GERMANYFREIGHTINNOTRANS 2014

Mastering complexity
Quality must underpin DB’stechnical development, says
Dr Heike Hanagarth

Bold moves needed
European rail freight needs
behavioural change toachieve modest growth

The global gathering
The international railway
industry comes together inBerlin this month

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September 2014

Railway Gazette International | September 2014

105

Track Components FREIGHTPUR pads protect heavy
freight corridors

Increasing axleloads and tonnage are placing increasing demands on tracksuperstructures on the busiest freight corridors. Getzner Werkstoffe has beendeveloping its range of PUR under-sleeper pads to give an optimised combination ofelastic and plastic properties designed especially for freight routes.

Following the introduction of
hardened rail steels and prestressed concrete sleepers,
ballast has arguably become
the weak link in track design. Sleeper
padding is a long-established practicethat can offer significant ballast protection; this is essentially achieved byexpanding the contact area between
the sleeper bottom and the top ballast layer. This in turn requires undersleeper pads with highly plastic properties to enable the ballast stones to beembedded as gently as possible.
On the other hand, achieving an
ideal load distribution in the track by
increasing its material flexibility calls
for under-sleeper pads with highlyelastic properties that can be sustained
over the long term, even when subjected to decades of stress. Designing pads
that combine both long-term elasticity and a high degree of plasticity isa daunting task for today’s materialscientists.
Heavy freight challenge
The need for under-sleeper pads thatcombine these properties is especiallyacute on those railways which carrylarge amounts of freight, whether aspart of a conventional mixed-traffic
network or as dedicated heavy-haullines serving mines and other bulk

DR HARALD LOYHead of System Development
ING ANDREAS AUGUSTIN
Member of the Management Board
Getzner Werkstoffeindustries. Typically this definition applies to operation of 40 million grosstonnes or more over a given length of
track in a year.1 This criterion is independent of the form of traffic, whether
passenger, bulk or mixed freight.

Alongside broken rail clips andsleeper cracks, the deterioration of theballast, starting from the contact areawith the sleeper, is one of the main
factors that shorten the lifespan of
track on these heavily-used networks.
Problems can occur at transition zones
from an open stretch of track to abridge structure; such areas of discontinuity in a highly-uniform track
structure can result in the exertion ofa particularly intense load, leading to
the fracture and pulverisation of ballast grains, of which white spotting isa clear indicator.
High-quality under-sleeper pads,often used in conjunction with suitable
rail pads, can effectively counteract sucheffects. Under-sleeper pads are implemented as a flexible spring layer located

The iron ore flows
from Rotterdam
to the Ruhr Valleyin Germany areamong the heaviest
freight trains to use
Europe’s mixedtraffic network.

White spots show
the destruction ofballast beneathconcrete sleepers(left). Broken clips(middle) and sleeper
cracks (right) result
from excessive
dynamic loads.

September 2014 | Railway Gazette International106

FREIGHT Track Components

Fig. 1. Polyurethane
under-sleeper pads
protect the ballastlayer.

Right: Fig. 2.
Recorded amplitude(peak-to-peakmovement) from thelong-term fatigue
test in the ballast
box.

Simulation of highaxleload stress inthe ballast box ofthe large test rig(left). Following theend of the test, theballast grains remain
embedded in thePUR material (right).

subgrade. The
fracture of ballast
grains due to overloading is prevented and settlement is minimised, while the formation of hollows and cavities can beeffectively mitigated.
To quantify the long-term effect ofunder-sleeper pads, extensive studies
have been carried out since 2001 tomeasure the rate of track deterioration
where they have been installed.2 The
results demonstrate a positive influence on the superstructure under different conditions, and the pads work
more efficiently as the load on thetrack increases. All the padded areasof track show a significantly reduced
deterioration rate. Even in sections
with lowered ballast height, the tamping interval has been at least doubled.
The use of under-sleeper pads brings
about a significant reduction in maintenance requirements, thus offering
life-cycle cost savings. Under-sleeper
pads are commonly used today for thefollowing applications:• reduction of settlement in all ballasted track structures;• limiting propagation of rail corrugation in tight curves;• adjustment of track stiffness in areas

with reduced ballast height;• improvement of transition zones
between different types of trackstructure;• mitigation of vibration;• avoidance of hollow areas underneathsleepers;• levelling different deflections withinturnouts.
Not all materials are equally suitable for these tasks. One material thathas been proven to be extremely toughand durable is Sylomer, which is madefrom PUR, a specialised polyurethane.
In PUR production, a diverse range ofproperties can be adjusted to meet thedesired requirements by mixing reactiveingredients. Polyol and isocyanate are themain components by proportion, having
a significant impact on the mechanicalproperties and comparatively high tensile strength. In the development of under-sleeper pads for ballast protection,
an optimum mixing ratio and the rightproduction process are critical if a balanced ratio of elastic and plastic properties is to be achieved. As a result of many
years of research, Getzner has developed
a novel PUR under-sleeper pad, whichaims to cushion extremely heavy loads
in freight applications while maintaining high technical performance.

