Glass flake technology is central to the paint systems that are being used on the Forth Rail Bridge in Scotland Dave Bottomly explains.
The famous Forth Rail Bridge in Scotland is set in an aggressive, sometimes hostile environment, spanning the Firth of Forth between North and South Queensferry.
The bridge is 117 years old and is constructed from 53,0001 of steel, held together by 6,5 million rivets, It was opened in 1890 and was built using some of the first Bessemer steel produced in quantity in the United Kingdom, The structure is 110m high from top to high water level and is 2,467m long; it took seven years to build.
The bridge carries around 150 trains each day which equates to approximately three million passengers and eight million tonnes of freight per year.
The steel structure is very complex in that it includes a significant number of rivets, back-to-back angles, sandwiched steel plates and corrosion traps. In total 230,000m² of steel must be painted to protect it from corrosion. As well as the challenges faced by painting contractors in terms of access, encapsulation, logistics and so on. Network Rail specified a system that would offer 25 years' protection against corrosion, The steel on this bridge is subject to coastal conditions with moderate to high salinity and needs protecting against a very aggressive environment; including high winds and sea mists - which can also significantly reduce the number of days suitable for painting.
When the structure was owned by British Rail, the number of painting days per year was around 100. However, this work was carried out by British Rail personnel and without the benefits of modern scaffolding and encapsulation.
Traditionally the Forth Rail Bridge has been protected with single-pack paints that reflect the development of such materials by the UK paint industry, This ranges from drying oil-based materials, such as red lead in linseed oil, via oleo-resinous paints to synthetic alkyds, When the steel was first produced it is believed that all the members and plates were thoroughly cleaned with steel scrapers and then given a coat of boiled linseed oil, which was applied as hot as possible. Following this, and in some cases before erection, two coats of red lead paint were applied. This in turn was followed by two further coats of iron oxide pigmented paints, a dark brown undercoat and a bright red finish.
Clearly the use of steel scrapers had little or no effect on the millscale and large areas of this were still intact prior to the most recent repainting contract being let.
For the first 100 years or so of the bridge's life, British Rail employed its own painting crew to maintain the structure and virtually everything was applied by brush - surface preparation during this period also tended to involve hand tools. Paints were relatively low volume solids materials and consequently multi-coat systems were specified. In later years, British Rail concentrated its efforts on the worst-affected areas.
The manual preparation methods used at each of the repainting periods resulted in very little of the existing paint work being removed each time. Clearly this resulted in a significant build-up of paint thickness and each additional repaint increased the stress within the system. At a certain point, these stresses exceeded the cohesive strength of the coating system causing failure at the weakest point.
This type of failure pattern became more evident during the late 1960s and early 1970s.
Privatisation of British Railways was probably tile biggest factor in ensuring that the structure was repainted properly, although a group of Scottish Railway civil engineers had already considered looking beyond the traditional railway specifications and methods of working. Systems and practices used by the offshore industry were seen as the way forward.
Around this time British Rail introduced blast-cleaning of the structure and several of the southern approach spans were prepared this way. However the paint system used was still a British Rail standard, comprising five coats of single pack materials with a total minimum dry film thickness of 190µm and an expected service life of up to 10 years.
The challenge facing newly-formed Railtrack was to specify a paint system that would protect the structure against corrosion for 25 years. Clearly the cost of access and labour is significant when compared with the cost of a paint system. It had long been recognised that the only way to achieve the service life required would be to totally remove the existing paint work by abrasive blast-cleaning.
Railtrack had a choice of two paint systems both based on epoxy systems: one using micaceous iron ore as the barrier pigment and the other using glass flake. The big advantage at that time came with the glass flake epoxy system - this could be applied at a higher film build and resulted in a system comprising three coats compared with four coats for the epoxy MIO system.
This not only offered significant savings in labour costs but also offered the potential for increased production. Railtrack finally opted for the glass flake epoxy based system. For the first time in its history at the ripe old age of 112, the Forth Bridge was about to be repainted with a modern two pack paint system that would protect it for at least 25 years to come. Leighs Paints was awarded the contract and selected as the paint manufacturer for the job.
First meetings saw a Railtrack standard/approved system being proposed, consisting of a two-pack epoxy blast primer (Metagard L574), an epoxy glass flake build coat (Transgard TG123) and an acrylic urethane finish (Transgard TG168). The system also included a stripe coat of epoxy glass flake between the first and second coats. A further coat of epoxy glass flake between the stripe coat and intermediate coat was also recommended for the areas close 10 the water line for extra protection.
Corrosion-Proof Glass
Coatings containing glass flake have been used for decades for the protection of structural steelwork, but the thickness of the coatings, which were developed to provide service lives of up to 25 years in very aggressive environments, meant that they were not considered economical for bridges and similar structures.
Recently, manufacturers have taken up the challenge to develop glass flake coatings offering a lower system cost per square metre.
This has been achieved by formulating new high solids coatings that have a lower dry film thickness but stilt retain the high barrier performance of traditional systems. These systems offer both commercial and technical advantages when compared with alternative systems pigmented with micaceous iron oxide.
In industrial use, glass flake systems have been found much faster to spray as they can be applied at a higher film build. This increase in productivity can give additional benefits such as reduced coating time and a reduction in the number of coats required. Once applied, glass flake coatings offer much longer service lives, typically 25 years to major maintenance, providing significant savings over the life of the structure.
