Ballasted track, while providing economical and practical advantages, is associated with high costs and material consumption due to frequent maintenance. More sustainable alternatives to conventional ballasted trackbeds should therefore aim at extending its durability, particularly considering ongoing increases in traffic speed and loads. In this regard, the authors have investigated a solution consisting of bitumen stabilised ballast (BSB), designed to be used for new trackbeds as well as in reinforcing existing ones. This study presents the idea behind the technology and then focuses on a specific part of its development: the optimisation of bitumen emulsion properties and dosage in relation to ballast field conditions. Results showed that overall bitumen stabilisation improved ballast resistance to permanent deformation by enhancing stiffness and damping properties. Scenarios with higher dosage of bitumen emulsion, higher viscosity, quicker setting behaviour, and harder base bitumen seem to represent the most desirable conditions to achieve enhanced in-field performance.
The railway system represents one of the most attractive modes of transportation worldwide because of its high efficiency, speed, capacity and low environmental impact compared to other systems. These factors lead to continuously increasing demand on train speed and load transported, which represents an important challenge especially for the infrastructure. In this regard, ballasted track, which is by far the most widely-used track form due to its economical and practical advantages, is particularly affected by deterioration and the need for maintenance (
The ballast layer contributes significantly to these problems. Typically, 50–70% of trackbed settlement is thought to be due to permanent deformation in the ballast layer (
To reduce ballast-related maintenance costs (approximately 30% of annual maintenance expenditure (
Solutions towards trackbed substructure modification, such as elastic elements, geosynthetics, ballast stabilisation by polymers or resins among others, aim to provide a balanced level of stiffness and damping properties especially in railway hotspots and transition areas where a smooth variation of these properties (
In this regard, bitumen stabilised ballast (BSB) is an alternative solution, relatively economic and easy to apply, proposed for either newly constructed track or track requiring maintenance (
This study aims to further develop this solution by optimising bitumen emulsion characteristics in relation to bitumen type, proportion, breaking behaviour and dosage, in terms of their influence on its flowability through the ballast layer and on BSB performance (for both clean and degraded ballast states).
For this study three different bitumen emulsions were selected (
Physical and rheological properties of bitumen emulsions used
Property | Standard | N1 | N2 | R1 |
---|---|---|---|---|
Particle surface electric charge | - | Positive | Positive | Positive |
Binder content [%] | EN 1428 or EN 1431 | 60 | 67 | 70 |
Breaking behaviour [s] | EN 13075-1 | > 170 | < 110 | < 110 |
Bitumen type | - | Neat | Styrene-Butadiene-Styrene polymer modified | Styrene-Butadiene-Styrene polymer modified |
Penetration [dmm] | EN 1426 | 47 | 160-220 | 45 |
Softening point [°C] | EN 1427 | 52 | 40 | 70 |
The ballast used for this study was granite aggregate sourced from Bardon Hill quarry in Leicestershire, United Kingdom (
Bitumen emulsion dosage and properties (
The three different emulsions selected with two dosages and two different gradations for ballast (clean and fouled) were combined to give a total of 12 BSB configurations and compared with two unbound materials (clean and fouled), used as reference. The dosing ranges of the bitumen emulsions were selected based on the authors’ previous work in this domain (
Testing plan
Variables | Material tested | Properties tested | Test | Main parameters |
---|---|---|---|---|
Dosage | Clean BSB (N1) | Flowability of BE | Flowability test | Penetration time |
BE Viscosity | Clean BSB (N2) | Quantity of BE lost | ||
BE breaking behavior | Clean BSB (R1) | Flowability Index | ||
Ballast gradation | Fouled BSB (N1) | |||
Fouled BSB (N2) | ||||
Fouled BSB (R1) | ||||
Dosage | Clean ballast | BSB mechanical behaviour | Confined compression test (PUMA) | Plastic strain |
Bitumen type | Clean BSB (N1) | Plastic strain rate | ||
Ballast gradation | Clean BSB (N2) | Resilient Modulus | ||
Clean BSB (R1) | Dissipated Energy per cycle | |||
Fouled ballast | ||||
Fouled BSB (N1) | ||||
Fouled BSB (N2) | ||||
Fouled BSB (R1) |
One of the most important factors influencing BSB application is the viscosity of the bitumen emulsion (BE), since the BE should be able to penetrate the aggregates, ‘gluing’ the contact points, but should not drain through the layer. The viscosity should therefore be an optimum to allow penetration to the bottom of the layer but should minimise the percentage of material that reaches the interface with underlying materials. Variables involved are the bitumen content in the emulsion (the higher the bitumen content the more viscous the BE), the dosage and the breaking behaviour (
Thus, a specific test, similar to the determination of penetration power of bituminous emulsion (
Flowability test apparatus and specimen set up.
