|Recycled-Plastic Lumber Standards
From Waste Plastics to Markets for Plastic-Lumber Bridges
by Dr. Prabhat Krishnaswamy and Richard Lampo
SN is proud to publish this paper on ASTM standards for recycled
plastic lumber, which won first place in the World Standards Day
paper competition on Standards and the Environment.
This paper presents a case study that illustrates the integral
link between standards development work and benefits to the environment.
Specifically, the diversion of large amounts of post-consumer
waste plastics from landfills to useful products involves the
development of both new technologies and standards that enable
the adoption of these products by the marketplace. The paper highlights
activities that have been undertaken over the past several years
to develop industry-consensus, performance-based standards that
enable the market acceptance of recycled-plastic lumber (RPL)
in structural applications. The results from these activities
have three major benefits to the environment: (1) reduction of
municipal solid wastes being landfilled, (2) an alternative to
pressure-treated lumber that leaches toxic chemicals into waterfront
environments, and (3) substitution of wood in certain specific
applications with an environmentally friendly alternative such
INTRODUCTION AND BACKGROUND
Pollution Prevention and Plastics in Waste Streams
Pollution prevention is an integral part of the management of
the nations municipal solid waste (MSW). Integrated management
of MSW involves source reduction; recovery and recycling of materials;
composting, if possible; and environmentally sound disposal of
waste streams through combustion and/or landfills. Of these alternatives,
source reduction and materials recycling are the preferred options.
One significant component of the waste stream is discarded plastic
products and packaging, which continues to be a growing portion
of the MSW. As detailed in a recent EPA report, (1) plastics contribute
22.4 million tons (20 x 106 metric tons) (or 10.2 percent) to
the total waste stream with only five percent of this total currently
recovered. More importantly, due to its low density, the volume
of plastics in landfills reaches almost 25 percent of the total
volume. And the amount of plastics being discarded annually is
not expected to decrease any time in the foreseeable future.
Over the past decade, local, state, and federal agencies have
addressed the recycling of plastic discards to reduce MSW. These
efforts have focused on making recycling of plastics both technically
feasible and economically viable.
Overview of Recycled-Plastic Products
The adoption of an identification code by manufacturers of plastic
products and packaging has greatly enhanced the recovery, characterization,
and sorting of the variety of plastics in the waste stream. Six
polymer codes identified numerically as: 1-Polyethyleneterephthalate
(PET), 2-High-Density Polyethylene (HDPE), 3-Polyvinyl Chloride
(PVC), 4-Low-Density Polyethylene (LDPE), 5-Polypropylene (PP),
and 6-Polystyrene (PS) constitute the majority of post-consumer
plastics. Other types of plastics fall under Type 7.
Figure 1 shows the various steps in the materials flow diagram for plastics
recycling. Considerable work has been conducted in characterizing
the quantity and types of plastics in different waste streams,
and in evaluating and improving collection methods, separation,
and sorting, as well as technologies for processing post-consumer
mixed plastics. (2-6) There are two broad categories of recycled-plastics
products: resin substitution products and material substitution
products. The following summarizes the state of the art in these
Resin substitution products involve applications where the same
polymeric product is made by substituting recycled resin for virgin
material, either wholly or partially. The technology to ensure
product quality and consistency when recycled resin is used is
quite advanced, especially in packaging applications. Certain
specific additives, ultraviolet-stabilizers, and compatibilizers
are added to the feedstock, and some minor modifications in processing
parameters are made to ensure both processibility and performance.
On a typical material flow diagram (Figure 1), the resin substitution
applications have come full circle.
In the second category, post-consumer resins (PCRs) are used as
a substitute for the raw materials in products made traditionally
from other materials, such as wood, metals, and concrete. One
prime example of this application is recycled-plastic lumber (RPL).
The manufacture of RPL from post-consumer and post-industrial
resins is promising as it consumes large quantities of waste plastics
that would otherwise be destined to landfills, and converts the
waste into useful, durable products.
Success in the second category of products was limited primarily
due to a lack of performance-based standards that would ensure
proper and successful use of the material. The lack of standards
is a particular barrier for the use of RPL by the construction
industry, especially for structural applications. This paper highlights
some of the major accomplishments in standards and specification
developments of RPL over the past few years by ASTM under Subcommittee
D20.20.01 on Manufactured Recycled Plastic Lumber and Shapes,
part of Committee D20 on Plastic. These standards have contributed significantly to
the over 40 percent per year growth rate of the recycled-plastic
lumber industry in recent years. As seen in Figure 1, performance-based
standards play a critical role after re-processing of the waste
State of the Recycled-Plastic Lumber Industry
In the early 1990s, prior to the development of any standards
for RPL, a number of small entrepreneurs created the plastic lumber
industry in the United States by importing some extrusion technologies
from Europe. (7-8) The variation in feedstock materials was significant
depending on the source of the PCRs. Therefore, the performance
of the RPL varied significantly between lots from the same manufacturer
and even more from one manufacturer to another.
