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Defining the process of including sustainable rubber particles under sleepers to improve track behaviour and performance
Journal of Cleaner Production 227 (2019) 178e188
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Defining the process of including sustainable rubber particles under sleepers to improve track behaviour and performance nchez a, *, F. Moreno-Navarro a, R. Pe rez b, M.C. Rubio-Ga mez a M. Sol-Sa a b
Laboratory of Construction Engineering at the University of Granada, C/ Severo Ochoa s/n, 18071, Granada, Spain Signus, C/Caleruega, 102, 5º, 28033, Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2019 Received in revised form 23 March 2019 Accepted 11 April 2019 Available online 17 April 2019
The inclusion of rubber particles derived from end-of-life tyres (ELTs) under the sleepers has been demonstrated in previous studies to be a potential solution to improve the behaviour and durability of railway tracks, and then, minimizing the negative impacts associated with the need for maintenance. Nonetheless, more in-depth studies are still required to define the optimal procedure prior to its application in real tracks. The present paper therefore focused on assessing the main parameters for defining the procedure for adding rubber particles to create an elastic layer under sleepers through a two-step stoneblowing process. Such parameters included the size of the rubber particles; the position of the elastic layer (above or below the conventional small aggregates used in the stoneblowing process); the need to compact the first layer before injecting the subsequent one; the effect of rubber dosage; and the impact of service conditions (traffic levels and temperatures). For this purpose, a number of laboratory tests were carried out on testing boxes simulating the effects of train passages, assessing diverse properties such as particles percolation, section settlement, stiffness and dissipated energy. The results revealed that the inclusion of rubber particles (ranging between 14 and 20 mm) as an elastic layer over the compacted small stones through a two-step stoneblowing process, provides an effective solution to graduate and optimize global track behaviour by using different quantities of rubber, which could reduce the need for track maintenance and its environmental impacts. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Railway Elastic elements Rubber particles End-of-life tyres Stoneblowing
1. Introduction Ballasted tracks for railway transportation have traditionally been the most common type of section used for the infrastructure due to a range of benefits such as lower construction costs, higher damping capacity, and easier maintainability (at a relatively low cost per operation) in comparison with other types of tracks such as concrete slab tracks. However, it is also important to consider the fact that the continuous passage of trains leads to the degradation of the geometrical track quality due to differential ballast settlement and displacement of the sleepers from its original position, which is more marked under conditions of high dynamic overloads (which is a result of increased train speeds required to reduce travel time, along with the higher loading capacity of vehicles) (Selig and pez-Pita, 2006). Therefore, to guarantee the safe Waters, 1994; Lo
* Corresponding author. E-mail addresses: [email protected] (M. Sol-S anchez), [email protected] (F. Morenorez), [email protected] (M.C. Rubio-Ga mez). Navarro), [email protected] (R. Pe https://doi.org/10.1016/j.jclepro.2019.04.122 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
and efficient running of trains, periodic profile corrections are required, where ballast tamping has been traditionally the most popular technique to recover track geometry and damping capacity, among other benefits. Nonetheless, these operations are also associated with the progressive deterioration of ballast layer due to the breakage of particles, while occurring the phenomenon of ballast memory (leading to a quick return of the track to the level previous to maintenance) (Evans, 1992; Selig and Waters, 1994). These interventions for the profile corrections (necessary during the service life of ballasted tracks, but also before its first usage) also have a large weight on the total environmental impacts during the operation phase (which represents up to 87e94% of the life cycle assessment of railway tracks) (Shinde et al., 2018), while leading to considerable costs (10e90 thousands of euros per year and kilometre, depending on line characteristics) (Baumgartner, 2001; UIC, 2008 and 2009), temporary closure of the line, and acceleration of track degradation, among others (Counter, 2015; Pires and Dumont, 2015). Thus, alternative solutions are required to reduce the negative environmental and economic issues related to maintenance of ballasted tracks. In this regard, various solutions are
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currently being developed to reduce the need for maintenance, highlighting from innovative elements such as metallic pads (Carrascal et al., 2018) and Under Sleeper Pads (USPs) made of nchez et al., 2014) to alternative techniques to wastes (Sol-Sa improve geometrical track quality. In particular, the process of stoneblowing (consisting mainly of including aggregates around 14e20 mm into the gap between sleepers and the settled ballast) has shown some advantages in reference to tamping such as reduction of particles breakage while limiting the quick settlement after intervention, among others (Nutbrown and Nicholas, 1999; McMichael and McNaughton, 2003). nchez et al., 2017) has Additionally, a previous study (Sol-Sa demonstrated that the combination of rubber particles from endof-life tyres with the conventional aggregates used during the stoneblowing process could be an appropriate option to reduce some of the main concerns associated with such technique like stiffening of the ballast layer and reduction of the damping capacity, while providing higher dissipation of stress (up to 35% more) and lower breakage of particles (passing from a Ballast Breakage Index-BBI of 1.89%e0.25%), among other benefits. Also, it was proven the potential use of this solution to reduce track settlement after maintenance while allowing for modifying the global track stiffness, recording similar effects to those obtained with standardized Under Sleeper Pads (USPs). Thus, the proposed technique (stoneblowing including rubber particles together with the small stones) could also be an effective alternative to USPs, which can be considered as appropriate to improve track behaviour and durability (mainly in transition sections by using pads with different flexibility) (Dahlberg, 2010; Insa et al., 2011; Wilk et al., 2016), but also presenting some limitations such as the costs of these materials and their limitation to be used in operative tracks (where it would be necessary the replacement of sleepers). Therefore, the main aim of this solution is to reduce the environmental and economic impacts associated with conventional track interventions, by improving the technique of stoneblowing, particularly in transition sections (points with higher probability for differential settlement) where it can also represent an alternative to USPs, reducing then their main negative issues. Nonetheless, so far, the study into this solution was limited to understand the effect of mixing rubber particles with the small stones used during the stoneblowing process. Thus, before its application in real tracks, more in-depth studies are required to establish the optimal process while more information is needed regarding the properties and dosage of the rubber and its effect on the global track section. In this context, the objective of the present paper is to define the procedure of a two-step process to provide an elastic solution under the sleepers through adapting the technique of stoneblowing (consisting of blowing a layer of stones and another of rubber). For this purpose, this study uses a range of laboratory tests to determine the most influential parameters on global track behaviour, analysing factors such as the size of rubber particles, the steps needed for injecting such material (before or after blowing the natural stones, and with or without compaction of stones), and the impact of external conditions. 2. Materials and methods 2.1. Materials For this study, three types of rubber particles were used to analyse their suitability for application as flexible aggregates under the sleepers. All of the rubber materials were obtained by cutting end-of-life tyres into particles within three different size ranges (which correspond to those commonly used by certain companies concerned with the recycling of end-of-life tyres through elastic
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particles): 4e14 mm (referred to in this study as R4); 8e16 mm (R8); 14e20 mm (R14). Fig. 1a shows the granulometric curve for each type of rubber particle as well as their visual appearance (Fig. 1b, c and 1d), presenting a particle density of approximately 1.15 Mg/m3 whilst the bulk density of the particles was close to 0.40 Mg/m3. Further, the small stones (referred to in this study as SS) commonly used during the stoneblowing process (to be partially replaced by the rubber particles) were obtained from ophitic rocks, presenting a Los Angeles coefficient (EN 1097-2) of approximately 7%, indicating that this material has adequate resistance to fragmentation. This material was selected with a size between 14 and 20 mm (Fig. 1a also shows its granulometric curve), which is the size most commonly used during stoneblowing interventions (McMichael and McNaughton, 2003; Sol-S anchez et al., 2016a). The density of the particles (EN 1097-6) was equivalent to 3.29 Mg/m3 and the bulk density was near to 1.60 Mg/m3. To complete the railway section for full-scale tests, a conventional ballast was used (ophitic aggregates described as uniform grade, crushed, and hard stone). According to UNE-EN 13450, the granular material presented appropriate physical (a density of 3.24 Mg/m3, EN 1097-6) and mechanical properties (Los Angeles coefficient below 7.9%, EN 1097-2) for application as ballast in railway tracks while its gradation fit the requirements of the European Standard EN 13450. In addition, a section of pre-tensile concrete sleeper (type DW commonly used in Spanish, with dimensions of 300 mm wide and 360 mm in length for this study) was employed for the superstructure, including a flexible fastening system type VM (with a rail pad of 7 mm thickness, composed of TPEE, and with static stiffness close to 105 kN/mm) and rail section type UIC-54. 2.2. Methods In order to understand the effect of rubber particles as well as the optimal process for their inclusion through the use of stoneblowing during maintenance interventions, the testing plan listed in Table 1 was followed. The aim of this plan was to evaluate the influence of (i) rubber particle size, (ii) position of the elastic layer, (iii) impact of compacting the layer of small stones before the injection of rubber, and (iv) susceptibility to service conditions. 2.2.1. Impact of rubber size Firstly, the effect of the size of rubber particles was analysed to establish the minimum values required to generate the most homogeneous elastic layer by injecting a higher number of particles (in comparison with higher sizes). However, due to the possible percolation of the particles through the ballast voids (which should be limited to avoid fouling of the granular layer), the optimal size range was determined by using two different tests: a vibratory permeameter test (Fig. 2a) and a loading test on a ballast box (Fig. 2b), which simulate the forces and movements acting between the sleeper and ballast (where the rubber particles are to be placed), and quantify the number of particles lost into the ballast layer due to such conditions. In the first case, a square section permeameter (300 mm 300 mm in horizontal dimensions, and 500 mm in height) was filled with a ballast layer of 300 mm thickness, and a concrete block (150 mm 150 mm and 50 mm of height) was placed over it (acting as the sleeper). The size of this first box was selected considering that this should fit a vibratory system used to simulate the vibrations of ballast layer (the possible boundary effects were considered as negligible by evaluating the different solutions under similar conditions for a comparative analysis). For this study, between the ballast and the concrete block, a 2 cm layer
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Fig. 1. Granulometric curves of the rubber and stone particles (a), and the visual appearance of R4 (b), R8 (c) and R14 (d).
