European Rail Technology Review | April 2011

Use of synthetic wood for railway
sleepers
The Hochbahn in Hamburg and the MVV in Munich have laid their first points on
sleepers made of FFU synthetic wood. This report looks into the requirements for
such sleepers, how they are laid and analyses of their life-cycle costs
Fig. 1: Points installed
in a depot of the
Hamburg Hochbahn
(photo: J. Böcker)
On 29 May 2010, the Hamburg Hochbahn
laid its first set of points on sleepers made
of synthetic wood, to be precise fibre-reinforced
foamed urethane (FFU), in its Farmsen
depot (Fig. 1). Prefabrication of the
points took place at BWG in Butzbach. The
FFU sleepers are dimensionally stable with
plane surfaces, and their assembly in the
factory can be done quickly and economically.
The Hamburg Hochbahn is a railway
with very little clearance, which made it necessary
to dismantle the point into its individual
parts for delivery to the construction
site and, once there to assembly it again
quickly. The sleepers, which had been prepared
with precision, were speedily laid in
Dipl.-Ing. Dr. techn.
Günther Koller
Managing director
koocoo technology & consulting GmbH,
Vienna (Austria)
office@koocoo.eu
place at the construction site. The point
was then assembled on them accurately
and professionally. As soon as the ballast
had been poured, the point was ready for
Hochbahn trains to run over the first synthetic
wooden sleepers laid in their network.
1 Railway applications for FFU
synthetic wood
The use of FFU synthetic wood was approved
by the German Federal Railway Authority
(Eisenbahn-Bundesamt, EBA) on 8
July 2009. Since then, Deutsche Bahn, the
MVV in Munich (the city’s integrated transport
and fares association) and the Hochbahn
in Hamburg have looking intensely
into the potential of FFU, which is a material
based on polyurethane reinforced with
oriented glass fibres and whose salient features
include longevity and very good electric
insulation. Deutsche Bahn, for instance,
sees potential for the use of FFU synthetic
wood both for projects where the heights of
sleepers have to be kept as low as possible
and for curved tracks on railway bridges.
Their use with points is to be concentrated
primarily on groups of points carrying heavy
loads in marshalling yards. The MVV sees
the very good electrical insulation of FFU
as its most attractive property. Sleepers
must have an electrical resistance (specified
in DIN EN 13481-5 as “R33”) of equal
to or greater than 5 kΩ. According to the
findings of research performed at the Munich
University of Technology back in 2008,
FFU synthetic wood was found to have for
a mean of 71.9 kΩ for the R33 electrical
resistance. That is also the reason why the
MVV has already equipped five of its points
with sleepers made of FFU synthetic wood
since the start of 2010.
The Austrian Federal Railways (Österreichische
Bundesbahnen, ÖBB) had their first
positive experience with the use of FFU
synthetic wood on a number of bridges in
spring 2010. That prompted them to place
an order with the Japanese company of
Sekisui for their first double slip using that
material. It was laid in the station throat
of Vienna’s new Hauptbahnhof in autumn
2010.
FFU synthetic wood has been a standard
material for use on railways in Japan since
1985. The main applications for it in that
country are on groups of points subjected
to heavy loads, on bridges and in particular
tracks in railway stations.
RTR 4/2011 41

• Use of synthetic wood for railway sleepers
high volume of traffic carried and the short
headways (Fig. 2).
It was not until 2004 that the first use in
Europe followed. That was near Zollamtsbrücke
on the Vienna metro network. Its operator,
Wiener Linien, found FFU synthetic
wood to be the sort of high-grade material it
was in search of for its superstructure and
capable of satisfying its targets as regards
maintenance intervals:
replacement of rails > 30 years
corrosion protection > 30 years
replacement of bridge timbers
> 50 years, and
overhaul work on steel structures
> 50 years
Fig. 2: Points mounted on FFU synthetic wood on a slab track in Japan
Fig. 3: Artificial bridge timbers for the
Ostbahn Bridge of ÖBB in Vienna,
with a cant as delivered from the factory
(Source of Figs 3-7: the author)
Prior to that, the Japanese railways had
analysed their statistical records and had
established that more than 70% of their
railway sleepers made of natural wood had
only a very short service life after laying on
account of weathering. They started to look
(source: Sekisui)
Wiener Linien have been laying FFU sleepfor
an alternative, and Sekisui developed
this in 1978 in the form of FFU synthetic
wood. FFU is a material that is similar to
natural wood as regards its positive service
properties in railway tracks as well as
simplicity of handling and processing. Its
specific mass is almost identical to that
of natural wood, but its life expectancy is
very considerably longer, and it is much
tougher in its weathering resistance. The
Railway Technical Research Institute computed
the service life to be more than fifty
years on the basis of an extrapolation of
100 000 000 load cycles applied to an FFU
sleeper after it had been in daily use for
15 years.
