Polyvinyl chloride (PVC) has become one of the dominant construction materials for sewer systems over the past decades, as a result of its reputed merits. However, since PVC sewer pipes have operated for decades in a hostile environment, concern over their longevity has been lately raised by sewer managers in the Netherlands. Towards that direction, the main factors and mechanisms that affect a PVC pipe’s lifetime are discussed in this article, along with the current lifetime prediction methods and their limitations. The review of relevant case studies indicates that material degradation, if any, occurs slowly. However, inspection (CCTV) data of three Dutch municipalities reveals that severe defects have already surfaced and degradation evolves at an unexpected fast rate. A main reason of this gap between literature and practice is the fact that comprehensive material testing of PVC sewer pipes is rarely found in the literature although it proves to be essential in order to trustfully assess the level of degradation and its origins.
eventually their results, which are the defects found in practice. An overview of these mechanisms and their origins is included in this article. Lifetime prediction methods for UPVC pipes are also utilised to describe specific types of failure, while their ability to provide trustful lifetime prediction is discussed.
manufacturing technique is called extrusion and is extensively used to form pipes. Fittings, such as joints, are formed by the injection moulding technique. In the injection moulding process, the melted plastic is injected in a mould, which gives the desired form to the PVC fitting, and after cooling the product is ejected.
During the production process, several additives and fillers may be incorporated in the polymers structure in order to enhance its chemical and physical properties, respectively. Plasticisers and stabilisers are the main additives as they affect the behaviour and degradation rate of the material through its lifecycle. Plasticisers are utilised in order to replace some monomers of the polymer chain, offering a higher degree of mobility and, hence, more flexibility. For sewer applications unplasticised rigid PVC pipes are used. Stabilisers are added for increased resistance to e.g.: UV rays, chemical attack and other relevant external factors (Cardarelli, 2008). For pvc pipework in Europe, lead has been used until the early 2000s, when it was replaced by calcium-based stabilisers in most countries (Anders, 2014).
Every step within the production of PVC pipes and furniture PVC fittings can have an effect on the long-term performance of the final product. The levels of water and oxygen during polymerisation could influence the formation and quality of the produced PVC particles (Butters, 1982). Subsequently, the gelation process, already affected by the degree of polymerisation (Fujiyama & Kondou, 2004), plays a major role in the mechanical properties (Mandell, Darwish, & McGarry, 1982; Moghri, Garmabi, & Akbarian, 2003; Truss, 1985; Van der Heuvel, 1982). These properties are determined by the morphology of the material (Benjamin, 1980; Kuriyama, Narisawa, Shina, & Kotaki, 1998) and by the polymer’s orientation and molecular mobility (Fillot, Hajji, Gauthier, & Masenelli-Varlot, 2007). Additionally, impurities and voids in the polymer structure, frequently referred to as inherent defects, are introduced during production, resulting in crack initiators, and their presence seems to be inevitable (Johansson & Törnell, 1987). The wear observed at the polymer pipes extruders (Gladchenko, Shevelya, Kiyanitsa, & Derkach, 1997) might also contribute to the occurrence of inherent defects.