Table 1 Testing plan. Study step
Variable
Impact of rubber size
Layer Layer Layer Layer
Tests
Parameters
- Vibratory permeameter - Loading ballast box
- Percolation of particles
Rubber layer position
SS SSþ50R14 SSþ50R8 50R14 þ SS 50R8þSS
- Vibratory permeameter - Testing box
-
Effect of layer compaction
SS SSþ25R SSþ50R SSncþ25R SSncþ50R
- Testing box
- Settlement - Stiffness - Dissipated energy
Impact of service conditions
SS SSþ25R SSþ50R
- Multi condition loading test (temperature and stress)
- Stiffness - Dissipated energy
of of of of
small stones (SS) R14 R8 R4
- Multi-condition loading test (stress and frequency)
was placed with each type of rubber (R14, R8 and R4) in comparison with the conventional small stones. This allowed for simulating the recovery of 20 mm of track settlement by lifting the sleeper with and interposing a layer with each granular material.
Percolation Settlement Stiffness Dissipated energy Stiffness Dissipated energy
The number of particles initially used in each case was measured, and the permeameter was placed over a vibratory system with a frequency of around 50e100 Hz for 2 min (this range of frequency being in consonance with the oscillation of rail-sleeper over ballast
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Fig. 2. Scheme of the vibratory permeameter test (a) and the ballast box-loading test (b) to determine percolation of the particles.
as well as the global rail-sleeper-ballast over substructure) (Ferreira, 2010). After the vibratory process, the particles resting on the surface were collected by using an industrial pump (but avoiding the movement of ballast particles), thereby making it possible to calculate the loss of particles by using the difference between the quantity of particles included before the test and those collected after the test. Similarly, a loading ballast box (with horizontal dimensions of 400 mm 750 mm, and 500 mm of height) was used to evaluate the parameter of particles loss under more realistic conditions (although it must still be considered that the boundary conditions of the box could limit the performance of the ballast layer in comparison with real tracks). This testing box included a ballast layer of 300 mm, a piece of concrete sleeper, a piece of rail UIC-54 and the soft fastening system described in the materials section. Between the ballast and the sleeper a layer of 20 mm of thickness was again placed for each granular solution (SS, R14, R8, R4), and the system was tested by simulating the passage of trains with a level of stress under the sleeper close to 200 kPa and a frequency of 5 Hz (as used in other studies like Indraratna et al., 2006; Bach and Veit, 2013; Ho et al., 2013) for 10,000 cycles. The percolation of particles was then calculated as the difference between the number of particles before and after the loading test. 2.2.2. Definition of rubber layer position A second study step analysed the effect of placing, through a two-step stoneblowing process, the layer of rubber particles above or below the layer of small stones (conventionally used), that is, this step determined whether rubber is to be injected before or after the conventional stones during the stoneblowing process. In a previous study (Sol-S anchez et al., 2017), it was found that the mix of natural stones with rubber (acting as flexible stones) could have a number of beneficial effects on track section performance, improving the effectiveness and durability of the stoneblowing process. However, due to the difference in density between rubber and natural stones, which hinders the inclusion of a mix of both materials at the same time, it was decided to include the rubber as an extra granular layer (placed before or after the stones) to facilitate its addition through a traditional stoneblower machine, but applying a two-step process.
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The critical process is therefore defined as a two-step stoneblowing technique, which has shown to be effective in correcting section geometry (Abrashitov and Semak, 2017). Therefore, in order to select the most appropriate position of the flexible layer, four different solutions were analysed (in the loading box of 400 mm 750 mm x 500 mm) in reference to the conventional solution (Fig. 3b): Two sizes of rubber (R14 and R8, defined as the most appropriate according to results, see section 3.1) for two positions (rubber over small stones or vice versa), placing 50% of rubber per volume of stones (this quantity produces comparable effect to very soft USPs e 0.2 N/mm3, and thus represents the maximum volume of rubber to be used in railway tracks) (Solnchez et al., 2017). These solutions were referred to as Sa SSþ50R14 and SSþ50R8 (when stones were applied prior to the rubber); 50R14 þ SS and 50R8þSS (when the stones were placed over the rubber); and SS, composed of only stones to be used as a control to determine the effect of rubber. To determine the optimal position, a vibratory permeameter test was first carried out to evaluate the loss of rubber depending on whether it was above or below the stones. This test was developed in a similar way to that of the previous step, including the two-layer systems with a total height of 20 mm. Further, to evaluate the effect of each size of rubber particle and position on track behaviour, a laboratory test was carried out over a track section, simulating the passage of trains and including the granular materials as a maintenance intervention, as occurs in real tracks. This test consisted of applying a series of dynamic loads (at 5 Hz and an amplitude close to 200 kPa under sleeper) over a conventional track section (consisting of compacted ballast, sleeper, rail and fastening system) until the settlement was close to 10 mm (around 50,000 cycles). At that point, the test was stopped, the rail-sleeper system was lifted by a laboratory device (until obtaining a sleeper-ballast gap of around 30 mm), and an injector (Fig. 3a) was inserted up to the ballast surface. - A layer of particles was first added (small stones or rubber); - Later, the rail-sleeper was lowered to compact the first layer (applying 50 cycles at 200 kPa); - Next, the rail-sleeper was lifted again; - Then, the second layer of particles was injected, in order to lower the sleeper to the correct position over the layer of particles; - Finally, before continuing with the dynamic test, the two-layer system was compacted by applying 50 cycles at 200 kPa. Following the maintenance process, the dynamic test was continued up to 200,000 cycles, assessing the change in position of the sleeper after maintenance (profile correction), and the modification of track performance (stiffness and capacity to dissipate energy). The number of total loading cycles was selected in consonance with previous studies where it was seen that this number was enough to obtain stable performance of the section, and then, being possible to analyse the influence of each solution under similar conditions. This process was carried out twice for each solution, injecting particles from two corners (in a diagonal position) of the sleeper. Additionally, for a more in-depth analysis of the impact of each solution on track performance under various loading conditions, multi-condition tests were carried out for each section. These tests consisted of applying a series of 1000 load cycles of varying amplitude (corresponding to 125 kPa, 190 kPa, and 250 kPa of stress under sleeper) and frequency (1 Hz, 5 Hz, and 10 Hz), which are loads that commonly occur in real service conditions (Indraratna et al., 2006; Bach and Veit, 2013; Ho et al., 2013).
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Fig. 3. Scheme of test procedure and injection of particles (a); configurations analysed (b).
2.2.3. Effect of compaction of the layers Having defined the most appropriate rubber size range and the position of this material in reference to traditional stone layers, the impact of compacting the first injected layer was evaluated (with rubber or stones), before the inclusion of the other component. This was analysed to prove whether this parameter is essential for obtaining a more stable behaviour and a greater capacity to modify the performance of the track by using the rubber to graduate parameters such as global stiffness. For this study step, five different solutions were assessed: (i) use of only stones, as the reference case (SS); (ii - iii) stones þ 25% or 50% of rubber over non-compacted stones (SSncþ25R and SSncþ50R); and (iv - v) 25% or 50% of rubber over the layer of compacted stones (SSþ25R and SSþ50R), the compaction being carried out by applying 50 loading cycles (at 5 Hz and a stress under sleeper of around 200 kPa). Thus, the effect of compacting the stones before the application of rubber (optimal position of rubber according to results, see section 3.2) was evaluated for two different quantities of this material. The test consisted of simulating train traffic over a conventional section until the settlement reached a value of around 10 mm. The sleeper-rail was then lifted (creating a gap of around 30 mm) to firstly inject the layer of stones. The aggregates were then compacted (when necessary for the solution) by applying the cited loads, after which the sleeper-rail was again lifted to inject the rubber. Finally, the test was continued until reaching 150,000 cycles. Thus, the change in section performance depending on the distribution of rubber was monitored, measuring parameters such as stiffness, dissipated energy, and settlement. 2.2.4. Impact of service conditions: temperature and load level Due to the rheological behaviour of rubber, the global track response was assessed in a ballast box at three room temperatures (20 C, 40 C, and 60 C as environmental temperatures), two loading conditions (125 KPa and 250 kPa), and for two different quantities of rubber (a layer with 50% of rubber over a compacted layer of stones - SSþ50R, and a layer of 25% of rubber over compacted stones e SSþ25R), in reference to the conventional solution with only small stones. The tests consisted of firstly applying dynamic loads until ballast settlement was around 10 mm, and later including each maintenance solution (two with rubber and one with only aggregates) at 20 C. Two dynamic tests were then conducted at stress amplitudes of 125 kPa and 250 kPa, both at 5 Hz, for
each solution. To increase the temperature to 40 C, and later to 60 C, infrared lamps were placed at different heights over the ballast surface. Temperature evolution on the ballast surface and in the particles under the sleeper (where rubber was placed) was monitored by thermocouples and an infrared camera. Once the temperature of the ballast and rubber was stable, the tests were carried out for each solution to measure its susceptibility to changes in temperature by assessing the effect on global track stiffness and dissipated energy values.
