V - Performance and Safety Issues Regarding the Use of Plastic

8/14/2019 V - Performance and Safety Issues Regarding the Use of Plastic

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PERFORMANCE AND SAFETY ISSUES REGARDING THE USE OF PLASTICCOMPOSITE CROSSTIES

Richard Lampo

U.S. Army Engineer Research and Development CenterConstruction Engineering Research Laboratory

P.O. Box 9005Champaign, IL 61826-9005

(217) 373-6765(217) 373-6732 (fax)

[email protected]

Thomas NoskerRutgers University

Building 3529, Busch CampusPiscataway, NJ 08855(732) 445-3631

(732) 335-0777 (fax)[email protected]

Barry Gillespie

Norfolk Southern CorporationResearch & Tests Laboratory110 Franklin Road, Box 77

Roanoke, VA 24042(540) 981-4678

(540) 981-5628 (fax)[email protected]

Raymond Schriks

Chicago Transit Authority120 North RacineChicago, IL 60607

(312) 733-7000, Ext. 7007(312) 432-7104 (fax)

[email protected]

Number of Words

Title Page: 95Abstract: 210

Text: 6446Plus: 3 Figures


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ABSTRACT

While wood has long been the standard material for crossties, several factors have in recent years

increased interest by the railroad community in possible alternative materials. These factors

include increased wear due to higher loads, and environmental concerns associated with

preservative chemical treatments. Several manufacturers have entered the market with a variety

of plastic composite material designs. Some of these plastic composite ties have to various

degrees been subjected to laboratory and field tests including the FAST in Pueblo, CO. Several

thousands of plastic composite ties have been installed in track, ranging from mass transit to

Class 1 revenue service applications. While these products have some performance attributes

that make them attractive alternatives to wood, they also have some properties that are somewhat

different than traditional wood materials and are not fully understood. Performance and safety

issues arising from these properties include: fracture, low tie-ballast interaction, spike-holding

power, tie plate cutting, creep (increased gage), stress-relaxation (spike loosening), and effects of

environmental exposures. This paper briefly describes the development of this new technology

and then addresses the most critical performance issues relative to the safe application of plastic

composite ties. Research needs are listed relative to the further understanding of the long-term

performance and safety issues associated with their use.

INTRODUCTION

Background

Since the early days of building railroads in the United States, wood has been the traditional

material used for crossties. In recent years, however, several factors related to material wear and

environmental sustainment have led to the railroad industry interest in alternative crosstie


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materials. Trains now carry up to 39 tons (35,400 kg) per axle, compared to 36 tons (32,650 kg)

just a few years ago. In many cases this increased loading has accelerated the wear of wood ties.

Also, the railroad industry is closely watching environmental regulations and how they may

restrict the future use of creosote, which is needed as a preservative treatment to provide

reasonable service life of wood especially in certain aggressive exposures.

Origins of Plastic Composite RR Ties

In the early 1990s, a new industry making plastic lumber materials from recycled waste plastics

emerged in the United States. Several of these early plastic lumber manufacturers had the idea to

produce and market plastic RR ties. However, it was quickly learned that it takes more than just

a crosstie-sized block of plastic to provide proper track performance. Using grant funds from a

State of Illinois economic development program, plastic lumber RR ties made from recycled

high-density polyethylene (HDPE) by an Illinois plastic lumber firm were installed on a

Chicago-area short line during this period. The installation was not completely successful due to

some mechanical property limitations of the unreinforced HDPE. However, some researchers

believed the mechanical property shortcomings could be overcome with relative ease by

incorporating reinforcement elements into the recycled-plastic matrix. By the mid-1990s, at least

two independent groups were developing engineered plastic composite RR ties using recycled

HDPE mixed with other materials for property enhancements (1). Apart from mechanical

property limitations, HDPE offers both performance and environmental advantages for use as

RR crossties. Wood can be particularly troublesome in warm, moist soils, where biological

organisms can attack wood ties and greatly shortening their service life. To fight rot and insect

attack, wood ties must be pressure-treated with creosote. HDPE needs no such treatment,

however, because it is inherently resistant to rot and insects. Also, given that several million


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crossties are replaced each year by the railroads, and considering the volume of plastic needed to

make each tie, considerable amounts of waste plastics could be diverted from landfills and put to

beneficial use if plastic crossties were to achieve any significant market penetration.

