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Japanese Twenty Five Years Experiences and Standardization of Synthetic Sleeper

Hideyuki Takai1, Yoshihiko Sato2, Kimiatsu Sato3

1Railway Technical Research Institute, Tokyo, Japan, 2Railway Track System Institute, Tokyo, Japan, 3Sekisui Chemical, Tokyo, Japan

Abstract To ameliorate the characteristics of wooden sleeper, especially durability, and to preserve the natural environment, the synthetic sleeper has been developed and fabricated in Japan since 1980. In these twenty five years 1.3 million of sleepers are in tracks replacing the wooden sleepers in turnouts points and on steel bridges. About 100 thousands synthetic sleepers are supplied every year now. They are made of hard type foamed poly-urethane and glass filament fiber, having much more superior characteristics than the wooden sleeper. They have been automatically fabricated in the factory. It is verified that the synthetic sleeper can maintain its physical strength for more than 50 years. Actually it has been in service for 25 years. As such, the demand of synthetic sleeper by railway companies in Japan is constantly increasing because the sleeper has resistance to deterioration and water absorption which afford long lasting strength and result in nonnecessity of exchange. Work of standardization of the synthetic sleeper to a JIS (Japan Industrial Standards) is now under way, for the convenience of users considering employment of the synthetic sleeper. It will assist the overseas use of synthetic sleepers, as well. Introduction The railway sleepers are necessary to have not only mechanical strength, but also elasticity and electric insulation. Further, they must be easy to work with and to handle, and must be durable. To ameliorate the characteristics of wooden sleepers, the monomer is impregnated in the wooden sleeper so as to decrease the absorption of water, to increase the strength and to protect from rotting. They were laid on Igarashi-gawa Bridge between Sanjo and Higashi-sanjo on Shin-etsu line. In parallel with this the sleepers on bridge of hard type foamed urethane reinforced with wood or steel were fabricated as trial, but they showed problems in holding shapes, material strength and durability. In such a situation the hardened foamed polyurethane combined with glass filament fiber was considered suitable for the material of synthetic sleeper [2, 3].

Photo 1: Synthetic sleepers on a steel bridge

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What is FFU? Synthetic sleeper is one of the applications of a polymeric material called FFU which is the abbreviation of Fiber-reinforced Formed poly-Urethane. FFU is widely used in various fields of application such as water treatment plant, civil engineering etc., where good mechanical properties, light weight, high durability and good water resistance are essential. FFU is a composite material made of hard type poly-urethane foam and glass filaments, as shown in Figure 1 and has the durability of plastics and the light weight and the workability of lumber. Since there is no knots, fluctuation in mechanical property is very small, giving constant and stable quality. It has good chemical resistance against acids, alkalines, saltwater etc., and the synthetic sleepers can maintain its excellent properties for a long period under various conditions. Figure 1: FFU – Fiber-reinforced Formed poly-Urethane Fundamental features The fundamental tests on physical characteristics were performed based on JIS (JIS Z 2101 Lumber testing methods). Their results are given in Table 1. The synthetic sleeper has nearly the same bending strength as beech and is much more superior in electric insulation. The results of the endurance test with use of a sunshine weather-meter are given in Figure 2. The synthetic sleeper does not change its adhesive shear strength, compressive strength and bending strength after the irradiation of 5000 hours. On the other hand the beech shows reduction of 45% in compressive strength and 25% in bending strength after the test.

Property Unit Synthetic Beech Test Density - 0.67 - 0.82 065 - 0.84 JIS Z 2101

Bending strength MPa 142 80 JIS Z 2101

Young's modulus MPa 8.1x103 7.1x103 JIS Z 2101

Compressive strength MPa 58 40 JIS Z 2101 Shear strength MPa 10 12 JIS Z 2101

Material strength

Adhesive shear strength MPa Cohesive breakage - JIS K 6852

137(Original) 24 hours mg/cm2 3.3 50

(With creosote) 590(Original) Water absorption

110 days mg/cm2 13 280 (With creosote)

JIS Z 2101

Dry kV >25 3 AC Breakage boltage Wet kV 13 <3 JIS C 2110

Dry Ohm 1.6x1013 6.6x107 Electrical

characteristics Insulation resistance Wet Ohm 1.4x108 5.9x104 JIS K 6911

Spike kN 27 25 Screw spike kN 65 43 Pull-out

strength Spike lateral kN 19 15

RTRI Method

Table 1: Physical characteristics of synthetic sleeper

Hard foamed urethane

Glass fi lament

Hard foamed urethane

Glass fi lament

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Figure 2: Weathering test results Through such tests, the synthetic sleepers proved to have the same physical characteristics as those of the new wooden sleepers and maintain the characteristics after laying. Concretely speaking, they are as follows: (1) They have enough mechanical strength. (2) With low water absorption, they will not rot. (3) They are stable in form and dimensions. (4) They have high electric insulation. (5) They hold spikes and screw spikes securely enough. (6) They have enough weather resistance. (7) The large size sleepers with high precision are supplied. (8) They are easily workable. (9) As a whole, they have a long life.

