The Evolution of Elastomeric Joints in Plastic Pipe Systems and the use of Long Term Stress Relaxation Testing to Predict Service Life

Until the late 1950’s, water and sewerage pipe lines used predominantly rigid joints. These were often made from tarred rope and cement mortar. The introduction of ‘flexible’ elastomeric joints allowed angular deflection, transverse shear load and localized pipe deformation without leakage or failure of the joint. This paper presents a brief history of elastomeric seals in water and sewerage pipeline systems, looking at how the existing materials and designs of joints have evolved. It considers the current elastomeric material standards and how these specifications relate to the long-term durability of the joint. 

The contact pressure in a joint, decreases over time due to relaxation of the rubber seal. In this time-dependent relaxation process, the rubber relieves a part of the imposed compressive stress by reorientation of the polymer chains in the rubber material. This reorientation is non-elastic and will result in a permanent reduction in contact pressure. 

Using recognized and proven ISO methodology, the relaxation threshold value may be chosen as the maximum relaxation acceptable for the joint design and application. The time taken to reach this threshold may be estimated.