Fibre Reinforced Polymer (FRP) composites have been used in decks and superstructure members of bridges since the 1970s ^[1][2][3][4][1,2,3,4]. The first pedestrian bridge is reported to have been built by the Israelis in 1975 [5]. FRP footbridges can be truss, cable-stayed or girder bridges [6]. The first all-composite vehicular bridge is Miyun Bridge, built in 1982 in Beijing, China [7]. Since then, many FRP pedestrian and road bridges have been constructed in Europe, China, Japan, and the USA. FRPs have various advantages for bridge engineering, such as lightweight, pre-fabrication, corrosion resistance, mouldability, fast installation in inaccessible areas and electrical insulation—glass FRP ^[8][9][8,9]. Although major impediments to the wider application of FRPs in construction are lack of design codes, material specifications and recycling, the high cost is still the main barrier to their widespread growth.

2. History of FRP Bridges

Steel and concrete materials still dominate bridge construction. Fibre-reinforced polymer (FRP) materials provide an alternative to traditional materials in new bridges. Applications of FRP are in all–FRP or hybrid–FRP pedestrian and road bridges. In all–FRP bridges, the substructure (piers and abutments) is usually constructed using traditional materials. While the superstructure (decks, girder and cables) is made of FRP. In hybrid–FRP bridges, the main FRP components are girders, decks, external cables and parapet elements ^[10][36]. Prototype FRP composite bridges are believed to have been first conceived in Europe and North America around the 1970s. It is hard to establish when the first ever FRP bridge was constructed. The first all–FRP road bridge in the world, Miyun Bridge, was constructed in 1982 in Beijing, China. The bridge had a span of 20.7 m with six hand-laminated glass fibre/polyester sandwich girders. At the same time, the first hybrid–FRP bridge, Ginzi Highway Bridge, 11.9 m long was constructed in Ginzi, Bulgaria, in 1981/82. It used FRP composite beams and other bridge parts were constructed with traditional materials. Both bridges used energy and the labour-intensive hand lay-up method ^{[8][10][11][12]}[8,36,38,39]. The major developments in FRP bridges happened in the 1990s when Aberfeldy Footbridge and Bonds Mill Lift Bridge were built in the UK and the No-Name Creek Bridge in the USA. Many FRP bridges were constructed after that using pultrusion, filament winding, wet or hand lay-up and resin infusion processes. In the following sections, selected examples of all–FRP and hybrid–FRP bridges are presented.

2.1. Examples of All–FRP Bridges

2.1.1. Aberfeldy Footbridge, Scotland, 1992

The Aberfeldy pedestrian bridge was the UK’s first major footbridge completed in 1992 in Scotland. This was a key development towards large-scale all–FRP bridge construction. The bridge connects two halves of a golf course on either side of the River Tay. The bridge comprised a GFRP deck, suspended by Parafil aramid ropes (cables) from two A-shaped GFRP towers. This cable-stayed bridge had a major span of 64 m, which was then believed to be the longest span in the world. It had an overall length of 113 m with a load capacity of 10 kN/m. Aberfeldy Footbridge was an all-composite bridge except for concrete foundations. The bridge was initially designed to carry pedestrians but later strengthened with CFRP to carry motorised golf buggies ^{[4][11][13][14]}[4,34,38,40]. Structural health monitoring of the bridge was carried out by Stratford ^[15][41] after 20 years of service.

2.1.2. Bonds Mill Lift Bridge, UK, 1994

Lightweight FRP offers great benefits when moveable bridges are required. The lifting machinery is significantly reduced with the lightweight components of the bridge ^[14][40]. Bonds Mill Lift Bridge, was the first UK all-composite road bridge, installed in Gloucestershire in 1994. This 8 m single bascule bridge provides access to a private industrial estate for heavy trucks over the Thames–Severn canal. The bridge used the same cellular pultruded GFRP system—Advanced Composite Construction System (ACCS) as in Aberfeldy Footbridge. To resist local bending under wheels, the upper portion of the cellular system was filled with structural foam. The bridge was designed to carry full highway loading including a 38-tonne truckload. After eight years of service, an inspection was carried out by Hollaway ^[16][43] with no deterioration in GFRP. However, there was a wearing surface at the south end of the bridge due to the impact of lorries running at a steep slope from the industrial estate ^[8][14][16][8,40,43].

