FRP materials offer bridge designers the advantages of high stiffness-to-weight and high strength-to-weight ratios when compared to conventional construction materials such as steel and reinforced concrete. FRP can be preformed into complete structural units, thus reducing construction time. In addition, FRPs have excellent corrosion resistance particularly against de-icing salts.
For vehicle carrying bridges in the UK, particularly across motorways, reinforced concrete or steel is the norm. FRP bridges can be provided as complete bridges in fully moulded units, and have particular applications in deck replacement on existing structures, where corrosion resistance and speed of installation are important, and in bridge enclosure systems where they provide environmental protection.
FRP bridge decks comprise sandwich profiles, spanning transversely or longitudinally between supporting elements (such as steel beams) or suspended from tension cables. Several variants have been developed. Most comprise pultruded multi-cell sections which have been glued or bolted together to form a compound structure. For example:
- ACCS system (Advanced Composites Construction System) uses pultruded multi-cell box section with connecting toggles and gluing
- Superdeck system uses bridge truss sections with hexagonal shear keys
- ASSET system (Advanced Structural System for Tomorrow’s infrastructure) uses a two-cell prismatic profile.
Alternatively complete bridges can be hand-moulded as complete structures, a process that eliminates any bonding or bolting between different sections.
Bridge decks are usually constructed from pultruded glass fibre reinforced polyester or vinylester sections, although some types are hand-laminated as single piece mouldings. Decks are prefabricated and craned onto bearings as a single unit leading to considerable savings in installation time. Principal advantages of this construction method are improved durability and lightness and FRP offers particular advantages for moving bridges. FRP bridge solutions are available to match spans and loading requirements for almost all standard bridge designs.
The wear surface or top surface can be post applied, polymer concrete (often applied before installation) and asphalt, or can be integrated into the hand-moulding process as a bonded top layer.
Bridge enclosures are hybrid bridge systems and are an example of an application where composites on their own would not provide the most effective solution. A system has been developed for high performance bridges which feature the combination of several materials utilised in an optimum complementary manner. The hybrid system features a lightweight tubular steel space frame, steel cast nodes, an aerodynamically profiled FRP enclosure shell, and reinforced concrete roadway slab. The structure may be widened by additional space framing and post-tensioning cables. The space frame is enclosed by an aesthetic, aerodynamically profiled shell which provides permanent protection for the steelwork and safe access for inspection and maintenance of the superstructure and bearings.
Quality and overall efficiency are achieved by assembly of factory-built modular units on site, and the space frame affords savings in steel and fabrication content. For long span viaducts, whole span space frames up to 150m long may be erected in a single lift, and for cable-stayed bridges having main spans of up to 750m, SPACES decks can be economically constructed from large segments. For suspension bridges, the aerodynamic shape and good torsional stiffness of a SPACES deck make it an attractive system. On a smaller scale, the SPACES system is ideal for motorway widening over bridges. Slender decks can cross 60m carriageways in a single span. The enclosed space frame can be installed rapidly and the concrete deck slab added with minimum disruption to traffic. SPACES decks may also be used to construct high-speed and urban railway bridges which are quiet.
FRP bridge parapet solutions are designed to tackle the problem of corrosion and on-going asset maintenance. Light weight makes for easier installation.