The invention refers to a track bed for a railroad track, comprising track ballast made from particulate matter and a multitude of railroad ties supported on the track ballast, wherein at least one region of the track ballast is stabilized by means of a bonding agent that bonds together particles of the particulate matter.

Further, the invention refers to a method of stabilizing a track bed for a railroad track, said track bed comprising track ballast made from particulate matter and a multitude of railroad ties supported on the track ballast.

Track ballast forms the track bed, upon which railroad ties are laid. The track ballast is packed between, below and around the ties and is typically made of crushed stone.

With regard to the shape of the particles, it is important that the stones are irregularly cut and have relatively sharp edges, so that they properly interlock and grip the ties in order to fully secure them against movement. ^'

The mechanical strength of the track ballast is essential for the stability of the railroad track bed. Since ballast consists of granular material, its strength and failure properties are determined by the frictional contact

interactions between the ballast particles. There have been many attempts to improve the mechanical stability of track ballast so as to improve its lifetime arid reduce

maintenance operations.

Some solutions comprise cohering the individual particles of the particulate matter into a coherent elastic structure with a polymeric bonding agent. For example, it is known from EP 1 619 305 Bl to foam up the cavities of a ballast bed of a railway track with polyurethane (PU) . For this, the reactants isocyanate, polyol and additives are mixed up as foaming agent and introduced into the cavities of the ballast bed where they react to form polyurethane foam.

Alternatively, it is also known to spray or pour polymeric resins onto the surface of the track ballast. This solution is effective technically, but is complex to implement because of the exposure of the workers and the environment to reactive and often harmful chemicals. This requires particular precautions on the jobsite to protect the workers, and to minimise that excess product is released into the environment, which would typically contaminate ground water. Furthermore, the resins that are used are relatively expensive.

The advantages of binding the individual particles of the particulate matter of the track ballast into a coherent structure by means of a bonding agent comprise enhanced track stability, resistance to rock displacement and improved riding qualities. With these advantages, however, a major drawback has been the inability to release the ballast from its consolidated condition for repairs to the road bed without major destruction of the consolidated material. Further, the particulate matter, once removed from the track for maintenance purposes, cannot be easily recycled due to its material composition comprising the polymeric bonding agent.

Therefore, the invention aims at providing improvements in stabilizing track ballast and to overcome the drawbacks of the prior art solutions as described above.

To solve these and other objects, the invention according to a first aspect thereof provides a track bed for a railroad track, comprising track ballast made from

particulate matter and a multitude of railroad ties supported on the track ballast, wherein at least one region of the track ballast is stabilized by means of a bonding agent that bonds together particles of the particulate matter, characterized in that the bonding agent is based on a hydraulic binder, in particular cement, and that the bonding agent leaves free voids between the bonded

particles so that the track ballast has a water draining capability in said at least one stabilized region.

By using a bonding agent that is based on a hydraulic binder, a strong bond can be achieved between the

individual particles of the particulate matter of the track ballast. Hydraulic binders are widely used in the

constructional industry and are known in a great number of different variations and mixtures so that the invention makes available a proven technology for stabilizing track ballast. The handling of hydraulic binders at the jobsite is safe and does not require specific safety measures.

Neither the workers nor the environment is exposed to harmful chemicals. Furthermore, hydraulic binders are much less expensive than polymers, such as polyurethane foam or resins .

Another advantage is that the bonding agent is mineral- based instead of organic-based so that the coherent

structure consisting of the particulate matter (in

particular crushed stone) and the bonding agent is altogether a mineral material that can easily be recycled.

Furthermore, using a bonding agent based on a hydraulic binder enables the stabilisation of the track ballast without modifying the actual track design.

According to the invention, the bonding agent leaves free voids between the bonded particles so that the track ballast has a water draining capability in said at least one stabilized region. Therefore, the bonding agent is applied in such a way that fluid flow can occur through the track ballast so that rainwater can drain, in particular towards the outer sides of the track ballast. The draining capability requires that the voids arranged between the particles of the particulate matter are interconnected resulting in a certain degree of open porosity of the track ballast .