3. Analysis of results 3.1. Influence of rubber size Fig. 4 displays the influence of rubber size (14/20 mm, 8/16 mm, and 4/14 mm, represented as R14, R8, and R4 respectively) on the percolation of particles into the ballast layer (loss of particles from the surface), during the vibratory permeameter test (Fig. 4a) and the simulation of loads in a testing box (Fig. 4b), in reference to the conventional solution with only aggregates (SS). The results show that the placement of particles on the surface of the ballast already leads to the percolation of particles through ballast aggregates (before any vibratory effect, Fig. 4a), and regardless of the type of material. This effect is more marked with smaller sized rubber particles (with a particle loss of around 10e20 increasing to more than 100 when using rubber size 4/14). Further, the results show that the percolation of particles was more accentuated following the vibration process, enhancing the increase ratio due to vibration, regardless of the material type, but particularly in the case of rubber with a particle size of between 4/ 14 mm (increasing from 120 to higher than 500). The other two sizes (R14 and R8) presented particle loss values comparable to the reference SS (close to 90e120), or even reducing this parameter as in the case of load simulations (Fig. 4b). These results thus indicate that particles of the latter sizes could be used as elastic particles during the stoneblowing process without increasing the loss of particles through the ballast, which is in consonance with the results of other authors that have defined the size range for small stones (Selig, 1999; Fair, 2003). Then, these last sizes of rubber (R8 and R14) were used for the rest of the study as they shown to be the most appropriate.
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Fig. 4. Results of particle percolation measured in the vibratory test (a) and loading test (b).
3.2. Effect of rubber layer position In order to determine the most appropriate position for the elastic layer, Fig. 5 firstly represents the loss of particles for each system with two layers (small stones þ rubber, or vice versa), during the vibratory test. The results show that, regardless of the rubber size e R14 or R8 (which, on the basis of previous results were shown to be appropriate for application), the placement of rubber before the stones (thus, the rubber placed below the layer of stones eR þ SS) generally led to higher level of particle loss, particularly due to rubber loss. This effect was more marked in the case of the smaller rubber particles (increasing by around 15%), which showed even slightly higher total values than in the case in which only small stones (SS) were used, and therefore being less appropriate for application than the 14/20 mm rubber. Fig. 6 represents the effect of the position of the rubber layer (assessed for two sizes of rubber, R8 and R14) on the ability of this solution to modify the mechanical performance of the track section, representing the settlement at 150,000 cycles from maintenance intervention and the average value of stiffness modulus at that point. This last parameter was calculated as stiffness (for this study measured as N/mm) by area of sleeper section (in mm2) to limit the influence of the sleeper size used in the test (Fig. 6a). The average density of dissipated energy is also shown in Fig. 6b, which was calculated as the loss of energy per loading cycle (obtained from the area of hysteresis loops) divided by the volume of the layer of ballast used in this study, limiting then the effect of the size of this device.
Fig. 5. Results of particle loss for different two layer systems under the effect of vibration.
The results indicate that the inclusion of small stones (SS) could lead to a stiffening of the track section (higher values of modulus than those observed pre-maintenance) and loss of damping capacity (lower values than those observed pre-maintenance), which is in consonance with other studies (Anderson and Fair, 2008). However, the inclusion of rubber allowed for decreasing the stiffness of track, these changes being higher when the rubber was placed over the small stones (i.e. injected after placing the natural stones e solutions SS þ R), which also led in this last case to a higher capacity to dissipate energy from loads.
Fig. 6. Effect of rubber position and size on global track performance.
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In particular, the highest changes in track performance occurred when using the biggest size of rubber (R14), obtaining a reduction in track stiffness of around 50%. This finding is in line with the nchez et al., 2017), thus reinresults of a previous study (Sol-Sa forcing the idea that the use of this solution (50% of rubber 14/ 20 mm over stones) could lead to an effect that is comparable to that observed when using a conventional soft USP (with bedding modulus around 0.2 N/mm3), which is also in consonance with other authors (Jayasuriya et al., 2019). Then, this solution would allow for grading the behaviour of the track by reducing the quantity of rubber employed (and thus optimizing track performance). Moreover, it was observed that the use of smaller sized rubber particles (also over the stones, solution SSþ50R8) led to lower modification of section stiffness and capacity to dissipate energy, presenting a similar effect to that observed when using medium USP (bedding modulus around 0.4 N/mm3), and therefore confirming the influence of the size of the rubber particles. These results are in accord with those of other studies (Abrashitov and Semak, 2017) showing that a two-step process (in this case, firstly injecting stones and later rubber) is effective in modifying track performance. However, the other cases studied (the rubber placed under the stones e solutions R þ SS) resulted in a lower capacity to change behaviour, and therefore a lower ability to optimize track performance. This could be linked to a less homogeneous distribution of rubber and the (partial) percolation of the particles, thereby obtaining a poorer elastic layer in comparison with the case of using rubber over stones (particularly R14), which appeared to be the most effective solution. In fact, it was observed that the solutions with rubber over the stones allowed for reducing the track settlement (up to 30% when using R14, in reference to SS), obtaining a more positive effect when reducing the stiffness of the section and increasing its capacity to dissipate energy, which is related to the elastic behaviour of rubber, nchez et al., 2016a; as seen in other studies using USPs (Sol-Sa Jayasuriya et al., 2019). However, when the rubber was injected before the stones, higher settlement values were recorded, even in comparison with the conventional solution without rubber (SS). This negative effect was more marked when reducing the flexibility of the solution, which indicates the heterogeneous distribution of rubber, and therefore leading to greater recompaction of the section (this effect being more pronounced when using the larger rubber particles due to a poorer distribution when it is injected
before placement of the stones). This is demonstrated in Fig. 7, which represents the settlement at different steps of the section behaviour after each maintenance solution (with various rubber sizes and positions in reference to the conventional section without stones). Additionally, Fig. 7 shows the ratio of settlement when the system behaviour was stable (from 100,000 cycles), aiming to evaluate the long-term performance of the solutions. This figure shows that just after the interventions (only 200 cycles), the values of recovered settlement were lower than 100% due to the recompaction of particles despite the fact that after the maintenance intervention the layers were compacted (50 cycles at 200 kPa and 5 Hz) before continuing the test, which is in consonance with other studies (McDowell and Li, 2016; Ngo et al., 2017). Nonetheless, it was seen that this phenomenon was limited by using rubber over the stones (injected after the small stones), which allowed for the greatest reductions in track settlement, regardless of the stage of measurement (at both the short and longterm). Particularly, it was observed that these solutions led to an increase in the capacity to reduce settlement by around 30% in reference to the case of conventional stoneblowing, which has been shown to allow for increasing track durability (Cope and Ellis, 2001; Abrashitov and Semak, 2017). Therefore, the use of rubber over stones could limit the phenomenon of settlement due to irregularities associated with material recompaction following the process of stoneblowing (Ball, 2003). On the contrary, it should be considered that in the cases in which rubber is placed under the stones, a lower reduction in track settlement was obtained than in the case of the conventional solution (SS), whilst a higher tendency towards settlement also was observed. This confirms that placing the rubber over the stones could be regarded as a more appropriate solution (and then, this solution was used to continue with the study), particularly when using R14 due to its higher flexibility and capacity to dissipate stresses. Nonetheless, for a more in-depth understanding of the effect of rubber on track behaviour, and the influence of the position of the elastic layer, Fig. 8 shows the variation in stiffness and dissipated energy when modifying the level of stress (Fig. 8a) and the loading frequency (Fig. 8b). The results show that, regardless of the loading condition, the use of rubber over stones (solutions SSþ50R14 and SSþ50R8) is more effective in modifying the response of the track by reducing track stiffness and increasing the dissipation of energy
Fig. 7. Impact of rubber position and size on the capacity to reduce the section settlement after maintenance interventions.
