Strain corrosion

A unique and important performance requirement for GRP gravity pipes used in sewer applications, is the chemical testing of the pipe in a deflected or strained condition. The strain corrosion testing is carried out in accordance with ASTM D3681, and requires a minimum of 18 ring samples of the pipe to be deflected to various levels and held constant. These strained rings are then exposed at the invert of the interior surface to 1.0N (5% by weight) sulphuric acid (see Figure below). This is intended to simulate a buried septic sewer condition. This has been shown to be representative of the worst sewer conditions including those found in the Middle East, where many FLOWTITE pipes have been successfully installed.

The time to failure (leakage) for each test sample is measured. The minimum extrapolated failure strain at 50 years, using a least squares regression analysis of the failure data, must equal the values shown for each stiffness class. The value achieved is then relatable to the pipe design to enable prediction of safe installation limitations for GRP-pipes used for this type of service. Typically this is 5% in-ground long-term deflection.

Stiffnes class

Scv. Strain, %

SN 2500

.49 (t/d)


.41 (t/d)


.34 (t/d)