This index increases as %BE lost and penetration time decrease, giving information about the ability of the BE to quickly penetrate the aggregate layer and start setting.
In order to assess the mechanical properties of BSB, a dynamic confined compression test (PUMA) was used. This test consists of the application of a repeated compression load and records the resulting vertical displacement as described in (
PUMA frame and loading apparatus (after Thom et al. (
The test conditions were the same as in D’Angelo et al. (
In order to assess and compare different BSB configurations many responses associated with measured parameters need to be taken into account. A method that allows these different properties to be optimised across the different configurations analysed in this study had to be established. For this purpose an optimisation method was used that introduces desirability functions (DFs) that transform the parameters into desirability in the range [0,1], where 0 values are unacceptable whereas 1 means the most desirable properties (
A range of acceptable values, given by lower and upper limits, or a single boundary value that should not be exceeded, were specified. These limits were established according to information obtained from other studies referred to below. It is acknowledged that much further work is required before these limits can be considered robust, but they illustrate the use of the optimisation technique. Flowability index ranged from 0 to 1200 (the total test duration expressed in seconds). Plastic strain ranged from 0% to 3.2%, a value intended to represent that at which a maintenance intervention is likely to be needed (
Derringer’s desirability functions for the selected BSB properties.
For each BSB configuration, applying the DFs to the parameters under investigation provides a set of desirability values (DVs) which, being now homogenous, can be used to compare all the configurations by means of any specified method, for example the geometrical mean. In this study a desirability index (DI) was used that valued all the properties equally, as in equation [
where the subscripts stand for flowability index, plastic strain, resilient modulus and dissipated energy, respectively.
This index allows for a straightforward comparison of the different BSB configurations. It should be noted that if even one of the DVs is zero then the DI comes to zero, disqualifying any configuration that does not meet the established requirements.
Flowability test results
Gradation | Clean | Fouled | ||||||||||
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BE | N1 | N2 | R1 | N1 | N2 | R1 | ||||||
Dosage | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% |
Penetration time [sec] | 16 | 15 | 180 | 134 | 55 | 77 | 52 | 22 | 210 | 98 | 155 | 135 |
BE lost [g] | 52.4 | 76.3 | 2.5 | 20.3 | 2 | 2.7 | 41 | 59.6 | 1 | 10.58 | 0.5 | 0.8 |
BE lost [%] | 81.9% | 79.5% | 3.9% | 21.1% | 3.1% | 2.8% | 64.7% | 62.0% | 1.6% | 11.0% | 0.8% | 0.8% |
Flowability index | 215 | 243 | 980 | 841 | 1109 | 1092 | 406 | 447 | 975 | 981 | 1037 | 1056 |
Results also show that, for the specific materials analysed, BE R1 had the best scores in terms of flowability index.
Final plastic strain after 200,000 repetitions in the PUMA test for clean (a) and fouled (b) ballast.
With the aim of analysing the influence of BE on long-term ballast behaviour,
Influence of BE stabilisation on long-term behaviour (PSR) of clean and fouled ballast
PSR: *109 [mm/cycle] | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Clean | Fouled | ||||||||||||
N1 | N2 | R1 | N1 | N2 | R1 | ||||||||
Ref. | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% | Ref. | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% |
2.12 | 0.75 | 0.70 | 1.20 | 1.43 | 0.94 | 1.81 | 3.79 | 1.53 | 1.16 | 1.99 | 1.63 | 1.12 | 2.67 |
These results confirm the potential of this technology to improve trackbed resistance to geometry degradation and consequently to reduce the need for maintenance due to ballast settlement (
Influence of stabilisation on resilient modulus (RM) and dissipated energy per cycle (DE) for clean (a) and fouled ballast (b).
