## 3 Results and Discussion
### Mechanical Characterization of Specimens
#### 3.1.1 Relative Humidity (RH)  
Figure 8 presents the mass gain curves as a function of the number of days of exposure at 85% RH for the manufactured samples. In general, the UR-FREI specimens exhibited a higher mass gain compared to the UN-FREI specimens from the beginning to the end of the exposure period. The porosity of the recycled rubber compound may be directly related to the material's water absorption capacity. Furthermore, the results show that the stabilization of mass gain during exposure depends on the size and shape of the specimens. While the tension (Figure 8a) and compression (Figure 8b) specimens reach stabilization after 20 days, this trend is not maintained for the prototypes (Figure 8d,e). In addition, the results of the compression set test exhibited greater dispersion in the recycled rubber material (Figure 8c), which comprised the smallest-sized samples evaluated. For the prototypes (Figure 8d,e), a mass gain occurred in the UR-FREI samples as expected, whereas the UN-FREI samples lost mass throughout the exposure period. Processes such as hydrolysis could result in the breaking of chemical bonds, allowing the release of substances and, consequently, a decrease in the material's mass [56]. \begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Symbol** & **Value** \\ \hline \multicolumn{3}{c}{Vertical displacement characteristics U1} \\ \hline Effective stiffness (N/m) & Kv & 146,303,417.64 \\ Effective damping (N\(\cdot\)s/m) & Cv & 76,525.09 \\ \hline \multicolumn{3}{c}{Horizontal displacement characteristics U2 = U3} \\ \hline Effective stiffness (N/m) & Keff & 850,121.57 \\ Effective damping (N\(\cdot\)s/m) & Ch & 41,266.82 \\ Elastic stiffness (N/m) & Ke & 2,151,572.72 \\ Yield stress (N) & fy & 46,473.97 \\ Post-yield stiffness ratio & Kp/Ke & 0.35 \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._The tensile stress of the UN-FREI and UR-FREI specimens is shown in Figure 9a for 500% and 7% strains, respectively. The UN-FREI specimens were tested up to the maximum displacement capacity of the testing machine; however, they did not reach failure, whereas all UR-FREI specimens failed at approximately 7% of their initial length. The tensile strength of UR-FREI was lower than that of UN-FREI in all evaluated cases. For the reference specimens, UR-FREI reached only 6.4% of the tensile strength of UN-FREI. This behavior occurred because the recycled rubber particles exhibit low cohesion, facilitating the early failure of the material. Figure 8: Curves of mass gained over time according to the type of specimen: (**a**) tension, (**b**) monotonic compression, (**c**) residual compression, (**d**) shear compression prototypes, and (**e**) cyclic compression prototypes. The RH condition resulted in a reduction in the tensile strength of the UN-FREI specimens by 9.50% and 10.48% at 14 and 30 days, respectively, indicating that as exposure time increases, strength loss slightly increases. Exposure to high RH can increase the brittleness of polymers compared to low RH because water absorption affects chemical bonds and weakens the polymer structure [56]. In the UR-FREI samples, tensile strength increased by 4.02% at 14 days but decreased by 0.09% at 30 days. Thus, there is no clear evidence of any significant effect of RH, possibly due to the low cohesion of the recycled rubber particles. Figure 9b shows the compressive stress required to reach a deformation of 70% of the initial height in the UN-FREI and UR-FREI specimens. In general, the UR-FREI specimens, which exhibited higher water absorption and greater porosity due to the heterogeneous structure of the recycled rubber sample (Figure 8b), demonstrated lower strength than the UN-FREI specimens. The UN-FREI specimens exposed to RH had 15.00% and 9.67% less compressive strength than the reference value at 14 and 30 days, respectively, while the UR-FREI specimens showed a strength reduction of 16.72% and 6.23%, respectively.