The use of PCR in products through material substitution can significantly
accelerate the manufacture and use of recycled-plastics products.
RPL, if manufactured to a consistent quality with useful properties,
can potentially re-use a very high proportion of plastics in the
waste stream, thus decreasing the volume of wastes going to landfills.
A few years ago, the total RPL production was approximately 16
million board-feet (38 000 m3), which is equivalent to about 40 million pounds (18 000 metric
tons) of waste plastics. The current annual growth rate for this
industry has been around 40 percent. Today it is estimated that
the total RPL production is over 300 million pounds (136 000 metric
tons) or about 120 million board-feet (280 000 m3). Assuming a 50 percent recycling rate for all waste plastics,
the total production of RPL is estimated to reach 25 billion board
feet (59 x 106 m3) per year as this industry matures to its full potential. (9)
In comparison, the current consumption of softwood lumber is about
34 billion board feet (80 x 106 m3) annually. Therefore, the importance of the RPL industry in recycling
of plastics cannot be overemphasized.
There are approximately 25 to 30 manufacturers of RPL and related
products across North America. Quality control is done primarily
by visual checking and sometimes by density measurements. No other
properties are measured on the finished product nor are samples
from each batch retained for records. Although processing deficiencies,
product inconsistency, PCR availability and price volatility,
and lack of market penetration have all contributed to RPLs limited
use in construction, the most important factor is the absence
of standards and specifications. Local, state, and federal agencies
that could potentially be large consumers of RPL have been unable
to purchase the materials because no established performance-based
specifications and procurement guidelines exist.
As a substitute for treated wood, recycled-plastic lumber (RPL)
products offer the advantages of being resistant to insects, rot,
moisture, and many chemicals. Plus, plastic lumber materials are
benign to the environment since they do not need chemical treatments
to achieve or maintain their properties.
Although fabricated in typical dimensional sizes, one realizes
very quickly when handling RPL products that they have some physical
and mechanical properties that are much different than their wood
counterparts (on a size-by-size comparison). One of the most obvious
differences is the much lower stiffness (modulus of elasticity)
of RPL materials. Pines and oak typically have moduli of at least
6.9 x 106 kilopascals (1 million pounds per square inch). Unreinforced
RPL typically exhibit moduli at least an order of magnitude lower
RPL is also viscoelastic in terms of its mechanical properties.
This means that the mechanical properties are time-temperature
dependent and subject to permanent deformation (creep) under sustained
loads. The rate of creep depends on the magnitude of the stress,
the duration of the stress, and the temperature at which the stress
is applied. Furthermore, dimensional changes due to temperature
are greater in RPL than in wood. (Wood experiences similar dimensional
dependence but more as a result from the uptake and release of
Initial applications of RPL were typically for picnic tables,
park benches, trash receptacle covers, and other non-critical
load bearing outdoor applications. Some of the earliest product
designs performed well initially but then sagged due to creep.
The use of RPL as a decking over a treated wood substrate also
gained popularity. Due to the lesser stiffness of the RPL decking
boards and its propensity to creep under its own weight, if the
span was too great, joist spacings were decreased and/or thicker
deck boards were used (as compared to those used if the decking
was made from natural wood). As the plastic lumber industry was
struggling to make its way, the use of RPL in outdoor structures
such as decks, boardwalks, and docks was seen as a major opportunity.
RPL picnic tables and park benches are considered worthwhile,
but the demand is just not great enough to divert any significant
quantities of waste plastics from landfills.
RPL STANDARDS DEVELOPMENT
While knowledge of the engineering mechanical properties of RPL
materials is important for something as simple as a picnic table,
this knowledge is particularly important in order to gain common
use of these materials in construction applications. Being able
to accurately measure the bending stiffness of the boards, determine
creep properties and thermal expansion behavior and other mechanical
properties, is required for the design of effective and efficient
structures. Safety and liability are also a part of engineering
design. The lack of industry consensus standards and specifications
for RPL was viewed as a major barrier to increased use of these
materials by the construction industry.
