ABSTRACT
High damping natural rubber (HDNR) bearings provide a simple means of isolating structures from the damaging horizontal ground motions of earthquakes. Recently this technology has been applied to a low-cost four-storey apartment block in Indonesia. Though most isolation systems are designed so that bearings can be relatively easily replaced, it is obviously preferable that they should normally perform without maintenance or replacement for at least several decades. Natural rubber has been used in structural bearings for 35 years without problems. There is nevertheless a need to confirm the long-term stability of the dynamic shear modulus of the high damping natural rubber compounds used in earthquake isolators. Accelerated ageing tests have been performed on blocks of HDNR compound at temperatures ranging from 50°C to 100°C over periods up to a year. The depth of penetration of oxidative reactions is seen to be limited in relation to the size of full-scale isolators. Anaerobic changes are the predominant influence on the overall stiffness of the bearings. Dynamic shear modulus measurements on anaerobically aged samples show increases of up to about 10%, a figure within acceptable limits. Interestingly, the stiffening observed in quasi-static tensile tests, the type of test more usually performed though less obviously relevant to isolators, is much greater.
