## 4 Conclusions  
This paper presents an investigation conducted to evaluate the effect of environmental conditions on a series of relevant mechanical properties of a natural rubber mixture (UN-FREI) and a recycled rubber compound (UR-FREI), materials that can be used to manufacture low-cost seismic isolators in Colombia. Comparisons were made between the two materials to determine differences in their behavior. The size of the isolators plays a significant role in how environmental conditions affect their properties. The mechanical properties evaluated in UN-FREI and UR-FREI showed greater changes in the specimens than in the small-scale prototypes. Small samples have a higher surface-to-volume ratio, which facilitates greater interaction with the surrounding environment. The maximum change observed for all properties and conditions evaluated was 41.6% for UN-FREI specimens and 48.6% for UR-FREI specimens under saltwater exposure. In contrast, the maximum change for small-scale prototypes was significantly lower, with 11.6% for UN-FREI and 12.1% for UR-FREI under RH-31d exposure. Small-scale prototypes have a larger volume relative to their surface area, reducing the influence of environmental conditions on their properties. In the small-scale prototypes, horizontal stiffness increased in UN-FREI under the evaluated conditions, with the greatest gain occurring after 31 days of RH exposure, reaching 10.60% and 7.22% at 100% and 75% strain, respectively. The increase in stiffness may limit the ability of the devices to absorb and dissipate energy through the viscoelastic deformation of the elastomeric material. In other words, the damping capacity of the structural system may be reduced. This effect must be carefully considered in UR-FREI, as these isolators already exhibit high horizontal stiffness and a maximum deformation of 50%. A system with high  
Figure 20: Comparison of the demand-to-capacity ratio (DCR) between \(\text{D}_{\text{TM}}\), \(\text{D}_{\text{TM}}\) max, and \(\text{D}_{\text{TM}}\) min: (**a**) UN-FREI, and (**b**) UR-FREI. effective stiffness tends to have limited deformation capacity during a seismic event. This behavior implies that the dissipation system may reach its maximum deformation capacity more quickly, exposing the structure to direct seismic forces. Additionally, the high effective stiffness of UR-FREI made it difficult to observe significant changes under different environmental exposures. The modification factors in ASCE 7-16 [55] establish a range of structural design safety against exposure conditions throughout the building's lifespan. However, the environmental modification factors obtained in this study are lower than those defined in ASCE 7-16 [55], emphasizing the importance of manufacturers conducting tests to quantify the effects of aging, environmental conditions, heating, loading rate, and manufacturing variations. Performing such testing may justify the use of values different from the standard, which could be more conservative and lead to oversizing the isolators. It was observed that including the modification factors in the modeling of the isolated structure (\(\lambda_{\text{ae,max}}\)) \(y\)\(\lambda_{\text{(ae,min)}}\)) affects the building's response, particularly when applying the maximum factor. Drift, base shear, and DCRs increase due to the reduction in the period and the increase in accelerations, which may lead to a highly conservative structural design. Finally, for future research, it is recommended to improve the recycled rubber mixture to enhance the cohesion of the particles and allow greater deformations in tensile tests for specimens and shear-compression tests for small-scale prototypes. Additionally, extending the exposure time to the environmental agents examined in this study could provide further insight into the material's long-term behavior, ultimately enabling the establishment of property modification factors based on experimental results. Conceptualization, E.D.L.-M., I.C.V.-C., I.E.M.-S., M.A.R.-M., O.C., S.J.D.