2.1.3. No-Name Creek Bridge, USA, 1996

No-Name Creek Bridge is the first all–FRP composite road bridge in the USA. This 8 m span bridge was built in 1996 in Russel, Kansas, by Kansas Structural Composites. It consisted of a sandwich deck with GFRP laminated skins and a honeycomb core. It is a single-span bridge supported on steel abutments. Static and fatigue field tests were carried out on the bridge in 1997, 2004 and 2008 to investigate the environmental degradation of FRP material ^[17][44].

2.1.4. Kolding Bridge, Denmark, 1997

Kolding Bridge, Denmark is the first FRP footbridge over a busy railway line. The cable-stayed bridge was constructed completely of glass fibre-reinforced polymer in 1997. This 40 m-long footbridge had two spans of 27 m and 13 m. The bridge had 100 × 100 mm square hollow tube GFRP cables, a 1.5 m deep girder and 18.5 m pylons made of standard FRP profiles. The bridge weighed 12.5 tonnes, half of the steel alternative, and was installed in 18 h ^[18][45]. Kolding Bridge is an all–FRP bridge, except for abutments at the foundations; and bolts are made of stainless steel ^[19][46].

2.1.5. Pontresina Truss Footbridge, Switzerland, 1997

Pontresina Truss Footbridge is a 25 m long all–FRP temporary footbridge constructed in Pontresina Switzerland in 1997. The bridge crosses Flaz Creek in the Swiss Alps at an altitude of 1790 m. This temporary bridge is mainly used in winter for ski tourism. During summer, the bridge is removed due to the potential risk of high water and is stored on the bank. Each year the bridge is installed in the autumn and removed in the spring. The bridge used five different types of pultruded GFRP profiles assembled into two multi-layer truss girders connected by cross beams and stabilised by the bracing. It has two 12.5 spans with bonded joints in one span and bolted in the other ^[10][20][21][36,47,48].

2.1.6. Halgover Footbridge, UK, 2001

Halgover Footbridge is the first bridge using a different processing method than pultrusion. The bridge was installed in 2001 over the A30 in Cornwall, UK. This 47 m span suspension bridge used resin-infused glass FRP decking ^[22][49]. Later in 2015, a geometrically nonlinear static and dynamic analysis of the bridge was carried out by Gunaydin et al. ^[16][43] and the results of this all–FRP bridge were compared with an identical steel bridge.

2.1.7. Lleida Footbridge, Spain, 2001

Lleida Footbridge is a double-tied arch bridge spanning 38 m over a high-speed train line between Madrid and Barcelona in Spain. The bridge used Fiberline profiles ^[19][46]. It was completed in 2001 and officially opened in 2004. The arches and bridge deck girders used rectangular hollow sections. All joints were bolted using stainless steel bolts and brackets ^[23][50]. The bridge weighed 19 tonnes and all profiles used E-glass fibres with polyester resin. It was designed for a 4 kN/m² serviceability load as per Spanish bridge design codes. The partial safety factors for materials to verify ULS were 2 and 3 for normal and shear stresses. The design was mainly controlled by deflection with some elements of arches, where buckling stability governed ^[12][39].

2.1.8. West Mill Bridge, UK, 2002

West Mill highway bridge was constructed over the River Cole near Shrivenham in Oxfordshire, UK in 2002. It was the first all–FRP road bridge in Western Europe, incorporating glass and carbon fibre-reinforced composites. The bridge span was 10 m and the width was 6.8 m. The bridge contained four 10 m GFRP box beams stiffened by CFRP flanges. A total of 34 GFRP ASSET deck profiles (transverse) were bonded to these beams. The bridge used reinforced concrete abutments and reinforced concrete parapet beams with steel parapets ^[13][24][34,51]. Structural health monitoring and repair of the bridge were carried out by Canning et al. ^[25][52] and Sebastian et al. ^[26][53].

