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Investigation of particle degradation of railway ballast materials due to static and dynamic loadings

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Investigation of particle degradation of railway ballast materials due to static and dynamic loadings
Investigation of particle degradation of railway ballast materials
due to static and dynamic loadings
Juhász Erika1 Dr. habil. Fischer Szabolcs2
1Széchenyi István University,
Department of Transport Infrastructure and Water Resources
telephone: 96/503 451
e-mail: juhasz.erika@sze.hu
2Széchenyi István University,
Department of Transport Infrastructure and Water Resources
telephone: 96/503 451
e-mail: fischersz@sze.hu
Abstract: Nowadays the railway infrastructure has become significantly important regarding to the environmental
friendly and economic transportation. Ballasted railway tracks have to be improved to be able to
construct and maintain railway tracks economically and efficiently. In railway superstructure the
railway ballast plays a critical role as the main load distributor and energy damping element, as well as
it has the largest quantity in a railway superstructure. The degradation of ballast particles, i.e. the
resistance of mechanical impacts is a main issue. A simplified laboratory test method was developed for
determination of particle breakage of railway ballast materials. It contains a HDPE tube with closing
element, and geotextile covering in the inner side. The HDPE tube is important to be able to apply
computer tomography method during test procedure. Approximately 20-25 particle are filled into the
tube and uniaxial static and dynamic loading are applied to the samples with several loading steps and
loading cycles. Crumbling of grains can be determined by computer tomography and weight
measurements. As results it can be defined how the rock type, the particle size distribution, the particle
shape, etc. influence the mechanical degradation of the tested railway ballast samples.
Keywords: ballasted railway tracks, railway ballast, particle degradation, breakage, uniaxial compression test,
static and dynamic tests, computer tomography
Introduction
The most commonly used type of superstructure on railway tracks is crushed stone. This is true not only
for our country but also for the whole world and this fact is confirmed by literary sources.
Observing this structure, it can be visible that the largest dimensional element of the superstructure is the
crushed stone itself, which usually consists of depth magmatic rock particles (mainly andesite or basalt).
The crushed stone aggregation layer in the track plays an important role in supporting the track in a solid
but at the same time flexible manner, as well as carrying the load towards the substructure. Structural
stability is also important, as drainage of rainwater and simplification of track geometry, too.
Except for their intended purpose, particles and aggregates need to have certain properties: toughness,
abrasion resistance, shape, structure, roughness, weather resistance, economy, etc., except for
completeness requirements, all of which are decisive for the ‘performance’ of embedded rocks.
Of course, it is also important which fraction of the rock material enters the superstructure itself. This
particle size distribution determines the drainage capacity and compactability of the rock mass, e.g. it
would be much easier to control the set of one-dimensional particles, but achieving the right compactness
would be a problem.
In the field of road construction layers with different fine grain contents are applied, while in Hungary
and almost everywhere in the world railway track structures have traditionally applied relatively large
granular bedding materials (31.5/50 and 31.5/63 mm; previously 20/65 mm). In contrast, in the hard-
surfaced track structure, a continuous (mainly fractured) granular base layer is deposited under the
reinforced concrete track, usually in terms of both load-bearing capacity and frost protection as well as
capillary interruption layers at high groundwater levels. The latter layers are also applied under the
bedding superstructure.
Investigation of particle degradation of railway ballast materials due to static and dynamic loadings
1. Presentation of research problem
According to the railway construction practice, on slopes with crushed stone structures, a hammer bearing
compaction method (e.g. Plasser & Theuer, Matisa machines) is used, but due to the technology, a larger
crushed stone bedding structure is required. One of the drawbacks of the technology is that the hammers
break the stone particles considerably during compaction, thus shortening the cycle time of the required
ballast tamping (together with the service life and maintenance costs). In addition to the compacted layers
are loosen underneath the concrete sleepers, which does not achieve perfect railway track geometry.
Technology as a solution currently in use, is recognized and appropriate in most countries of the world, so
during the authors’ research they focus on optimizing it as an innovation. Especially they investigate the
development of methods for measuring and reducing the aging processes of ballast particles by
fragmentation and degradation.
Degradation in the authors’ research means (as an unofficial definition) the cracking, obsolescence,
partial or total failure of railway crushed aggregate particles. Particles can be suffering matter and
abrasion. They usually occur simultaneously, in some cases reinforcing each other and at other times
reducing.
In terms of the operating conditions of the railways, fragmentation is the greater of the two processes. In
this case, smaller or larger pieces are detached from the particles mainly from their edges, corners and
the ballast particle may fall apart into several pieces. The wear is due to the surface friction of the
particles and typically has smaller volumetric effect. The product of the wear is stone powder.
During the research the authors investigate the triggering effects of the degradation of the particles. In
ballasted tracks, the aggregate exposed to static and dynamic effects, of which the most significant are as
net weight load, forces of vehicles, environmental impacts and last but not least maintenance machinery
effects.
