Certified Elasticity: Acceptance and Testing of Rubber Bearings (UIC 772-2)

UIC Leaflet 772-2, titled “Code for the use of rubber bearings for rail bridges,” outlines the comprehensive engineering standards for elastomeric bearings. The section regarding Tests, Control, and Acceptance (often referenced alongside inspection protocols) is the definitive guide for verifying that the manufactured rubber bearings possess the necessary physical and mechanical properties to safely accommodate bridge movements (thermal expansion, braking loads) without degradation.

The Criticality of Rubber Quality in Bridges

Unlike steel bearings, rubber bearings are subject to aging, ozone cracking, and creep. Therefore, UIC 772-2 imposes strict “Go/No-Go” criteria on the raw materials and the finished product. The goal is to ensure the Shear Modulus (G) remains within limits (typically 0.9 to 1.1 MPa depending on the grade) and that the steel plates inside the bearing are perfectly bonded to the rubber during vulcanization.

Key Acceptance Criteria

The standard divides testing into two main categories: checking the raw elastomer properties and testing the finished laminated bearing.

• Physical Properties: The rubber must meet specific tensile strength and elongation at break thresholds.
• Bond Strength: The adhesion between the elastomer and the steel reinforcing plates must be stronger than the tear strength of the rubber itself (cohesive failure required).
• Accelerated Aging: Samples are subjected to heat (e.g., 70°C for 72 hours) to ensure hardness changes (Shore A) do not exceed permissible limits (+5 points typically).

Test Methods and Tolerances

To accept a batch of bearings, the following tests are mandatory under UIC 772-2 protocols. The table below summarizes the key tests:

Inspection and Maintenance in Service

Beyond the factory acceptance, UIC 772-2 also dictates the in-service inspection (often detailed in Chapter 6). Engineers must regularly check for:

• Excessive bulging (indicating internal bond failure).
• Cracking of the rubber surface due to ozone or UV exposure.
• “Walking” or slipping of the bearing from its plinth.