Mobility is a fundamental prerequisite for growth and Denmark has like all other European countries a focus on shifting from road to rail. A recent survey revealed, that 50 % of the Danish citizens consider improving the Danish rail net as one of the most important infrastructural investments in the coming years, where faster travelling, shorter travel time and fewer interruptions will occur, and 60 % of the Danish citizens declare that they will use this form of transportation more often. To accommodate this, the Danish Government has allocated 28.5 billion DKK over the next 10 years, to electrify and prepare the Danish rail net for increased speed and operational reliability. Even though trains, signals, operation systems etc. have been steadily improved over the past decades, the track is still constructed by rails on sleepers placed on a layer of unbound ballast stones – a system introduced more than a century ago.

The Executive Director for the European Rail Infrastructure Managers (EIM) stresses the importance of bearing in mind, that faster rail operation requires significantly more maintenance, compared to traditional systems, and no one expect the budgets for maintenance to increase.

Because of the plans for upgrading the Danish rail net, with the aim of reaching increased speeds and reliability, the timing is right to re-engineer the traditional structure upon which the tracks are laid. This addresses the challenge of determining the most cost-efficient construction method from an LCC-perspective. Rail Net Denmark has hypothesized a new system based on a fixed, solid and impervious ballast and/or subballast as the upgrade with the greatest potential for cost optimization. Such a system could be based on asphalt as the material of choice, which is already in use in countries like Germany, USA, Japan, and Austria etc. with good preliminary results. The challenge in the traditional construction system is the limited durability and geometric stability, and regular need for maintenance works, which highly influences the overall life-cycle-costs and risk of traffic delays. Frequent maintenance also implies that used (polluted) track ballast regularly needs to be discarded. In addition, only a limited protection of the substructure from penetrating rainwater is obtained. This leads to softening, isolated defects and rapid deterioration of the sub-structure due to rainwater soaking into the ground, as well as risk of pollution of the subsoil and shallow groundwater from leaching of polluted run-off from the track surface through the ballast. Especially in relation to future climatic changes with more extreme rainfall, this could be a severe challenge, whereas an alternative construction based on a dense material like asphalt will ensure good rainwater drain-off and protection of the sub-structure.

Based on literature studies and interviews with leading international railway experts, it seems that the motives for choosing the different construction principles in the different countries are multiple, although the evolution from a simple trail-and-error based approach generally has been very conservative. In addition to the general conservative approach, factors such as climate, geotechnical conditions, availability of crushed rock directly at the construction site, economical evaluations/expectations – and political motives – may influence the typical patterns for selection of track construction method. Thus, the ballasted construction principle is still the predominantly used technique at present. A more technically well founded approach for optimization of the track construction is highly needed in order to take railway construction to a higher 21st century performance level.

There is a clear synergy between the stated industrial and public service challenges and thus a motivation to form a consortium to overcome the technical challenges that lies in changing the paradigm for how our railway infrastructure is designed, built and maintained.

The shared vision of the participants is to create the technical basis for a transition to high-speed rail operation without increasing maintenance costs and simultaneously exploit this to improve market conditions for the Danish asphalt industry.

The users of the expected outcome of the project i.e. model and new track super structure design are on a national scale primarily represented within the consortium. Rail Net Denmark will e.g. use the computational model to determine the most profitable track superstructure design for future implementation. In Denmark there are also a number of private railway operators which would use model and/or new track construction given this provides a cost optimization.

The specific research and innovation goals of the Roads2Rails are:
a)To develop an operative, generic, mathematical FEM model for evaluation and comparison of different structural designs in railway construction, and thereby enabling optimization of the preferred structural design system, including the effects from use of asphalt layers.
b)In relation to the model development to conduct a state-of-the-art study to disclose the causes and effects of rolling contact fatigue damage (RCF) on the rails, in order to ensure that the new system(s) does not introduce an increased vulnerability to RCF e.g. from the increased stiffness of the structure and materials used.
c)By use of a Life Cycle Cost analysis to make plausible that the new construction system will perform better than the traditional construction system, including focus on added value parameters such as reduced noise and ground-borne vibrations.
d)To develop new asphalt products, optimized for expected use in the structural design, with focus on flexibility, durability and circular asphalt production by use of recycled materials. The asphalt based systems will be used for validation of the model developed under a).
e)Construction of a test facility (mock-up or full scale test section in closed track environment) to be able to simulate and monitor the actual forces for further evaluation, documentation and model validation.
f)On the basis of the above mentioned steps to develop a complete new system for structural design, encompassing considerations for soil preparation, water drainage, maintenance and repair techniques etc.
g)By use of Life Cycle Assessment (LCA) to establish how the new construction system contributes to lower environmental impacts than the traditional ballasted system, including focus on lower leaching, asphalt recycling, reduced waste created to be disposed of, etc.

