Performance Characteristics of Profile Wall HDPE Drainage Pipes

Performance Characteristics
of Profile Wall HDPE
Drainage Pipes
By Darrell Sanders, P.E.
June 2008
Professional Development Series
Performance Characteristics of Profile Wall
HDPE Drainage Pipes
By Darrell Sanders, P.E.
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Learning Objectives
In the United States, high-density polyethylene
(HDPE) pipes are commonly used in a variety of applications, including agricultural drainage, storm drains,
natural gas transmission lines, irrigation, relining of
existing utilities, and various industrial processes.
Many of these applications include products utilizing
a profile wall design to enhance the material efficiency
of the product. After reading this article, you should
understand the advantages and limitations of profile
wall pipe designs, along with the performance characteristics of various HDPE resins commonly used in
profile wall pipe construction.
Professional Development Series Sponsor
CONTECH Construction Products Inc.
PS F
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T
he majority of plastic pipe currently installed in
the United States is manufactured using a solid
wall profile. The technology used to produce
solid wall pipes is relatively simple and the manufacturing equipment required to produce a solid wall pipe
product is available from a wide variety of producers. Little
research and development cost is required to enter into
the market with a solid wall product. Generally, existing
product and design specifications are already in place that
can be used to minimize marketing and selling expenses
for the product. Finally, the number of vendors available
to provide ancillary products such as fittings and manhole
connections is much greater when working with a solid
wall pipe versus a profile wall product.
Therefore, why would a producer want to spend extra
time and expense to develop and market a profile wall
product? The answer is because a well-designed profile wall
pipe can utilize material more efficiently, thereby decreasing material costs and potentially providing the product
with a competitive advantage in the marketplace.
Profile wall pipes are defined as “a pipe wall construction that presents a smooth interior wall in the waterway
but includes ribs, corrugations, or other shapes, which
can be either solid or hollow, that helps brace the pipe
against diametrical deformation” (ASTM F2306/F2306M).
By incorporating ribs, corrugations, or other shapes, the
desired pipe wall, moment of inertia, and moment capacity of the wall section can be achieved with less material
than for a comparable solid wall pipe profile.
For gravity flow pipes, often the fundamental performance requirement that plastic pipes have to meet for
acceptance in the marketplace is pipe stiffness. Pipe stiffness
is generally measured using a methodology described in
ASTM D2412, Standard Test Method for Determination of
External Loading Characteristics of Plastic Pipe by ParallelPlate Loading. This test method requires a pipe sample to
be conditioned to a room temperature of 73°F and placed
between parallel plates controlled by a hydraulic load cell.
The pipe is then deflected at a standardized rate of 0.5
inch per minute. The load required to deflect the pipe by
a given amount, generally 5 percent of the diameter, is
recorded. Pipe stiffness (PS) is calculated using Equation 1:
PS F
$y
(Equation 1)
where F is load per unit length required to deflect the
pipe to the deflection level in pounds per inch (lb/in.); and
$y is pipe deflection in inches.
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Performance Characteristics of
Profile Wall HDPE Drainage Pipes
For small deflections, the performance of a given pipe
wall in a pipe stiffness test correlates closely to its relative
wall stiffness, EI. The wall stiffness is the product of the stiffness of the material measured by its flexural modulus (E)
and the stiffness of the geometric wall profile (I). For thermoplastics, the modulus of a given material varies on its
temperature and the rate at which the material is strained.
Even within a common classification of thermoplastics such
as high density polyethylenes (HDPE), the flexural modulus
can vary significantly based on the density of the material.
The moment of inertia for an element is strictly a function of geometry. For a solid wall pipe, the moment of inertia is equal to the cube of the wall thickness (t) divided by
12 (t3/12). For profile wall pipes, the contribution of each
wall element must be determined separately and then
properly combined to arrive at the theoretical moment
of inertia for the pipe wall cross section. A typical cross
section can be broken into its individual elements as shown
in Figure 1.
