Bituminous Ballast & Applications in Railway Construction
Bituminous ballast consists of mixture of aggregate and bitumen. The mineral aggregate varies from very fine dust (filler) to a maximum particle size, which is usually around 40 mm. By varying the composition of the mixture, the ratio of the various constituents and the particle size distribution of the aggregate, the properties of the eventual mixture can be adapted to suit the specific requirements of the construction. Depending on the mix composition and the quality of the constituent bitumen and aggregates, the bituminous ballast mixture may be either stiff and ofhigh stability and almost impermeable
A bituminous ballast construction may consist of one or more separate layers of possibly different composition. Depending on the design the various layers each perform a specific role in the construction
The properties of bitumen offer good opportunities to apply this type of material in railway track construction. Bituminous Ballast as Sub Ballast Layer. The rail ballast absorbs the train weight and distributes it from the rails to the sub grade, thereby avoiding any deformation. The rapid decay of the railroad level which occurs with traditional ballast construction is mainly due to the unsatisfactory “fatigue behaviour” of the ballast; this is mostly due to embankment settling.
By interposing a special semi-rigid layer (the so-called “sub-ballast”) in the area between the ballast and the embankment, the behaviour of the overall structure is greatly improved. The sub-ballast is normally laid on a highly compacted embankment layer.
The sub-ballast functions
- to create a working platform on which subsequent work operations, such as ballast and rail laying, are more easily undertaken;
- to assist in distributing the loads transmitted by passing trains
- to protect the embankment body from the seepage of rain-water and from seasonal thermal extremes (frost and thaw cycles
- to eliminate contamination of the ballast from fine material migrating up from the foundation
- to distribute the concentrated pressures and eliminate any “rupture” of the embankment
- A railway structure with sub-ballast works almost exclusively on compression and, therefore, differs from a traditional structure
- This consequently eliminates fatigue cracking. Especially on high-speedtracks maintaining levels and profile is of high importance.
- A higher stiffness has as a consequence of better load distribution to the ballast and sub ballast material. This will prevent an early deterioration of the rail geometric. In this case the use of bituminous ballast in a sub-ballast layer can offer the solution
Application of Bituminous Ballast in Railway Construction
The relatively high stiffness of the bituminous ballast sub-ballast layer will make a positive contribution to the compaction of the layers on top of the bituminous ballast layer. This improves the total stability. So the bituminous ballast mix as sub-ballast contributes to keeping the railroad geometry unaltered
Resistance to Vertical Deformation
The relatively high stiffness of the bituminous ballast layer compared to granular material will lead to less permanent vertical deformation by trainloads. The vertical loading conditions and the relatively short loading time are relatively small, so there will be no permanent deformation in the bituminous ballast layer
When a layer of dense bituminous concrete is used as a sub-ballast layer, optimal drainage of the total structure will be realized. The impermeable bituminous ballast as sub-ballast layer can prevent possible contamination of the sub-structure by vertical hydraulic transport of mud and fines.
ballast by the bituminous ballast layer, the ballast layer is strengthened and deterioration of the ballast is reduced. The bituminous ballast as sub-ballast layer increases the foundation modulus, providing a more rigid foundation, with the effect that there is a reduction of tension and shearing stress inside the ballast material, with consequently less fatigue and less degradation and wear of the individual aggregate particles. Because of the low air voids in the mix (1 – 3%) and because the bituminous ballast layer is buried, weather effects (temperature changes, Ultra Violet radiation, oxygen) will not affect the hot mix, so no deterioration (aging) of the bitumen will take place. Even if limited deformation of the sub-soil does take place, this will not affect the bituminous ballast layer because it is capable of withstanding the deformation without loosing its integrity because of the visco-elastic properties of bitumen.
Noise and Vibrations
mechanical properties of the bituminous ballast layer will lead to a reduction in the vibrations and noise produced by passing trains. The use of modified bitumen (polymer modified bitumen, rubber crumb) can further improve the vibration dampening effect of the sub-ballast