Some countries have already implemented the use of track constructions involving asphalt layers in stead of the traditional purely ballasted track. In general it is agreed that for the traditional ballasted track construction, increased train speed and loads will drastically increase the need for maintenance work and repairs. In many of the countries who have switched to using asphalt in their track construction only use the material as water protection and stress relieving of the sub-structure, by placing a layer of asphalt beneath the traditional ballast top-layer. This upgrade takes care of most of the water related issues, but significant increases in general deterioration as well as noise and vibration issues are still very much present. These solutions has been used for several years and is today a part of the local standards (Austria, parts of USA, France (TGV) etc.). In other countries, a.o. Germany (e.g. the GETRAC system), a ballastless asphalt system has successfully been used for high speed train sections. The international expectation to the service life of a ballastless construction is according to leading researchers estimated to around 60 years (P. Veit, TU Graz), and may thus more than double the durability of a conventional track. Especially if considering future novelty changes of the contractual operational responsibility (e.g. P.P.P.) it is important to be able to obtain a very long durability.

The increase of the operating speed of the railway networks has raised the concern of the ground-borne noise and vibration due to railway traffic during the past decades in Denmark. The source of dynamic loading is primarily the movement of the train load which generates surface and body waves within the soil medium. Vibrations are secondarily produced during the rotation of train wheels on rails with imperfections on their surface, and due to the movement of the unsprung masses (vehicle-track interaction). Several numerical modelling techniques (3D FEM, coupled BEM-FEM, Perfectly Matched Layer) have been developed to analyze the complex train-track-ground system and assess the induced traffic vibrations [1]. Field measurements have been used for the validation of numerical models not only with the application of Green’s functions combining the beam theory, [2, 3], but also with the spectral element method in 2D and 3D models [4] and the FEM in 2D models [5]. More recently the 2.5D finite-infinite element method has been implemented, where the geometry of the track and the soil is often assumed to be invariant in the longitudinal direction of the track while soil material nonlinearity is considered by applying the equivalent linear approximation [6].

The mitigation of train induced vibrations can be controlled at the source (train-track), at the transmission path or at the receiver. In regards with the mitigation at the source techniques, rail profiling can be used for minimizing track unevenness, or modification of the rolling stock characteristics could be applied. However the isolation of the track superstructure from the ground appears as a promising solution. Ballast mats or floating slab tracks are an example of resilient elements, which can modify the dynamic load and the transfer function [1, 7, 8]. Nevertheless ballasted tracks intended to high speed trains exhibit geometrical disorders that evolve fast with traffic, requiring frequent maintenance and increasing the direct and indirect costs.

The international experience with asphalt based solutions comprise two different approaches. Either an asphalt layer is introduced as replacement of an upper part of the unbound subballast layer, or the asphalt layer is completely replacing the ballast. As leading railway construction experts estimate that the ballastless construction principle may provide an extreme long durability of 60 years, this principle will be the primary aim in the project. The ballastless solution has many advantages including:
•Short construction time
•Capability of using conventional road and track-construction equipment
•High degree of mechanization and small number of work steps in laying the track span
•Long life cycle with little maintenance
•Great stability
•Unimpeded drainage of rainwater and other precipitation
•Fast availability of the track after repairs

For Danish climate and conditions, it is important to develop a new system that will benefit from the extreme long service life of the ballast-free asphalt construction, but with special emphasis on development of a dense and durable asphalt base that will protect the lower layers from water softening and pumping effects. Also, it is extremely important for the new solution to be developed as a complete concept, encompassing the total structure and related details, including related environmental aspects. This should include measures to ensure sufficient and uniform strength of the often challenging Danish subsoil (e.g. by adding lime or cement as a stabilizing agent, as known from modern Danish motorway construction), effective protection of the lower layers from penetration of rain water, effective drainage systems, and related possibilities of reducing ground pollution etc. Furthermore, new techniques must be developed (as a mitigation tool) in order to ensure efficient maintenance of the asphalt pavement, in case that unpredicted settlements and unevenness may occur. Also the transition zones between differently constructed sections must be addressed (See also section 2.5, Key risk elements).

The application of asphalt in railway construction is as mentioned not new, but employing a systematic approach to re-engineer the railway construction, based on a computational design tool, that can model cause and effects of different combinations of materials and that will serve as a decision making tool is completely novel. Asphalt will be used to verify the model through lab trials.
By the introduction of an innovative generic model approach, it will be possible to determine the most optimal layer/material compositions for the structure. In other words, by using a new model optimized structural design there is a significant potential to lower the total life-cycle cost for the railway track infrastructure, and thus to make railway transportation more competitive and attractive, and at the same time increase the level of environmental protection.

The asphalt industry sees a great economical potential in this new railway market. The total length of the Danish railway infrastructure is approx. 2100 km, and as some sections are double/parallel tracked, the total track length is approx. 3200 km. If these (theoretically) are all reconstructed using an asphalt based construction with a thickness of 20-30 cm and e.g. 3.5 m width, the total market potential is more than 5 mio. tons of asphalt. At the same time asphalt for rail construction supports the focus of a circular resource economy, where production is based on a circular material flow, as the annual quantities of highly valuable reclaimed asphalt (milled-off or broken-up from old roads) by far exceed the amount recycled in asphalt production for roads. Recycled asphalt is expected to be a valuable material for production of new asphalt for rail constructions, which implies both savings in relation to avoided disposal of recyclable asphalt, and savings in raw materials and energy for new asphalt production in relation to utilization of reclaimed asphalt. The environmental and socioeconomic improvement by implementing reclaimed asphalt in the solution will ensure a positive LCA effect on the railway construction, as the reclaimed and crushed old asphalt is an already available material source from stockpiles available at the vaious asphalt plants. By recycling the asphalt, the need for import of crushed rock (e.g. from Norway/Sweden) will be reduced. Also, the need for import of bituminous binder processed from crude oil (limited fossil resources) will be reduced. Finally, the use of a ballastless asphalt solution will remove the need for import of crushed rock ballast aggregates (from Norway/Sweden).