Figure 1: Typical cross section elements of a profile wall pipe
The contributions of each individual element are
summed together to compute the moment of inertia for
the cross section using Equation 2:
I X X ¤ I x x Ad 2 Other considerations for profile wall pipe
design
While designing a pipe wall section that will provide
the required section properties to compete in a particular market is important, other considerations must be
accounted for to properly qualify the adequacy of the
design. One of the principle checks that needs to be
performed is a stability check for the profile at the performance limits for the product.
In the past, some pipe sections have been designed and
tested only to discover that, when the pipe deflected or
heavy service loads were applied to the pipe, portions of
the pipe wall would buckle and the pipe wouldn’t perform
at the level the section properties implied that it should.
This is because some of the members were not fully effective at strain levels that could occur within the service limits
for the product. The result was elements that buckled or
deformed in service. This can be a particular problem for
elements that include the waterway wall, which needs to
be smooth to maintain the pipe’s hydraulic capacity.
Recognizing that local stability problems were a potential problem for profile wall pipes, the current AASHTO
Load and Resistance Factor Design (LRFD) Bridge Design
Specifications for thermoplastic pipes, Section 12, includes
a methodology that allows a wall profile to be evaluated to
determine if the wall elements are fully effective. If they are
not, the specification provides a methodology for determining what portion of the element can be considered
effective in the computation of the profile’s section properties. This effective width concept is shown in Figure 2.
(Equation 2)
In this formula, the overall moment of inertia (I) is calculated by summing the moment of inertia of each element
about its own centroid plus the product of the element’s
cross sectional area (A) and the square of the distance from
the element’s own centroid to the centroid of full cross
section (d). The key here is that the area of each individual
element contributes to the overall moment of inertia by
the square of the distance from it’s own centroid to the
centroid of section. By creating a tall wall profile that has
elements far away from the centroid of the cross section, a
very high moment of inertia can be developed with far less
wall area when compared to a solid wall profile section.
Since pipe stiffness correlates closely to the EI stiffness
of a wall profile, assuming pipe sections are made from the
same material, a profile wall that achieves
the same moment of inertia as a solid wall
pipe with less wall area will provide a more
efficient cross section and lower material costs for the
producer.
Figure 2: A portion of the element’s width may be determined
to be ineffective, and therefore ignored in calculating section
properties.
For hoop compressive loadings, any portion of the
element that is considered to be ineffective at the strain
levels for the anticipated performance limits for the pipe
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Performance Characteristics of
Profile Wall HDPE Drainage Pipes
is ignored and the
effective
section
properties are computed
as if the material considered ineffective does not exist.
AASHTO also considers that
strains caused by overburden
loads are not the only strains that
need to be considered. Because
thermoplastic pipes are flexible, bending strains due to pipe
deflections are also considered. The LRFD code provides
methodologies to check both the total compressive and
tensile strains within the pipe wall. The primary concerns
about excessive compressive or tensile strains are for different reasons. Excessive compressive strains are primarily a
concern because they can lead to buckling of the pipe wall
members. Tensile strains are a concern because they can
lead to premature cracking in the pipe wall.
Bending strains can be estimated using the AASHTO
design methodology using Equation 3:
¥ c ´¥ $ ´
E b D f ¦ µ¦ µ
§ R ¶§ D ¶
(Equation 3)
in which:
$ 0.05D TL D
Aeff E50
(Equation 4)
where:
Eb
= bending strain (inch/inch)
Df
= shape factor
R
=
c
= distance from the neutral axis to extreme fiber
(inches)
$
= deflection of pipe, reduction of vertical diameter due to bending (inches)
D
= diameter to centroid of pipe wall (inches)
TL
= thrust in the pipe wall (kip/foot)
Aeff
= effective area of the pipe wall (square inches/
foot)
radius to the centroid of the pipe wall profile
(inches)
Table 1: Pipe shape factor (Source: AASHTO)
E50 =
50-year modulus of elasticity for the HDPE material (ksi)
A quick analysis of the calculation of the deflection of
the pipe shows that AASHTO is suggesting that that pipe
be checked for a 5-percent in-service deflection level. The
in-service deflection includes the decrease in the pipe wall
diameter due to the overburden load and the subsequent
foreshortening of the pipe wall. Obviously, other deflection
levels could be evaluated by a designer if desired.