Minimum Performance Requirements

One of the above-mentioned groups developing plastic composite RR ties included personnel

from Rutgers University, Norfolk Southern and the former Conrail railroads, the U.S. Army

Construction Engineering Research Laboratory, and a major plastic lumber manufacturer. In

1994, this team established a set of performance target goals for both physical and mechanical

properties of plastic RR ties to serve as a guide for developing plastic RR ties for Class 1 rail

service. These goals are as follows:

Dimensions / Appearance

Cross-section of 7 x 9 in. (17.8 x 22.9 cm) +/- 0.125 in. (0.318 cm)

Surface flatness to within 0.0625 in. (0.0159 cm) peak-to-peak in the area of the tie plate.

Mechanical Properties

Under the following conditions, the track will maintain gage within + 0.125 in. (0.318

cm)

Lateral load of 24,000 lbf (106.8 kN)

Static vertical load of 39,000 lbf (173.5 kN) and dynamic vertical load of 140,000 lbf

(622.7 kN)


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A modulus of elasticity of greater than 170,000 psi (1,172 MPa).

General Performance Requirements

Less than 5% water absorption

Exposure to diesel fuel and grease will not affect the properties by more than 10%

Electrically non-conductive

Surface degradation due to ultraviolet light will not exceed 0.003 in. (0.0076 cm) per year

Installation of the ties can be accomplished using standard equipment

The ties must be compatible with standard rail fastening hardware.

Types of Plastic Composite Ties

In a little more than a decade since the first plastic ties were installed in the Chicago-area short

line railroad, several manufacturers have entered the market with a variety of plastic composite

tie designs. Generically these new plastic composite ties include such compositions as:

Glass-fiber reinforced HDPE matrix

Glass-fiber reinforced rubber-modified HDPE matrix

Polymer-fiber reinforced HDPE matrix

HDPE and mineral by-product combination

Hybrid steel, concrete, and plastic composite design


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Variations on the above -- or even other entirely different compositions or design types -- may be

possible in the future as new manufacturers start competing for a share of the tie replacement

market.

In October 2000, a new Subcommittee on Engineered Composite Ties was formed under the

American Railway Engineering and Maintenance of Way Association (AREMA) Committee 30

on Crossties to develop recommended engineering standards and practice for the use of these

new materials by the railroads. The performance goals noted above form the basis for guidance

being developed by the Subcommittee. (Please also note: while not specifically covered in the

scope of this paper, the Engineered Composite Ties Subcommittee is also responsible for

developing guidance for laminated wood and related wood-polymer composite ties.)

TIE PERFORMANCE

Laboratory Testing

Bending

Flexural tests have been performed on full-sized plastic composite ties using a modified four-

point flexural test apparatus routinely used in the railroad industry (2). The support span is 60 in.

(152.4 cm) and the load span is 6 in. (15.2 cm). Some plastic composite ties tested in this

manner have shown ultimate strengths exceeding 4,000 psi (27.6 MPa) and elastic moduli

(stiffness) as high as 300,000 psi (2,069 MPa).

Fastener Holding Power

It was determined early through laboratory testing that screw spike holding power in plastic

composite ties was comparable to that in wooden crossties. However, the results with cut spikes


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in plastic ties were not as promising. Laboratory tests performed with cut spikes revealed their

holding power in plastic ties to be significantly lower than in a new wood tie -- relative pull-out

forces of approximately 3,500 lbf (15.6 kN) versus 8,000 lbf (35.6 kN), respectively. With cut

spikes being the most commonly used fasteners by U.S. railroads, including the military, this

finding raised some economic issues if it were concluded that new tie plates and fasteners would

be required in order to safely use the plastic ties. This new hardware would raise the tie unit cost

of installing plastic ties.

However, it was recognized that these spike-holding results were on short-term tests using new

materials not subjected to weathering and service loads. It is known that cut spikes will loosen

considerably in wood over time (3). What is not currently known is how much spike-holding

ability is actually required for different service loads, and whether (and how rapidly) cut spike

holding ability in wood and plastic ties will converge with time. Researchers continue to explore

these issues. Meanwhile, several field installations using cut spikes in plastic ties have been

initiated with satisfactory results as discussed below.

Field Testing and Demonstrations

Facility for Accelerated Service Testing

In April 1996, two plastic crossties were installed in a 5-degree curve in the Facility for

Accelerated Service Testing (FAST) at the Association of American Railroads (AAR)

Transportation Technology Center, Inc. (TTCI) in Pueblo, CO. After 130 million gross tons

(MGT) (117.9 billion gross kg) of traffic at 40 mph (64.4 km/hr), one of the ties was removed for

laboratory testing. This removed tie was subjected to a rail seat abrasion test to determine the

sensitivity of the tie material to tie plate cutting. The testing machine broke down after 900,000


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cycles with no evidence of tie plate cutting up to that point. Based on satisfactory performance

of the originally installed ties, 24 additional plastic composite ties were installed in March 1997.

