A test study was undertaken to determine the effects of a sustained exposure to sulphuric acid solution on pipe stiffness of PVC sewer pipes held at fixed 5% deflection. This study was part of a multi-participant round-robin program to evaluate and develop a test method for long-term effects of deflection in various environments on plastic sewer pipe. Sections of 6 in. (152.4 mm) diameter SDR 35 PVC sewer pipe ASTM D 3034 — Specification for Type PSM Poly (Vinyl Chloride) (PVC) Sewer Pipe and Fittings 6 in. (152.4 mm) long were tested for pipe stiffness by the parallel plate method ASTM D 2412 — Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading, as received from three suppliers. Pipe specimens were then immersed in a 1.0N sulphuric acid solution for 1 week and retested for pipe stiffness. The pipe sections were then held at 5% deflection in special test fixtures and continuously immersed in a 1.0N solution of sulphuric acid at room temperature. The pipe stiffness of each specimen was measured at various time intervals over a period of two years, without removing them from the acid bath. Results were analyzed to evaluate the effects of the acid immersion, coupled with the fixed deflection. One control pipe specimen was fixed at 5% deflection in an air-only environment and tested for pipe stiffness at the same time intervals as the acid-immersed specimens.
Test data indicates a minimal effect of fixed deflection and acid immersion over time on short-term pipe stiffness. Analysis of load data illustrates the significant relaxation response of PVC pipe under constant deflection, i.e. the decrease in wall stress over time. Two year data exhibit good correlations to the design-oriented viscoelastic predictions derived from Findley's power law. The influence of filler content on stiffness behavior is examined but no conclusive relationship is apparent. The influence of the test fixture on interpretation of results is discussed and recommendations for improvements are presented.