In order to keep the water draining capability of the track ballast, a certain amount of bonding agent should not be exceeded, since the bonding agent would otherwise

completely fill the voids between the particles, which in turn would impair the draining capability. According to a preferred embodiment, the stabilized region has a mass ratio of bonding agent to track ballast of 1:10 - 1:20, in particular about 1:12.

Hydraulic binders are substances used in construction that set in the presence of water and harden. By adhering to the particles of the track ballast, the bonding agent based on a hydraulic binder binds the particles together. According to a preferred embodiment, the bonding agent is a hardened cement slurry, in particular a hardened cement paste or cement mortar. A cement paste is a mixture of cement and water as well as optionally admixtures. A mortar is a mixture of cement, water and fine aggregates as well as optionally admixtures .

Preferably, the hydraulic binder is a Portland cement binder, wherein the Portland cement preferably is a cement of the type CE I, CEM II or CEM III.

In order to provide stabilisation to the track ballast, it is not necessary to have all the particles of the entire track ballast bonded with each other. Rather, stabilising only specific regions of the track ballast may be

sufficient, in particular with a view to stabilising the track ballast when it is already placed with railroad ties already being supported on the track, ballast and rails fixed to the ties. Further, the bonding of particles is not desired directly below the railroad ties, because the particles should remain free in this region to move in order to provide a lateral confinement of the structure.

Therefore, according to a preferred embodiment, the track ballast comprises a central region located below said railroad ties and side regions arranged on both sides of said central region, wherein only the side regions are stabilized by said bonding agent. In particular, the side regions correspond to the outer slopes of the track ballast on either side of the ties.. The track ballast in the central region can be easily replaced and/or compacted without breaking the bonding in the side regions.

According to a second aspect the invention provides a method of stabilizing a track bed for a railroad track, said track bed comprising track ballast made from particulate matter and a multitude of railroad ties supported on the track ballast, wherein particles of the particulate matter are bonded together by means of a bonding agent that is based on a hydraulic binder, in particular cement, said bonding agent being applied to the particles of the particulate matter so as to leave free voids between the bonded particles so that the track ballast has a water draining capability.

Preferably, the bonding agent comprises or consists of a cement slurry, in particular a cement paste or a cement mortar .

Preferably, the hydraulic binder is a Portland cement binder. Portland cement as used in the invention may be any type of Portland cement, whatever its chemical composition is. Suitable cements used in the invention preferably are the cements described according to the European EN 197-1 Standard of April 2012 or mixtures thereof, whereby

Portland cement clinker is defined as being "a hydraulic material, which shall consist of at least two-thirds by mass of calcium silicates, (3 Ca0-Si02, and 2 CaO-Si02), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of CaO to Si02 shall not be less than 2.0. The magnesium oxide content ( gO) shall not exceed 5.0% by mass".

Preferably cement of the types CEM I, CE II, CEM III, CEM IV or CEM V can be used. Preferably, the Portland cement is a cement of the type CEM I, CEM II or CEM III. The European norm EN 197-1 defines five classes of common cement that comprise Portland cement as a main constituent.

- CEM I (Portland cement) comprising Portland cement and up to 5% of minor additional constituents

- CEM II (Portland-composite cement) comprising Portland cement and up to 35% of other single constituents

- CEM III (Blastfurnace cement) comprising Portland cement and higher percentages of blastfurnace slag

- CEM IV (Pozzolanic cement) comprising Portland cement and up to 55% of pozzolanic constituents

- CEM V (Composite cement) comprising Portland cement, blastfurnace slag or fly ash and pozzolana.

In another embodiment, the hydraulic binder contains high amounts of aluminate phases, namely where the cumulated alumina content of the binder is between 10wt.-% and 50wt.- %. These aluminate phases can be provided by a specific cement, such as a calcium sulphoaluminate cement, a calcium aluminate cement, or a specific binder such as that

described in EP 1781579. In a preferred embodiment, such a binder can be a mixture of 50wt.-% CEM I, 25wt.-% gypsum, and 25wt.-% calcium aluminate cement, such as Ciment Fondu produced by Kerneos.

