Detection of Prestressed Concrete Cylinder Pipe Thinning from Hydrogen Sulfide Deterioration

Abstract

In service evaluation of internal hydrogen sulfide deterioration of a 14.5 km (9 mile) long sewage force main constructed of 152 cm (60-inch) prestressed concrete cylinder pipe (PCCP) was conducted using sonic/ultrasonic resonant frequency data. One of the causes of concrete waste water pipe deterioration and failure is the thinning and weakening of concrete due to hydrogen sulfide generation at the high points of the line. In May 2003, it was reported that a large piece of sheet steel had been discovered on a bar screen at the wastewater treatment plant supplied by the 152 cm (60-inch) PCCP sewage force main. It was the second time in six months sheet steel had been recovered at the treatment facility. A current manufacturer of PCCP inspected the recovered piece of steel and confirmed that it appeared to be part of the steel cylinder from a large diameter PCCP. It was also reported that a large quantity of concrete and aggregate was being recovered at the sewage treatment plant and the upstream pump stations. The implications of the discovery of the sheet steel at the treatment facility are that the interior concrete core of the pipeline has completely deteriorated in places and the steel cylinder of one or more pieces of PCCP has corroded and dislodged from the pipe. It was probable that the deterioration of the pipeline was caused by hydrogen sulfide corrosion and that the pieces of steel cylinder came from the 152 cm (60-inch) PCCP sewage force main. Non-destructive stress wave measurements in the sonic/ultrasonic frequency range were made on the exterior of the pipe at excavated locations. The sonic/ultrasonic data is a direct measurement of the transmission velocity of both the compression and shear waves and the reflected phases of the compression wave from the inside face of the pipe. The transmission velocity values determine the elastic deformational characteristics of the concrete, including Young's, shear, and bulk moduli, Poisson's Ratio, and calculated strength values. The reflected signals resonate at a frequency that is related to the thickness and compression wave velocity of the concrete liner. Since the longitudinal velocity is measured directly and average thickness of the pipes are known, the average strength of the concrete pipe core as well as the presence of delaminations, spalls or pipe thinning can be determined from the measured resonant frequency. Structural analysis of the 152 cm (60-inch) PCCP was performed to determine the impact of pipe wall thickness reduction through hydrogen sulfide attack. This case study provides a summary of the non-destructive testing performed and the results of structural analysis of the 152 cm (60-inch) PCCP sewage force main.