Variation of the mechanical properties of the natural rubber used in bridges’ bearings and seismic isolators in Canada

Abstract

Seismic base isolation is a widely used earthquake-resistant system to protect structures from earthquake-induced damage, focusing on mitigating the seismic demand. Elastomer-based isolators are one of the common systems used in seismic bridge isolation. Elastomers used in the isolators are mostly classified into two main categories: Polyisoprene (natural rubber) and polychloroprene (synthetic rubber known as neoprene). The mechanical properties of these elastomers play a crucial role in the performance and behaviour of the isolation system. However, these properties are variable, being influenced by several factors, notably low temperatures, aging, as well as fabrication and material source. Exposure to low temperatures increases the stiffness and hysteresis of elastomers, causing variations in their key mechanical properties, which alter the seismic response of the seismic isolators and affect the seismic performance of the structure. In this study, the variation of mechanical properties of natural rubber, commonly used in seismic isolation applications and laminated bearings for bridges in Canada, is experimentally studied. Four different sources of natural rubber, including aged rubber extracted from recuperated laminated bearings of the original Champlain Bridge and new rubber, are considered to establish the variation of mechanical properties due to source, age, conditioning duration, and test temperature as well as the frequency of cycling. Specimens from each source of natural rubber are conditioned at different temperatures, namely 23ºC, -8ºC, and -30ºC, for different durations, going from 1 hour to 28 days. Experimental tests are conducted, at the conditioning temperatures, on quadruple shear samples. They consist of imposing a sequence of three cyclic shear loading at an increasing strain amplitude ranging from 25 to 150%. Tests were conducted at different frequencies (0.1, 0.25, and 0.5 Hz). Test results are used to extract the key characteristic properties of hysteresis, notably the effective shear modulus and the equivalent viscous damping, as a function of the studied parameters and shear deformation level. Instantaneous stiffening and crystallization curves are constructed. The effects of the studied parameters are investigated and statistical distributions of the mechanical properties of natural rubber are identified. The experimental results show that increasing conditioning time intensifies the stiffening of natural rubber. However, this effect is minimal at -8ºC but more pronounced at very low temperatures (-30ºC). The frequency of cycling has a negligible effect, within the studied range, while aging induces a notable stiffening of the rubber. Additionally, statistical analysis shows that the variations of mechanical properties due to sources and low temperatures (-30ºC), specifically for prolonged conditioning time, are statistically significant. Finally, it is found that the generalized extreme value and lognormal distributions provide the best fit to the mechanical properties (shear modulus) of natural rubber at room (23ºC) and low (-8ºC and -30ºC) temperatures.

Date	12 Aug 2024
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Lotfi Guizani (Supervisor)