Another performance consideration is lateral track stability. TTCI performed lateral tie push-out

tests on some of the plastic composite ties when newly installed. The maximum force needed to

push out the plastic tie was approximately 1,000 lbf (4.45 kN). This is in the range of a newly

installed wood crosstie of equivalent size. After 15 20 MGT (13.6 18.1 billion gross kg) of

traffic, a wood tie will begin to lock into the ballast and the lateral push-out forces will

increase to around 2,500 3,000 lbf (11.1 13.3 kN). The push-out tests performed on the

plastic ties after approximately 15 MGT (13.6 billion gross kg) showed the plastic ties to retain a

value similar to when they were first installed. Apparently the plastic composite tie is too hard

and friction resistant to achieve any appreciable mechanical locking into the ballast.

To increase the tie-to-ballast interaction and consequently increase the lateral tie push-out forces,

a checkerboard pattern was heat-embossed into the sides and bottom of ties (Figure 1). Norfolk

Southern conducted field tests using some of these embossed ties during November 1997. The

ties were installed, tamped, and ballast placed around the ties as in a conventional wood tie

installation. Up to 4,500 lbf (20 kN) force was needed to push out the ties with the embossed

pattern. This result is consistent with additional tests performed by TTCI on plastic ties in the

FAST. These results indicate that checkerboard-embossed plastic ties could have a performance

advantage over wood ties. Where new wood ties are installed, train speeds are typically reduced

until sufficient traffic has accumulated to seat the new ties into the ballast. Plastic ties with a

surface embossed pattern may not require such a seating period.


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To date, plastic composite ties from a few different manufacturers have been installed in the

FAST. Some of the oldest plastic ties still in track have accumulated over 350 MGT of traffic.

A 100-tie section of plastic composite ties was recently installed in a 6-degree curve using all cut

spike fasteners. With well over 100 MGT (90.7 billion gross kg) of accumulated traffic, TTCI

reports no track alignment or vertical profile problems.

Field Installations

Plastic composite ties are not only accumulating traffic at the Pueblo test center, but in revenue

and mass transit applications as well. Union Pacific has the largest number of plastic composite

ties installed of a Class 1 railroad. Most of these ties were installed using cut spikes. To date,

the Chicago Transit Authority (CTA) has installed the largest number of plastic composite ties of

any railroad. Over the years, the dripping of creosote onto people and property from elevated

track has been a constant problem for the CTA. The organization was initially interested in the

plastic composite ties as a replacement for creosote-treated ties in elevated track. The CTA also

looked to the plastic ties as a way to reduce the problem of stray-current electrical corrosion of

track fasteners (and the safety hazard such corroded fasteners present). After a successful trial of

a small group of plastic ties in one of its elevated stations, the CTA decided to install plastic

composite ties not only in their elevated (open) track but in ballasted track as well. Performance

has been successful enough that procurement of tens of thousands of additional plastic ties is

currently in process. Continued growth in the use of plastic ties by all classes of railroads is

expected in the foreseeable future.

First Plastic Tie Turnout


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In December 1998, the U.S. Naval Surface Warfare Center in Crane, IN, installed a set of 64

recycled-plastic composite ties in a #10 turnout (1). This installation was undertaken to

demonstrate that plastic ties could perform well in a turnout under service conditions on a

military base. This is the first known installation of plastic ties in a turnout. A variety of

fasteners was used, including Pandrol plates and screw spikes as well as a hybrid of 5/8 in. lag

screws and cut spikes in a standard cut spike tie plate.

Since being placed back into service, the turnout has been subjected to its regular traffic loadings

some 30 moves a day in both directions by light engines alone and engines and loaded cars.

Track maintenance personnel are closely monitoring the performance of the turnout. To date, no

component failures or major problems have occurred. Two additional turnouts using plastic ties

from two other manufacturers are planned for installation at the Navy center before the end of

2001. Reportedly, TTCI is planning in the near future to install a plastic tie turnout in the Heavy

Axle Loop of the FAST.

SAFETY ISSUES

Background

As established by their successful performance in track to date, plastic composite RR ties have

demonstrated that they can be used as a replacement for wood ties in many applications. Of

course, the performance history of these new materials is relatively short compared to wood ties.