Preferably, the track ballast comprises a central region located below said railroad ties and side regions arranged on both sides of said central region, wherein the bonding agent is only applied to the particles of the side regions.

The bonding agent can be applied to the track ballast in various ways. According to a first alternative, which is preferred, the bonding agent is poured or sprayed onto the track ballast from above in at least one region to be stabilized. Preferably, compressed-air spraying is used, which allows to deeply spray bonding agents having

different fluidity. The bonding agent that is poured or sprayed onto the surface of the track ballast will also be distributed within the track ballast under the influence of gravity .

According to a second alternative, the bonding agent is mixed with the particles of said particulate matter and the resulting mixture is placed as track ballast on a track formation. This embodiment can be suitable for the

construction of new lines, or for when the aggregates of the ballast are replaced in large maintenance operations.

In order to safeguard that the bonding agent, in particular when it is sprayed or poured onto the track ballast, effectively impregnates the bed of track ballast and bind the particles throughout the depth of the bed on the one hand and does not clog the voids between the particles on the other hand, the bonding agent should have a low

viscosity and/or a high flowability when being applied. In this connection, a preferred embodiment provides that the flowability of the bonding agent, at the time of

application, is 2-7 seconds, preferably 2-4 seconds, with a specific funnel test method that is identical to the method described in ASTM D6910/D6910M-09, and with two

modifications :

- The funnel's dimensions used are: internal top cone diameter: 149 mm, internal bottom cone diameter:

17.3 mm, bottom tube height 30.3 mm, total height 190 mm (comprising cone and bottom tube) .

- The time measured to characterize the flowability

corresponds to the flow of 0.6 L of the product, and not 1 L as in ASTM D6910/D6910M-09.

Alternatively, the flowability may also be expressed in terms of the spread of the slurry, wherein a preferred embodiment provides that the bonding agent has an initial spread of at least 90 mm, measured according to a test method as described in EN 12350-8 referring to the testing of fresh concrete/self-compacting concrete, but with the dimensions being adjusted for slurries (height 57 mm, internal diameter at top: 21 mm, internal diameter at bottom: 37 mm), 5 min after mixing.

European Standard EN 12350-8 specifies the procedure for determining the slump-flow and t500 time for self- compacting concrete. The test is suitable for specimens having a declared value of D of the coarsest fraction of aggregates actually used in the concrete (Dmax) not greater than 40 mm.

The adjustment of the rheological properties of the bonding agent, in particular depending on the application method (pouring or spraying) , may be achieved by methods known to the person skilled in the art, such as admixture selection and dosage and/or water to cement ratio variations .

In order to increase the flowability of the bonding agent, the Portland cement, according to a preferred embodiment of the invention, has a specific surface (Blaine) of 3000 - 10000 cm ² /g, preferably 3500 - 6000 cm ² /g.

Preferably, the bonding agent has a water/binder ratio of 0.4 to 0.6, where the mass of binder includes the Portland cement and, if any, mineral particles, such as slag, fly ash, silica fume, natural or synthetic pozzolans, limestone fillers, siliceous fillers, calcined clays, or mixtures thereof .

Preferably, the bonding agent, in particular the cement paste or the cement mortar, comprises a water reducer, in particular a plasticiser or super-plasticiser, such as a polycarboxylate based or a polynaphthalene sulfonate based water reducer. A water reducer makes it possible to reduce the amount of mixing water for a given workability by typically 10-15% or to increase flowability for a given water/binder ratio. By way of example of water reducers, mention may be made of lignosulphonates, hydroxycarboxylic acids, carbohydrates, and other specific organic compounds, for example glycerol, polyvinyl alcohol, sodium alumino- methyl-siliconate, sulfanilic acid and casein.

Super-plasticisers belong to a new class of water reducers and are capable of reducing water contents of mixing water, for a given workability, by approximately 30% by mass. By way of example of a superplasticizer, the PCP super- plasticisers without an anti-foaming agent may be noted. The term "PCP" or "polyoxy polycarboxylate" is to be understood according to the present invention as a

copolymer of acrylic acids or methacrylic acids and their esters of polyoxyethylene (POE) .

