Drainage and Sewerage
Plastic Pipes Conference Association # 1974 Southampton
Marshall, Williams
During the last ten years, it has often been predicted that plastics will be used in increasing quantities as true engineering materials, Whilst there has been an increased acceptance of GRP as a positive alternative to metals in many applications, the use of thermoplastics for major load bearing structures has been more restricted, with the exception of the piping. industry, where the use of thermoplastics pressure and drain pipe has increased very significantly.
Manufacturers’ data sheets on various types of plastic would suggest that they are ideal materials for pressure piping, since very large “extensions to failure” are quoted (~100%) and uPVC in particular has a ee high “ultimate tensile strength” (~50 MN/m2 [~8000 Ibf/in2]). These properties, coupled with cost advantages (c.f. metals), acknowledged resistance to many environments which corrode metals, increased speed of pipe laying and easier handling characteristics have made thermoplastics attractive alternatives for use in sewage, water carrying and general chemical plant applications. The range of use has been extended recently into higher pressure piping by employing thermoplastics (particularly uPVC) overwrapped with GRP, the thermoplastic effectively acting in place of a gell coat resin.
It must be emphasised that with the majority of plastic pipe which is at present in service use, the materials have behaved in a satisfactory manner and there have been no problems apart from leakages at joints, etc., which are to be expected on a random basis, However, it is unfortunate to have to report that there have been a number of cases where uPVC pressure pipes have failed by cracking of the main section of pipe and various UK water and sewage undertakings have suffered failure problems which have been extremely expensive to repair. Indeed, in two cases the whole lines have been scheduled for replacement with metal pipes.
Of over 100 cases of failures known to the authors, the majority of fractures have occurred on a few systems which would suggest that in these cases either a faulty batch of material had been used or the line had not been laid according to specification. By careful excavation of failed sections, it is possible to check on the degree of care with which the pipe had been laid (backfill, etc.), but it is not quite so easy to check on the condition of the material since, as yet, there are no written standards by which to judge the fracture behaviour of plastic materials. Indeed, the present ASTM and British Standards demand elongations to failure of 80% as a requirement for uPVC, suggesting that the material is highly ductile and should not be subject to brittle failure. Unfortunately, in practice this 1s not the case, since the failures are invariably brittle in nature as shown in figures 1 and 2, which illustrate a practical failure on uPVC sewage pipe. In the close-up view of the failure (figure 2) it can be seen that there is no sign of any macroscopic deformation or yielding and the extension on the material at fracture must have been less than 5% (there was no likelihood of impact failure in this case).
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