In this contribution, results of long-term strain corrosion tests on glass fiber reinforced unsaturated polyester (GRP) pipe ring samples are reported. The pipe sections tested had a nominal diameter of 500 mm, an external diameter of about 520 mm, and a wall thickness of about 11 mm. The pipes had been produced by a filament winding process. The nominal ring stiffness was 10 000 N/m2. In the inverse creep tests, each ring section was subjected to a specified constant diametric deflection, which was applied by a deformation-controlled device constructed for the purpose. The samples were exposed to 5% sulfuric acid on the inside of the pipe at the bottom zone of the ring. Stress relaxation tests were carried out under specified constant diametric deflection up to the failure of the sample. At the beginning of each test, the equivalent diametric compressive force in the samples was measured and, in addition, the time to failure of each sample was recorded. For long-term extrapolation of the experimental data, a regression analysis was used and the trend of relative vertical deflection versus time to failure of test samples was established. The long-term 1000 h tests showed that the deformation capacity of the GRP pipe section in acid environment would be reduced by about 75% compared with the deformation at break of the dry samples under short-term static loading. The theoretical correlation showed that under the influence of constant diametric deflection and sulfuric acid, the maximum strain after 1000 h would be reduced. Furthermore, the long-term extrapolation showed a strain at failure after 50 years under continuous exposure to the acid environment of less than 0.2%.