An independent estimate by a design and engineering company in the UK suggested that the use of appropriate glass flake coatings can reduce the cost of corrosion protection for new-build projects from 7% to 5% of total construction costs.
These new coatings have already been used successfully on a number of projects in the UK and this has lead to the UK Highways Agency approving them as a standard system. They have been used on a number of new build and refurbishment projects; as well as the Forth Rail Bridge, they have been used on the Tay Rail Bridge in Scotland and the Gateshead Millennium Bridge among others.
Many of the coating systems that protect large structures such as bridges have been used for very many years. But since they were first applied, there have been changes in users' expectations: traffic growth has been dramatic, and the cost of delays has become more widely understood and debated, reflecting the greater concern of the public at large. As a result lifecycle cost is becoming increasingly relevant to the construction of major pieces of infrastructure.
Additionally, the regulations regarding the reduction of volatile organic compounds have had a significant impact on the coatings used for the protection of bridges. High solids low voc coatings containing glass flake have become an increasingly popular option providing protection for both the structure and the environment.
At this stage, Railtrack raised a couple of concerns. Firstly the thickness of the blast primer; despite excellent encapsulation of the work areas, there are times when the blast primed surface has to be left exposed for extended periods; particularly during cold, windy or wet conditions. At this point an alternative primer was proposed: Transgard TG223 Blast Primer which can be applied at 60µm dft – a thickness deemed sufficient to protect the substrate from corrosion during periods of delays. However, the increased film build of the primer brought with it a further concern; the intercoat adhesion between primer and intermediate coats. Some thought that the surface profile which resulted from the blast-cleaning process was necessary to ensure good adhesion by the glass flake epoxy coating and that by filling the surface profile with primer, some adhesion could be lost. But these fears were dispelled by a series of laboratory tests and practical trials, and consequently Railtrack accepted the higher build primer.
The second worry was the flexibility of the glass flake epoxy coating. There were concerns that a thick film of epoxy glass flake may be unable to cope with the movement of the structure and that it would crack, particularly around rivet heads and so on, especially over a period of time as trains crossed it. An in-house 'flexometer' test was carried out at Leighs Paints involving a specially-designed test rig.
The rig flexes a 90cm-long coated steel panel, plus and minus 2.5cm either side of the centre line. The test is severe as the forces created both compress and extend the coating, Test data and experience suggest that if a coating is going to tail it will always happen within the first 10,000 cycles and usually in under 1,000 cycles - a typical failure mode is cracking and detachment of the coating.
The 5cm deflection is the maximum that can be achieved without prematurely fatiguing the steel panel, which tends to break at deflections of more than 3cm each side of the centre line.
Leighs Paints Transgard TG123 Epoxy Glass Flake coating was subjected to 100,000 cycles at one cycle per second. On completion there was no evidence of cracking or detachment and Railtrack accepted the test data evidence.
The modified system, including the higher build primer, was then subjected to an accelerated weathering test programme by Railtrack's independent test house. The tests included: 5,000 hours exposure to salt spray: 50 cycles to test resistance to sulphur dioxide; 5,000 hours to test resistance to water immersion, humidity/condensation and resistance to UVA lighVcondensation and 2,000 hours resistance to UV light condensation.
Ultimately, the glass flake epoxy system was selected because of excellent adhesion and corrosion capabilities acting as a powerful barrier against harsh weather conditions. The primer Transgard TG223 has excellent adhesion to blast cleaned steel and includes zinc phosphate which is an anti-corrosive pigment. It can be applied at 60 µm dft. This provides additional protection to the blast cleaned surface, without adversely affecting the intercoat adhesion of the system.
Transgard TG123, the high-build epoxy glass flake coating, is a high solids material with the glass flake particles providing excellent barrier protection. The particles align themselves within the film, parallel to the substrate, to give barrier and physical reinforcement. It is formulated using a micronised glass flake and this has several benefits over equivalent products that include coarser glass flake particles. It is easier to apply as it can be applied through smaller spray tips, giving a smooth finish; the dry film thickness is variable from 200-1,000µm in one coat, with a continuous film; and it has excellent mechanical properties and corrosion resistance. Transgard TG123 also includes zinc phosphate so not only does it include a barrier pigment but also an anti-corrosion pigment.
Because the product forms a continuous film at 200µm dft, it is ideal for stripe coating and can be applied by brush. Stripe coating is an important part of the painting process and is applied to every single rivet head as well as other specified areas such as sharp edges.
It was very important to maintain the red oxide colour of the Forth Rail Bridge and together the team ensured the primer, stripe and intermediate coats had a sufficient colour contrast for cover purposes, but ones that would also complement the traditional Forth Rail red finish.
The finish coat, Transgard TG168, is based on an acrylic urethane resin system manufactured to the required gloss level and colour shade - the colour is the same as that used traditionally on the bridge. It has good colour and gloss retention properties and is indefinitely recoatable, which is advantageous for future repaints as surface preparation can be kept to a minimum.
At this stage, part way through the project, Leighs Paints has been involved for six years and has supplied technical support and material to protect in excess of 120,000m2 of the bridge.
Dave Bottomley is technical manager for transport at Leighs Paints
Back to Technical Information