The degradation of the granules is influenced by several factors, e.g. rock physical properties of the
aggregation (material, mechanical resistance); the magnitude of the load (axle load, speed); material and
geometry of the rails, fastening systems, track system; the surface of the ballast particles; thickness of the
ballast bedding layer, etc.
In Hungary, the suitability of railway rock material is determined by the examination required by the
same product standard (MSZ EN 13450:2003):
Micro-Deval wear according to MSZ EN 1097-1:2012, as well as
Los Angeles fragmentation test in accordance with MSZ EN 1097-2:2012.
The examinations are well suited to determine the wear and tear resistance properties of a given
aggregate, which is essential to ensure the consistency of the manufactured product. However, the forces
and stresses caused by the operational conditions in the plant are not accurately simulated in the tests. In
connection with this, laboratory studies have been developed that can later serve as a basis for estimating
the real aggregation effects and stress ratios, and their uniqueness takes a new approach to the research
topic.
2. International literature research
The authors did a wide range of literature research in the topic. Notable material is available on this topic,
especially in laboratory and field studies, and also DEM and FEM modelling. The authors have collected
a number of relevant findings that have been applied in our research in the appropriate areas.
Parameters introduced by acknowledged foreign researchers, such as Among other parameters, LARB,
MDERB, FV, BBI [1], M, and λ [2] values were used to predict the prognosticated ballast tamping cycle
times for the prior “shear loading tests”. The authors also found valuable resources for granular shape
examinations. [3]
At present, the authors have found only a limited number of articles and publications on CT scans, which
show that so far there has been little research on the subject. In our opinion, combining CT scans with
other methods found in the literature research can be very useful.
Investigation of particle degradation of railway ballast materials due to static and dynamic loadings
3. Presentation of laboratory tests
The current test was preceded by several other laboratory measurements. The examinations needed to be
rethought and improved.
Single-particle fracture tests (shown in Figure 1-2.) did not yield accurate results because the shape,
surface, size and rock mechanical properties of the broken particles were significantly different from each
other. For these reasons, I decided to investigate the ballast aggregates counter to only one particle.
Figure 1-2. Single particle failure testing with ZD-40 crushing machine
In addition, the types of test for larger aggregates included many factors that significantly influenced the
results, so the results were not close to those predicted („shear box test”). [4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
14, 15, 16, 17, 18]
After that it was necessary to develop a method and technique that can be used to examine a manageable
and traceable amount of aggregate.
For the current testing method, an approximately 140 mm outside diameter, 10 mm wall thickness HDPE
water pipe was applied, which is 300-400 mm high, with a pipe closure made for it.
A 1200 g/m2 high-strength geotextile piece (Viacon GEO TC 1200) was placed inside the tube due to the
reduction of friction between the inner surface of the HDPE tube and the crushed stone particles, and the
steel loading plate and the particles also. The loading plate is 120 mm diameter, 3289 grams, and
approximately 40 mm thick steel disc and the uniaxial (vertical) static load were applied to the
aggregation through this. The tube was filled with railway ballast particles approximately the height of
140-160 mm limited by the CT X-ray equipment measuring, because the height of the sample to be
screened optimally equal to the width (in the case, the inside diameter of the tube was a guideline). The
amount of material entering the HDPE tube is about 13-15, sometimes 20-25 pieces. The aggregates were
tested to uniaxial compression loading which combined with CT examination. The arrangements are
illustrated in Figure 3-4.
Figure 3-4. Uniaxial static load (left) in HDPE tube and CT (X-ray) equipment (right)
Investigation of particle degradation of railway ballast materials due to static and dynamic loadings
The developed measuring model cannot be paralleled to the real demands, nor can the standard tests
mentioned in the previous chapter, but presumably a good direction in our research.
With this method, even degradation of more particles can be observed simultaneously with high-precision
digital 3-D image capture and can be processed as particle or aggregate, as needed, to determine the exact
position of the particle and their contact with the wall or with other particles. For the time being, the
research is in its initial phase, but based on the results so far, it is encouraging to continue. The shape
model is shown in Figure 5. below, based on the initial measurements (for a three-step load, the four shots
are from left to right at a static pressure of 0, 300, 600 and 900 kPa).
Figure 5. Recordings made by CT equipment (in case of three-step loading)
The above image was not taken in this semester, currently the CT device is undergone maintenance,
anticipated to March 2020, so priorate to that the authors can only load the samples statically with the
ZD-40 crusher machine (which is a good basis for developing the exact method).
The colleagues helped us of the Material Testing Laboratory (dr. István Kozma and Imre Fekete) of the
Department of Material Science and Technology of the Faculty of Vehicle Engineering of the Széchenyi
István University, where a 450 kV tube of Yxilon Modular CT X-ray equipment was utilized. The is
device shown in Figure 4. The device is not suitable for static and dynamic load, but can be used
effectively to create high-precision 3-D digital surface and volume models in addition to these tests. The
surface models consist triangles, and the size of the triangles depends on the accuracy of the image. The
resulting models can be evaluated in the software provided with the device or the software of GOM
Inspect (presented in Figure 5.).