Preferably, the cement slurry comprises 0.05 to 1%, more preferably 0.05 to 0.7% of a water reducer, a plasticiser or a superplasticizer, percentage expressed by mass

relative to the dry cement mass.

The setting time can be adjusted depending on the requirements. A fast setting mortar is for example suitable in maintenance operations that are carried out during night time maintenance slots of existing lines, and when the railway tracks need to be reopened the following morning. The reduction of the setting times is then carried out by using known strong set time accelerators regularly

purchased from admixture suppliers. These products may be formulations based on calcium or sodium nitrates, nitrites, chlorides, thiocyanates, or aluminium sulphates.

Preferably, the bonding agent used in the invention

comprises 0.05 to 2.5 wt.-% of an accelerator, expressed as dry mass relative to dry cement mass.

According to an embodiment of the invention, other

additives may be added to the bonding agent. Such additives may be setting retarders, coloured pigments, film forming agents, hydrophobic agents or de-polluting agents (for example zeolites or titanium dioxide) , latex, organic or mineral fibres, mineral additions or their mixtures.

According to an embodiment of the invention, the bonding agent used in the invention may further comprise mineral particles. Preferably, the bonding agent may comprise 15 75% of mineral particles, more preferably from 15 to 65%, most preferably from 20 to 55%, the percentages being expressed by mass relative to the dry mass of cement.

The suitable mineral particles have a maximum particle size of 200 micrometres are selected from calcium carbonate, silica, ground glass, solid or hollow glass beads, glass granules, expanded glass powders, silica aerogels, silica fume, slags, ground sedimentary siliceous sands, fly ash or pozzolanic materials or mixtures thereof.

The mineral particles used according to the invention may be slags (for example, as defined in the European NF EN 197-1 Standard of April 2012, paragraph 5.2.2), pozzolanic materials (for example as defined in the European NF EN 197-1 Standard of April 2012, paragraph 5.2.3), fly ash (for example, as described in the European NF EN 197-1 Standard of April 2012, paragraph 5.2.4), calcined schists (for example, as described in the European NF EN 197-1 Standard of April 2012, paragraph 5.2.5), material

containing calcium carbonate, for example limestone (for example, as defined in the European NF EN 197-1 Standard paragraph 5.2.6), silica fume (for example, as defined in the European NF EN 197-1 Standard of April 2012, paragraph 5.2.7), siliceous additions (for example, as defined in the "Concrete" NF P 18-509 Standard) , metakaolin or mixtures thereof .

Fly ash is generally pulverulent particles comprised in fume from thermal power plants which are fed with coal. Fly ash is generally recovered by electrostatic or mechanical precipitation .

Slags are generally obtained by rapid cooling of molten slag resulting from melting of iron ore in a blast furnace.

Silica fume may be a material obtained by the reduction of very pure quality quartz by the coal in electric arc furnaces used for the production of silicon and alloys of ferrosilicon . Silica fume is generally formed of spherical particles comprising at least 85% by mass of amorphous silica . The pozzolanic materials may be natural siliceous and/or silico-aluminous materials or a combination thereof. Among the pozzolanic materials, natural pozzolans can be

mentioned, which are generally materials of volcanic origin or sedimentary rocks, and natural calcined pozzolans, which are materials of volcanic origin, clays, shale or

thermally-activated sedimentary rocks.

With regard to the amount of bonding agent used to bond the particles of the particulate matter with each other, a preferred embodiment provides that the bonding agent is poured or sprayed onto the track ballast in an amount of 40-70, preferably 50-70, more preferably 55-65, litres per m ² of track ballast surface and per m of track ballast thickness .

Preferably, the bonding agent is poured or sprayed onto the track ballast in such an amount that a homogeneous bed of said bonding agent is formed at the bottom of the track ballast, said bed preferably having a height of 5-20 mm. In this embodiment the bonding agent is applied in excess so that the bonding agent forms a bed at the bottom of the track ballast so as to increase the stability of the ballast. In this particular embodiment, the fluidity of the bonding agent must be so high that the voids between the particles remain at least partially free from the bonding agent so that the track ballast remains permeable to water.

