A review on the durability of PVC sewer pipes
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.
Plastics are used for a wide range of commercial and industrial piping applications. The most known are polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), acrylonitrile–butadiene–styrene (ABS), polybutylene (PB) and glass–fibre-reinforced polyester (GRP or FRP). Concerning piping systems for drinking water supply, gas distribution and sewage disposal, PVC, PE and PP are the most popular polymer materials (PlasticsEurope, 2017). Especially for gravity sewer pipes, PVC has been extensively used over the past decades and has become the dominant construction material. Cost efficiency, ease of installation, range of available diameters (40–630?mm) and its reputed chemical resistance favour its wide acceptance by decision makers in urban drainage (Davidovski, 2016).
Since there are PVC sewer pipes in operation for at least four decades, concern over their longevity has been lately raised in the Netherlands. It is still unknown whether the expectations of long-lasting PVC pipes (Folkman, 2014) will prove realistic or new asset management strategies should be established in the near future. Knowledge of the current structural integrity of sewer systems is a key issue for establishing successful asset management strategies, leading to better decision making and more affordable investments. In practice, sewer managers currently base their strategies mainly on visual (CCTV) inspections (Van Riel, Langeveld, Herder, & Clemens, 2014). Subsequently, decisions are taken whether replacement, rehabilitation or a near future inspection should take place. However, linking the observed defects in CCTV to the actual physical state of a pipe is challenging (Van Riel, 2017). A necessary condition for achieving this is comprehensive understanding of the mechanisms that affect a PVC pipe’s lifetime, their combined effects and 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.
The main aim of this article is to present case studies of PVC sewer pipes found in the literature and to compare the derived conclusions on PVC durability with findings in inspection (CCTV) data. Emphasis is given on the studies that investigate the properties that define the structural integrity and overall performance of a sewer system. The inspection data concerns three different municipalities in The Netherlands: Almere, Amstelveen and Breda. The main discrepancies between literature and inspection data are discussed, as a step towards bridging results from scientific research and observations from practice.
Suspension polymerisation is the most applied process for PVC particles production (80%), whereas emulsion and mass polymerisation provide 12 and 8% of the world production, respectively (Fischer, Schmitt, Porth, Allsopp, & Vianello, 2014). Although the specific details of the PVC particles size slightly differ in the literature (Benjamin, 1980; Butters, 1982; Faulkner, 1975), the microstructure follows the same pattern. This can be described in three stages (Butters, 1982): the stage III-PVC particle (～100–150?μm), the stage II-primary particle (～0.1–2?μm) and the stage I particle (～10?nm). The conversion of the material to a homogeneous product requires that the boundaries of the primary particles disappear and a new continuous entanglement network is developed (Visser, 2009). This procedure is known as the gelation process and its quality is expressed by the gelation level. There are several methods to obtain information about the gelation level (Castillo, 2016; Choi, Lynch, Rudin, Teh, & Batiste, 1992; Fillot, Hajji, Gauthier, & Masenelli-Varlot, 2006; Gilbert & Vyvoda, 1981; Gramann, Cruz, & Ralston, 2010; Johansson & T?rnell, 1986; Kim, Cotterell, & Mai, 1987; Marshall & Birch, 1982; Real, Jo?o, Pimenta, & Diogo, 2018; Terselius, Jansson, & Bystedt, 1981; Van der Heuvel, 1982).
A general accepted opinion suggests optimum gelation levels of 60–85% (Benjamin, 1980; Breen, 2006). A temperature of >250?°C is needed for this purpose (Guerrero & Keller, 1981), much higher than the degradation temperature of PVC which is ～205?°C (Wypych, 2015). Due to this fact, thermal energy is complemented with mechanical energy (high shear stresses) by the use of twin rotating screws, so as to accelerate this process without extensive exposure of the material to high temperatures (Visser, 2009). Subsequently, the molten material is introduced in a die so that the final pipe is shaped and cooled. This 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 & T?rnell, 1987). The wear observed at the polymer pipes extruders (Gladchenko, Shevelya, Kiyanitsa, & Derkach, 1997) might also contribute to the occurrence of inherent defects.
Residual stresses are also introduced during production, as a result of different cooling rates between the inner and the outer pipe surface (Siegmann, Buchman, & Kenig, 1981), and constitute another parameter that affects the mechanical properties of the produced pipe (Siegmann, Buchman, & Kenig, 1982). Relevant research on residual stresses in PVC pipes (Breen, 2006; Meerman, 2008; Scholten, van der Stok, Gerets, Wenzel, & Boege, 2016) has revealed that their magnitude is in a range of 0.9–4.8?MPa for tensile and 3.9–9.4 for compressive stresses (Table 1). In principle, a faster cooling rate or a thicker pipe wall thickness will lead to higher levels of residual stresses (Janson, 2003; Scholten et al., 2016). However, irrespective of their magnitude, residual stresses affect the crack propagation as they change the stress profile through the pipe (Burn, 1992; Chaoui, Chudnovsky, & Moet, 1987), increase the brittle–ductile temperature (Scholten et al., 2016), and, consequently, they seem to have a tremendous effect on the lifetime of pressurised plastic pipes (Huta? et al., 2013; Podu?ka et al., 2016).
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