The use of an asphalt based solution also provides a more environmentally “green” effect, improving the protection of the surrounding environment. By introducing dense asphalt types with very low permeability in the railway structure, it will not only have a structural positive effect on the protection of the subsoil from softening by penetrating rainwater. In addition, it will also prevent polluted water drain-off from the track surface to leak through an unbound pavement into the ground and maybe pollute the ground water. In the planned project solution it is the intention that the surface water will be led through filter aggregates at the sides of the embankment (“cribbing”) before led to drainage ditches/system at the foot of the track embankment. The filter system may not be able to catch all rain water run-off created by intense (or even future extreme) rain falls, but will be able to collect the initial rain water run-off from the surface, where the polluting materials have been collected and concentrated at dry weather conditions. As an additional benefit, the need for expensive cleaning of discarded, used ballast aggregates will be avoided as no ballast is used. Thus, the proposed solution possesses several environmental benefits and also addresses the critical topic of preparing the important infrastructure to the future climate changes – which is an essential aspect when designing the railway for the future.

Danish positions of strength in business, industry and research
Atkins Rail Denmark, which is a part of Atkins Global, is a worldwide leader in rail engineering and systems design, providing experience and in-depth knowledge of the rail and engineering domains. Atkins as a global company will ensure, that the activities outlined in this project are based on state-of-the-art railway engineering practices and experiences with existing systems, both in terms of the traditional ballasted tracks and the proposed ballast-less asphalt based tracks currently in use. Rail Net Denmark has recently started using computational tools to calculate the need for adjustment of the rails, which is quite novel in the European rail sector, thus Rail Net Denmark is accustomed to develop and implement computational tools to optimize maintenance and as decision making tools.

The members of the Danish Asphalt Industry are amongst the leading companies in Europe in terms of development of new sustainable asphalt products. Furthermore, the Danish Asphalt Industry is a pioneer when it comes to including long term durability and maintenance, when developing new products, due to the extensive use of performance contracts over the recent years. Arkil has the specific advantage of having cross-disciplinary specialist skills within asphalt and rail construction alike, which will be especially beneficial for the outlined project.
DTU has internationally renowned competences within the fields of computational and numerical modeling, geotechnics and materials, roadconstruction etc, which will through the newly introduced Railtech section be utilized when developing the mathematical model tool, which is the core of the project. DTI has extensive expert knowledge in a.o. road construction and pavements, concrete and other building materials, including product development for the industry. Furthermore, DTI is participating in national as well as international standardization work in testing, development, optimization and production of building materials (including asphalt), and has for many years been a key player when it comes to participating in innovation projects within the construction sector.

The use of asphalt in railway tracks as previously mentioned been evaluated in some countries for several years. In some of these countries, this has led to the establishment of a greater number of kilometers of track superstructure with asphalt. Still there are no international rules or regulations for this subject – for example is a track superstructure with asphalt is not included in the "Technical Specifications for Interoperability" (TSI). By implementing this discipline in Denmark in the near future, Denmark might have the possibility to influence which specific systems to the development of international rules, which is likely to be produced in future for this subject.

In Denmark there will be significant investments in railway projects over the next 10-15 years, most notably in connection with “Togfonden”. However, not all of these projects are relevant for the solutions contained in this project (not related to tracks). Some of the most relevant projects could be the new lines on respectively West Funen, Hovedgaard-Hasselager and a new bridge at Vejle Fjord. When the new track construction has proven ist suitability, these projects would be able to benefit from it, under condition that it will be ready by the time when detailed design of the above mentioned projects starts. In addition to the construction of new lines, the outcome of this project will be relevant for maintenance and renewal of existing tracks. Rail Net Denmark renews about 2.5% of its network per year (corresponding to 80-100 km), and when the new super structure has proven better than the current, it will probably be used as new standard, and hence used for future track renewal. In the maintenance area, Railnet Denmark expects on a time horizon of year 2030 an increase in required track adjustment and maintenance costs of 70% - solely caused by the increased train speeds. The introduction of a ballastless asphalt slab-track system will enable to reduce the total structural height, which will be an enormous benefit in relation to the electrification works and works in tunnels, at train station platforms etc., where the total height available is restricted/limited.

The asphalt based solutions will encompass the use of reclaimed asphalt from roads, in order to take a large step in the direction of circular materials flow – one of the primary goals for Miljøstyrelsen. Furthermore, the introduction of a construction system with a less permeable, more dense structure will reduce the risk of soil and groundwater pollution and finally the improved rainwater drain-off systems will ensure a better preparation for the future climatic changes.