Performance of Prestressed Concrete Cylinder Pipe in North Africa: Case Study of the Water Transmission Systems in the Tafilalet Region of Morocco

摘要

This paper describes the most important results of investigations conducted to determine the causes of the degradation of underground prestressed concrete cylinder pipes (PCCPs) used for the drinking water supply in the Tafilalet region of Morocco. Diagnostic, laboratory, and in situ studies were conducted to locate and visually inspect the damaged areas as well as characterize the damaged PCCP, soil, and groundwater. The results obtained show that chloride-induced corrosion of the prestressing wires is the main cause of the deterioration of the PCCP pipes. The high concentration of chloride ions in the mortar coatings' capillary system is found to originate from the soil and groundwater surrounding the damaged pipes. This is because the soil and groundwater in the damaged areas were found to be highly corrosive. The presence of high amounts of sulfate ions in the mortar coatings, soil, and groundwater further accelerated the corrosion process. The rapid corrosion attack on the PCCPs is also partly attributed to the cyclical wetting and drying exposure conditions in the region, which tends to increase the chloride and sulfate concentrations in the capillary system of the PCCP mortar coating and thus initiate corrosion on the prestressing wires. Improper design and manufacturing defects (such as irregular mortar coating thickness, inconsistent prestressed wire spacing, and low quality of mortar coating) also significantly affected the durability of the PCCPs. Operational factors, however, had a minimal impact on the performance of the PCCP pipes. Based on the results obtained, the durability exposure class for the Tafilalet region was identified as XA3 (i.e., highly aggressive chemical environment) according to international standards. In such circumstances, it is essential to use sulfate-resisting cement. However, the mortar coatings evaluated were neither sulfate resistant nor chloride resistant; therefore, the study concludes that the PCCPs installed were not adequately designed for the exposure conditions in the Tafilalet region.