www.railwaygazette.com | February 2013
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TRACK TECHNOLOGY
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TECHNOLOGY | TRACK

Photo: Wikipedia/Störfix

Understanding the lifeof ballast mats

RESILIENCE A ballast renewal programme on theHannover – Würzburg high speed line provided a valuable
opportunity to examine the condition and performance ofa resilient under-ballast mat installed 21 years earlier.
Mirko Dold and Stefan Potocan
Product Management Rail Division
Getzner Werkstoffe GmbHAlthough railways havebeen installing resilient
under-ballast mats formany years, there havebeen few opportunities to assess
how well they are performing after decades of service. However,when renewing the ballast on theAbove: The
Bartelsgraben
bridge carries theHannover – Würzburg line over avalley between
Zellingen andLeinach.
Right: The
indentations fromballast stones can
be clearly seenon the removed
sections of mat.

Bartelsgraben bridgeon its Hannover –Würzburg high speed
line a few years ago,
DB Netz removed atest sample of the Sylomer D 220ballast mat that had been installed
in 1987. This provided the opportunity for a detailed examination of
the performance of the mat, which
had carried an estimated 384 million
gross tonnes over a 21-year period.

The beneﬁts of ballast mats

Installation history

ELASTICITY: Resilient under-ballast mats can provide an
economical method to increase the elasticity of ballasted track,offering a long-term improvement in trackbed quality and ride
comfort, together with extended tamping intervals and higher
availability of the track which have a positive impact on life-cycle
costs. They can also be used to reduce the secondary airborne
noise radiation from bridges, and are often deployed to provide
structure-borne sound insulation in dense urban areas, or toprotect particularly sensitive structures and buildings.
Ballast mats typically have a two-layered structure. A load
distribution layer on the side exposed to the ballast helps to protect the spring layer below from the sharp-edged ballast stones.
It also ensures an even load distribution, as stones become
embedded in the mat, increasing the load transfer area, preventing the premature deterioration of the ballast and protecting thetrackbed. The actual spring layer is made of microcellular polyurethane materials, giving the mat a compressible volume and,negating the need for any profiling or cavities to achieve thedesired elasticity. The thickness of the microcellular material isselected to deliver the desired static and dynamic stiffness. l

Located at Km 312 between Zellingen and Leinach northwest ofWürzburg, the 1 160 m long Bartelsgraben bridge carries a double-track
section of the high speed line 55 m
above the valley floor. Completedin 1986, it is a prestressed concrete
box-girder bridge with a longest
span of 58 m. Located on a 1·25%
gradient falling towards the south,
the line initially describes a 10 000
m radius curve before straighteningout. South of the bridge, the trackpasses over a short section of embankment before continuing onto
the adjoining Leinachtal bridge.
The ballast mat was installed in
1987. The material was delivered tosite in rolls and laid out on thecleaned subsurface. They were thenfolded in half lengthways (p46), andeach half in turn was spot-bonded tothe bridge with a two-part adhesivein line with DB’s installation standards. This bonding ensured that themats did not move during the subsequent ballast laying process, butbonding is generally not essential toguarantee effective noise mitigation.
During installation, TU München
undertook an initial study of themat on behalf of DB’s central research institute (BZA) in Minden.
This was commissioned to investigate the deflection of the rails on
sections of ballasted track with different sub-surface elasticities during
the passage of a train. Data was obtained for deflections in:t track laid directly on the bridget track laid on the bridge with a resilient under-ballast matt track on a subgrade.
TU München was also involvedin testing the recovered sample, toassess the performance of the matfollowing a lengthy period of service

Railway Gazette International | February 2013

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TRACK | TECHNOLOGY
and exposure to different weather
conditions.
Inspecting the sample

Stiffness testing

The static and dynamic stiffness ofthe sample were also measured, in order to compare them against the values recorded when the mat was new.
These variables can be used to show
the ageing behaviour and the performance of the ballast mat.
Static stiffness was determined according to the procedure set out in
the DB specifications from 1987,
using 500 mm square samples ofthe mat. Secant stiffness was evaluated between the load points 0·02 and0·1 N/mm². The static bedding modulus was calculated at 0·0529 N/mm³,
which still met the original specification of 0·06 ± 0·01 N/mm³. Compared with the value of 0·0571 N/mm³
for the new mat as measured by TU
München, this represents a change of
7·9% after 21 years (Fig 1).
Dynamic stiffness was measured
using smaller 200 mm square samples,
according to the dynamic properties
measurement process described by
the Müller BBM 12506/1 report ofJanuary 1986. In this method, preloads
of 0·03 N/mm² and 0·10 N/mm²
were applied at a test frequency of
40 Hz. The mat sample returned values of 0·092 N/mm³ and 0·090 N/mm³
for the two cases respectively, whichcorresponds to deviations of 11·2%
and 9·6% since 1986 (Table I).

Installation of theSylomer D 220
ballast mat on
the Bartelsgraben
bridge in 1987.

Fig 1. Comparisonof the static
stiffnessof the mat
sample, before
installation andafter removal.

the ballast mat had been exposed to
all the associated weathering effects,together with thousands of frost-thaw
transitions. But the water has had no
negative impact on the properties ofthe ballast mat. The testing confirmedthat the Sylomer mat was still proving
effective and did not exhibit any noteworthy signs of ageing or degradation.
The measured values were still
within the tolerances specified at thetime of installation and which remain
valid to this day. No cracks or perforations in the mat were found, evenwhen it was subjected to the closest
scrutiny. As a result, the expectation isthat these ballast mats will continue toremain completely effective for at least
another 30 years. l0·250·2Good to continue
Static bedding modulus

The sample tested showed no relevant changes greater than 15% to
any of its properties after 21 years ofuse and 384 million tonnes of traffic.
Having been installed on a bridge,

Pressure N/mm2

Over the 21-year period, the maton the Bartelsgraben bridge had
withstood an operational loadingof approximately 384 million grosstonnes. This is far in excess of theload stipulated for the fatigue strength
testing. At the time of installation themats were only required to support
2·5 million load cycles. But based on
a 22 tonne axleload, the fatigue stress
was calculated at 17·5 million cycles,
which is seven times the figure definedin DB’s 1978 standard TL 918071.
Visual inspection of the mat sample
revealed some plastic indentationsin the load distribution layer fromindividual ballast stones. These
typically occur in the loading area
directly below the sleepers, which was
in line with expectations. The sameeffect was observed during the fatigue
behaviour testing undertaken at thetime of installation.
The indentations indicate thatthe ballast stones had been properly
embedded into the load distribution
layer, helping to avoid any load peaksin the contact area between the ballast
and the concrete bridge structure.
As a result, ballast degradation wasreduced, leading to lower maintenancecosts and longer tamping intervals.
No signs of damage or perforationof the load distribution layer could beestablished. It was therefore concluded
that the ballast mat had successfully
withstood the high mechanical loads
and prevailing weather conditions,and would continue to do so.

0·15
After 0·0529 N/mm30·1

Table I. Dynamic performance of the D 220 ballast mat

Before 0·0571 N/mm30·05

Beforeinstallation

After 21years and384 MGT

ChangePreload 0·03 N/mm³0·083

0·092

11·2

Preload 0·10 N/mm³0·082

0·090

9·6

Dynamic bedding modulus
cdyn at room temperature

46

Railway Gazette International | February 2013

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