Photo: Norwegian National Rail Administration
Stone wool based anti-vibration mats exhibit outstanding efficiency and durability even after decades of heavy-duty main line operation. This has been verified, for instance, by an exhaustive Norwegian study of the long-term Insertion Loss performance and material characteristics of stone wool based anti-vibration mats under harsh climatic conditions.
RockDelta Stone Wool Based Anti-vibration Mat: The Perpetual Spring
At first visual encounter with high-resilience RockXolid® and RockBallast® anti-vibration mats, stone wool may appear to be a delicate material. However, in reality, nothing could be further away from the truth as stone wool fibres exhibit outstanding resistance to mechanical stress fatigue and resistance to influences from the environment in which the track is located. Concerning the latter, of special interest is the fact that this includes a uniquely high degree of resistance to dissolution and functional performance degradation when in contact with or even when fully submerged in a liquid. From acid rain to pH neutral liquids stone wool fibres are durable to a degree that is hardly matched by most other full-contact, sheet-type, elastic materials for efficient track vibration mitigation treatment.
Long-term Experiences from Norway
Owing to the excellent and ultra-stable material properties, stone wool based anti-vibration mats have been applied in track vibration mitigation applications for decades. The Norwegian national railways, NSB, have been among the first to make use of the many benefits of stone wool anti-vibration mats in track design.
For instance in 1978, when optimising measures by which to protect the Oslo cathedral against structure-borne vibrations from a new tunnel track beneath the cathedral, NSB chose stone wool based sub ballast mats.
During a 1996 field study, stone wool anti-vibration mat samples were excavated from the Oslo cathedral tunnel track and subjected to thorough measurement and analysis. The analysis indicated that the sub-ballast mats had retained their excellent functional characteristics and had exhibited a constant insertion loss throughout the entire operational life span of the stone wool material.
Important Findings of the 1996 Field Study
Over the full period of 18 years, it was clear that track sections with stone wool based sub ballast mats showed no need for additional maintenance compared to track sections without stone wool sub ballast mats:
- No “setting” of the ballast could be observed
- No contamination of the inside of the mats had occurred (e.g. due to ballast disintegration and the forming of sand deposits)
- Static loading from ballast and track superstructure and dynamic loading from passing trains had had no impact on the degradation of the stone wool fibres
- Building upon such valuable long-term experiences, RockBallast® and RockXolid® stone wool based anti-vibration mats have – under the scrutiny of leading European test institutes – been developed, tested and approved for use within the field of track vibration isolation.
The fatigue life cycle test – designed by The Technical University of Munich, Chair and Institute for Road, Railway and Airfield Construction and performed at the Technical University of Denmark – included RockDelta stone wool based anti-vibration mat samples exposed to an incredible 100 million load cycles in a spring-mass system with a static load of 15 kN/m2. Careful control measurements of the dynamic stiffness at Ingemansson Technology AB in Sweden showed no significant change of the dynamic stiffness of samples when comparing aged product to virgin state level.
Large-scale Fatigue Life Test at The Technical University of Denmark
A hallmark of the RockBallast® and RockXolid® product lines is the exceptional long product life cycle, owing, amongst other things, to the unique fatigue resistance of stone wool fibres.
This has been verified by a large-scale life cycle test performed by The Technical University of Denmark, Laboratory of Building Physics and Design in co-operation with Ingemansson Technology AB of Sweden and with control and final conclusion of the test by the Technical University of Munich, Chair and Institute for Road, Railway and Airfield Construction.
RockXolid® samples were exposed to an incredible 100 million load cycles in a spring-mass system with a static load of 15 kN/m2. For 97,5 million cycles the maximum (sinusoidal) load was 30 kN/m2 and for 2,5 million cycles the maximum load was 40 kN/m2. The response deflection of the mass was sampled and the response remained constant during the period of testing.
The conclusion was clear and unambiguous: Virtually no change of the dynamic stiffness of the samples when comparing aged product samples to virgin state level could be detected.