The failure modes of these products in track service are not yet well known, nor are the

implications for track safety. In the fall of 1998, the U.S. Army Engineer Research and


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Development Center started working with the Federal Railroad Administration to help address

safety issues that may arise with the use of plastic composite RR ties.

Expected Failure Modes

Based on more than 32 years of cumulative experience studying the mechanical properties of

plastic lumber and the substitution of plastic lumber for treated wood applications (including six

years specifically related to railroad ties), Lampo and Nosker have determined that the failure

modes most likely to occur with plastic-based railroad ties are as follows:

1. Failure to Meet Recommended Minimum Performance Requirements

2. Fracture

3. Low Tie-Ballast Interaction

4. Fire

5. Tie Plate Cutting

6. Creep (Increase of Gage Due to Axial Tie Loading)

7. Stress-Relaxation, Resulting in Spike Loosening

8. Deterioration of Properties Due to Exposure to the Elements.

These potential failure modes are ranked on the basis of criticality and operational safety, with

number 1 considered to be the most critical and number 8 considered to be the least critical.

Some of the failure modes listed above could be catastrophic, with essentially no early warning,

while others would be more gradual in nature. A breakout of the failure modes into these two

categories is shown below.


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Potential Catastrophic Failures

Failure to meet recommended minimum performance requirements

Fracture

Low tie-ballast interaction

Fire.

Gradual Failures

Tie plate cutting

Creep

Stress-relaxation

Deterioration of properties via environmental exposures.

The most undesirable failures would be the unexpected, sudden types of failures that would most

likely occur from the first item on the list: Failure to Meet Recommended Minimum

Performance Requirements. Each of the above-listed failure modes is discussed in greater detail

below in terms of safety, prospective performance issues, and future research requirements.


The previously listed minimum performance requirements were based on Class 1 freight

applications, considered to be the most demanding. These requirements provided valuable

guidance as to what the research targets would be in terms of product performance. The

philosophy taken by the development team at that time was to develop a plastic composite tie

that could withstand the most demanding situation a tie could be expected to endure. For

example, that situation might be defined as track on a mountain curve carrying heavily loaded


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coal cars. An alternative approach would have been to develop multiple specifications for

different loading conditions, but that approach was not taken because tracking different grades of

plastic-based ties was considered unfavorable at the time.

Not specifically addressed by these performance requirements were the fastener pullout

requirements for cut and screw spikes. Apparently, not much information has been collected by

the railroads to date on the required value for cut-spike fastener pullout forces for ties in service.

However, it is known that the holding power of a cut spike will decrease from the time when first

installed until some future time in service. The average value of that force, or what it should be

based on some time in-service for a given type of tie material, is not currently known. In the

interim, the performance of a screw spike is considered acceptable if the pullout force in plastic

is about equal to the pullout force in a new oak tie.

The inability of a recycled-plastic composite tie to meet the minimum performance requirements

as described above is considered the most likely reason for sudden catastrophic failure of the

track system (resulting from excessive deflections) that may lead to a derailment. Conversely, if

the minimum performance requirements are met (while noting that they have not yet been fully

developed for cut spikes), the probability of a catastrophic failure in service is considered quite

low.

Fracture

Some plastic composite ties have been observed to form small cracks or fracture during or after

fastener installation. This effect has been most often observed to be related to the fastener

installation method (e.g., continuous ram-driven versus hand- or impact-driven methods) and


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preparation of the tie (i.e., size and depth of a pre-drill). An instance of complete fracture failure

occurred in a plastic composite tie while it was being installed using standard hydraulically

operated equipment and too great a bending force was applied to the tie as it was levered under

the track.

The fracture failures described above occurred during initial tie installation. It is also possible

that plastic ties could fracture in the field under severe operational conditions, particularly in the

case of a center-bound tie and heavy dynamic loading. Derailment, with the combination of

shear and stress concentrations of the wheel flanges acting on the ties, is another possible cause

for tie fracture.

Low Tie-Ballast Interaction

Ties are expected to interact with the ballast and provide resistance to lateral rail movement.