Initially RPL manufacturers specified existing test methods developed
for neat plastic resins to measure such properties as compressive
strength and modulus and flexural strength and modulus. However,
materials researchers quickly realized that there was a shortcoming
using these existing test methods for RPL. These methods specify
coupon-sized specimens that are either molded or cut from the
bulk sample material. This works well for thin-section, homogeneous
materials such as plastic sheets or rods. However, RPL materials
have thick cross-sections (typically greater than 19 millimeters
(3/4 inch)) that are not homogeneous. RPL made from commingled
(mixed) plastics may contain material inclusions and impurities.
Even if made from 100% virgin polymer, the thermodynamics of processing
leads to a non-homogeneous cross-section with a density gradient
that is higher at the outer surfaces and lower in the center.
The existing test methods for plastic materials are not applicable
to RPL materials with non-homogeneous cross-sections. The results
could vary significantly depending on where within the cross-section
the specimen coupon is prepared. If taken from the outer surface,
the results could be greater than the bulk, and if taken from
the center, the results could be significantly less than the bulk
product. The solution was to test the material in its original,
as-manufactured state (at least, relative to the original cross-section).
In July 1993, the ASTM Subcommittee D20.20.01 on Manufactured
Recycled Plastic Lumber and Shapes was formed to develop the needed
test methods and specifications for plastic lumber materials.
The nucleus of this group was comprised of academic and government
researchers, private sector engineers, non-profit research organizations,
and plastic lumber manufacturing representatives, all of whom
had been working cooperatively to further the commercialization
and applications of RPL materials. (10-12) The newly formed Plastic
Lumber Trade Association (PLTA) coordinated its meetings with
the ASTM D20 meetings to maximize the interaction of the association
membership with the standards development activities of the ASTM
plastic lumber subcommittee. This cooperative spirit has led to
work with a focus on developing test methods, specifications,
and building code acceptance criteria. Each of these is detailed
The ASTM D20 activities in recycled-plastic lumber and shapes
have led to the establishment of seven test methods to date. These
D 6108, Standard Test Method for Compressive Properties of Plastic Lumber
D 6109, Standard Test Method for Flexural Properties of Unreinforced
and Reinforced Plastic Lumber;
D 6111, Standard Test Method for Bulk Density and Specific Gravity of
Plastic Lumber and Shapes by Displacement;
D 6112, Standard Test Methods for Compressive and Flexural Creep and
Creep-Rupture of Plastic Lumber and Shapes;
D 6117, Standard Test Methods for Mechanical Fasteners in Plastic Lumber
D 6341, Standard Test Method for Determination of the Linear Coefficient
of Thermal Expansion of Plastic Lumber and Plastic Lumber Shapes
Between 30 and 140°F
(-34.4 and 60°C); and
D 6435, Standard Test Method for Shear Properties of Plastic Lumber
and Plastic Lumber Shapes.
Given the differences between RPL and wood lumber, and the fact
that it is non-uniform in its cross-section, the development of
the above test methods was an essential first step before developing
any specification that may be used for purchasing. These test
methods took almost four years and were published between 1997
Simultaneously with the development of test methods, ASTM Committee
D20 also undertook the development of purchasing and distribution
specifications for RPL. For each end application of RPL in structures,
such as decking board, joists, marine fender piles, pallets, etc.,
a separate specification needed to be developed per the end-use
and performance requirements. Since residential decking boards
from RPL promised to be the most significant market, the first
of these specifications was targeted toward this market. ASTM
D 6662, Standard Specification for Polyolefin-Based Plastic Lumber Decking
Boards was completed and published in March of 2001. As is done
for plastic piping for water, gas, and sewer applications, manufacturers
can now use an ASTM stamp on plastic lumber decking boards that
meet the specifications.
In the development of D 6662 several major issues concerning the
use of RPL in decking boards were resolved. These included the
Dimensional Tolerances: Acceptable tolerance for dimensional RPL
was not available. Tolerance limits that would meet industry requirements
and performance consideration were developed.
Creep: The most significant difference between wooden lumber and RPL
is sensitivity to elevated temperature. As stated earlier, the
viscoelastic nature of RPL makes it susceptible to creep at sustained
loads at elevated temperatures. A methodology was developed to
use creep data per ASTM D 6112 to define design limits to avoid
excessive deflection and creep in the decking boards.
Flammability: The question of plastic lumbers ignitability properties was
also addressed. The fire test method described uses a small ignition
source, which is appropriate for expected sources on a deck such
as hot charcoal briquettes from a tipped over barbecue grill.
Allowable Material Properties for Structural Design: A complete methodology is presented in the standard to determine
allowable maximum span lengths for decking boards based on the
material properties determined from the test methods listed above.
