least equal to 8 kN, 16 kN and 28 kN, respectively. The experimental results shown in Tables 2-7 are reported in Figures 7-9, together with the relative minimum tensile strength provided by [15]. It should be noted that the minimum strength for samples of the group with G = 0.8 MPa (12.60 kN, sample 6) is lower than 16 kN. Moreover, the strength of another sample in this group, sample 5 (14.62 kN), is also lower than this value. This suggests that these two samples do not satisfy the standards required by UNI EN 15129, as can also be seen in Figure 8. On the contrary, the strengths of all the other samples  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Refreshed Elastomer** & **Bridge Layer** & **F\({}_{max}\) [kN]** & \(d_{F_{max}}\) [mm] & \(d_{u}\) [mm] & **Type of Failure** \\ \hline
14a (cs) & No & No & 35.34 & 1.41 & 2.91 & Mixed \\
14b (cs) & No & No & 33.42 & 1.71 & 3.12 & Mixed \\  & & _Average 14a and 14b_ & 34.38 & 1.56 & 3.02 & \\
16a & Yes & No & 34.29 & 123.34 & 123.61 & Abnormal \\
16b & Yes & No & 34.16 & 1.51 & 4.52 & Adhesive \\
17a (cs) & No & Yes & 34.77 & 3.31 & 2.32 & Adhesive \\
17b (cs) & No & Yes & 32.83 & 2.61 & 4.91 & Adhesive \\  & & _Average 17a and 17b_ & 33.80 & 2.96 & 3.62 & \\
18a & Yes & Yes & 29.50 & 2.41 & 16.63 & Mixed \\
18b & Yes & Yes & 32.33 & 1.61 & 36.46 & Cohesive \\  & & _Average 18a and 18b_ & _30.92_ & 2.01 & 26.55 & \\ \hline \hline \end{tabular}
\end{table}
Table 6: Experimental results for samples 14a, 14b, 16a, 16b, 17a, 17b, and 17a, 16b.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Sandblasting Material** & \(F_{max}\) [kN] & \(d_{F_{max}}\) [mm] & \(d_{u}\) [mm] & **Type of Failure** \\ \hline
1 (cs) & Grit & 37.18 & 1.60 & 7.82 & Cohesive \\
10 & Corundum & 36.32 & 1.00 & 3.21 & Mixed \\ \hline \hline \end{tabular}
\end{table}
Table 4: Experimental results for samples 1, 14a and 14b.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Covering Layer** & \(F_{max}\) [kN] & \(d_{F_{max}}\) [mm] & \(d_{u}\) [mm] & **Type of Failure** \\ \hline
1 (cs) & Yes & 37.18 & 1.60 & 7.82 & Cohesive \\
14a & No & 35.34 & 1.41 & 2.91 & Mixed \\
14b & No & 33.42 & 1.71 & 3.12 & Mixed \\  & _Average 14a and 14b_ & _34.38_ & 1.56 & 3.02_ & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Experimental results for samples 14a, 14b, 15a and 15b.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Oily Component** & \(F_{max}\) [kN] & \(d_{F_{max}}\) [mm] & \(d_{u}\) [mm] & **Type of Failure** \\ \hline
14a (cs) & Yes & 35.34 & 1.41 & 2.91 & Mixed \\
14b (cs) & Yes & 33.42 & 1.71 & 3.12 & Mixed \\  & _Average 14a and 14b_ & _34.38_ & 1.56 & _3.02_ & \\
15a & No & 37.22 & 1.20 & 30.45 & Cohesive \\
15b & No & 35.23 & 1.28 & 17.80 & Cohesive \\  & _Average 15a and 15b_ & 36.23 & 1.24 & 24.13 & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Experimental results for samples 14a, 14b, 15a and 15b.