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Thermoplastics at Work: Comprehensive Review of Municipal PVC Pipe Products
Thermoplastics at Work: Comprehensive Review of Municipal PVC Pipe Products
Thermoplastics At Work
A Comprehensive Review Of Municipal PVC Piping Products
By Shah Rahman, EIT, Regional Vice President, Underground Solutions, Inc.
Technological advances in the piping unique quality of thermoplastics enable In the set of curves, B, in Figure 1, it can
industry over the course of a century them to be heated, processed, formed be seen that the stress-strain relationship
have made wooden pipes and brick sew- and reshaped many times, without any is somewhat different for viscoelastic
ers a concept of the past, while opening permanent changes taking place in the materials than it is for elastic materials.
the door to better engineered piping material’s physical/mechanical proper- Clearly, you can no longer see a directly
products which provide the user with ties. Thermoset plastics on the other linear relationship between stress-strain,
more cost-effective solutions. Traditional hand, are processed by a combination of and the gradients of the curves depend
piping materials for water and sewer sys- chemicals and heat, and once formed, on the loading time. In other words, for a
tems are steadily being replaced by mod- can not be reshaped. given stress level, the longer the loading
ern materials which provide better per- Thermoplastics, such as PVC used in time, the larger the strain reached. Creep
formance, ease of installation, and over- pipe manufacture, are referred to as rigid is defined as continuing deformation
all, a better “bang for the buck.” plastics. The term rigid indicates that (increasing strain) with time when the
Environmental effects such as corro- these materials do not contain any plasti- material is subjected to a constant stress.
sion pose a dramatic threat to the buried cizers, which would make them more The consequence of creep is that failure
pipe infrastructure in North America. It ductile, and hence unsuitable for buried of the material will occur after load is
has been estimated in a study performed municipal applications. Both thermoplas- applied for a certain amount of time. So
by the Federal Highway Administration tic and thermoset pipes are flexible con- time dependency is a major factor to con-
(FHWA) that corrosion accounts for duits (the pipe/soil interaction is identi- sider in viscoelastic material behavior.
more than $36 billion of loss annually in cal) and are designed accordingly. An important fact is that the time to
water and sewer systems throughout failure is inversely proportional to the
the U.S. Properties of viscoelastic applied stress. In thermoplastic pressure
Today, polyvinyl chloride (PVC), a pipe, it is therefore possible to find and
materials
thermoplastic material, is the most wide- apply a stress level that is low enough to
ly used product in water and sewer sys- Thermoplastics such as PVC are vis- ensure that the theoretical time to failure
tems in the U.S. and Canada. This article coelastic. Viscoelastic materials exhibit will surpass the design life of the
will provide design engineers and speci- elastic as well as viscous-like characteris- pipeline.
fiers with an information base on various tics. A material that deforms under
PVC pressure and gravity piping stan- stress, but regains its original shape and
dards and products, for open-cut and size when the load is removed is classi-
trenchless construction and rehabilita- fied as elastic. Viscous materials, on the
tion. The contents should contribute to other hand, after being subjected to a
the decision-making process for standard deforming load, do not recover their orig-
and product selection when PVC has inal shape and size once the load is
been specified or is under consideration. removed. In reality, all materials deviate
Other pertinent information such as from the linear relationship between
available joint-types is also discussed. stress and strain (Hooke’s Law) at some
point in various ways.
Plastic pipes Defining the direct relationship Figure 1: Stress-Strain Relationship in Elastic
The availability of plastic pipe in the between stress and strain when a load is and Viscoelastic Materials.
late 1950’s in North America was the applied to a material is the most common
beginning of an industry that would way to evaluate the strength of that In thermoplastic pipe applications,
eventually revolutionize the field of material. Graph A in Figure 1 illustrates creep is not free, but is prevented
municipal water and sewer piping. the linear relationship between stress- because the deflection of the pipe is
Plastics are formed by the polymeriza- strain in elastic materials. In an ideal kept constant, as is the case in buried
tion of molecules containing hydrogen elastic material, strain returns to zero as PVC gravity (or pressure) pipe.
and carbon. The three main types of soon as the material is unloaded, and the Consequently, it can be seen from
plastic pipes widely in use in North linear relationship is not typically time- Figure 1 that the initial stress decreases
America include polyvinyl chloride dependent. But it should be noted that in with time, and is referred to as the relax-
(PVC), high density polyethylene all materials, this behavior is valid only ation property of thermoplastic piping
(HDPE), and glass-reinforced pipe (GRP, up to a certain stress point, called the materials. These basic properties of vis-
also called fiberglass pipe). PVC and yield point, after which the strain in the coelastic materials such as PVC and
HDPE fall into the group of thermoplas- material will increase dramatically by HDPE enable engineers to design
tics, while GRP is a thermoset pipe. The creep, before finally failing. pipelines that ensure both structural