According to a preferred embodiment of the invention, the flexural strength of the bonding agent after having set and hardened is selected to be 1-5 MPa, in particular 1-2 MPa (measured on prisms having a dimension of 4*4*16 cm according to EN 196-01 24h after mixing) . In this way, the cohesion provided by the bonding agent is enough to stabilize the track ballast, but not too high, so that ballast replacing machines are able to break the bonding points and allow the replacement of treated ballast.

The appropriate thickness of a layer of track ballast depends on the size and spacing of the ties, the amount of traffic on the line, and various other factors. The

thickness of the track ballast preferably is greater than 150 mm. With high-speed railway lines 'the thickness of the track ballast may be up to 500 mm.

The invention will now be described in more 'detail with reference to an exemplary embodiment illustrated in Fig. 1. Fig. 1 shows a cross section of a track bed 1 for a

railroad track. The track bed 1 comprises a sub-ballast layer 2 and track ballast 3 made from particulate matter. A multitude of railroad ties 4 are supported on the track ballast 3, wherein rails 5 are fixed to the railroad ties 4. In side regions 6 of the track ballast a bonding agent based on a hydraulic binder has been applied so that the particles of the track ballast are bonded together.

The invention will also be described in more detail with reference to the following examples.

Example 1

This example illustrates the bonding capacity of the bonding agent of the present invention and the possibility to bond a specific given thickness of the track ballast by depositing the suitable quantity of bonding agent per unit area of ballast.

The following materials were used for the test:

Bonding agent: A Portland cement mortar with the following composition:

CE I 52.5 R (Lafarge France Le Teil plant): 750 parts by weight

Limestone filler (BL 200) : 375 parts by weight Sand with a particle size of 0-1 mm: 833 parts by weight

Water: 420 parts by weight

The cement mortar was prepared by mixing the cement, the filler and the sand in a Perrier planetary mixer during 15 sec. Thereafter, water was added to the mixture during a time period of 30 sec and the mortar was mixed during 2 minutes at a slow speed.

The fresh cement mortar has the following properties:

Slump flow (method inspired from EN 12350-8 referring to the testing of fresh concrete/self- compacting concrete, wherein the dimensions are adjusted for slurries (height 57 mm, internal diameter at top: 21 mm, internal diameter at bottom: 37 mm) 5 min after mixing) : 110 mm Modified Marsh Funnel: 2.5 seconds

The cement mortar once hardened had a 24h compressive strength of 6 MPa and a 24h flexural strength of 1 MPa (measured on prisms having a dimension of 4*4*16 cm according to EN 196-01 24h after mixing) .

Ballast: Glensanda Ballast 35-65 having particle sizes of 35-65 mm. 3 +/-1 kg of ballast was placed in a bucket (30 cm deep,0 cm diameter) and compacted 30 seconds on a vibrating table. The ballast height after compaction was obtained by the average value of 4 height measurements (Hi) . The effective mass of ballast was measured (Ml) . The apparent specific weight of the ballast (Rl) and the ballast porosity (PI) were calculated.

A given mass of bonding agent (M3) was deposited

homogeneously at the top surface of the ballast and then sealed in order to prevent any water loss which may cause weight measurement artefacts. This sealing was done purely for the purpose of this test. 24 hours after the

application, the bonded ballast was unmoulded and the mass of the bonded ballast (including the bonding agent) was measured (M2). The fraction of the ballast bonded was evaluated first by calculating (M2-M3)/Ml.

If any, the excess of bonding agent was quantified by determining the height of the bonding agent layer (H2) at the bottom of the bucket. The volume of excess bonding agent per surface unit was calculated: H2/P1

Three tests were performed, wherein the test differed primarily in the mass of the bonding agent (M3) used.