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Fig. 8. Influence of load level and frequency depending on rubber position and size.
from the reference (SS), particularly with R14. Further, it is observed that for all solutions, the increase in load level leads to a stiffening of the track and an increase in dissipated energy (since higher hysteresis loops were recorded) while the change in frequency had a weaker effect on track flexibility. Nonetheless, the trend towards change (in the case of systems using rubber) was quite similar to the reference, which indicates comparable susceptibility to variations in track behaviour under various loading conditions, which is also in consonance with other studies using elastic elements under sleepers (Jayasuriya et al., 2019). 3.3. Effect of compacting the layer of stones before applying the rubber Fig. 9 represents the changes in track behaviour (relationships between settlement and stiffness or dissipated energy, Fig. 9a and b, respectively) when including rubber over the stones previously blown in the first step of the stoneblowing process. This figure analyses the influence of compacting (or not) the small stones before blowing the rubber (solutions SS þ R and SSnc þ R, respectively), this study being carried out for two quantities of rubber (25% and 50% over the volume of stones). The type of rubber corresponded to R14 since in previous steps this was shown to be more appropriate for using as flexible aggregates during the stoneblowing process, while the position over the stones also shown to be the optimal one. Firstly, the results confirmed that, regardless of the quantity of rubber and the state of the SS layer, the inclusion of rubber allowed
for reducing settlement in comparison with the conventional process of stoneblowing with only aggregates. This was particularly marked when obtaining a higher reduction in stiffness and an increase in the capacity to dissipate energy, showing a relationship between superstructure behaviour and the tendency towards setnchez et al., tlement, as established in previous studies (Sol-Sa 2016b). Therefore, this indicates that the higher the effect of the rubber, the higher the reduction in track deformation. The results also show that a relatively low volume of rubber (25% over stones) presented little effect (or even a negative effect) in terms of modifying track performance when applied over noncompacted stones (SSnc). This is in accord with the results of other studies (Abrashitov and Semak, 2017). However, the case in which the rubber is placed over compacted aggregates did lead to a positive impact in terms of reducing track stiffness and increasing its damping capacity (and thus decreasing settlement), which could be linked to a more homogeneous distribution of rubber since there is higher free gap between compacted stones and the bottom of the sleeper to facilitate the inclusion of rubber. This effect is particularly marked when increasing the volume of rubber (50%) to be injected after stones, obtaining a limited effect with non-compacted stones, while its use over the compacted small stones produced a significant variation in track performance and reduction in settlement, which is in consonance with similar nchez et al., measurements recorded in a previous study (Sol-Sa 2017) where the rubber was homogenously placed by hand over the stones. This indicates a good distribution of the flexible particles when the stones are compacted previously, presenting a
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Fig. 10. Impact of temperature on track performance with different quantities of rubber under various load levels. Fig. 9. Effect of compacting stones prior to rubber injection on changes in track performance.
similar effect to that observed when using soft (0.2 N/mm3) and medium (0.4 N/mm3) USPs for the cases with 50% and 25% of nchez et al., 2016a). rubber, respectively (Sol-Sa 3.4. Impact of service conditions Fig. 10 shows the changes in track performance when modifying the test temperature, for two loading levels (125 kPa and 250 kPa) and two quantities (25% and 50% over the volume of stones) of rubber R14 over compacted stones. It is clear that in spite of fixing the external testing temperature at 20 C, 40 C, and 60 C, the temperature recorded under the sleeper (where the rubber is placed) was around 20 C, 27 C, and 37 C since the sleeper serves to protect the particles below. This can be observed in Fig. 11, which displays the temperature on the ballast surface (higher than 60 C in this example) and particles under the sleeper (around 37 C), which was recorded by removing the sleeper just after the test and using an infrared camera to verify the change in temperature in reference to ballast. Regarding the effect of temperature on section behaviour, the results indicate that the increase in temperature led to a slight decrease in stiffness and an increase in dissipated energy in the cases in which only stones were used (SS), regardless of the level of stress. This could be a consequence of the softening of the rail pads (in spite of the fact that the rail-sleeper system was partially isolated from temperature changes). However, for the solutions using rubber, the stiffness of the entire section increased slightly, regardless of the quantity of rubber and level of load. This fact could be a consequence of the softening of the rubber when increasing the temperature, and then, the weight of the sleeper and rail led to higher static deformation of this material, reducing its ability to
Fig. 11. Visual appearance of the temperature on the ballast surface and particles under the sleeper.
deform when applying the dynamic loads, and thus recording slightly higher stiffness values while obtaining a more remarkable effect on the capacity to dissipate energy. Nonetheless, it should be noted that the changes in track behaviour at higher temperatures were less marked than those measured when modifying other factors such as, for instance, stress level or rubber quantity. In these last cases, it was obtained again a strong decrease in stiffness and increase in dissipation of energy when rising the dosage of rubber (particularly at medium temperatures due to a lower softening of rubber) while the increase in load level led to higher variations in track performance and higher
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susceptibility to changes. Thus, this analysis indicates moderate susceptibility of this solution to temperature in reference to other parameters, which should be related to the fact that the sleeper protects the rubber from changes in temperature. 4. Conclusions The focus of the present paper was to contribute towards the development of an alternative maintenance process that aims to reduce the negative impacts associated with profile operations to conserve track geometry. For this purpose, the study focused on analysing some of the main factors involved in the application of rubber particles (derived from end-of-life tyres) as flexible aggregates during a two-step stoneblowing technique. The chief aim was to search for a way of optimizing the mechanical performance and durability of existing tracks where the inclusion of standard elastic elements such as USPs is expensive and technically challenging. On basis of the results obtained in the present work, the following conclusions can be drawn: - Rubber particles above 8 mm appear to be suitable for use as flexible aggregates under the sleeper without increasing the fouling of ballast due to the percolation of fine particles. Nonetheless, the results indicate that particles within the range of 14e20 mm showed the lowest values of percolation, and are thus the most appropriate. - Injecting the rubber particles over a layer of small stones (routinely used in the stoneblowing process) appears to be the most effective process for modifying the mechanical performance of the track section, and therefore the behaviour of the track can be optimized according to line and traffic characteristics. - The above is more marked when using rubber particles with a size between 14 and 20 mm, with the latter also leading to a greater reduction in section settlement in both the short and long-term, yielding more positive results in comparison with the case in which only stones were used (i.e. the traditional stoneblowing process). - The results confirmed that the compaction of small stones (injected by the stoneblowing technique) prior to the inclusion of rubber emerged as a key factor for improving track performance and durability, particularly where a high volume of particles is required. - Therefore, a track intervention that includes rubber particles as flexible aggregates could consist of a two-step process where the traditional stones are firstly injected and compacted, after which rubber is injected at a volume that depends on track requirements. - The results indicate that an increase in temperature and load level leads to track stiffening and a reduction in damping capacity, with stress level being the most influential factor. Nonetheless, it is important to note that the pattern of changes in section behaviour is comparable to that obtained for the conventional stones only solution. The present findings therefore suggest that the stoneblowing process could be used to include rubber particles under the sleeper in existing lines by adding an extra layer to traditionally compacted small stones, which allows for the possible gradation of the track and improvement of its behaviour and durability. This would allow for important environmental and economic benefits associated with reducing the need for periodic maintenance. Nonetheless, further studies are still required to completely understand this alternative technique for improving railway track performance, such as the effect of rubber on lateral resistance, factors defining
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the process of automatic stoneblowers, and the volume of particles to be injected depending on settlement level. References Abrashitov, A., Semak, A., 2017. Experimental study of stoneblowing track surfacing technique. Procedia Eng. 189, 75e79. Anderson, W.F., Fair, P., 2008. Behavior of railroad ballast under monotonic and cyclic loading. J. Geotech. Geoenviron. Eng. 143 (3), 316e327. Bach, H., Veit, P., 2013. Evaluation of Attrition Tests for Railway Ballast. PhD Dissertation. Graz University of Technology, Austria. Ball, M., 2003. Railway Ballast Maintenance. Final Year Project. School of Science and the Environment, Coventry University, 2003. Baumgartner, J.P., 2001. Prices and Costs in the Railway Sector. Laboratoire des Transports Et de Planification. Ecole d'Intermodalite Polytechnique Federale de Lausanne. ~ o, D., Martín, J.J., Carrascal, I., Perez, A., Casado, J.A., Diego, S., Polanco, J.A., Ferren 2018. Experimental study of metal cushion pads for high speed railways. Constr. Build. Mater. 182, 273e283. Cope, D.L., Ellis, J.B., 2001. British Railway Track e Design Construction and Maintenance. The Permanent Way Institution, Nottingham. Counter, B., 2015. Maintenance of Ballasted Track Systems in the UK the Challenges of Whole Life Asset Management. Presented at Railway Engineering Conference, Edinburgh. Dahlberg, T., 2010. Railway track stiffness variations-consequences and countermeasures. Int. J. Civ. Eng. 8 (1), 1e12. EN 1097-2, 2010. Tests for Mechanical and Physical Properties of Aggregates - Part 2: Methods for the Determination of Resistance to Fragmentation. AENOR, n Espan ~ ola de certificacio n y Normalizacio n, Madrid, Spain. Asociacio EN 1097-6, 2006. Tests for Mechanical and Physical Properties of Aggregates - Part n 6: Determination of Particles Density and Water Absorption. AENOR, Asociacio ~ ola de certificacio n y Normalizacio n, Madrid, Spain. Espan n Espan ~ ola de EN 13450, 2003. Aggregates for Railway Ballast. AENOR, Asociacio n Normalizacio n, Madrid, Spain. Certificacio Evans, E., 1992. The design of railway track foundations. In: Presented at 4th RIA Track Sector Course. Nottingham. 1; Section 3: Design; Paper 4.3.3. Fair, P., 2003. The Geotechnical Behaviour of Ballast Materials for Railway Maintenance, PhD Dissertation. University of Sheffield. cnica de Lisboa. In: Modelling Ferreira, P.A., 2010. Ph.D. dissertation. Universidade Te and Prediction of the Dynamic Behavior of Railway Infrastructures at Very High cnico, Portugal. Speed. Instituto Superior Te Ho, C.L., Humphrey, D., Hyslip, J., Moorhead, W., 2013. Use of Recycled Tire Rubber to Modify Track Substructure Interaction. Transportation Research Board, Annual meeting, Washington D.C., U.S.A. Indraratna, B., Khabbaz, H., Salim, W., Christie, D., 2006. Geotechnical properties of ballast and the role of geosynthetics. J. Gr. Improv. 19, 91e102. Insa, R., Salvador, P., Inarejos, J., Medina, L., 2011. Analysis of the influence of under sleeper pads on the railway vehicle/track dynamic interaction in transition zones. J. Rail Rapid Transit. https://doi.org/10.1177/0954409711430174. Jayasuriya, C., Indraratna, B., Ngo, T.N., 2019. Experimental study to examine the role of under sleeper pads for improved performance of ballast under cyclic loading. Transport. Geotech. 19, 61e73. pez-Pita, A., 2006. Infraestructuras Ferroviarias. Edicions UPC, Universidad PoliLo ~ a, Barcelona. tecnica de Catalun McDowell, G.R., Li, H., 2016. Discrete element modelling of scaled railway ballast under triaxial conditions. Granul. Matter 18 (3), 66. McMichael, P., McNaughton, A., 2003. The stoneblower e delivering the promise. Development, testing and operation of a new track maintenance system. In: 82sd Annual Meeting, Transortation Research Board, Washington, U.S.A, CDROM. Ngo, N.T., Indraratna, B., Rujikiatkamjorn, C., 2017. Simulation ballasted track behavior: numerical treatment and field application. ASCE Int. J. Geomech. 17 (6), 04016130. Nutbrown, D., Nicholas, R., 1999. Stoneblower Experiences. PWI Autumn Technical Seminar. Pires, J., Dumont, A.G., 2015. Railway ballast degradation. In: 15th Swiss Transport Research Conference. Monte Verti a/Ascona april 15-17. Selig, E.T., 1999. Stoneblowing Principles. University of Massachusetts. Selig, E.T., Waters, J.M., 1994. Track Geotechnology and Substructure Management. Thomas Telford, London. Shinde, A.M., Dikshit, A.K., Slingh, R.K., Campana, P.E., 2018. Life cycle analysis based comprehensive environmental performance evaluation of Mumbai Suburban Railway, India. J. Clean. Prod. 188, 989e1003. Sol-S anchez, M., Moreno-Navarro, F., Rubio-G amez, M.C., 2017. An alternative sustainable railway maintenance technique based on the use of rubber particles. J. Clean. Prod. 142, 3850e3858. mez, M.C., 2014. Viability of using Sol-S anchez, M., Moreno-Navarro, F., Rubio-Ga end-of-life tire pads as under sleeper pads in railway. Constr. Build. Mater. 64, 150e156. Sol-S anchez, M., Moreno-Navarro, F., Rubio-G amez, M.C., 2016a. Analysis of ballast tamping and Stone-blowing processes on railway track behaviour: the influence of using USPs. Geotechnique 129. https://doi.org/10.1680/jgeot.15. ISSN 0016-8505.
