Forth Road Bridge

Project summary

The Forth Road Bridge is a suspension bridge north of Edinburgh which originally opened in 1964. Its main deck consists of a steel orthotropic system with a number of plates welded to a network of transverse beams, longitudinal stiffeners and girders while its approach viaducts carry a reinforced concrete slab. The running surface consists of a nominal 37mm layer of mastic asphalt which contributes to the structural stiffness of the deck itself. The viaducts were originally surfaced with hot rolled asphalt and more recently with a proprietary material called Masterflex.

The most recent resurfacing of the steel deck occurred in 2004 and consideration needed to be given to the next resurfacing cycle, due to be undertaken within five years. The investigation sought to assess the continued suitability of mastic asphalt and to understand whether viable and more cost-effective solutions were available in the market. Options explored included the use of TS2010 stone mastic asphalt and the incorporation of a polymer modifier to the binder itself.

Scope of assessment

A number of input parameters had to be considered at the outset: the physical behaviour of the main deck and the viaducts; the nature of the vehicular traffic – frequency, type and concentration; the availability of plant, material and labour; and the historical context of each significant resurfacing works previously undertaken. Parameters assessed included the resistance to deformation, fatigue performance and the overall durability of any asphalt considered.

With highly concentrated running loads in the form of HGVs, there was significant risk of rutting on an already thin surface course material. Similarly, while the deck had a waterproofing layer immediately below the asphalt surface course, the already minimal material thickness meant the opportunity for corrosion of the deck to take hold remained significant. Nevertheless, the material laid needed to be flexible enough to accommodate the significant movement of the deck in adverse weather conditions and still fulfil the performance criteria.

Health and safety aspects were also taken into consideration. Mastic asphalt is typically mixed at 220 degrees centigrade and more often than not hand-laid by specialist labour. Although proprietary plant exists in the marketplace, its suitability for use on the Forth Road Bridge deck was questionable on account of the arrangement of the steel plates and the typically thinner layer required.

Benefit

The investigation enhanced the employer’s understanding of the technologies available in the marketplace and outlined the steps necessary to select a suitable material for the next resurfacing project. It identified that asphalt technology had significantly advanced in the fifteen years since the last deck resurfacing and that a small range of options could feasibly be considered. These primarily centred around the re-use of mastic asphalt, either machine laid or hand laid, or the consideration of the more commonly specified TS2010 stone mastic asphalt (SMA). The incorporation of an appropriate polymer modifier to TS2010 was seen as key in conjunction with the undertaking of laboratory trials and a full-scale mock-up on an existing deck panel.