IN Europe, approximately 2.5 million tonnes of the 3.2 million tonnes of end-of-life tyres generated every year are recycled. Whether they are re-grooved and reused, shredded and used for other rubber applications, or for fuel and glassification, their natural flexibility means they remain an overtly useful material.
End-of-life tyres also have potential for track applications. Rubber polymers have been used for decades beneath track structures as a means of reducing vibrations from passing trains and a range of manufacturers now offer various under sleeper pad solutions to meet this need which is a requirement of many railways. Yet these products do not tend to use recycled materials.
Recognising this potential for end-of-life tyres in these applications, the three-year Recytrack project was established in 2011. Funded by the European Commission under the Life+2010 initiative, the project aims to demonstrate that an elastomeric eco-friendly material made of end-of-life tyres blended with resin can satisfy railway vibration reduction requirements.
The Recytrack system consists of a continuous mat which is installed under the ballasted superstructure, and as a rubber boot for isolated block systems installed as sleepers in concrete-based superstructures.
Initially work focused on developing the design of potential Recytrack product. An FEM model was developed in order to study the mechanical properties. This was subsequently fed into the simulation program, Ansys, which includes several theoretical models with the tools to evaluate various models using experimental data. Material constants are obtained by least square adjustment, and it was found that the model that best reproduces the existing material’s behaviour is the Mooney-Rivlin’s 5-parameters model.
In addition, engineers developed a superstructure model which takes into account the two excitation types that generate the axle loads produced at the wheel/rail interface:
- quasi-static excitation, which is caused by the static component of axle loads moving along the track, and
- dynamic excitation, which is the result of the spatial variation of the support stiffness along the track as well as wheel and rail roughness.
The superstructure is modelled as a two-layer continuously-supported model where the rail is assumed to be an Euler-Bernoulli* beam, the sleepers are considered as completely rigid elements and the contact force between sleepers and soil is a continuous function in the longitudinal track direction. In the transversal track direction, it is considered a rectangular function.
The elastomeric products effectiveness in terms of vibration reduction is evaluated by Insertion Loss (IL), the parameter used to define the difference of vibration level in a specific location due to the insertion of an under ballast material (UBM) or isolated block system (IBS) in the railway superstructure. IL is calculated:
In order to characterise the properties that define the materials, different mechanical tests were developed at Ladicim laboratory. The parameters obtained were the Poisson’s Ratio v, the static and the dynamic modules, Csta and Cdyn, the horizontal static module Csta,h, dynamic fatigue, and dynamic characterisation between 1 and 80Hz in order to obtain the elastomeric material’s equivalent dynamic parameters, kbm, cbm and nbm.
Tests were developed using a servo-hydraulic actuator with a ±100kN load cell that applies the necessary load cycles to obtain the aforementioned parameters. Equivalent dynamic parameters of the material are obtained from the experimental hysteretic loops adjustment to a 1 DOF system.
With the researchers satisfied with the system, the project moved into the field testing phase, which encompassed two specific trials on the Spanish network: installation of an under-ballast mat demonstrator beneath an at-grade ballasted 1668mm-gauge track with concrete sleepers at Aranda de Duero in Burgos, which is used solely by freight trains; and an isolated-block demonstrator which was installed in the El Cabañal tunnel on the Mediterranean Corridor in the suburbs of Valencia. This section has dual-gauge track, and is used by passenger and freight trains. Here the track superstructure is based on a concrete slab directly attached to the tunnel invert with concrete blocks which act as sleepers isolated from the invert slab by a Recytrack boot.
The under-ballast mat demonstrator was also installed in new railway infrastructure which is yet to open to commercial traffic. Here a self-developed excitation prototype capable of generating the vibration created by passing trains was developed and used. During operation, the prototype system is placed on the rails and uses an electric motor which rotates to generate a variable frequency in the range 1-80Hz which reflects a passing train.
To measure the impact on vibrations, force sensors are installed between the excitation machine supports and the rails, with different types of accelerometers placed on the rail, sleepers and the ground. In order to verify the under-ballast mat’s vibration attenuation capability, measurements using the excitation system were carried out at two sections of the same railway infrastructure: a section with under-ballast mat and another one without under-ballast mat. Using the results of these measurements and an inversion model, the vibratory behaviour of the Recytrack under-ballast mat is obtained.
In the case of the isolated-block demonstrator, a temporal vibration signal from 12 seismic accelerometers is acquired simultaneously from the rail, block, slab and tunnel wall during regular commercial railway operations with vibrations from the operation of freight, commuter and regional trains recorded.
The results of the tests show similar vibration levels for Recytrack track and standard track, although for rail and block, spectra corresponding to Recytrack track is slightly higher (+3 dB in the 1-316Hz frequency range.) Likewise, when using Recytrack rail-tunnel wall transmission loss is higher than that from the standard track (+4.5dB in the 1-316Hz frequency range.) These differences are particularly high at low frequencies, particularly in tunnels where it is important to minimise the impact on nearby buildings.
The Recytrack project has developed two elastomeric products based on end-of-life tyres to reduce ground-borne vibrations caused by railway traffic. The experiments have demonstrated that its vibratory behaviour is similar to that of existing products which are not based on recycled products. It has therefore successfully demonstrated that Recytrack elastomeric products can be used as vibration abatement solution in railway infrastructure.
This article was written by: Joan Cardona, AV Ingenieros; Robert Arcos, Acoustics and Mechanical Engineering Laboratory (LEAM), Technical University of Catalonia, Barcelona; Faiver Botello, Acciona Infraestructuras; José A Casado, Materials Science and Engineering Laboratory (Ladicim), University of Cantabria, Santander; José Conrado with Spanish infrastructure manager Adif.