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nchez, M., Moreno-Navarro, F., Rubio-Ga mez, M.C., 2016b. A study into the Sol-Sa mechanical performance of different configurations for the railway track section: a laboratory approach. Eng. Struct. 119, 23e33. UIC e Union Internationale des Chemins de fer, 2008. High Speed Rail e Fast Track to Sustainable Mobility. ISBN 2-7461-1452-6. UIC, 2009. Railway applications/Track - Concrete Sleepers and Bearers - Concrete
Sleepers and Bearers with under Sleeper Pads. CEN-document TC 256 WI 00256597. Wilk, S.T., Stark, T.D., Moorhead, W., 2016. Under-tie Pads to Improve Track Resiliency in Railrod Transition Zones. Transportation Research Board 96th Annual Meeting, Washington, D.C., USA.
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Defining the process of including sustainable rubber particles under sleepers to improve track behaviour and performance nchez a, *, F. Moreno-Navarro a, R. Pe rez b, M.C. Rubio-Ga mez a M. Sol-Sa a b
Laboratory of Construction Engineering at the University of Granada, C/ Severo Ochoa s/n, 18071, Granada, Spain Signus, C/Caleruega, 102, 5º, 28033, Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2019 Received in revised form 23 March 2019 Accepted 11 April 2019 Available online 17 April 2019
The inclusion of rubber particles derived from end-of-life tyres (ELTs) under the sleepers has been demonstrated in previous studies to be a potential solution to improve the behaviour and durability of railway tracks, and then, minimizing the negative impacts associated with the need for maintenance. Nonetheless, more in-depth studies are still required to define the optimal procedure prior to its application in real tracks. The present paper therefore focused on assessing the main parameters for defining the procedure for adding rubber particles to create an elastic layer under sleepers through a two-step stoneblowing process. Such parameters included the size of the rubber particles; the position of the elastic layer (above or below the conventional small aggregates used in the stoneblowing process); the need to compact the first layer before injecting the subsequent one; the effect of rubber dosage; and the impact of service conditions (traffic levels and temperatures). For this purpose, a number of laboratory tests were carried out on testing boxes simulating the effects of train passages, assessing diverse properties such as particles percolation, section settlement, stiffness and dissipated energy. The results revealed that the inclusion of rubber particles (ranging between 14 and 20 mm) as an elastic layer over the compacted small stones through a two-step stoneblowing process, provides an effective solution to graduate and optimize global track behaviour by using different quantities of rubber, which could reduce the need for track maintenance and its environmental impacts. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Railway Elastic elements Rubber particles End-of-life tyres Stoneblowing
1. Introduction Ballasted tracks for railway transportation have traditionally been the most common type of section used for the infrastructure due to a range of benefits such as lower construction costs, higher damping capacity, and easier maintainability (at a relatively low cost per operation) in comparison with other types of tracks such as concrete slab tracks. However, it is also important to consider the fact that the continuous passage of trains leads to the degradation of the geometrical track quality due to differential ballast settlement and displacement of the sleepers from its original position, which is more marked under conditions of high dynamic overloads (which is a result of increased train speeds required to reduce travel time, along with the higher loading capacity of vehicles) (Selig and pez-Pita, 2006). Therefore, to guarantee the safe Waters, 1994; Lo
* Corresponding author. E-mail addresses: [email protected] (M. Sol-S anchez), [email protected] (F. Morenorez), [email protected] (M.C. Rubio-Ga mez). Navarro), [email protected] (R. Pe https://doi.org/10.1016/j.jclepro.2019.04.122 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
and efficient running of trains, periodic profile corrections are required, where ballast tamping has been traditionally the most popular technique to recover track geometry and damping capacity, among other benefits. Nonetheless, these operations are also associated with the progressive deterioration of ballast layer due to the breakage of particles, while occurring the phenomenon of ballast memory (leading to a quick return of the track to the level previous to maintenance) (Evans, 1992; Selig and Waters, 1994). These interventions for the profile corrections (necessary during the service life of ballasted tracks, but also before its first usage) also have a large weight on the total environmental impacts during the operation phase (which represents up to 87e94% of the life cycle assessment of railway tracks) (Shinde et al., 2018), while leading to considerable costs (10e90 thousands of euros per year and kilometre, depending on line characteristics) (Baumgartner, 2001; UIC, 2008 and 2009), temporary closure of the line, and acceleration of track degradation, among others (Counter, 2015; Pires and Dumont, 2015). Thus, alternative solutions are required to reduce the negative environmental and economic issues related to maintenance of ballasted tracks. In this regard, various solutions are
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currently being developed to reduce the need for maintenance, highlighting from innovative elements such as metallic pads (Carrascal et al., 2018) and Under Sleeper Pads (USPs) made of nchez et al., 2014) to alternative techniques to wastes (Sol-Sa improve geometrical track quality. In particular, the process of stoneblowing (consisting mainly of including aggregates around 14e20 mm into the gap between sleepers and the settled ballast) has shown some advantages in reference to tamping such as reduction of particles breakage while limiting the quick settlement after intervention, among others (Nutbrown and Nicholas, 1999; McMichael and McNaughton, 2003). nchez et al., 2017) has Additionally, a previous study (Sol-Sa demonstrated that the combination of rubber particles from endof-life tyres with the conventional aggregates used during the stoneblowing process could be an appropriate option to reduce some of the main concerns associated with such technique like stiffening of the ballast layer and reduction of the damping capacity, while providing higher dissipation of stress (up to 35% more) and lower breakage of particles (passing from a Ballast Breakage Index-BBI of 1.89%e0.25%), among other benefits. Also, it was proven the potential use of this solution to reduce track settlement after maintenance while allowing for modifying the global track stiffness, recording similar effects to those obtained with standardized Under Sleeper Pads (USPs). Thus, the proposed technique (stoneblowing including rubber particles together with the small stones) could also be an effective alternative to USPs, which can be considered as appropriate to improve track behaviour and durability (mainly in transition sections by using pads with different flexibility) (Dahlberg, 2010; Insa et al., 2011; Wilk et al., 2016), but also presenting some limitations such as the costs of these materials and their limitation to be used in operative tracks (where it would be necessary the replacement of sleepers). Therefore, the main aim of this solution is to reduce the environmental and economic impacts associated with conventional track interventions, by improving the technique of stoneblowing, particularly in transition sections (points with higher probability for differential settlement) where it can also represent an alternative to USPs, reducing then their main negative issues. Nonetheless, so far, the study into this solution was limited to understand the effect of mixing rubber particles with the small stones used during the stoneblowing process. Thus, before its application in real tracks, more in-depth studies are required to establish the optimal process while more information is needed regarding the properties and dosage of the rubber and its effect on the global track section. In this context, the objective of the present paper is to define the procedure of a two-step process to provide an elastic solution under the sleepers through adapting the technique of stoneblowing (consisting of blowing a layer of stones and another of rubber). For this purpose, this study uses a range of laboratory tests to determine the most influential parameters on global track behaviour, analysing factors such as the size of rubber particles, the steps needed for injecting such material (before or after blowing the natural stones, and with or without compaction of stones), and the impact of external conditions. 2. Materials and methods 2.1. Materials For this study, three types of rubber particles were used to analyse their suitability for application as flexible aggregates under the sleepers. All of the rubber materials were obtained by cutting end-of-life tyres into particles within three different size ranges (which correspond to those commonly used by certain companies concerned with the recycling of end-of-life tyres through elastic