This is analogous to having a design guide or a handbook that
is available for other construction materials such as steel, wood,
Outdoor Weathering and UV Exposure: Another key factor unique to outdoor applications of plastics
is degradation due to exposure to ultraviolet radiation during
outdoor weathering. A key finding during the development of this
specification was that the mechanical properties did not degrade
for RPL even though there is surface discoloration of the material.
RPL decking boards that were installed in a demonstration project
in 1989, and for which original properties were available, were
removed from service and tested in 2000 after 11 years. The flexural
modulus and strength showed no degradation whatsoever.
Slip Resistance: To date, no in-depth studies have been done to compare the slip
resistance or coefficient of friction between wood and plastic
lumber. ASTM test methods exist for various materials but the
results can vary greatly, depending on factors such as the type
of shoe sole material used in the test (e.g., leather vs. rubber)
and whether the test sample is wet and/or rough. There are no
known instances of personal injury due to excessively slippery
plastic lumber surfaces. However, the ASTM plastic lumber subcommittee
recognizes the concern and is continuing to study the issue.
ASTM D20 still has ongoing activities for developing other standards
and specifications for RPL. The nine draft standards that are
under development at various stages of balloting within the organization
Guide for Testing of Plastic Lumber,
Guide for Plastic Decking Construction,
Specification for Plastic Lumber,
Flexural Properties of Marine Piles,
Specification for Plastic Lumber Joists and Beams,
Engineered PVC Decking Boards,
Radial Compression of Fender Piles,
Plastic Lumber for Bulkhead Systems, and
Plastic Marine Fendering Systems.
Building Code Activities
Once test methods and specifications for RPL were developed, the
final step in the standards development process was to incorporate
them into acceptance criteria for new materials in the three major
building codes in the United States. The PLTA and individual RPL
manufacturers have approached the Building Officials and Code
Administrators (BOCA), the International Conference of Building
Officials (ICBO), and the Southern Building Code Congress International
(SBCCI). To date, the standards activities within ASTM have led
to the acceptance of several RPL products for decking board applications
by ICBO and BOCA.
Another key aspect of developing standards to promote the use
of RPL in structures consisted of conducting several demonstration
projects with increasing degrees of complexity and sophistication.
The two main objectives of these projects were to demonstrate
successful use of RPL and to derive information and data that
enhances the standards activities. To this end, five projects
have been undertaken:
Decking boards in a boardwalk at Kelleys Island on Lake Erie,
Bridge at Fort Leonard Wood, Mo.;
Floating docks for the Op Sail 2000 event in New York, N.Y.;
Elevated platforms at the bob-sled and luge track, Lake Placid,
An arched truss bridge near Albany, N.Y.
Some unique features of each project are highlighted next.
Boardwalk at Kelleys Island
To educate the public on the uses of recycled materials and at
the same time test plastic lumber in an extreme environment, the
construction of a 600-foot long (180 m) boardwalk in a wetlands
area was selected as a demonstration project. The boardwalk is
located at the North Pond area of Kelleys Island in Lake Erie,
Ohio. This area is owned by the State of Ohio and provides a wonderful
habitat for birds and waterfowl among other fauna and flora. It
is a major resting area for migratory birds, many of which, otherwise,
we would never observe. Before construction of this boardwalk,
there was no access into the marshland and to the lakeshore. The
boardwalk provides public access to this area and is a perfect
application to highlight the durability and rot resistance of
the recycled-plastic lumber in a wet environment. This area will
experience wet and damp conditions, extreme temperature changes
from summertime (more than 90°F (32°C) to winters subzero), and
heavy snowfalls and ice heaves. Figure 2 shows the completed boardwalk. Since this was one of the first
demonstration projects, the substructural joist system was made
of wood (see Figure 3). Plastic lumber from six different manufacturers was used for
the decking boards over this wood frame. The boardwalk was constructed
using volunteer labor in coordination with the Ohio Department
of Natural Resources staff.
Bridge at Fort Leonard Wood, Mo.
With the help of funding from the USEPA, plans for a plastic lumber
bridge were developed to demonstrate the structural capabilities
of plastic lumber. An existing wood timber bridge at Fort Leonard
Wood, Missouri, was selected to demonstrate the structural applications
of plastic lumber. The 25-foot long by 26-1/2 foot-wide (7.6 m
by 8.1 m) plastic lumber bridge sits on six steel girders that
had supported the original wooden bridge. Although the bridge
is now used primarily for pedestrian traffic, the original wood
bridge was designed for light vehicular traffic. The replacement
plastic lumber bridge was also designed to carry vehicular loads.
