Flexible pipe comparision
Facts about flexible (non-rigid) pipe line materials
Strength
Flexible pipes such as GRP and PVC-U cannot be defined by crushing strength and a nominal stiffness classification is used. This nominal stiffness is equal to the initial ring stiffness of the pipe which is known to significantly degrade through the life of the pipe.
The draft European standards for PVC-U and GRP make provisions for this by specifying a minimum long term stiffness, usually after 50 years of continuous loading. In the case of GRP the draft EN specifies that the long term value (extrapolated from the results of a 10,000 hour test) should be not less than 40% of the initial stiffness.
The values of suitable long term strengths as low as 40% of the initial value are written into some national standards for this products.
So the engineer is left with the uncertainty of long term strength but the certainty that it will be significantly reduced from the initial strength.
Vitrified clay pipes are rigid pipes and strength does not deteriorate over the service life.
Installation
Durplastic (Thermoset) pipes such as GRP are considered to have zero inherent strength for design purposes and rely entirely on the type and compaction of the bedding material.
This normally means that the bedding materials have to be imported to the site as the in-situ excavated material is unsuitable. Where ground conditions are poor it may be necessary to widen the trench so that the necessary side support can be provided by the installed bedding (rather than the trench walls). This adds considerably to the already increased costs of installation of thermoset pipes.
Vitrified clay pipes because of their inherent strength can normally be laid with a minimum of bedding material.
Maintenance
Abrasion resistance is often ignored in pipe specifications but can be a significant contribution to reduce service lift. For GRP pipes the abrasion resistance of the barrier layer depends more on its thinness than the resistance of the material. An increasing use of high pressure water jetting techniques for cleaning drains and sewers and removes blockages. GRP has shown itself to provide inadequate resistance to the effects of this high pressure jets.