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particles): 4e14 mm (referred to in this study as R4); 8e16 mm (R8); 14e20 mm (R14). Fig. 1a shows the granulometric curve for each type of rubber particle as well as their visual appearance (Fig. 1b, c and 1d), presenting a particle density of approximately 1.15 Mg/m3 whilst the bulk density of the particles was close to 0.40 Mg/m3. Further, the small stones (referred to in this study as SS) commonly used during the stoneblowing process (to be partially replaced by the rubber particles) were obtained from ophitic rocks, presenting a Los Angeles coefficient (EN 1097-2) of approximately 7%, indicating that this material has adequate resistance to fragmentation. This material was selected with a size between 14 and 20 mm (Fig. 1a also shows its granulometric curve), which is the size most commonly used during stoneblowing interventions (McMichael and McNaughton, 2003; Sol-S anchez et al., 2016a). The density of the particles (EN 1097-6) was equivalent to 3.29 Mg/m3 and the bulk density was near to 1.60 Mg/m3. To complete the railway section for full-scale tests, a conventional ballast was used (ophitic aggregates described as uniform grade, crushed, and hard stone). According to UNE-EN 13450, the granular material presented appropriate physical (a density of 3.24 Mg/m3, EN 1097-6) and mechanical properties (Los Angeles coefficient below 7.9%, EN 1097-2) for application as ballast in railway tracks while its gradation fit the requirements of the European Standard EN 13450. In addition, a section of pre-tensile concrete sleeper (type DW commonly used in Spanish, with dimensions of 300 mm wide and 360 mm in length for this study) was employed for the superstructure, including a flexible fastening system type VM (with a rail pad of 7 mm thickness, composed of TPEE, and with static stiffness close to 105 kN/mm) and rail section type UIC-54. 2.2. Methods In order to understand the effect of rubber particles as well as the optimal process for their inclusion through the use of stoneblowing during maintenance interventions, the testing plan listed in Table 1 was followed. The aim of this plan was to evaluate the influence of (i) rubber particle size, (ii) position of the elastic layer, (iii) impact of compacting the layer of small stones before the injection of rubber, and (iv) susceptibility to service conditions. 2.2.1. Impact of rubber size Firstly, the effect of the size of rubber particles was analysed to establish the minimum values required to generate the most homogeneous elastic layer by injecting a higher number of particles (in comparison with higher sizes). However, due to the possible percolation of the particles through the ballast voids (which should be limited to avoid fouling of the granular layer), the optimal size range was determined by using two different tests: a vibratory permeameter test (Fig. 2a) and a loading test on a ballast box (Fig. 2b), which simulate the forces and movements acting between the sleeper and ballast (where the rubber particles are to be placed), and quantify the number of particles lost into the ballast layer due to such conditions. In the first case, a square section permeameter (300 mm 300 mm in horizontal dimensions, and 500 mm in height) was filled with a ballast layer of 300 mm thickness, and a concrete block (150 mm 150 mm and 50 mm of height) was placed over it (acting as the sleeper). The size of this first box was selected considering that this should fit a vibratory system used to simulate the vibrations of ballast layer (the possible boundary effects were considered as negligible by evaluating the different solutions under similar conditions for a comparative analysis). For this study, between the ballast and the concrete block, a 2 cm layer
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Fig. 1. Granulometric curves of the rubber and stone particles (a), and the visual appearance of R4 (b), R8 (c) and R14 (d).
Table 1 Testing plan. Study step
Variable
Impact of rubber size
Layer Layer Layer Layer
Tests
Parameters
- Vibratory permeameter - Loading ballast box
- Percolation of particles
Rubber layer position
SS SSþ50R14 SSþ50R8 50R14 þ SS 50R8þSS
- Vibratory permeameter - Testing box
-
Effect of layer compaction
SS SSþ25R SSþ50R SSncþ25R SSncþ50R
- Testing box
- Settlement - Stiffness - Dissipated energy
Impact of service conditions
SS SSþ25R SSþ50R
- Multi condition loading test (temperature and stress)
- Stiffness - Dissipated energy
of of of of
small stones (SS) R14 R8 R4
- Multi-condition loading test (stress and frequency)
was placed with each type of rubber (R14, R8 and R4) in comparison with the conventional small stones. This allowed for simulating the recovery of 20 mm of track settlement by lifting the sleeper with and interposing a layer with each granular material.
Percolation Settlement Stiffness Dissipated energy Stiffness Dissipated energy
The number of particles initially used in each case was measured, and the permeameter was placed over a vibratory system with a frequency of around 50e100 Hz for 2 min (this range of frequency being in consonance with the oscillation of rail-sleeper over ballast
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Fig. 2. Scheme of the vibratory permeameter test (a) and the ballast box-loading test (b) to determine percolation of the particles.
as well as the global rail-sleeper-ballast over substructure) (Ferreira, 2010). After the vibratory process, the particles resting on the surface were collected by using an industrial pump (but avoiding the movement of ballast particles), thereby making it possible to calculate the loss of particles by using the difference between the quantity of particles included before the test and those collected after the test. Similarly, a loading ballast box (with horizontal dimensions of 400 mm 750 mm, and 500 mm of height) was used to evaluate the parameter of particles loss under more realistic conditions (although it must still be considered that the boundary conditions of the box could limit the performance of the ballast layer in comparison with real tracks). This testing box included a ballast layer of 300 mm, a piece of concrete sleeper, a piece of rail UIC-54 and the soft fastening system described in the materials section. Between the ballast and the sleeper a layer of 20 mm of thickness was again placed for each granular solution (SS, R14, R8, R4), and the system was tested by simulating the passage of trains with a level of stress under the sleeper close to 200 kPa and a frequency of 5 Hz (as used in other studies like Indraratna et al., 2006; Bach and Veit, 2013; Ho et al., 2013) for 10,000 cycles. The percolation of particles was then calculated as the difference between the number of particles before and after the loading test. 2.2.2. Definition of rubber layer position A second study step analysed the effect of placing, through a two-step stoneblowing process, the layer of rubber particles above or below the layer of small stones (conventionally used), that is, this step determined whether rubber is to be injected before or after the conventional stones during the stoneblowing process. In a previous study (Sol-S anchez et al., 2017), it was found that the mix of natural stones with rubber (acting as flexible stones) could have a number of beneficial effects on track section performance, improving the effectiveness and durability of the stoneblowing process. However, due to the difference in density between rubber and natural stones, which hinders the inclusion of a mix of both materials at the same time, it was decided to include the rubber as an extra granular layer (placed before or after the stones) to facilitate its addition through a traditional stoneblower machine, but applying a two-step process.
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The critical process is therefore defined as a two-step stoneblowing technique, which has shown to be effective in correcting section geometry (Abrashitov and Semak, 2017). Therefore, in order to select the most appropriate position of the flexible layer, four different solutions were analysed (in the loading box of 400 mm 750 mm x 500 mm) in reference to the conventional solution (Fig. 3b): Two sizes of rubber (R14 and R8, defined as the most appropriate according to results, see section 3.1) for two positions (rubber over small stones or vice versa), placing 50% of rubber per volume of stones (this quantity produces comparable effect to very soft USPs e 0.2 N/mm3, and thus represents the maximum volume of rubber to be used in railway tracks) (Solnchez et al., 2017). These solutions were referred to as Sa SSþ50R14 and SSþ50R8 (when stones were applied prior to the rubber); 50R14 þ SS and 50R8þSS (when the stones were placed over the rubber); and SS, composed of only stones to be used as a control to determine the effect of rubber. To determine the optimal position, a vibratory permeameter test was first carried out to evaluate the loss of rubber depending on whether it was above or below the stones. This test was developed in a similar way to that of the previous step, including the two-layer systems with a total height of 20 mm. Further, to evaluate the effect of each size of rubber particle and position on track behaviour, a laboratory test was carried out over a track section, simulating the passage of trains and including the granular materials as a maintenance intervention, as occurs in real tracks. This test consisted of applying a series of dynamic loads (at 5 Hz and an amplitude close to 200 kPa under sleeper) over a conventional track section (consisting of compacted ballast, sleeper, rail and fastening system) until the settlement was close to 10 mm (around 50,000 cycles). At that point, the test was stopped, the rail-sleeper system was lifted by a laboratory device (until obtaining a sleeper-ballast gap of around 30 mm), and an injector (Fig. 3a) was inserted up to the ballast surface. - A layer of particles was first added (small stones or rubber); - Later, the rail-sleeper was lowered to compact the first layer (applying 50 cycles at 200 kPa); - Next, the rail-sleeper was lifted again; - Then, the second layer of particles was injected, in order to lower the sleeper to the correct position over the layer of particles; - Finally, before continuing with the dynamic test, the two-layer system was compacted by applying 50 cycles at 200 kPa. Following the maintenance process, the dynamic test was continued up to 200,000 cycles, assessing the change in position of the sleeper after maintenance (profile correction), and the modification of track performance (stiffness and capacity to dissipate energy). The number of total loading cycles was selected in consonance with previous studies where it was seen that this number was enough to obtain stable performance of the section, and then, being possible to analyse the influence of each solution under similar conditions. This process was carried out twice for each solution, injecting particles from two corners (in a diagonal position) of the sleeper. Additionally, for a more in-depth analysis of the impact of each solution on track performance under various loading conditions, multi-condition tests were carried out for each section. These tests consisted of applying a series of 1000 load cycles of varying amplitude (corresponding to 125 kPa, 190 kPa, and 250 kPa of stress under sleeper) and frequency (1 Hz, 5 Hz, and 10 Hz), which are loads that commonly occur in real service conditions (Indraratna et al., 2006; Bach and Veit, 2013; Ho et al., 2013).