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Fabrication The synthetic sleeper is an industrialized product and manufactured in the automatic processing line which has been improved in the past 20 years and is now a well-established process. Individual sleepers are made under well-controlled conditions and quality management. Hence, its consistency in properties and dimensional accuracy is the great advantage of the synthetic sleeper. The synthetic sleepers are made by laminating the FFU plates formed by a pultrusion process. The fabricating process of FFU is given in Fig. 3. Two liquids are mixed at the mixing jet nozzle, one of which consists of polyol prepolymer, catalyst and foaming adjuster, and the other consists of diisocyanate. Strands of glass filaments, of which diameter is 10 x 10-6 m to 20 x 10-6 m, are impregnated with the mixed liquid and become the precursor. Tens of thousand filaments per square cm cross-section will exist in the synthetic sleeper. The precursor is continuously fed into the forming zone where foaming and curing take place. The formed strand is automatically cut to the size of FFU plate. The continuous production of FFU is shown in Photo 2.

IsocyanatePolyol

Glass filaments

Impregnation FormingResin charge Take-off Shaping Cutting

IsocyanatePolyol

Glass filaments

Impregnation FormingResin charge Take-off Shaping Cutting

Figure 3: Fabricating process of FFU

Photo 2: Continuous production of FFU plate In the lamination process, the skin of FFU plate is ground off to remove the releasing agent and other contamination. FFU plates are laid and adhered together by epoxy adhesive to the size of sleeper and placed in a hot press for curing of epoxy. Then the sleeper is finished by machining before being coated with acrylic urethane paint. Each sleeper will carry a marking of producer and other details. Sampling inspection for flexural strength, longitudinal compressive strength, shearing strength and adhesive shearing strength, is due. Products are routinely inspected for dimension, shape, appearance, density, load resistance and cut spike or screw spike holding.

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Application (1) - in Japan Since the launch in 1980, the demand of synthetic sleeper in Japan grew steadily to mid 1990s, with a focus on the bridge sleeper. The present domestic demand level is about 100 thousand sleepers a year. Recent overseas adoptions make us expect a great increase of future demand. Here, we like to discuss the characteristics of major two types of sleeper, one of which is the bridge sleeper and the other is the turnout sleeper (1) Bridge sleeper The synthetic sleeper is light in weight and is about 1/5 of the concrete sleeper, which fact allows good workability. It becomes very effective for the work on bridges where heavy machines can not be used but manpower. It is also suitable for quick replacement during the short night pause of train operation. In cases where different height of sleepers are required, such as curved tracks and installation over varying base levels, the height of synthetic sleeper can be adjusted partially or individually by laminating extra layer of FFU material. Further, it has greater freedom of providing special shapes such as a groove for cable installation. (2) Turnout sleeper Longer sleepers than bridge or standard sleepers are required for laying of turnouts. While the material of synthetic sleeper is produced continuously as mentioned above, it is possible to obtain any lengths as long as the space of production allows. Yet the quality is consistent regardless of the length contrary to the natural wood which often contains week spots such as knots. The chemical resistance and oil resistance of synthetic sleeper will maintain the mechanical properties against the oil which is used to lubricate the friction between rail and base plate at the time of point turning. Application (2) – in foreign countries Synthetic sleeper has proved to perform its full function for short or long period as the result of being adopted in the high speed tracks of Tokaido Shinkansen Line. By the performance of 15 years in Japan, it is highly appreciated and recent adoption of synthetic sleeper in foreign countries is in smooth progress. The first foreign employment of synthetic sleeper was in Taiwan in maintenance depots of new high speed line. Photo 3 shows the tentative assembly of turnout sleepers. In Vienna Subway (Winer Linen) of Austria, the synthetic sleeper was fully adopted for bridges at the time of periodical replacement, recognizing its long life span and contribution to the environmental conservation. See Photo 4. In Guang Zhou, China, the synthetic sleeper is adopted in the linear motor subway mainly in turnouts, of which complex structure has to be overcome by the sleepers’ freedom of fabrication. See Photo 5.

