Wax Build-Up on the Surfaces of Natural Rubber Bridge Bearings

The problem of excessive slip and walking of natural rubber bridge bearing pads was investigated as part of the overall research project 0-1304, Elastomeric Bridge Bearings. One of the principal factors related to this phenomena was the waxy residue found on the bearing pads and their contact concrete surfaces from problem bridges. Spectroanalysis established that the surface material on the natural rubber bearings taken from two Texas bridges was paraffin wax. This wax is added primarily to permit the natural rubber required by AASHTO are neither accurate in assessing true behavior in the full-size bearing nor relevant to the bearing's performance in practice so that the wax additive may not be necessary. For the bearings studied, it was found that the wax migrates to the surface at a rate of 3 to 9 mm after six months. Other researchers have found that only 1 mm is necessary to protect rubber and that over 3 mm is excessive bloom. It is not clear what thickness could be tolerated without causing slip. A survey was conducted of all states to determine the extent of bearing pad slip. The survey found 55% of the states (or 28 states) were aware of problems with bearing pad movement in their state. If the states who frequently connect their bearings against movement are excluded, we find 63% of the states (or 24 states) were away they had experienced problems. Only Texas, Florida, and Idaho knew of more than 10 slipping cases in their state. A total of 22 states felt the bearing pad movement they had experienced was structurally significant. An attempt to correlate the use of natural rubber or the use of tapered bearing pads with slippage were inconclusive. Because many states attributed the bearing pad movement to improper design, incorrect placement, large roadway skews or long bridge spans, the survey could not conclude the natural rubber or tapered bearings were susceptible to slipping. The Texas district survey was more conclusive showing older bearing pad slipping was cused by problems with design, alteration and maintenance and newer bridge bearing movement was related to the use of natural rubber.