Fatigue strength
The fatigue strength of the newly developed pad was tested on a large rig at
the Getzner Werkstoffe laboratory in
Bürs, Austria.3,4 A test rig allows thesimulation of maximum track-borne
loads generated by heavy freight trains,
including curving forces. Other factors,
such as increased train speed and overall
track condition, can also be modelled.
The applied static axleload of 36 tonnes
was increased in line with these additional dynamics by a factor of 1·52.
For such specific fatigue tests, theunder-sleeper pad is attached to theunderside of a concrete slab measuring
300 mm x 300 mm, which represents
the sleeper, using a plastic mesh. Half of
this mesh is inserted into the concreteand the other half into the pad. The longterm test was carried out in two loadstages at a test frequency of 5 Hz on ballast with a height of 300 mm. The firststage saw 5 million load cycles, which
was increased by a further 3 million cycles in the second phase, giving a totalloading of 8 million cycles over a continuous period of about 3·5 weeks. The
progression of the vibration amplitude isshown in Fig 2, reaching 1·2 mm at theend of the test. The padding exhibitedno destruction from cracks, perforations

Progress of amplitude (peak-to-peak movement)
2·0

1·6

Amplitude mm

underneath the sleepers, which increases the contact area with the ballast
from between 2% and 8% to more than30%. It has already been widely demonstrated that a larger contact area improves the load transmission to the ballast bed and puts less of a stress on theload
loadstageIstage I ĺ loadloadstagestageIIII
1·20·80·4
10100

1 000

10 000100 000

Load cycles

1 000 000

10 000 000September 2014 | Railway Gazette International

108

FREIGHT Track Components
Fig 3. Digital analysisof the surface
structure withballast indentations.
These are shown by
chalk spray (left) andpore scan (right).
The analysis showeda contact area for
the PUR padding of
between 25% and33%.

or punctures, and the ballast grains embedded themselves to a maximum depthof approximately 80% of the thicknessof the pad. This confirmed the satisfactory long-term behaviour of the PUR
material under loading typical of heavy
freight railways.

Fig. 4. Averageballast contactpressures
underneath sleepers.
Comparison of an
unpadded concrete
sleeper with EVAand PUR padded
sleepers.

Contact area and contact pressure
The plasticity of the under-sleeper
pad enables the uppermost layer of ballast to embed in the padding material.
This is an important safety feature, as it
ensures a greater degree of lateral trackresistance; lateral resistance is generally
higher for concrete sleepers equippedwith pads than those without.5 From
the outset, it was a requirement of theproduct development process that there
should be no negative impact on trackstability, and this objective was confirmed in testing undertaken by TU
München.6
However, the main benefit affordedby the flexibility of the padding material
is that the forces are transmitted to theballast superstructure more homogeneously. Getzner has developed a proprietary method for quantifying the contact
area quickly and very precisely (Fig 3).
This post-test analysis showed a contact
area for the PUR padding of between
25% and 33% — Fig 4 shows the contact areas under a concrete sleeper without padding, with EVA ethylene vinyl
acetate padding and with PUR padding,
all at the same stiffness.
As expected, the contact area with
non-padded sleepers is the smallest(generally <5%), the EVA padding material is in the middle (5·9%), while thePUR material demonstrates the largest

contact area (27·8%). The test procedure
for each material was identical, including the identical nominal material bedding modulus. The larger the effective
contact area of the ballast, the more
uniform the load transmission, with aconsequent reduction in ballast contactpressure.
The effectiveness of using Sylomer
as a material for under-sleeper pads has
been demonstrated in numerous fieldtests.7 Individual sections have beenequipped with semi-plastic PUR pads
for comparison against sections withno padding. Track settlement behaviour
was recorded by means of precision levelling. Fig 5 shows the average settlement progression; trends were clearlyvisible after just 377 days. The average
track settlement in the unpadded zone
was 12·5 mm, while in the padded zone
the track had only settled by an averageof 7·5 mm. So the settlement of the unpadded ballast superstructure was morethan 65% greater, reflecting the positive results seen in other installations,despite the relatively short observation
period. Indeed, the greater homogeneity of the padded trackform can in somecase be discerned by the naked eye. Q
Below: Fig. 5. Settlement progression withand without under-sleeper pads after 377
days of operation.
Bottom: The different quality of track isevident by comparing track sections withpads (left) and without (right).

Ballast contact pressure N/mm²3·5Comparison of track settlement

3·002·52·0

with USP-2·3-2

1·50

non-padded
concrete sleeperEVAPURSettlement mm0·5without USP-4·4-4

1·0-6·7-4·6-6-8-7·0-7·5-7·0-10-10·8-12-11·8-12·5-14
contact area <5%

5·9%

27·8%

050

100

150200250(image shows 1·4%)Time days

References:
1. Barresi F, Kinscher W and Lorenz G. Design and Maintenance Experiencefor Heavy Haul Turnouts including feedback on the use of Austenitic
Manganese Steel for Fixed and Swing-Nose Crossings. Proceedings of 10thInternational Heavy Haul Association Conference, 2013.
2. Schilder R. Under Sleeper Pads — Schwellenbesohlungen. Getzner
Bahnfachtagung 2007.
3. DIN 45673-6: Mechanical vibration — Resilient elements used in railway
tracks — Part 6, Laboratory Test Procedures for Under-Sleeper Pads ofConcrete Sleepers, 2010.
4. Loy H. Determination of Long-Term Fatigue Strength of USP for Heavy
Haul Applications. Getzner Werkstoffe Report No. 2012-55, 2012.
5. Iliev D. Die horizontale Gleislagestabilität des Schotteroberbaus mit
konventionellen und elastisch besohlten Schwellen. Doctoral Thesis,
Technische Universität München, 2012.
6. Freudenstein S, Iliev D and Stahl W. Querverschiebewiderstandsmessungen an un- und besohlten Schwellen. Der Eisenbahningenieur,
July 2013.
7. Loy H and Heim M. Measurements of Track Settlement for IR — Trials ofSleepers with Under-Sleeper Pads. Getzner Werkstoffe Report SE03, 2013.

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