a protective solution

It goes without saying that the railway relies heavily upon its supporting structures to ensure a safe and reliable service. Protecting steel bridges with today’s high performance anti-corrosive coating systems helps to minimise maintenance downtime and limit the number of disrupted journeys.

Rail bridges have a significant life expectancy. But environmental factors can have a damaging impact on steel structures if they are not adequately protected against corrosion – this can ultimately make the steelwork unsound, unsafe for use and a danger to the surrounding area. If, as a consequence, a temporary speed restriction has to be imposed, there are obvious implications for the timetable. Carrying out maintenance and repairs to bridges can not only disrupt journeys but can be time-consuming and highly costly to both the infrastructure owner and train operators.

As well as requiring long term corrosion protection, rail bridges are often iconic structures within their local landscape. For this reason, a durable and aesthetically pleasing finish is generally required from the protective coating system.

Coating formulation changes
Massive changes have been made in coating formulation for the heavy duty market. The two key drivers for these changes are the increasing levels of legislation and regulation to protect the environment by reducing the amount of volatile organic compounds (VOCs) emissions and higher customer expectations for coating performance, resulting from a need to increase the lifetime of their assets.

To meet these challenges, existing single-component technologies have been advanced and two-pack technologies introduced. Formulators realised that the performance obtained from two-pack epoxies and polyurethanes was significantly better then the majority of single-pack products and, as a result, anti-corrosive primers, high-build epoxies and durable finishes were among the new products introduced.

Modern technologies
A typical three-coat system consisting of a primer, intermediate and topcoat is popular in the infrastructure market for the likes of bridges. Epoxy primers and intermediates provide excellent steel protection in most environments, thus reducing the need for maintenance. Rapid overcoating properties help to improve productivity and low VOC contributes fewer solvent emissions. Low temperature cure versions of products allow painting all year round. The topcoat of a system is traditionally used for the cosmetic appearance on iconic structures and to provide a long-lasting finish.

Depending on the project requirements, a polyurethane or polysiloxane topcoat will offer a quality finish. Polyurethanes, when used as part of a system, provide resistance against the detrimental effects of natural sunlight. Such protection will maintain the cosmetic appearance for any bridge over an extended period of time. Alternatively, acrylic polysiloxanes have a colour and gloss retention which lasts up to four times longer then traditional polyurethanes. The absence of free isocyanate makes acrylic polysiloxanes ideal for offsite application, reducing health and safety concerns. This also avoids the high costs of applying traditional site coats and any related disruption to the schedule for new construction projects.

Advanced primers and intermediates can be applied as single coats and have the ability to be colour-tinted so there is no need to topcoat, particularly in areas where they are not exposed to UV light. This delivers increased efficiency as fewer coats have to be applied for both new construction and maintenance requirements.

Coatings supplier expectations
Rail companies should choose a coatings supplier with a full product range including the latest technologies to provide a single source supply, so minimising disruption and interfaces. Coating suppliers who have an extensive track record in supplying quality systems will have the expertise needed to offer the best guidance when it comes to choosing the right specification.

The specifying engineers who decide which coating system to use on a project expect it to provide a design life of 25 years, so choosing a system which offers long term corrosion protection is imperative and will minimise the level of maintenance required over the structure’s lifetime. Specifying engineers require a full set of approvals to allow them to choose a specification for Network Rail structures. Each tested and approved system needs to be certified on the correct surface preparation, stating a 25-year minimum service life requirement. Systems will be tested and approved over a combination of one or more of three surface preparation standards – mechanical/hand tool, wet blast and dry blast.

Additionally, rail companies should expect dedicated technical support from the coatings supplier to ensure that the coatings are applied correctly and that regular inspection intervals are carried out on the system.

Article courtesy of the rail engineer magazine.