In evaluating the formula for computing the bending
strain (Equation 3), there are two primary factors that need
to be considered. First is the ratio of the distance from the
outer fiber of the pipe wall section to the neutral axis of
the section to the radius of the pipe. This demonstrates
one disadvantage of using tall profiles to gain pipe stiffness; a tall profile also leads to higher tensile strains when
the pipe is deflected. Therefore, composite HDPE products
that use steel ribs or other stiffening features generally
have an advantage over profiles manufactured exclusively
with HDPE material because the higher modulus of the
stiffening features generally allows them to achieve the
required stiffness for the pipe profile with a lower overall
wall height. A lower wall profile height translates to lower
bending strains at the same pipe deflection.
The second factor that contributes directly to the bending strain within a pipe wall is what AASHTO defines as the
pipe’s shape factor. The shape factor for a given pipe is as
shown in a table from the AASHTO specification (Table 1).
The shape factor is an empirical value that is affected by
the stiffness of the pipe being considered, as well as by the
type and level of compaction of the structural fill directly
around the pipe. Therefore, pipes with greater pipe stiffness will deflect less and therefore experience lower bending strains. This is most significant in considering the tensile
strains because those can lead to premature cracking in the
pipe wall. AASHTO currently has a 5-percent allowable
limit on long-term strain.
HDPE resins
HDPE is frequently chosen as a pipe material because
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Performance Characteristics of
Profile Wall HDPE Drainage Pipes
it has many physical properties that make it effective for
common piping applications. It has exceptional abrasion
and chemical resistance; good structural characteristics;
and can be easily extruded, injection, or rotationally molded
into pipe profiles and parts. HDPE pipes are commonly
used in pressurized gas and water transmission systems,
gravity flow drainage systems, and many industrial applications.
All polyethylene resins are made of molecules composed
of long chains of carbon atoms with hydrogen atoms
bonded to the sides of the chain. A typical chemical
diagram is shown in Figure 3.
Figure 3: Typical molecular structure of a polyethylene resin
Some branching of short polymer chains off the sides
of the long polymer chain can occur. The amount of side
branching greatly affects the density of the resin. Variances
in resin density can affect many physical properties. Some
of these changes are listed in Table 2.
Table 2: Change in physical properties as resin density increases
From a pipe-performance standpoint,
higher-density resins provide higher tensile
strength and stiffness properties. However,
higher-density resins are also more strain sensitive and
have less crack resistance. Therefore, pipe manufacturers
need to balance the need for strength and crack resistance
when selecting the resins for use in their pipe products. A
higher-density resin allows a pipe manufacturer to obtain
the required strength and stiffness for its pipe product
with less material. However, the product is more prone to
stress cracking over time, compared with a lower-density
material. Composite pipes that use steel or other alternate materials to gain the structural strength
required for the product can choose a slightly
lower-density material that provides superior
crack resistance under long-term loadings.
To establish long-term design properties,
some resins undergo controlled tests that
subject resin materials to various constant
stress levels and determine the time to
rupture for each specimen. This data is used
to develop the long-term physical properties
for the material that can be used in the design
of pipe products. Testing is performed in accordance with
procedures described in ASTM D1598, Time to Failure of
Plastic Pipe Under Constant Internal Pressure. This data
is frequently submitted to a committee (the Hydrostatic
Stress Board) to review and issue a hydrostatic design
basis (HDB) for that material. These material properties are
published, and the resin is designated as a pressure-rated
resin. Because the testing process takes a long time and
is quite expensive to perform, generally only the highest
quality resins are tested to establish a pressure rating.
From a designer’s perspective, specifying the use of
products that use HDB-rated materials means that the resin
being used in the manufacture of the pipe has a known set
of long-term design properties. Certainly, many products
use polyethylene resins that do not have a pressure rating,
but the mechanical properties for those resins have to
be extrapolated or inferred from other sources. Typically,
higher performing pipe applications such as pressurized
gas lines require the use of HDB materials in the manufacture of the pipe. While pressure-rated resins are generally more expensive, for long-term structural applications,
many designers insist on the use of HDB-rated materials to
provide a more predictable service life.