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Fig. 3. Scheme of test procedure and injection of particles (a); configurations analysed (b).
2.2.3. Effect of compaction of the layers Having defined the most appropriate rubber size range and the position of this material in reference to traditional stone layers, the impact of compacting the first injected layer was evaluated (with rubber or stones), before the inclusion of the other component. This was analysed to prove whether this parameter is essential for obtaining a more stable behaviour and a greater capacity to modify the performance of the track by using the rubber to graduate parameters such as global stiffness. For this study step, five different solutions were assessed: (i) use of only stones, as the reference case (SS); (ii - iii) stones þ 25% or 50% of rubber over non-compacted stones (SSncþ25R and SSncþ50R); and (iv - v) 25% or 50% of rubber over the layer of compacted stones (SSþ25R and SSþ50R), the compaction being carried out by applying 50 loading cycles (at 5 Hz and a stress under sleeper of around 200 kPa). Thus, the effect of compacting the stones before the application of rubber (optimal position of rubber according to results, see section 3.2) was evaluated for two different quantities of this material. The test consisted of simulating train traffic over a conventional section until the settlement reached a value of around 10 mm. The sleeper-rail was then lifted (creating a gap of around 30 mm) to firstly inject the layer of stones. The aggregates were then compacted (when necessary for the solution) by applying the cited loads, after which the sleeper-rail was again lifted to inject the rubber. Finally, the test was continued until reaching 150,000 cycles. Thus, the change in section performance depending on the distribution of rubber was monitored, measuring parameters such as stiffness, dissipated energy, and settlement. 2.2.4. Impact of service conditions: temperature and load level Due to the rheological behaviour of rubber, the global track response was assessed in a ballast box at three room temperatures (20 C, 40 C, and 60 C as environmental temperatures), two loading conditions (125 KPa and 250 kPa), and for two different quantities of rubber (a layer with 50% of rubber over a compacted layer of stones - SSþ50R, and a layer of 25% of rubber over compacted stones e SSþ25R), in reference to the conventional solution with only small stones. The tests consisted of firstly applying dynamic loads until ballast settlement was around 10 mm, and later including each maintenance solution (two with rubber and one with only aggregates) at 20 C. Two dynamic tests were then conducted at stress amplitudes of 125 kPa and 250 kPa, both at 5 Hz, for
each solution. To increase the temperature to 40 C, and later to 60 C, infrared lamps were placed at different heights over the ballast surface. Temperature evolution on the ballast surface and in the particles under the sleeper (where rubber was placed) was monitored by thermocouples and an infrared camera. Once the temperature of the ballast and rubber was stable, the tests were carried out for each solution to measure its susceptibility to changes in temperature by assessing the effect on global track stiffness and dissipated energy values.
3. Analysis of results 3.1. Influence of rubber size Fig. 4 displays the influence of rubber size (14/20 mm, 8/16 mm, and 4/14 mm, represented as R14, R8, and R4 respectively) on the percolation of particles into the ballast layer (loss of particles from the surface), during the vibratory permeameter test (Fig. 4a) and the simulation of loads in a testing box (Fig. 4b), in reference to the conventional solution with only aggregates (SS). The results show that the placement of particles on the surface of the ballast already leads to the percolation of particles through ballast aggregates (before any vibratory effect, Fig. 4a), and regardless of the type of material. This effect is more marked with smaller sized rubber particles (with a particle loss of around 10e20 increasing to more than 100 when using rubber size 4/14). Further, the results show that the percolation of particles was more accentuated following the vibration process, enhancing the increase ratio due to vibration, regardless of the material type, but particularly in the case of rubber with a particle size of between 4/ 14 mm (increasing from 120 to higher than 500). The other two sizes (R14 and R8) presented particle loss values comparable to the reference SS (close to 90e120), or even reducing this parameter as in the case of load simulations (Fig. 4b). These results thus indicate that particles of the latter sizes could be used as elastic particles during the stoneblowing process without increasing the loss of particles through the ballast, which is in consonance with the results of other authors that have defined the size range for small stones (Selig, 1999; Fair, 2003). Then, these last sizes of rubber (R8 and R14) were used for the rest of the study as they shown to be the most appropriate.
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Fig. 4. Results of particle percolation measured in the vibratory test (a) and loading test (b).
3.2. Effect of rubber layer position In order to determine the most appropriate position for the elastic layer, Fig. 5 firstly represents the loss of particles for each system with two layers (small stones þ rubber, or vice versa), during the vibratory test. The results show that, regardless of the rubber size e R14 or R8 (which, on the basis of previous results were shown to be appropriate for application), the placement of rubber before the stones (thus, the rubber placed below the layer of stones eR þ SS) generally led to higher level of particle loss, particularly due to rubber loss. This effect was more marked in the case of the smaller rubber particles (increasing by around 15%), which showed even slightly higher total values than in the case in which only small stones (SS) were used, and therefore being less appropriate for application than the 14/20 mm rubber. Fig. 6 represents the effect of the position of the rubber layer (assessed for two sizes of rubber, R8 and R14) on the ability of this solution to modify the mechanical performance of the track section, representing the settlement at 150,000 cycles from maintenance intervention and the average value of stiffness modulus at that point. This last parameter was calculated as stiffness (for this study measured as N/mm) by area of sleeper section (in mm2) to limit the influence of the sleeper size used in the test (Fig. 6a). The average density of dissipated energy is also shown in Fig. 6b, which was calculated as the loss of energy per loading cycle (obtained from the area of hysteresis loops) divided by the volume of the layer of ballast used in this study, limiting then the effect of the size of this device.
Fig. 5. Results of particle loss for different two layer systems under the effect of vibration.
The results indicate that the inclusion of small stones (SS) could lead to a stiffening of the track section (higher values of modulus than those observed pre-maintenance) and loss of damping capacity (lower values than those observed pre-maintenance), which is in consonance with other studies (Anderson and Fair, 2008). However, the inclusion of rubber allowed for decreasing the stiffness of track, these changes being higher when the rubber was placed over the small stones (i.e. injected after placing the natural stones e solutions SS þ R), which also led in this last case to a higher capacity to dissipate energy from loads.
Fig. 6. Effect of rubber position and size on global track performance.
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In particular, the highest changes in track performance occurred when using the biggest size of rubber (R14), obtaining a reduction in track stiffness of around 50%. This finding is in line with the nchez et al., 2017), thus reinresults of a previous study (Sol-Sa forcing the idea that the use of this solution (50% of rubber 14/ 20 mm over stones) could lead to an effect that is comparable to that observed when using a conventional soft USP (with bedding modulus around 0.2 N/mm3), which is also in consonance with other authors (Jayasuriya et al., 2019). Then, this solution would allow for grading the behaviour of the track by reducing the quantity of rubber employed (and thus optimizing track performance). Moreover, it was observed that the use of smaller sized rubber particles (also over the stones, solution SSþ50R8) led to lower modification of section stiffness and capacity to dissipate energy, presenting a similar effect to that observed when using medium USP (bedding modulus around 0.4 N/mm3), and therefore confirming the influence of the size of the rubber particles. These results are in accord with those of other studies (Abrashitov and Semak, 2017) showing that a two-step process (in this case, firstly injecting stones and later rubber) is effective in modifying track performance. However, the other cases studied (the rubber placed under the stones e solutions R þ SS) resulted in a lower capacity to change behaviour, and therefore a lower ability to optimize track performance. This could be linked to a less homogeneous distribution of rubber and the (partial) percolation of the particles, thereby obtaining a poorer elastic layer in comparison with the case of using rubber over stones (particularly R14), which appeared to be the most effective solution. In fact, it was observed that the solutions with rubber over the stones allowed for reducing the track settlement (up to 30% when using R14, in reference to SS), obtaining a more positive effect when reducing the stiffness of the section and increasing its capacity to dissipate energy, which is related to the elastic behaviour of rubber, nchez et al., 2016a; as seen in other studies using USPs (Sol-Sa Jayasuriya et al., 2019). However, when the rubber was injected before the stones, higher settlement values were recorded, even in comparison with the conventional solution without rubber (SS). This negative effect was more marked when reducing the flexibility of the solution, which indicates the heterogeneous distribution of rubber, and therefore leading to greater recompaction of the section (this effect being more pronounced when using the larger rubber particles due to a poorer distribution when it is injected
before placement of the stones). This is demonstrated in Fig. 7, which represents the settlement at different steps of the section behaviour after each maintenance solution (with various rubber sizes and positions in reference to the conventional section without stones). Additionally, Fig. 7 shows the ratio of settlement when the system behaviour was stable (from 100,000 cycles), aiming to evaluate the long-term performance of the solutions. This figure shows that just after the interventions (only 200 cycles), the values of recovered settlement were lower than 100% due to the recompaction of particles despite the fact that after the maintenance intervention the layers were compacted (50 cycles at 200 kPa and 5 Hz) before continuing the test, which is in consonance with other studies (McDowell and Li, 2016; Ngo et al., 2017). Nonetheless, it was seen that this phenomenon was limited by using rubber over the stones (injected after the small stones), which allowed for the greatest reductions in track settlement, regardless of the stage of measurement (at both the short and longterm). Particularly, it was observed that these solutions led to an increase in the capacity to reduce settlement by around 30% in reference to the case of conventional stoneblowing, which has been shown to allow for increasing track durability (Cope and Ellis, 2001; Abrashitov and Semak, 2017). Therefore, the use of rubber over stones could limit the phenomenon of settlement due to irregularities associated with material recompaction following the process of stoneblowing (Ball, 2003). On the contrary, it should be considered that in the cases in which rubber is placed under the stones, a lower reduction in track settlement was obtained than in the case of the conventional solution (SS), whilst a higher tendency towards settlement also was observed. This confirms that placing the rubber over the stones could be regarded as a more appropriate solution (and then, this solution was used to continue with the study), particularly when using R14 due to its higher flexibility and capacity to dissipate stresses. Nonetheless, for a more in-depth understanding of the effect of rubber on track behaviour, and the influence of the position of the elastic layer, Fig. 8 shows the variation in stiffness and dissipated energy when modifying the level of stress (Fig. 8a) and the loading frequency (Fig. 8b). The results show that, regardless of the loading condition, the use of rubber over stones (solutions SSþ50R14 and SSþ50R8) is more effective in modifying the response of the track by reducing track stiffness and increasing the dissipation of energy
Fig. 7. Impact of rubber position and size on the capacity to reduce the section settlement after maintenance interventions.