The models can be used to predict the degree of fragmentation during stepwise loading and to estimate
the magnitude (or proportion) of stresses within the set.
In the recent laboratory measurements, 183 pieces of crushed ballast particle were weighed individually,
according to a standard method. In the selection of the granules, the authors tried to select the granules of
all shapes and sizes (by visual inspection) so that they would have different shape and size particles, but
according to our experience, the result of particles’ distribution from the mine of Szob was mainly based
on one type of grain.
Before placing the washed and dried stones in the HDPE tube, the authors measured them with a digital
Mitutoyo calliper the two orthogonal axes by tenth of millimetre accuracy. One axis represented the
longest extent of the particle (h), the other one the smallest (s), the third value the mesh size (v).
Interestingly, one of the sources found in the literature research used a similar method, However, the
limits FI (flaky index) and the EI (elongated index) calculated from the axial values are used: if FI < 0.6,
the particle is flaky, if EI > 1.8, the particle is elongated, and if not elongated and not flaky, it is cubic:
surprising, but the limit values on my aggregate particles were exaggerated. [19]
Figure 7-9. clearly show the classification possibilities of the 2.5.1-3. points of already withdrawn
standard MSZ 18288/3-78, which taken together, could only interpreted in a “mixed” way to classify the
aggregate. Figure 6. illustrates that, despite of the variety of classification possibilities, most of the
particles are cubic and elongated at the same time, and only a small portion can be classified as obviously
elongated, cubic or flaky. In terms of numbers, only 7.1% of the selected particles were cubic and 19.7%
were flaky. This is due to the peculiarity of railway ballast.
Investigation of particle degradation of railway ballast materials due to static and dynamic loadings
Figure 6. Classification of weighed particles according to axis ratios in accordance with MSZ 18288/3-
78 standard
Figure 7-9. Classification possibilities according to MSZ 18288/3-78 based on axle ratios
Investigation of particle degradation of railway ballast materials due to static and dynamic loadings
To elaborate the refinement of the measurement method, three different aggregates of samples were
created, of which four measurements were made with uniaxial compressive load (most of the rock
composition is composed of cubic and elongated particles):
1130-1490 grams of rock with 50-69% flaky and flaky-like cubic/elongated content,
1245-1630 grams of rock with 12-30% clearly cubic and cubic-like cubic/elongated content,
1274-1685 grams of rock with only cubic and elongated particles.
The measurements were carried out by a ZD-40 crusher machine on the total of 12 samples (aggregates).
The authors performed uniaxial static compression load on the formed aggregate formations in the stress
range of 0 ... 1800 kPa with a statically increased load value.
4. Experimental results of the investigations
On the basis of the fracture tests in can be stated that the most broken/degradated particles were the flaky
elements, and the least broken particles were the cubic ones. This can be explained by the fact that
tensions wake up most along sharp edges and thin “protrusions”, while cubic particles are somewhat
exaggerated but more like spherical shapes.
The results are shown in Table 1.
Table 1. The propensity to degradation depending on particle shapes
(rather)
FLAKY
(rather)
CUBIC
ELONGATED &
CUBIC
Fragmentation average (%
by weight relative to the
original mass)
7.01%
Average aggregate mass
1311.6 g
Average fragmentation
90.3 g
For each of the 12 samples, the grain size distribution curves were drawn for the pre- and post-load
condition, which is shown Diagram 1. The grain size distribution curves also confirmed the degradation
values shown in Table 1, which show that the flaky aggregate samples are more fragmented that the cubic
aggregate samples. The elongated ones (and partly cubic) are between the two curves.
Diagram 1. Grain Size Distribution diagrams for different aggregate samples
Investigation of particle degradation of railway ballast materials due to static and dynamic loadings
Conclusions
In this article the authors described the latest laboratory tests carries out on the research topic and results,
findings. The aim of the authors’ research is to determine the exact time course of the degradation of
railway ballast particle materials with the help of the performed investigations with CT (X-ray)
equipment, and with this new method one of the authors (Erika Juhász) propose the application conditions
and limits in her dissertation.
Acknowledgements
The publishing of this paper was supported by EFOP 3.6.1-16-2016-00017 project.
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... Part of our research to date has examined ballast (crushed stone) fragmentation using various visual imaging devices 3D scanner, in which the ballast bed itself was loaded mainly into closed vessels or boxes of various sizes (e.g., shear box or closed-ended, capped HDPE tube). In the authors' previous publications, they have already measured deformations and changes with the GOM system, focusing on the fragmentation of smaller aggregates (Juhász & Fischer, 2019a;Juhász & Fischer, 2020). However, load tests (static and dynamic) in closed boxes do not resemble real ballast bed conditions, so a new experiment methodology has been developed and applied. ...
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