Elstomeric Bridge Bearings: Ozone Protection, Leachate Analysis and a National Survey on Movement

Elastomeric bearing pads have been used extensively in bridges for the last forty years. Recent problems in the state of Texas with excessive movement of these pads have lead to an investigation of the probable causes. The problem appears to be caused by walking of the pads, a phenomena explained as significant overall bearing pad movement in one direction caused by the accumulation of small movements during shear cycling. One factor found to be conducive to movement was the build up of wax on the bearing surface. The wax, used as ozone protection in vulnerable elastomers, can migrate to the surface of the rubber in large quantities and produce a slick contact surface. Studies not only question the validity of using wax as an antiozonate but also the necessity of the stringent ozone-resistance test required by current codes. This study shows that bearing pad leachate is composed predominately of wax and that use of excessive amounts of wax may be common in bearing pad elastomers. A national survey wasc conducted and indicates that bearing pad movement was experienced by 63% of the states not frequently fixing the movement of their bearing pads. No conclusions could be drawn from the Texas districts survey or the national survey regarding the contribution of natural rubber and tapered bearings to slipping problems.