These results jointly suggest the potential of BSB for reducing track deterioration and maintenance costs: a relatively small increase in stiffness could be beneficial for reducing fatigue and deterioration of track components when dynamic loads are adequately damped (
In the sections above the potential benefits of stabilising ballast with bitumen emulsion have been highlighted. This section will provide a comparison of the BSB configurations analysed in this study using the optimisation method illustrated in Section 2.6. This method was carried out based on the data obtained from flowability and confined compression tests. The evaluated parameters, namely flowability index, plastic strain, resilient modulus and dissipated energy reflect some of the most important objectives to be achieved by this new technology for its application in a railway.
Detailed desirability values of flowability index, plastic strain, resilient modulus, dissipated energy and resulting desirability index for all BSB configurations
Gradation | Clean | Fouled | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
BE | N1 | N2 | R1 | N1 | N2 | R1 | ||||||
Dosage | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% | BSB 2% | BSB 3% |
Flowability index | 0.18 | 0.20 | 0.82 | 0.70 | 0.92 | 0.91 | 0.34 | 0.37 | 0.81 | 0.82 | 0.86 | 0.88 |
Plastic strain | 0.86 | 0.87 | 0.78 | 0.80 | 0.74 | 0.85 | 0.81 | 0.83 | 0.71 | 0.78 | 0.82 | 0.83 |
Resilient modulus | 0.88 | 0.85 | 0.89 | 0.87 | 0.79 | 0.80 | 0.77 | 0.90 | 0.91 | 0.84 | 0.76 | 0.68 |
Dissipated energy | 0.58 | 0.53 | 0.52 | 0.55 | 0.51 | 0.52 | 0.55 | 0.57 | 0.57 | 0.57 | 0.55 | 0.58 |
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Desirability index results of (a) clean and (b) fouled stabilised specimens as a function of BE dosage and type.
In can be observed that the best scores, ranging from 0.72 to 0.75, were obtained by N2 and R1, regardless of the gradation considered. BE N1, in contrast, reached noticeably lower values, especially in the case of clean ballast; despite general improvements in terms of mechanical properties of BSB, BE characteristics negatively influenced its flowability through the ballast, allowing a high quantity of material to be lost during stabilisation. This parameter had, in fact, the highest impact on the optimisation process. This suggests that a more viscous BE is preferred for stabilisation of ballast having a relatively low level of degradation.
The present paper provides an insight into the optimisation of BSB. Different solutions in terms of type of bitumen emulsion, dosage, bitumen properties and ballast gradation have been compared, using an optimisation method, in terms of mechanical performance and effectiveness of BE application. From the analysis carried out in this study, the following conclusions can be drawn:
Overall, bitumen stabilisation improved ballast properties in terms of plastic strain and plastic strain rate (long-term behaviour), confirming the potential of the technology for enhancing ballast layer mechanical performance, in a similar way to other stabilisation technologies (
The type of emulsion and its dosage seem to play an important role in BSB properties: increasing the %BE provided a better resistance to permanent deformation and changed the mechanical properties of BSB; increasing the viscosity of bitumen emulsion decreased the percentage of material lost, thereby providing improved stabilising behaviour. Nevertheless, depending on the field application and ballast depth to be stabilised, a specific BE could be designed to fulfil specific requirements.
Ballast gradation is another important factor to take into account: results showed that overall clean specimens exhibited better performance than fouled ones. Nevertheless, it has to be noted that this study was carried out using a scaled grainsize distribution.
Comparison of all BSB configurations, with the optimisation method used, indicates that configurations with N2 and R1 emulsions, which obtained the highest desirability values, are likely to mark the path to follow for further development of this new technology.
The results obtained give important guidance on the influence of the factors studied here, but further investigation of full-scale BSB is still needed to support the results obtained in this study and provide a better understanding of the potential of this new technology.
The research presented in this paper was carried out as part of the Marie Curie Initial Training Network (ITN) action, FP7-PEOPLE-2013-ITN. This project (
The authors would like to acknowledge also Nynas Bitumen and Repsol for providing the materials necessary for this study.