While a lack of sufficient interaction may not fit under the classification of a tie failure, this

factor can lead to a system failure. Tie-ballast interaction can be enhanced, as needed, by

varying the surface roughness of the bottom and sides of the plastic composite ties. As noted

previously, it has been found is that recycled-plastic ties with low surface roughness and low

mechanical interaction with the ballast do not "lock into" the ballast over the same loading

history, as do wooden ties. This characteristic could be considered a serious shortcoming of

plastic ties, but it can be corrected with relative ease by fabricating plastic ties with engineered

surface patterns to increase the interaction with ballast. When this is done, single tie push-out

values, at initial installation, have gone from as low as 700 lbf (3.1 kN) for a smooth-sided

plastic tie up to 6,000 lbf (26.7 kN) for a modified surface plastic tie. In fact, the ability to easily

modify the surface pattern of plastic ties may now represent a big performance advantage


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because it can eliminate the break-in period otherwise required for wood ties to achieve an

acceptable lateral stability where train speeds do not have to be reduced.

Fire

Plastic composite ties are combustible as are wood ties. The burn energy associated with HDPE

is approximately 19,500 BTU/lb (45,600 kJ/kg). Composite materials made with HDPE are

somewhat less dense in burn energy than the pure material. The question of whether plastic

composite ties catch fire more easily than creosote-treated wood ties has not yet been formally

studied. However, it is known is that the ignition temperature of wood ranges from 536 to 932

oF (280 to 500 oC), according to the Wood Handbook (4). Creosote-treated ties are likely to

ignite easier than untreated wood ties. Even in their most susceptible powder form, HDPE,

polypropylene, rubber, and polystyrene materials have ignition temperatures of 770, 788, 608,

and 932 oF (410, 420, 320, and 500 oC), respectively, according to Marks Standard Handbook

for Mechanical Engineers (5). Experiments conducted by Underwriters Laboratories in support

of standards development for plastic lumber indicate that plastic lumber decking made from

HDPE does not represent a greater fire hazard than wooden decks. These experiments were

conducted with Class C burning brands placed on the deck specimens with air flowing over the

surface, as detailed in Appendix X.4 of ASTM D 6662-01 (6).

The use of rubber as a component in plastic composite ties has raised fire-related concerns

among some observers. The primary concern has been the possible release of toxic gases upon

ignition which could prove to be fatal to individuals in a confined space such as in a tunnel.

HDPE is the only insulating plastic allowed as a wire insulator in New York City subways due to

unfortunate incidents in the past related to the combustion of PVC wire coverings, one


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combustion by-product of which is hydrochloric acid. It should be noted, however, that any

track fire in a confined space, involving either creosote-treated ties or plastic ties, with or without

a rubber component, would likely create a highly noxious or toxic environment. Therefore, the

verdict is still out relative to the increased hazard, if any, arising from the use of ties containing

rubber in such confined spaces.

Tie Plate Cutting

Over long periods of time, tie plates can be observed to cut into wooden ties. In addition to

compromising the useful mechanical properties of the tie, this phenomenon changes the normal

rail cant and the associated wheel-rail interface. Heavy wheel loads and heavy traffic density can

accelerate this action by breaking the wood under the plate into small fibers. Tie plate cutting

may also occur with some of the plastic composite ties, but the toughness of HDPE is considered

to be an advantage in minimizing the occurrence of this phenomenon in plastic ties.

Creep (Increase of Gage Due to Axial Tie Loading)

It is well established that polymers are viscoelastic in terms of their mechanical properties. That

is to say, there is an immediate response by the material when stress is applied to it, followed by

a time-dependent or viscous response. This response is generally attributed to the chemical-

mechanical interaction of covalently bonded long main chain molecules, and the time required

for main-chain molecular bond rotations to occur.

The long-term creep performance of any plastic composite railroad tie will obviously play a

major role in whether the rails will stay in gage over the long term. The creep in this case would


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be deformation due to stresses acting to separate the rails, acting in some fashion over a long

time and/or over a large number of loading cycles.

A lateral load of up to 24,000 lbf (106.8 kN) (with train speed, track geometry, and truck

performance all playing a part in the actual service loads) will occur during the period of time

that a train is moving over the ties. This loading, coupled with the viscoelastic nature of plastics

and composites, will lead to an eventual stretching of the ties between the rails, increasing the

gage. Creep is not expected to be a problem in vertical loading situations, due to the low stresses

imparted by these loads (thanks to the tie plates). All of the plastic composite ties tested to date

have equal or higher compressive strengths and modulus values than oak in the vertical load

orientation, but the same is not true of the lateral load modulus orientation.

Stress-Relaxation, Resulting in Spike Loosening

In plastics, when a fixed strain is imparted on the material, the stress decreases with time. This is

called stress-relaxation. When a spike is inserted into a tie, the tie material is displaced a fixed

amount from its original location under (mostly) compressive stress. The compressive stress

acting on the spike-tie interface, together with the coefficient of friction between these two

materials, acts to prevent removal of the spike. Over time, however, stress-relaxation will occur.