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Fig. 8. Influence of load level and frequency depending on rubber position and size.
from the reference (SS), particularly with R14. Further, it is observed that for all solutions, the increase in load level leads to a stiffening of the track and an increase in dissipated energy (since higher hysteresis loops were recorded) while the change in frequency had a weaker effect on track flexibility. Nonetheless, the trend towards change (in the case of systems using rubber) was quite similar to the reference, which indicates comparable susceptibility to variations in track behaviour under various loading conditions, which is also in consonance with other studies using elastic elements under sleepers (Jayasuriya et al., 2019). 3.3. Effect of compacting the layer of stones before applying the rubber Fig. 9 represents the changes in track behaviour (relationships between settlement and stiffness or dissipated energy, Fig. 9a and b, respectively) when including rubber over the stones previously blown in the first step of the stoneblowing process. This figure analyses the influence of compacting (or not) the small stones before blowing the rubber (solutions SS þ R and SSnc þ R, respectively), this study being carried out for two quantities of rubber (25% and 50% over the volume of stones). The type of rubber corresponded to R14 since in previous steps this was shown to be more appropriate for using as flexible aggregates during the stoneblowing process, while the position over the stones also shown to be the optimal one. Firstly, the results confirmed that, regardless of the quantity of rubber and the state of the SS layer, the inclusion of rubber allowed
for reducing settlement in comparison with the conventional process of stoneblowing with only aggregates. This was particularly marked when obtaining a higher reduction in stiffness and an increase in the capacity to dissipate energy, showing a relationship between superstructure behaviour and the tendency towards setnchez et al., tlement, as established in previous studies (Sol-Sa 2016b). Therefore, this indicates that the higher the effect of the rubber, the higher the reduction in track deformation. The results also show that a relatively low volume of rubber (25% over stones) presented little effect (or even a negative effect) in terms of modifying track performance when applied over noncompacted stones (SSnc). This is in accord with the results of other studies (Abrashitov and Semak, 2017). However, the case in which the rubber is placed over compacted aggregates did lead to a positive impact in terms of reducing track stiffness and increasing its damping capacity (and thus decreasing settlement), which could be linked to a more homogeneous distribution of rubber since there is higher free gap between compacted stones and the bottom of the sleeper to facilitate the inclusion of rubber. This effect is particularly marked when increasing the volume of rubber (50%) to be injected after stones, obtaining a limited effect with non-compacted stones, while its use over the compacted small stones produced a significant variation in track performance and reduction in settlement, which is in consonance with similar nchez et al., measurements recorded in a previous study (Sol-Sa 2017) where the rubber was homogenously placed by hand over the stones. This indicates a good distribution of the flexible particles when the stones are compacted previously, presenting a
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Fig. 10. Impact of temperature on track performance with different quantities of rubber under various load levels. Fig. 9. Effect of compacting stones prior to rubber injection on changes in track performance.
similar effect to that observed when using soft (0.2 N/mm3) and medium (0.4 N/mm3) USPs for the cases with 50% and 25% of nchez et al., 2016a). rubber, respectively (Sol-Sa 3.4. Impact of service conditions Fig. 10 shows the changes in track performance when modifying the test temperature, for two loading levels (125 kPa and 250 kPa) and two quantities (25% and 50% over the volume of stones) of rubber R14 over compacted stones. It is clear that in spite of fixing the external testing temperature at 20 C, 40 C, and 60 C, the temperature recorded under the sleeper (where the rubber is placed) was around 20 C, 27 C, and 37 C since the sleeper serves to protect the particles below. This can be observed in Fig. 11, which displays the temperature on the ballast surface (higher than 60 C in this example) and particles under the sleeper (around 37 C), which was recorded by removing the sleeper just after the test and using an infrared camera to verify the change in temperature in reference to ballast. Regarding the effect of temperature on section behaviour, the results indicate that the increase in temperature led to a slight decrease in stiffness and an increase in dissipated energy in the cases in which only stones were used (SS), regardless of the level of stress. This could be a consequence of the softening of the rail pads (in spite of the fact that the rail-sleeper system was partially isolated from temperature changes). However, for the solutions using rubber, the stiffness of the entire section increased slightly, regardless of the quantity of rubber and level of load. This fact could be a consequence of the softening of the rubber when increasing the temperature, and then, the weight of the sleeper and rail led to higher static deformation of this material, reducing its ability to
Fig. 11. Visual appearance of the temperature on the ballast surface and particles under the sleeper.
deform when applying the dynamic loads, and thus recording slightly higher stiffness values while obtaining a more remarkable effect on the capacity to dissipate energy. Nonetheless, it should be noted that the changes in track behaviour at higher temperatures were less marked than those measured when modifying other factors such as, for instance, stress level or rubber quantity. In these last cases, it was obtained again a strong decrease in stiffness and increase in dissipation of energy when rising the dosage of rubber (particularly at medium temperatures due to a lower softening of rubber) while the increase in load level led to higher variations in track performance and higher
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susceptibility to changes. Thus, this analysis indicates moderate susceptibility of this solution to temperature in reference to other parameters, which should be related to the fact that the sleeper protects the rubber from changes in temperature. 4. Conclusions The focus of the present paper was to contribute towards the development of an alternative maintenance process that aims to reduce the negative impacts associated with profile operations to conserve track geometry. For this purpose, the study focused on analysing some of the main factors involved in the application of rubber particles (derived from end-of-life tyres) as flexible aggregates during a two-step stoneblowing technique. The chief aim was to search for a way of optimizing the mechanical performance and durability of existing tracks where the inclusion of standard elastic elements such as USPs is expensive and technically challenging. On basis of the results obtained in the present work, the following conclusions can be drawn: - Rubber particles above 8 mm appear to be suitable for use as flexible aggregates under the sleeper without increasing the fouling of ballast due to the percolation of fine particles. Nonetheless, the results indicate that particles within the range of 14e20 mm showed the lowest values of percolation, and are thus the most appropriate. - Injecting the rubber particles over a layer of small stones (routinely used in the stoneblowing process) appears to be the most effective process for modifying the mechanical performance of the track section, and therefore the behaviour of the track can be optimized according to line and traffic characteristics. - The above is more marked when using rubber particles with a size between 14 and 20 mm, with the latter also leading to a greater reduction in section settlement in both the short and long-term, yielding more positive results in comparison with the case in which only stones were used (i.e. the traditional stoneblowing process). - The results confirmed that the compaction of small stones (injected by the stoneblowing technique) prior to the inclusion of rubber emerged as a key factor for improving track performance and durability, particularly where a high volume of particles is required. - Therefore, a track intervention that includes rubber particles as flexible aggregates could consist of a two-step process where the traditional stones are firstly injected and compacted, after which rubber is injected at a volume that depends on track requirements. - The results indicate that an increase in temperature and load level leads to track stiffening and a reduction in damping capacity, with stress level being the most influential factor. Nonetheless, it is important to note that the pattern of changes in section behaviour is comparable to that obtained for the conventional stones only solution. The present findings therefore suggest that the stoneblowing process could be used to include rubber particles under the sleeper in existing lines by adding an extra layer to traditionally compacted small stones, which allows for the possible gradation of the track and improvement of its behaviour and durability. This would allow for important environmental and economic benefits associated with reducing the need for periodic maintenance. Nonetheless, further studies are still required to completely understand this alternative technique for improving railway track performance, such as the effect of rubber on lateral resistance, factors defining
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