Photo 3: Taiwan Photo 4: Winer Linien Photo 5: China

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Activities for Standardization - JIS The synthetic sleeper was a product created by forming a composite material to the shape of sleeper which comprises glass filament fiber and hard polyurethane foam. As the synthetic sleeper deteriorates very little by aging or by water absorption and has good properties such as dimensional stability, and, in addition, facing the recent drying of wood supply sources, its demand of railway companies has increased rapidly since 1980. In view of the long history of use in tracks, its stable properties, well controlled quality and increase of demand, JIS standardization work for the synthetic sleeper officially started in 2004. The JIS draft was submitted to JICS (Japanese Industrial Committee) and is expected to be approved in 2006 fiscal year. Figure 4 shows the flowchart of JIS developing process. Figure 4: Flowchart of JIS developing process [1] JIS sleeper test methods (draft) Specified properties of synthetic sleeper in the JIS (draft) are shown in Table 2.

Property Unit Duration of test Bending strength (test specimen) N/mm2 70 min. Young's modulus (test specimen) N/mm2 6000 min.

Bending load resistance kN 170 min. Longitudinal compressive strength N/mm2 40 min.

Shear strength N/mm2 7 min.

Material strength

Adhesion shear strength N/mm2 7 min. (base material failure)

AC breakdown voltage kV 20 min. Electrical characteristics DC insulation resistance Ohm 1x1010 min.

Cut spike pull-out strength kN 15 min. Pull-out strength Screw spike pull-out strength kN 30 min.

Apparent density g/cm3 0.74 +/- 0.1 Water absorption mg/cm2 10 max.

Table 2: Specified properties of synthetic sleeper

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1) Bending strength and Young's modulus tests Flexural strength and flexural modulus tests shall be conducted in the following procedures in the atmosphere of specified standard conditions. Test pieces shall be cut out of the sleeper or the FFU material made in the same conditions as the sleeper, so that their length is in the parallel direction of filaments and the load is applied in the transverse direction of filaments. The concentrated load shall be applied in the center of two supports. The adding speed of load shall be less than 14.7 N/mm2/min. The dimensions of test piece and the span of supports shall be in accordance with Figure 5.

280

2050

400 unit: mm

280

2050

400 unit: mm

Figure 5: Dimensions of the test piece and the support span

2) Bending load resistance test In the bending load resistance test, maximum load at breakage shall be obtained by adding concentrated load to the test piece as shown in Figure 6. Steel plates shall be provided at the points of support and loading to prevent intrusion of the load blade or the support edges. The load lowering speed shall be 2+-0.5 mm/min.

140

1400200

1120

unit: mm

140

1400200

1120

unit: mm

Figure 6: Dimensions of the sleeper and the support span for the bending load resistance test

3) Fatigue test This test is designed to simulate the repeating flexural stress that takes place in the actual track, for the evaluation of fatigue resistance of synthetic sleeper. As shown in Photo 6, the same test piece and the same support span are used to apply 1010 cycles of repeating load in the center in 2 to 5 Hz frequency. The magnitude of load is selected so that it gives a maximum flexural stress of 28.0 N/mm2 and there should be no failure or breakage of the test piece till the end of test.

Photo 6: Fatigue test

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4) Spike pull-out test The maximum holding (pulling) force shall be determined while pulling the spike as shown in Photo 7. The spike has been socked or screwed in a drilled hole of 14 mm or 15 mm diameter and 110 mm depth for the cut spike, or of 18 mm diameter and 110 mm depth for the screw spike. The pulling speed shall be 2 +/- 0.5 mm/min. 5) Burning resistance The burning resistance test shall be conducted according to JIS K 6911, Method A. A test piece having the dimension of 12.7 mm by 12.7 mm by 127 mm shall be heated directly by gas burner flame for 30 seconds, as shown in Photo 8 and the burning distance shall be measured. If the burning distance is 25 mm or less, the material is deemed non-combustible, which is the criterion of qualification.

Photo 7: Pull-out strength test (Screw spike) Photo 8: Burning resistance test (JIS K 6911, Method A) Concluded remarks As the synthetic sleepers do not deteriorate or reduce its strength by water absorption, and they are consistent in quality assured by the industrial quality management system and light in weight giving good workability, they have been frequently employed in Japan. JIS standardization of the sleeper is anticipated in the near future and such official recognition will help to diffuse it in the domestic market. As the increase of overseas demand is also anticipated, we plan to standardize it as ISO. References [1] JISC website, http://www.jisc.go.jp/eng/jis-act/flow-dev.html [2] T. Nagafuji, N. Abe, "Performance of Synthetic Sleeper," QR of RTRI, Vol. 29, No. 3, pp. 107-113, (1988). [3] T. Nagafuji, N. Abe, "Profit in Usage of Synthetic Sleeper," QR of RTRI, Vol. 35, No. 1, pp. 2-4, (1994). [4] H. Takai, Y. Sato, K. Sato, "Japanese Experience in Using and Fabricating Synthetic Sleeper," Railway Engineering 2005, London, (2005).