Since 1985, more than 1000 km of track
have been laid with FFU sleepers. Taking,
for instance, the Shinkansen line between
Tokyo and Osaka, all its points and steel
bridges are equipped with such sleepers.
Shinkansen trains, which have headways of
between three and five minutes, operate on
that line at speeds of up to 300 km/h. The
private Japanese railway decided to use
top-grade, long-lived materials, prompted
by the need to assure excellent network
availability and reliability of the track superstructure
and also considering the very
Fig. 4: Direct fastening
with FFU sleepers with
a height of 10 cm in
Vienna
Since 2005, the Austrian Federal Railways
have also been using FFU synthetic wood,
predominantly on open steel structures and
on and near less important level crossings.
In 2008, the first point equipped with FFU
synthetic wooden sleepers was laid in Germany
in the Chemiepark railway complex in
Leverkusen.
2 The material and its
preparation in the factory
The technology used for manufacturing
FFU synthetic wood is pultrusion, subject
to strict ISO-certified quality assurance.
Continuous oriented glass-fibre strands are
pulled through a roller-type extruder, cast in
polyurethane and cured at an elevated temperature.
The outcome is a very high grade,
pore-free material.
If so ordered by its customers, the factory is
capable of producing, uniquely labelling and
delivering bridge timbers and railway sleepers
in synthetic wood as finished products,
precisely shaped and with a dimensional
accuracy in the millimetre range.
The following jobs are performed on the FFU
sleepers in the factory:
removal of the belt reinforcement by milling
milling work on longitudinal girders
milling countersinks for rivet heads
drilling holes for bolted connections
surface sanding, and
adding material to produce cants
The Austrian Federal Railways made use of
the possibilities just listed in 2009 in order
to be able to implement the target gradient
of bridge structures quickly and cheaply by
using FFU timbers that had been prefabricated
in the factory (Fig. 3). Every bridge
timber had a different height and, by being
able to compensate for the permanent deformation
of the bridge, this method resulted
in an optimum track position.
42
RTR 4/2011

Use of synthetic wood for railway sleepers •
Fig. 5: Drilling countersinks for rivet heads (ÖBB, Hainburg)
Fig. 6: Chiselling in the abutment zone of the bridge over the river Sava
(Belgrade)
ers on their network since 2007. The dimensions
used are:
width: 26 cm
height: 10 cm, and
lengths: 201 and 250 cm,
The shorter sleepers have been laid on
Floridsdorfer Bridge (Fig. 4), and the longer
ones in tunnels, where there are mass-andspring
systems.
3 In-situ processing of the
synthetic wooden sleepers
situations. The critical price was determined
for different wage levels, assuming
that the load-bearing structure would have a
residual service life of, firstly, more than 15
years and, secondly, more than 30 years.
This was done, since the input parameter
had originally been set at 15 years to equal
the mean practical service life of bridge timbers
made of natural wood. The imputed
rate of interest was forecast on the basis of
the extremely long expected service life of
FFU bridge timbers of more than 50 years
and, after performing a sensitivity analysis,
was then taken to be a real 3% higher than
the rate of inflation.
Having established this basis, it was then
possible to calculate the critical prices of
the FFU bridge timbers for various wage levels,
which showed that their economic performance
was better on the assumption of
one replacement of conventional bridge timbers
having been avoided (after 15 years)
or two such replacements having been
avoided (after 30 years). The price level of
FFU synthetic wood taken for the purpose
The drilling of holes for holding rail fastenings,
the cutting of FFU synthetic wood,
milling operations and chiselling are performed
with standard tools in a similar way
to natural wood (Figs. 5 and 6). Widia tools
(these are made of very hard steel, nearly
as hard as diamonds) are recommended for
these jobs or, alternatively, ones designed
for metal-working. Practice, however, shows
that experienced contractors’ workers
sometimes resort to simple power saws in
a similar way to the processing of natural
wood (Fig. 7). The drawback to using conventional
timber saws, however, is that their
chains are likely to wear down very quickly.
Any rail fastenings recovered from the old
wooden sleepers can be reused straightaway
in the new FFU ones.