The results are shown the following table:

Test Test 1 Test 2 Test 3

Ballast Mass Ml (kg) 22 23.55 22.44

Ballast Height HI (cm) 22 23 22

Specific weight Rl (kg/m3) 1415 1449 1443

Porosity (-) 46% 44% 44%

Mass of treated ballast M2 (kg) 23.06 25.78 25.56

Mass of bonding agent applied 1.06 2.23 3.12 M3 = M2 - Ml (kg) Volume of bonding agent per 7.5 15.7 22 unit area of ballast (L/m ^z )

Mass of bonded ballast with 10.32 25.52 25.34 bonding agent M4 (kg)

Mass of bonded ballast without 9.26 23.29 22.22 bonding agent M5 = M4 - M3 (kg)

Wt.-% of ballast bonded 42 99 99

Depth of bonding (cm) 9 23 22

Thickness of excess bonding 0 0.5 2 agent H2 (cm)

Volume of excess bonding agent 0 0.13 0.63 in the bucket (L)

Volume of excess bonding agent 0 1.8 8.9 per surface unit (L/m ² )

Test 2 shows that with the application of 15.7 L/m ² of bonding agent a thickness of 23 cm of ballast is bonded. An homogeneous layer of 5mm of bonding agent is found at the bottom of the bucket.

Test 1 shows that with a lower amount of bonding agent per surface unit the depth of bonding is reduced to 9cm. The percentage of ballast bonded is surprisingly closely proportional to the amount of bonding agent deposited (42% versus 47% = 7.5/15.7) .

Test 3 shows that with an amount of 22 L/m ² of bonding agent all ballast is fully bonded. A homogeneous layer of 2 cm of bonding agent is measured at the bottom of the bucket.

It is observed that in tests 2 and 3 the bonding agent was deposited in excess, When subtracting this excess volume and normalize it by the thickness of ballast bonded, one can calculate the minimal volume of bonding agent needed per ballast layer thickness unit:

For Test 2: (15.7-1.8) /0.23 = 60.7 L/m ² /m, i.e.

60.7 litres of bonding agent per surface unit and per metre (depth) of ballast.

For Test 3: ( 22-8.9 ) /0.22 = 59.7 L/m ² /m, i.e. 59.7 litres of bonding agent per surface unit and per metre (depth) of ballast.

Example 2

In example 2 a number of different cement slurries were prepared that are suitable as bonding agent according to the invention.

Table 1 illustrates the impact of different types of binder :

Table 1 Table 2 illustrates the impact of different types of sand:

Table 2

Table 3 illustrates the impact different types

admixtures :

Table 3

In the above examples, the following materials were used: Materials Source

CE I 52.5 R Lafarge France, Le Teil plant

CEM II/A 4.2.5 R Lafarge France Val D'Azergues plant

Holcim Croatia, Adria Cement Koromacno

CEM τττ/Α 42.5 N plant

Lafarge France, Le Teil plant, blaine

Finer CEM I 52.5 R fineness 6000 m ² /g

Mixture of 50wt.-% CEM I 52.5 R, 25wt.-% gypsum, 25wt.-% calcium aluminate cement

Aluminate Cement (Ciment Fondu from Kerneos)

Limestone Filler Omya BL 200

Fly Ash Cordemais plant

0-1 Sand Sibelco BE 01

0-2 Sand EN Normalized sand

Superplasticizer Chryso Premia 180

Accelerator CaCl ₂

Fibres Chryso Syntec 12

Latex Waker Etonis

Thickening agent CP Kelco - Kelco-crete

In the above examples, the bonding agent was prepared by the following method: The cement mortar was prepared by mixing the cement, the filler and the sand in a Perrier planetary mixer during 15 sec. Thereafter, water and

additives were added to the mixture during a time period oJ 30 sec and the mortar was mixed during 2 minutes at a slow speed.

The material properties were measured as follows:

The slump flow was measured with a method inspired from EN 12350-8 referring to the testing of fresh concrete/self- compacting concrete, wherein the dimensions are adjusted for slurries (height 57mm, internal diameter at top: 21mm, internal diameter at bottom: 37mm) 5 min after mixing.

The flow time was measured by the modified Marsh funnel test, using the protocol described above. The compressive strength and the flexural strength were measured on prisms having a dimension of 4*4*16 cm according to EN 196-01 24h after mixing.