Across the rail network, the deterioration of a bridge structure due to the action of water, exacerbated by the presence of chloride ions, can cause serious maintenance problems, incur major expense and lead to inconvenient line closures.

The installation of an effective waterproofing membrane is therefore an essential part of any bridge building or refurbishment programme.

It’s over 40 years since the UK rail industry first recognised the seriousness of these problems, following investigative work by the then British Rail Civil Engineers body into the degradation of their structures, which identified defective waterproofing as the major cause.

Its working party set about identifying the ideal specification which an effective new waterproofing system must achieve and arrived at the following key criteria:

• Long and effective life

• Quick and easy, all year round application

• Capable of load bearing after one hour

• Tough enough to resist random site

• Bridge shrinkage cracks in concrete decks over a wide temperature range

• High bridge deck bond strength

• Easily repairable.

Rail industry relationship

There followed an extensive laboratory and site-based testing programme to achieve this and it was during this time that Stirling Lloyd’s close involvement with the UK rail industry began.

Approached by the plastics development unit to utilise its knowledge of polymer chemistry, Stirling Lloyd’s three years of laboratory tests and four years of on-site trials eventually arrived at the Eliminator waterproofing system.

This methyl methacrylate-based, liquid membrane system, spray applied in one or two coats, was the only one that could meet all key criteria in both laboratory and on-site conditions. The many advantages the system offered included easy detailing with no vulnerable joints, a high bond strength to the substrate, ease and speed of application to minimise track possession time, toughness with no protection boards required, crack building capabilities and tolerance of difficult site and climatic conditions enabling all year round application.

The Eliminator has gone on to enjoy some 40 years of success and continues to protect hundreds of high profile rail bridge structures at home and abroad, with international recognition from some of the world’s leading rail authorities. Throughout this time, Stirling Lloyd has continued to invest in the development of both the Eliminator and other polymer-based systems.

Remaining concerns

In pursuit of continuous improvement and success, Stirling Lloyd has maintained its ongoing dialogue with the rail industry. Though generally accepted that the best waterproofing is provided by a tough, flexible, tightly bonded system, it emerged, through these regular rail industry discussions, that certain situations would require the use of a non- bonded system.

The general opinion amongst most engineers remained that, in the long term, a fully bonded system would have the benefit of restricting any water penetration caused by localised damage or failure of the waterproofing, and that liquid systems were the most adaptable, accommodating changes in plane with ease and requiring little special attention at details and terminations.

However, two concerns remained, both relating to refurbishment possession work where conditions would remain unknown until the work commenced. The first concern was that liquid systems could not be applied if the substrate was of loose, friable material such as masonry arch fill. Whilst this was not strictly true as Eliminator has been successfully applied onto such substrates using a geotextile base, it did establish the need for Stirling Lloyd to provide better guidance concerning the use of its materials.

The second concern was the one element of a contract that no-one has any control over; inclement weather. Whilst low temperatures and high humidity create no problems, wet weather does, requiring either a suspension of the waterproofing work or the provision of suitable temporary protection.

It seemed that even the Eliminator fully bonded system was not immune to the problems caused by rain during a possession. As a result, Stirling Lloyd set up a development team to produce a system which was not inconvenienced by wet weather and could be offered alongside Eliminator.

Non-bonded system

The result was its Hytec flexible polypropylene waterproofing system, designed especially for fast track rail possession working, where deck or climatic conditions are uncertain, or for masonry arch bridges where no continuous deck exists.

This loose laid sheet system can be seamed using traditional hot-air welding which is quick, clean and highly effective, or by hot wedge or extrusion welding. Sheets of Hytec membrane may be seamed on-site or pre-formed into large tailormade sheets off-site prior to the works. The material is produced from a compound of polypropylene and ethylene propylene (EP) rubber and combines the heat weldable seaming benefits of traditional thermoplastics with the superior mechanical properties and resistance to ageing of thermoset elastomers.

Flexible polypropylene materials were first developed to provide a superior, but environmentally less damaging, alternative to traditional thermoplastics. They are now used in all major industrial sectors in over 100 countries throughout the world.

Hytec offers exceptional resistance to chemical attack, UV, tear initiation and propagation and puncture, with its excellent physical properties also helping to ensure a long service. These are maintained across an unusually broad range of temperatures even when exposure is for long periods. A high melting point of 140°C and retained flexibility and impact resistance down to – 40°C ensures handling is possible and integrity is maintained under all weather conditions.

Hytec is inert and completely resistant to stress cracking. It has an unusually low co- efficient of thermal expansion which reduces dimensional change, simplifying installation and handling as well as reducing stresses on seams and anchor points.

In short, the Hytec loose laid system can be applied under unreliable installation conditions where the substrate condition is unknown and under variable weather conditions, including very wet conditions, with no loss of quality.

Alternative systems

New structures, replacement bridges, deck replacements and the refurbishment of existing structures can all now be accommodated with a combination of non-bonded and bonded deck waterproofing systems, regardless of site or climatic conditions. Both systems conform to Specification NR/L3/CIV/041 Issue 3 and are PADS Registered. Eliminator’s catalogue number is 028/97002 while Hytec’s is 028/980492.

Much of London’s recent rail infrastructure improvements, carried out as part of the capital’s public transport network development ahead of the Olympic Games, has, for example, involved these systems.

The East London Line extension stretching from West Croydon to Dalston, and a major project to enhance the overground rail service to the Olympic Park site, has seen both Eliminator and Hytec installed. Some 22 bridges in total were waterproofed using these systems to provide long term protection for a total area of over 32,000m2.

Elsewhere in London, following earlier successes at Waterloo International Terminal and on various bridge and viaduct structures along the Channel Tunnel Rail Link (now HS1), Eliminator was also chosen as the ideal waterproofing system during the major renovation project at St Pancras Station. Here too it was used on all rail bridges as well as to waterproof the track wells, providing essential long-term protection for the retail area below the refurbished station.