The effect is to allow easier removal of all types of spikes over time from a plastic composite tie.

This situation is undesirable because (1) the geometry of track structures can place stresses on

spikes that can act to remove the spikes and (2) spike hole elongation can lead to gage widening

and/orincrease the possibility of rail rollover.

Deterioration of Properties Due to Exposure to the Elements


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The properties of most plastic composite ties should not deteriorate rapidly in the field. The base

material utilized in these products is HDPE, which is moisture-proof, but which does slowly

degrade under the influence of ultraviolet (UV) light from the sun at rates up to 0.003 in. (0.0076

cm) per year. Two of the composite tie formulations contain at least 60% HDPE by weight, and

have been shown to not lose any mechanical properties when exposed to cyclic moisture,

temperature, and UV radiation at levels equivalent to 15 years of exposure for wooden ties.

Durability of plastic composite ties is a key issue. If these ties are not able to provide significant

increases in durability over traditional materials, their market will likely be quite limited.

PERFORMANCE ISSUES AND INVESTIGATIVE NEEDS


Mechanical property screening can determine if the short-term performance requirements can be

met. The experiments must include full-size flexure experiments to determine product stiffness

and strength, thermal expansion measurements, and fastener pullout experiments to compare

with wood. Unfortunately, critical fastener pullout performance numbers are not currently

available or easy to derive. Therefore, a comparison of initial performance with wood for

fastener pullout is the best that can be done at this point. Fastener performance with plastic

composite ties may be a major safety issue, so this subject calls for specific investigation and

testing.

Fracture

Tie fracture during installation cannot be simply modeled by any single material property. As

seen in the above discussion on creep and stress relaxation, plastics have rate-dependent


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properties. For example, some ties have had cut spikes driven in easily with no pilot hole by a

ram over a period of a minute or so, but these same ties will fracture when a cut spike is driven in

(with no pilot hole) with a spike driver. A properly sized pilot hole will allow the spike to be

hammer driven. Since the compressive stress generated by the driven spike relaxes in plastic

with time, fracture after installation is not likely unless the tie is subjected to another rapid stress.

This is consistent with observations to date. However, if large pilot holes must be utilized in

order to minimize the probability of tie cracking during spike driving, the pullout force required

to remove the spike may likewise be lower.

The study of fracture is complex, especially when large impacts are imparted to materials. In

these cases, fracture can occur at what seems like very low stresses as compared to a materials

strength (as in the observed cracking of these composite ties in the field during spike

installation). This is because impact blows on an object involve traveling stress waves within an

object, and these can become multiples of the original applied stress at interfaces. It is possible

that ties could be reformulated by the manufacturers, if necessary, to reduce the sensitivity to

impact situations. A study is necessary to better assess this and other impact-related phenomena.

Low Tie-Ballast Interaction

This property can be easily measured with the single tie push test, which yields rapid results.

This property should be measured whenever plastic composite ties are placed in field trials. Due

to the variability of the test conditions and procedures, hundreds of trials will be required to

establish a reliable technology database.

Fire


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The issue of whether plastic composite ties will catch fire more easily than creosote-treated ties

has not been systematically investigated. Experiments should be conducted to determine the

susceptibility of the ties to ignition when an open flame passes near the ties. Open flames are

sometimes used to clear overgrowth of weeds onto the tracks, and comparative experiments with

creosoted ties need to be conducted to properly assess these risks. Thermite welding and rail

heaters used during track installation and maintenance activities, as well as the use of switch

heaters, are other heat sources near ties that require consideration.

Tie Plate Cutting

A standard experiment has been devised to determine the susceptibility of wooden ties to tie

plate cutting. This experiment can also be used to determine the susceptibility of a variety of

plastic composite ties to tie plate cutting. One problem with this test with plastic ties, however,

is that plastic material under the plate can melt or soften during the cyclic loading. This material

response is due to the thermoplastic nature of the plastic materials used in the tie, and their

highly insulative properties. Friction produced during cyclic loading creates heat, but because

the plastic cannot dissipate the heat fast enough, the material softens. When this is a problem,

the test should be modified to lower the frequency of the loading, thus giving the material a

chance to dissipate the heat before it softens. The challenge is to make such modifications that

still allow an accelerated determination of the property while avoiding a setup that is too lax to

represent the material response in actual track service. It may be possible to model the materials

and predict the performance of different formulations based on the hysteresis of the load and

displacement cycling of the ties in this experiment.

