For this study three different bitumen emulsions were selected (Table 1). Since this is a new application for bitumen emulsion a wide range of possibilities, in terms of BE characteristics, was explored: different viscosities (depending on solid content); use of neat or modified bitumen; different setting speeds; and different types of bitumen. N1 is a high performance BE typically used for pavement surface courses (24). N2 and R1 were selected for their quick setting, high viscosity and suitability for this new application (25). All of them are cationic emulsions with affinity to a wide range of mineral aggregates and promote adhesion of bitumen to ballast particles.

The ballast used for this study was granite aggregate sourced from Bardon Hill quarry in Leicestershire, United Kingdom (23, 26). Due to the dimensions of the test apparatus, two scaled (approximately one third scale) gradations (clean and fouled ballast) were used as described in previous studies (23, 27, 28).

Bitumen emulsion dosage and properties (29), as well as the aggregate gradation used, can potentially affect the quantity of BE achieved during the application and could also influence BSB mechanical behaviour. Thus, with the aim of evaluating the influence of such variables on the application method and on the main parameters affecting in-field ballast performance, a series of laboratory tests were carried out: (i) Flowability tests and (ii) Confined compression tests using the Precision Unbound Material Analyser (PUMA) apparatus.

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 (23). Table 2 summarises the work carried out.

Table 3 shows results of the flowability tests, indicating penetration time, quantity and percentage of BE lost, and flowability index as determined by equation [1]. It can be observed that viscosity and breaking behaviour played the most important roles. In fact, passing from low binder content (60%) and slow setting (N1) to high binder content (70%) and fast setting (R1) (with an intermediate situation for N2) the percentage of material lost dropped drastically. The gradation had an important influence on the results to the extent that passing from a more open graded (clean ballast) to a well graded (fouled ballast) there was a 20–75% reduction in material lost, depending on the BE used. In the case of BE N2 the effect of dosage was also relevant: an increase from 2% to 3% BE led to an approximately tenfold increase in material lost. This analysis shows that, depending on field conditions, properties of BE such as viscosity, breaking behavior and dosage can be optimized to reach a desired penetration depth.

Results also show that, for the specific materials analysed, BE R1 had the best scores in terms of flowability index.

Figure 4 shows the plastic strain after 200,000 repetitions in the confined compression test (PUMA) for the 12 BSB configurations and 2 reference materials. Results indicate an overall improvement in resistance to permanent deformation (up to 3 times) for stabilised specimens compared to the unbound reference materials, in agreement with other studies on ballast stabilisation (33, 40, 41). It can be noted that gradation had an important influence on results since in almost every case clean ballast specimens (both stabilised and unbound) exhibited a lower plastic strain than their fouled counterparts. This result is in agreement with Keene et al. (33). The dosage of BE also influenced results to the extent that increasing %BE led to a lower final plastic strain, confirming the findings of D’Angelo et al. (23). Also the type of bitumen had a significant influence: almost all BSB specimens with harder bitumen (N1 and R1, Table 1) exhibited lower plastic strain than those with softer bitumen (N2).

With the aim of analysing the influence of BE on long-term ballast behaviour, Table 4 shows plastic strain rate (PSR) values calculated over the last 20,000 cycles as in (23). It can be observed that in general stabilised specimens had lower values of PSR than the reference materials. Also for this parameter gradation had an important influence, PSR values for clean ballast being lower than those for fouled ballast.

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 (23).

Figure 5 illustrates the influence of bitumen stabilisation on ballast resilient modulus (stress amplitude over the resilient strain) and its ability to dissipate energy (hysteresis loop area per cycle), averaged over the last 20,000 cycles. It can be observed that stabilisation typically provided an increase in resilient modulus of approximately 25% with respect to the reference cases without BE. At the same time the stabilisation process improved the ability of ballast to dissipate energy. BSB specimens dissipated on average 40% and 30% more energy than clean and fouled reference materials, respectively. In addition, with the exception of 2% BSB (N1), it can be noted that fouled specimens dissipated a higher quantity of energy, even if by a relatively small margin, than their clean counterparts. It is also interesting to highlight that a small increase in bitumen content (BE from 2% to 3%) corresponded for almost all BSB specimens to a small increase in dissipated energy, confirming the important role played by this variable. The influence of gradation and BE content was however less marked in the case of the resilient modulus values.

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 (6, 10). Nevertheless it is worth noting that these results may be partly a function of the confined conditions of the test used.

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.

Table 5 and presents results in terms of desirability values obtained using Derringer’s desirability functions with parameters analysed during flowability and PUMA tests. By using the geometrical mean the corresponding desirability index is obtained for each configuration. Figure 6 summarises these values for clean and fouled BSB.

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.