4 Examination of life-cycle costs
(LCCs) at Graz University of
Technology
In 2008, Graz University of Technology, working
on a contract from the Austrian Federal
Railways, performed an LCC analysis of FFU
bridge timbers on open steel bridges compared
with the use of natural wood for this
purpose [1]. It performed a similar analysis
again in 2009, but varying a number of key
input parameters. For instance, it reduced
the specific labour requirement when using
FFU sleepers in track curves by 20 %,
thereby better reflecting their advantage
over natural wooden ones in complicated
RAILWAY TECHNOLOGY
State of the Art
FFU SYNTHETIC SLEEPER CALMMOON RAIL FLAT FFU SLEEPER
SEKISUI CHEMICAL GmbH
Cantadorstr.3
D-40211 Düsseldorf
TEL: +49-(0)211-36977-0
FAX: +49-(0)211-36977-31
E-Mail: ffu@sekisui.de
www.sekisui-railwaytechnology.com
RTR 4/2011 43

• Use of synthetic wood for railway sleepers
Curve – 240 bridge timbers
Wage level
Imputed real interest rate = 3 % 110% 100 % 90 % 80 %
First replacement rendered unnecessary +29 +21 +13 +5
Second replacement rendered unnecessary +85 +73 +61 +49
Table 1: Critical price of material for different wage levels (+ = profit, – = loss)
FFU synthetic wooden sleepers are manufactured
in accordance with precise specifications,
and the products delivered deviate
from the dimensions ordered by only a few
millimetres’ manufacturing tolerance. FFU
synthetic wood stably maintains its shape
after positioning and lies flush on the
flanges of steel beams, with nearly 100 %
surface contact with them. Moreover, it retains
its homogenous structure. Thanks to
this stability of shape, sleepers made of
synthetic wood can be quickly and immeof
this calculation corresponds to the price
that would be typically paid for an order for
more than 20 m³. The wage level in Austria
was taken to be 100 %. The results (Table 1)
show that the use of FFU synthetic wood is
more economical than conventional bridge
timbers for the organisation responsible for
maintaining the railway line. The savings
are those values preceded by a plus sign.
Professor Veit (of Graz University of Technology)
summed up the outcome of the LCC
analysis as follows:
If a real value of 3 % is taken for an imputed
rate of interest, it is possible to
use FFU synthetic wood as a standard
solution at Austrian wage levels,
FFU bridge timbers demonstrate advantages
in technically demanding limiting
conditions, thanks to production flexibility,
since, with these, it is to be expected
that the specific labour requirement
would be significantly less. This, in turn,
would have a positive impact on the business
case; and
Fig. 7: Sawing work on
the construction site on
the bridge over the river
Sava (Belgrade)
FFU bridge timbers are a further step
towards a sustainable track superstructure,
a property which is going to become
even more important, especially
considering the increasing number of
trains and, as a consequence of that,
the reduced time available in future for
doing maintenance work.
5 Experience, advantages and
benefits
diately put into place on open steel structures.
In track curves, FFU sleepers, which
have been precisely prepared and clearly labelled
in the factory, can be very quickly and
professionally put into place. Their mass,
which is close to that of natural wood (FFU
74 = 740 kg/m³), ensures easy handling
at the engineering site and offers static
advantages, especially in combination with
steel structures. The load-dissipating structure
is clear, unequivocal and homogenous.
The material has no pores in it and thus
does not retain liquids. It does not contain
any insecticides. It does not undergo any
change on contact with greases and chemicals
used on the railways. Its ingredients
are simply polyurethane and continuous
glass fibres.
On ballasted railway tracks, the lower face
of FFU sleepers grips the ballast in a similar
way to natural wooden sleepers. Repair
work is simple, conventional cutting and
drilling tools can be used, and any skill
acquired in working with natural wood can
be put to use with synthetic wood too. The
outlay on maintenance work (which is a
key cost factor for railway operators) can
be generally minimised, as has been demonstrated
in practice, thanks to the good
load dissipation, the good, long-term adhesion
between sleeper screws and synthetic
wood, excellent resistance to weathering
and the permanently closed structure.
On the basis of the LCC analysis performed
in Graz, the initial capital outlay for creating
the track superstructure on bridges using
FFU synthetic wood is in the range of 1.35
to 1.55 times what it would be if natural
wood were to be used. The life expectancy
of FFU, on the other hand, is three to five
times that of the natural wood typically used
for bridge timbers today. Should it happen
that the FFU timbers need to be replaced
after a period of more than 50 years, industrial
methods of complete recycling are
available for them.
Reference
[1] Life Cycle Cost Analysis, Graz University of Technology,
a. o. Univ.-Prof. Peter Veit and Dr. Marschnig,
2008
¡Viva España!
Use our country feature RTR Spain as your advertising platform!
Strengthen your activities on the Spanish railway market.
Book your advert for this issue by 14th May 2012.
Revista Técnica de
los Ferrocarriles
Euro 16,50
www.eurailpress.de/rtr
ISSN 0079-9548
I look forward to your reply.
Tel: +49/40/237 14 - 171, E-mail: Silvia.Sander@dvvmedia.com
44
4437_anz_RTR_RTRSpain_182x64_en.indd 1 05.03.2012 10:48:11
RTR 4/2011