Figure 4 shows an Army Humvee crossing the plastic lumber bridge.
A typical treated wood bridge structure at Fort Leonard Wood would
need to be replaced every 15 years, with biannual inspections
and maintenance to replace deteriorated boards and loose fasteners.
The plastic lumber bridge is expected to last 50 years with minimal
maintenance. While the plastic lumber materials cost more than
double what they would for a treated wood bridge, a lifecycle
cost analysis showed the plastic lumber bridge would begin to
pay for itself in less than eight years. An added benefit is the
fact that the plastic lumber bridge used 13,000 pounds (5900 kg)
of waste plastics (equivalent to approximately 78,000 one-gallon
(3.79-liter), high-density polyethylene milk jugs and 335,000
eight-ounce (240-mL) polystyrene foam coffee cups) that had otherwise
been destined for landfilling. The bridge will not require application
of protective coatings or preservatives that can emit environmentally
damaging volatile organic compounds into the atmosphere.
Floating Docks at Op Sail 2000
One of the principal markets for RPL would be in marine and waterfront
applications, where chromium copper arsenate-treated lumber poses
an environmental hazard. Therefore, a demonstration project was
undertaken to show the viability of RPL for floating docks during
the Op Sail event with tall ships in New York Harbor in July 2000.
Seven docks were built as shown in Figure 5. These docks were successfully deployed and used during the event
and then donated to public boat clubs in New York City. The performance
of these docks has been monitored and to date they have all been
RPL Platform at Lake Placid, New York
In the winter of 2000, the platforms leading up to the start and
the end of the bobsled and luge track at Lake Placid, New York,
were being renovated for the Goodwill Games. In a project coordinated
with the Environmental Investment Management Group of New Yorks
Department of Economic Development, RPL platforms were designed
and installed in weather that reached lows of 40°F (40°C) in
time for the start of the games. This was the first major project
in which reinforced structural plastic lumber was used in joists,
beams, girders, and decking boards. Figure 6 shows one of these platforms. Since being constructed in February
of 2000, the RPL platform has performed very successfully and
has not had problems with temperature swings from 40°F to 90°F
(40°C to 32°C). The gaps between adjacent decking boards that
were left during installation to account for thermal expansion
and contraction have not changed at all. No creep or permanent
deformation of the material has occurred anywhere in the reinforced
The initial success of RPL at Lake Placid has prompted the department
to use the material for all other platforms at the bob-sled and
luge track including a 10-foot by 10-foot (3-m by 3-m) platform
at the end of the run made exclusively from reinforced RPL.
This particular platform is being tested for creep under sustained
loading. Sandbags that result in a 100 pounds-per-square-foot
(490 kg/m2) loading have been placed on this deck for a year; the results
will be used to further enhance the standards for structural RPL
designs by ASTM.
Arch Truss Bridge in Albany, N.Y.
One way that wooden structures are designed involves laminated
beams where smaller dimensional lumber such as two-by-sixes or
two-by-eights are used to make built-up beams. This results in
a more efficient and cost-effective use of materials. Therefore,
a 30-foot (9.1 m) span bridge was used as a demonstration project
to investigate if structural reinforced RPL may be used to construct
laminated beams. Figure 7 shows a picture of the structural RPL beams that consumed approximately
60,000 discarded milk jugs. The arched top chord of the bridge
consists of laminated two-by-eight curved members while the bottom
chord is a standard dimensional eight-by-eight reinforced RPL.
Although the bridge only needed to be designed for H-10 (10 ton
or 9 metric ton) emergency vehicular loading, it was designed
and tested for H-15 loading (15 tons or 30,000 pounds or 13.6
metric tons). As seen in Figure 7, a fully loaded dump truck weighing
almost 32,000 pounds (14.5 metric tons) was used for testing the
bridge. The maximum deflection was only 1.2 inches (30 mm), which
is more than acceptable for such structures. The data from testing
the bridge four times over a one-year period to investigate the
effect of temperature will be used again to develop further standards
for structural RPL designs.
RPL has come a great distance from the novelty material applauded
by environmentalists but largely ignored by the construction industry.
The new ASTM standards have played a critical role in pushing
RPL materials to the marketplace. As the international focus begins
to shift more and more toward a sustainable environment, RPL product
use is destined for increased growth. With increased understanding
of the materials performance through standard test methods and
specifications and the construction industrys growing comfort
with its use, RPL will one day be an integral part of the built
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Copyright 2001, ASTM