When traditional materials reached their limit, engineers turned to advanced fibre-reinforced polymers to revitalise a South Australian landmark.
Fibre-reinforced polymers (FRP) are increasingly favoured for structural repairs due to their high strength, low weight, and exceptional corrosion resistance. In harsh marine environments, these composites offer a distinct advantage over reinforced concrete, which can suffer from chloride-induced steel corrosion.
Despite these benefits, the primary challenge facing FRPs is the relative lack of established engineering standards compared to legacy materials such as steel or concrete. However, physical performance continues to drive selection in recent structural retrofits.
A great example of this application is the central double-leaf bascule span of the Birkenhead Bridge. Opened in 1940 it connects Port Adelaide to Birkenhead serving 15,000 vehicles daily. For years, the bridge’s required maintenance up to four times annually to patch defective asphalt and worn tiles. A more efficient solution was sought.
Dr Ali Mohammed, Technical Lead at Wagners Composite Fibre Technologies, notes that pultruded glass fibre reinforced polymer (GFRP) provided a sophisticated solution to this complicated problem.
“When it comes to FRP bridge decks, they offer significant advantages in terms of light weight and durability,” Mohammed said.
Pultrusion process
The manufacturing phase for the individual hollow sections utilised a pultrusion process where glass fibres were pulled through a heated die and simultaneously impregnated with a specific resin mix. This material was pulled over a heated stainless-steel mandrel to ensure the sections held their shape during the curing process.
To optimise performance, unique pull winders integrated into the line allowed for a novel fibre layup consisting of several layers of unidirectional and wound fibres. The specific ratio of 80 per cent unidirectional to 20 per cent wound fibres, set at a 50° off-axis angle, provided the high longitudinal strength and stiffness required to manage complex multi-axial stresses.
Structural assembly and offsite prefabrication
Turning individual sections into a unified deck unit required a meticulous bonding process that was completed entirely offsite to ensure quality control and minimal site disruption. This process began with sanding the contact surfaces by 0.5 mm to remove the top resin layer and expose the structural fibres. A two-part toughened epoxy resin was applied as an adhesive before the components were clamped together in bonding jigs and subjected to a thermal cure cycle.
To further enhance the integrity of the finished decking, both the top and bottom surfaces were hand-laminated with a layer of 900 GSM weft triax mat and epoxy resin. These prefabricated units were then fitted with a shear key layer on the top surface to ensure a robust interlocking connection with the road pavement layer.
Comprehensive load and fatigue validation
Because FRP is a relatively new material in the Australian civil sector, there were no pre-existing engineering equations to guide the design. This necessitated a regime of full-scale testing to validate the proposed decking system.
Wagners collaborated with Wallbridge Gilbert Aztec and the South Australian Department for Infrastructure and Transport to develop a prototype using 125 square hollow sections.
The structural design was validated against historical and modern standards, including the Austroads ‘92 T44 design vehicle and the AS 5100 SM1600 wheel load. Fatigue performance was rigorously evaluated through a series of two million cycles at loads between 10 and 100 kN, with 150 kN spike testing conducted every 100,000 cycles to confirm structural integrity remained consistent.
Additionally, to ensure the pavement would remain stable during bascule operation in high summer temperatures, engineers tested a panel heated to 60°C in a vertical position. This simulation confirmed the system’s ability to withstand braking loads without surface displacement even when the span is tilted at 90°.
The validation process included:
- Proof testing: 15 minutes at 145.7kN (4 per cent above factored ultimate load).
- Cyclic testing: 2 million cycles between 10 and 100kN to simulate long-term traffic.
- Failure testing: The deck finally failed at 378.2kN – 53 per cent higher than the ultimate test loading case.
Using these testing results, Wagners then developed a finite element model using Strand7 software to support future projects. This model accurately captured the prototype’s behaviour and will serve as a vital tool for parametric studies and future design validations.
Site-specific installation engineering
Because the heritage bridge utilised original mechanical and electrical plant, the new decking was required to be lightweight to reduce the load on the bascule. The GFRP system was successfully installed atop the existing structural steel girder beams using nominal high-density polyethylene packers to match the original roadway surface.
To ensure secure attachment and efficient shear transfer, the panels were fixed to the girders using bolted polyethylene terephthalate insert assemblies. The entire system was supported by industry-standard bridge-rated elastomeric rubber bearing pads, which protected the composite sections from direct contact and wear against the steel structure.
The successful installation and reopening of the bridge in late 2020 proved that composites can meet the rigorous demands of non-pedestrian traffic. Mohammed emphasizes that while Australia currently lacks 100 per cent applicable engineering standards for GFRP, the gap is bridged through international standards and extensive R&D facilities calibrated with bodies such as the National Association of Testing Authorities.
The Birkenhead Bridge now stands as one of the few non-pedestrian composite decks in Australia, demonstrating a durable and efficient way forward when heritage constraints meet modern engineering needs.
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