To compare and specify various polyethylene materials, many specifications use ASTM D3350, Standard
Specification for Polyethylene Plastics Pipe and Fitting
Materials. This specification categorizes polyethylene resins
based on six primary physical properties of the material —
density, melt index, flexural modulus, tensile strength, slow
crack growth resistance, and hydrostatic strength classification. The specification assigns a number to each physical
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Performance Characteristics of
Profile Wall HDPE Drainage Pipes
property of a given resin. The grouping of
these numbers is known as a cell classification. Many specifications indicate the minimum
performance limits required for resins used in the manufacture of the pipe by requiring a minimum cell classification
for the material.
Properly designed HDPE profile wall pipes that address
the strength and service life requirements for an installation can add real value to a project. Project engineers and
specifying agencies should make sure that the products
they approve take all of the applicable design parameters
into consideration.
References
• ASTM F2306/F2306M, Standard Specification for 12 to
60 in. [300 to 1500 mm] Annular Corrugated Profile-Wall
Polyethylene (PE) Pipe and Fittings for Gravity-Flow Storm
Sewer and Subsurface Drainage Applications
• ASTM D2412, Standard Test Method for Determination of
External Loading Characteristics of Plastic Pipe by ParallelPlate Loading
• AASHTO LRFD Bridge Design Specifications, Chapter 12
• Handbook of Polyethylene Pipe, Plastics Pipe Institute, First
Edition
• The Complete Corrugated Polyethylene Pipe Design
Manual and Installation Guide, Plastics Pipe Institute
Darrell Sanders, P.E., is chief engineer for CONTECH
Construction Products Inc. He holds a BS degree in Civil
Engineering from the University of Cincinnati and an MBA from
the University of Dayton. He has been a registered Professional
Engineer in Ohio since 1996. Sanders is a member of several
industry committees, including NCSPA, AASTHO, ASTM, and
Uni-Bell.
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Article Title: Performance Characteristics of Profile Wall HDPE Drainage Pipes
Publication Date: June 2008
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Professional Development Series Quiz
1. What is the greatest advantage of profile wall pipe
6. What is the current allowable limit for
designs versus solid wall pipe?
long-term strains for profile wall HDPE pipes
a) Greater material efficiency
b) More available equipment manufacturers
c) Use of lower cost materials
d) Lower cost of entry for the products
per the AASHTO specification?
a) 3 percent
b) 4 percent
c) 5 percent
d) 6 percent
2. The article indicates that wall stiffness is the best
predictor of pipe stiffness for a wall profile. The wall
stiffness of a pipe profile is the product of which two
physical properties of the section?
7. What physical property is most directly affected by
the amount of side branching that occurs within an
HDPE molecular chain?
a) Density
a) FyI
b) Melt index
b) EI
c) Tensile strength
c) ES
d) Flexural modulus
d) Ir
3. Tensile strains in an HDPE pipe wall are a concern
because of they can lead to what problem?
8. Because composite pipe profiles don’t rely heavily
on the structural contribution of the HDPE material,
manufacturers can use resins with lower densities. What
a) Thinning of the pipe wall
performance advantage does that provide?
b) Local buckling of the pipe wall
a) Better flow characteristics
c) Premature cracking of the pipe wall
b) Greater tensile capacity
d) Loss of pipe hydraulic efficiency
c) Superior long-term crack resistance
d) Greater resistance to ultraviolet degradation
4. In the AASHTO specification, the shape factor for an
installed pipe is an empirical value that is influenced by
9. To ensure that the pipe resin has known long-term
which two factors?
properties, the product should use resins with what kind
a) Pipe diameter and cover depth
of rating?
b) Pipe diameter and backfill type
a) Flexural rating
c) Pipe deflection and backfill compaction level
b) HDB pressure rating
d) Pipe stiffness and backfill compaction level
c) PPI quality rating
d) ISO rating
5. Local instability of the inner wall of a pipe profile
can have a negative impact on which performance
10. Cell classifications for HDPE materials are defined by
characteristic for the pipe?
which specification?
a) Pipe hydraulics
a) ASTM D2412
b) Joint performance
b) AASHTO Design Specification, Section 12
c) Resin performance
c) Plastics Pipe Institute Design Guide
d) Fitting compatibility
d) ASTM D3350
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