Creep and Stress-Relaxation


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The time-dependent mechanical properties of plastics and composites will have an effect on:

a. Whether track gage will change with time, especially at track curves (from creep)

b. When spikes will become easy to pull out (from stress relaxation).

Methods to predict long-term performance (e.g., creep, stress relaxation, etc.) of virgin plastic

materials quickly in a laboratory using easily produced stress-strain data have been developed

over the past 15 years. Most of this effort has been applied to high-priced engineering resins.

Unfortunately, these results are not directly applicable to recycled-plastic composite railroad ties

because these materials are not made with traditional engineering resins and because of the

composite nature of these products. Therefore, it is necessary to develop techniques for

predicting long-term performance using short-term measurements for these composite members

made with commodity-grade plastics (e.g., HDPE). Such methods would enable the timely use

of much smaller safety factors in construction design than would otherwise be possible, thus

allowing the efficient use of these materials with a high degree of confidence. Improving design

efficiency without sacrificing safety will improve the cost competitiveness of these materials and

the railroads that use them.

Common stress-strain experiments conducted at different strain rates (Figure 2) readily reveal the

viscoelastic properties of plastics. Plastic materials typically have a stiffness or modulus that is

different for each strain rate, with higher modulus values observed when plastics are strained


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more rapidly. Also, higher stresses for a given strain (or higher strength) occur when plastics are

strained more rapidly.

Another common mechanical property experiment is applied to determine creep. In this

experiment, a constant stress is applied to a sample of material and the strain is measured as a

function of time. There is an immediate strain induced on the plastic sample at time, t = 0, and

then a delayed response that occurs with the passage of time. Figure 3a shows a typical response

for one loading stress. Higher levels of stress lead to more rapid rates of creep.

Another common stress-straining experiment is used to determine a material's stress-relaxation

properties. In this experiment, a fixed strain is applied to a sample of material, and the stress is

measured as a function of time. As in the creep test, there is an immediate response in the plastic

material that generates an initial stress, which is followed by a time-dependent relaxation of the

material that produces lower states of stress over time (Figure 3b). Higher initial strain levels on

a given plastic lead to higher initial stress levels, and a similar family of curves can be generated

for any plastic material.

A number of problems, however, make the direct application of these types of data useless

without additional information and a significant analytical effort. The most obvious problems

relative to plastic ties are as follows:

The tie materials of interest are not a pure plastic, but a polymer-based composite.

The loading of a tie is not constant, but follows a cyclic pattern of loading and unloading.


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Stress levels in loaded ties can vary according to many variables.

Reliable predictable methods that do not yet exist are needed since 20-year controlled

laboratory experiments are not feasible.

A systematic approach for dealing with these problems could include the following:

Perform stress-strain tests at different strain rates to make sure the material behaves

consistently or predictably and be able to generate any family of desired curves.

Find and validate a method to use this family of curves as a basis for predicting creep and

stress-relaxation.

Develop a rule to account for the relaxation of the tie that occurs between loadings.

Develop a method to predict when a tie will go out of gage based on any given loading

specifications.

Researchers at Rutgers University are working on models to develop some of these predictive

methods.

Additionally, issues of spike hole plugging and the potential applicability to plastic ties must be

studied. Different materials and techniques for plugging would also need to be investigated.

Deterioration of Properties Due to Exposure to the Elements

An experiment has been developed by Dr. Poo Chow of the University of Illinois at Urbana-

Champaign to accelerate the effects of aging on wooden ties under controlled laboratory

conditions (7). This form of testing involves exposing railroad tie materials to moisture and


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temperature cycles, then performing destructive mechanical property testing. A correlation has

been established with wood ties of known species that were in the field for known lengths of

time. This type of test methodology also could be used on plastic composite ties for accelerated

testing in a qualitative way, but the correlation between the laboratory and field specimens is not

likely to be the same for wood ties as for plastic ties. Three composite tie types have been tested

using this method, with two of the types showing no deterioration of properties. The other tie

type exhibited what could be considered a significant reduction in mechanical properties as a

result of the test. In this latter case, some component of the tie composition is evidently heat or

moisture-sensitive under the test conditions. However, how this result correlates to long-term

performance of the tie in actual service is as yet unknown. Even with this shortcoming, this type

of testing can provide important performance information on plastic composite ties and it would

be beneficial to apply this test protocol to other types of plastic tie materials. However, there is a

need to develop a similar test method that is specifically designed for plastic composite crossties.

