The view from the top of the 45m high twin towers of the cable stayed Kessock Bridge on the A9 north of Inverness can’t fail to inspire, with the Beauly Firth and ever changing views to the Highlands on all sides. The view of the newly resurfaced deck directly below will be equally inspiring to all long span bridge owners and operators, but it is unlikely to change much for about 30 years, which is the hoped for lifespan of the surfacing that was laid in June.
As well as providing a durable surface the high quality surfacing material, Gussasphalt, promises to extend the life of the 30-year old bridge by providing such a smooth running surface that bridge movement under load will be significantly lessened. Welding maintenance on the slender, orthotropic steel deck will be reduced as a result and local residents can expect to experience less road noise from the 30,000 vehicles - 11% of them HGVs - that use it daily.
Gussasphalt has been used twice in the UK recently, on the Avonmouth and Tamar bridges where it has reportedly been a great success (NCE 29 June 2012).
This is the material’s first use in Scotland; it might not be the only use for long however as it is understood to be under consideration for the Forth road bridge. It has been used on Denmark’s Storebælt Bridge among others throughout Europe and could become the go-to solution for long span bridges of all types.
Gussasphalt is a dense mastic asphalt developed by Swiss company Aeschlimann International, with a high performance polymer modified binder from bitumen specialist Nynas, which relies on the properties of the bitumen mixed with crushed stone and limestone fines to provide the stiffness and durability needed for a bridge running surface. The binder is a high performance material, Nynas Endura N5, designed to ensure suitability for the Aeschlimann mixes.
The mix is designed specifically for each project but is known to include a blend of sands, limestone filler and a number of additives including Trinidad Lake Asphalt granules. A 6mm bitumen coated aggregate is embedded in the hot surface by rollers and provides surface texture.
This material is impermeable to air and water, which means a longer service life as it is permeation of water and oxygen that causes the ageing process. Other benefits include minimal interventions for repairs, resistance to water penetration, early skid resistance, and good ride quality.
The 1,056m-long bridge comprises two-lane dual carriageways and a pedestrian/cycle lane on each side. Transport Scotland let a main contract for the £13.25M of repairs and resurfacing to Balfour Beatty, with Bear Scotland supervising the works on their behalf. The works are being carried out in two phases, from February to June this year and the same in 2014. Stirling Lloyd is undertaking the deck waterproofing and resurfacing under subcontract to Balfour Beatty.
Surfacing material supplier is Leiths Group, which is bringing the Gussasphalt from its plant at Contin, some 30km north of the site, where it is made according to Aeschlimann’s specification. “The asphalt production is very closely monitored by both ourselves and Aeschlimann and transported by Aeschlimann tosite in special delivery trucks that can mix as well as heat the material at temperatures up to 220oC,” explains Leiths Group technical editor Neil Anderson.
“We supplied the surfacing material on a continuous basis over an extended shift of 16 hours to avoid making a transverse joint.”
Aeschlimann laid the 12,250m2 of material in two layers, each of 25mm but up to 27mm in any dips, which involves a surface finishing machine laid on steel rails to allow a high standard of regularity. Standing on the rather lively bridge, with two lanes still open to traffic, the pace of laying on the 240m central span at 1.4m /min may appear slow, but progress is sure, and so is the quality achieved.
Stirling Lloyd project manager Darren Holmes says the previous hot rolled asphalt surface had cracked in places and some areas of corrosion were found across the bridge deck.
Preparation was key.
“These areas have been prepared by shotblasting to provide a clean and keyed surface for our primer to go on top,” says Holmes. “Wet weather meant that we lost quite a few days as the steel deck would almost instantly oxidise and we would have to blast it again.”
Lane rental charges on the 17 week, Phase 1 contract mean that main contractor Balfour Beatty will be keen to make up this time during Phase 2 works next year, when among other works the cable stays will be retensioned for the first time since construction.
Balfour Beatty doubled grit blasting and waterproofing squads to make up lost time but all work was “winded off” three times, with winds of up to 80km/h buffeting the bridge.
The new surface gives the deck a tension that it hasn’t enjoyed before, creating a composite structure between the surfacing and the deck itself. “The deck is now properly protected from the elements in a way that it never has been,” says Holmes. “This is an extremely durable system that is being applied to the bridge, with our two layer Eliminator waterproofing system underneath the Gussasphalt.”
Bear Scotland senior bridge engineer Eric Coulshed says extensive tests were carried out ensure the products were suitable.
These included Transport Scotland’s TAIT skid resistance. “The system is specifically designed for this type of structure and TAIT trials were detailed in the specification for the project,” he says.
“This involved a production run prior to starting and a series of tests being run on samples. Further tests will be carried out on the finished carriageway, which ensures that the product meets with the specified performance requirements.
“Transport Scotland has a 10- year guarantee but we hope for a much longer life for the system. The previous mastic asphalt surface was down for 30 years, but it can’t be said to have lasted that long as there was some corrosion on the deck. Surface cracks in the surfacing told us that it had failed and interventions for repairs were causing traffic delays.”
The worst of the corrosion appeared on the south approaches where HGVs pull up the slope and caused wear to surfacing. Corrosion was found in a limited number of areas to as much as 4mm; the plate on the lane 1 where HGVs run is 14mm thick, and on lane 2 it is 12mm.
Fatigue analysis by Transport Scotland’s specialist consultants at Halcrow and Jacobs confirmed that the thick layer of surfacing bonded so tightly to the deck would reduce fatigue stresses, so it was possible to resurface without major repairs. “The deck needs a fully bonded surfacing for the composite effect,” says Coulshed. The deck bond to the waterproofing is important to this effect as well, and a pull off test revealed greater than two megapascals resistance.
Under the surfacing there is a single coat of primer, and two coats of Stirling Lloyd’s Eliminator waterproofing, keyed to receive the Gussasphalt. Balfour Beatty north of Scotland director of civil engineering Andrew Gordonsays has worked well.
“The Gussasphalt surfacing was selected after an extensive period of consultation, trials and planning at both the pre and post tender stages.
“I am delighted to see the results coming to fruition.”
Upgrading to modern standards
With these major works on the deck under way Transport Scotland has taken the opportunity to upgrade other aspects of the bridge to raise it to modern standards.
For example, 10 new gantry access platforms are being provided by specialist contractor Millar Callaghan as part of the contract, to end maintenance workers having to cross a live lane of traffic.
A new steel open-rail H4a vehicle restraint system provided by Tata Steel is being installed, as protection for the bridges’ stays and towers, able to resist impact by HGV’s travelling at speed.
The spreader plates for this system had to be welded to the orthotropic deck so great care is taken not to cause any damage to the deck. Magnetic particle inspection was used to check 12,250m2 of steel deck plate. Some 34 lighting columns are being replaced. New Maurer DS480 expansion joints are also being fitted at either end of the bridge.
All lighting cables have been replaced and a new cable tray installed on the bridge underside. The height of the parapet has been raised from 1m to 1.4m for cyclist safety on both sides of the bridge.
Traffic management has been a big part of the project, involving consultations with all interested parties and public meetings. There was a phase of pre main contract works to install traffic lights at roundabouts on the Inverness side of the bridge approaches as well as at a main junction, with traffic modelling to minimise the need for these works. It seems to have been successful as delays have been less than feared.
Wide loads have to be timed to minimise delays with a lot of manufacturing to the north of Kessock at Evanton, Invergordon and Nigg Bay.