CONCLUSIONS

The technology of plastic composite RR ties has advanced significantly over the past decade.

Successful performance of plastic ties in actual rail service demonstrates that the technology

works in various applications. Nevertheless, much is yet to be learned about the long-term

performance of these products. The most important performance issues affecting safety have

been identified. Further studies are needed to help fill gaps in knowledge and expand the

performance history database for these products. Working toward that end will help to assure the

safe, benefical application of these technologies and continued growth in their use.


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RECOMMENDATIONS

The following action items and investigations are recommended to promote better understanding

of safety issues related to the use of plastic composite RR crossties. These recommendations are

offered in descending order of priority.

1. On April 26, 2001, AREMA Committee 30 conducted a workshop on composite ties in

Arlington Heights, IL. The workshop goals were to: (a) introduce the concepts of engineered

composites, (b) introduce the different manufacturers composite tie products, and (c)

generate user feedback to help establish minimum requirements for use of engineered

composite RR ties in Class I, regional/short line, and mass transit applications. Given the

success of this first workshop, additional workshops are recommended to also address plastic

tie installation and maintenance procedures as well as the safety issues described in this

paper.

2. Conduct screening tests for all available plastic composite ties against the minimum

performance requirements being developed under the AREMA Engineered Composite Ties

Subcommittee. The various railroad laboratories, Rutgers University and the Construction

Engineering Research Laboratory would benefit by conducting these tests cooperatively.

The results of these tests should then be made readily available to all parties considering the

adoption of plastic tie technologies.

3. Continue current investigations on fastener holding power in plastic composite ties,

including:


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Short-term performance

Stress-relaxation long-term performance

Spike hole plugging.

4. Continue to assess lateral stability performance issues with the plastic composite ties,

including a determination of how best to conduct testing, discovering features (e.g., surface

patterns)that provide the best tie performance, and studying how variations in ballast

conditions impact the results and the means of comparing such results.

5. Investigate issues of creep and its effect on gage widening.

6. Assess tie plate cutting and tie plate deformation (both short-term and permanent

deformation) for the available plastic tie materials.

7. Investigate fracture-in-service issues, especially for uneven ballast situations and the more

brittle types of plastic ties.

8. Investigate the degradation of properties of plastic composite ties due to environmental

exposures and common chemical spills found on railroad track.

9. Assess performance/behavior of the various plastic ties in typical fire scenarios.


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ACKNOWLEDGMENT

Acknowledgment is given to Mr. Mahmood Fateh, in the Office of Research at the Federal

Railroad Administration, for his support enabling the further study of the safety issues relative to

this new technology.

REFERENCES

1. Lampo, R., T. Pinnick, and T. Nosker, Development, Testing, and Demonstration of

Recycled-Plastic Composite RR Crossties, proceedings of DoD-sponsored Transportation 2000

Conference, San Antonio, TX, February March 2000.

2. Gillespie, B., M. Lutz, T. Nosker, and D. Plotkin, Development of a Recycled

Plastic/Composite Crosstie, American Railway Engineering Bulletin, No. 760, May 1997,

Volume 98.

3. Zarembski, A., Aging of Wood Ties and Associated Loss of Strength, Railway Track &

Structures, August 1993, p 10.

4. Williams, R., "Finishing of Wood," Wood Handbook - Wood as an Engineering Material,

Forest Products Society, Chapter 15, p 15-3.

5. Avallone, E., and T. Baumeister III, Marks Standard Handbook for Mechanical Engineers,

Ninth Edition, McGraw-Hill, 1986, pp 7-31 to 7-33.


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6. D 6662, Standard Specification for Polyolefin-Based Plastic Lumber Decking Boards,

American Society for Testing and Materials (ASTM), West Conshohocken, PA, July 2001.

7. Davis, D., P. Chow, and R. Meimban, Performance Prediction and Specification of Wood

Ties for Revenue Service, #TD 96-010, Technology Digest, Association of American Railroads,

April 1996.


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.

Figure 1. Plastic tie with surface pattern for increased lateral stability.

Figure 2. Typical polymer material response to different rates of strain, (e).


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Figures 3a and 3b. Graphical illustrations of material creep and stress-relaxation.


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.

LISTING OF FIGURE CAPTIONS

Figure 1. Plastic tie with surface pattern for increased lateral stability.

Figure 2. Typical polymer material response to different rates of strain,(e).

Figures 3a and 3b. Graphical illustrations of material creep and stress-relaxation.

Documents

V - Performance and Safety Issues Regarding the Use of Plastic