2.1.9. Fredrikstad Bascule Footbridge, Norway, 2003

The Fredrikstad pedestrian bascule FRP footbridge in Norway was the first moveable lifting bridge in Europe. This 60 m long and 3 m wide pedestrian bridge was completed in 2003. The bridge did not have any counterweights to operate the opening mechanism. Instead, it had a large hydraulic cylinder at each side to lift the bridge. By using FRP material the weight of each moveable part was reduced to 20 tonnes, with FRP elements weighing 9 tonnes. The bridge uses double-curved boxed girders with longitudinal and transverse stiffeners inside. The deck is a sandwich structure with CFRP reinforcements and a balsa core. All FRP parts, except side panels, were manufactured by vacuum-assisted resin infusion ^[27][28][54,55].

2.1.10. St Austell Railway Bridge, UK, 2007

St Austell Railway Bridge was installed over the Paddington–Penzance railway line near St Austell station, Cornwall, UK in 2007. This all–FRP footbridge replaced an old corroded wrought-iron bridge built in the early twentieth century. It used the same FRP cellular system (ACCS) as in Bonds Mill and Aberfeldy FRP bridges. The bridge had three simply supported spans, with a main span of 14 m and two side spans of 6 m each. The main 14 m span weighted just 5 tonnes replacing the 26 tonnes old structure. It had a U-shaped cross-section made from pultruded FRP elements with an outer FRP moulded shell. The bridge had to be supported on existing masonry piers and abutments. This bridge is designed for a standard footbridge loading of 5 kN/m². It was fabricated off-site and installed in only eight hours ^[29][30][31][26,56,57].

2.1.11. ApATeCh Arched Footbridge, Russia, 2008

ApATeCh arched footbridge is an all-composite bridge installed by ApATeCh ^[32][58] in Moscow Russia in 2008. Except for metal hinges and fence fasteners, all parts are made of FRP composites. The bridge had a central arch and two beams. It is 22.6 m long, 2.8 m wide and weighs just 4.5 tonnes. The bridge was the first FRP bridge in Russia made by the vacuum infusion process. Using vacuum infusion reduced assembly and manufacturing time resulting in low cost. This production technology improved aesthetic possibilities with new pleasing structural forms.

2.1.12. Bradkirk Footbridge, UK, 2009

Bradkirk Footbridge is one of the first few bridges using moulded FRP composites. This all–FRP bridge was installed near Kirkham in Lancashire over the Preston–Blackpool North Line. The bridge contained two 12 m spans with a staircase at each end. Each span weighed just two tonnes. The bridge installation was completed in just six hours overnight. The moulded FRP composite used two layers of woven E-glass cloth with resin in between them. Then, a vinylester tie coat was applied with a polyester gelcoat. The material was then placed in the mould, bagged and cured at 70 °C ^[33][59]. Halgavor footbridge also used the same moulding process [8]. The structural health monitoring system was used to measure the effect of train buffeting on the dynamic behaviour of the bridge ^[33][59].

2.1.13. Dawlish Footbridge, Exeter, Devon, UK, 2012

Dawlish Footbridge was constructed in 2012 at Dawlish train station in Exeter, South Devon, UK. The bridge replaces the old, corroded steel bridge built in 1937. On completion, Dawlish bridge became the first Grade II listed FRP bridge. It uses standard pultruded FRP profiles using bonded and bolted joints, sandwich parapets made by film infusion and moulded FRP stair elements. The bridge has a span of 17.5 m and the walkway is 1.8 m wide. Due to its proximity to the beach, the bridge is constantly exposed to coastal erosion and salt spray-induced corrosion. The all–FRP bridge was proposed to reduce maintenance cost, and installation time and withstand the hostile coastal environment. The new FRP bridge replicated the form of the old steel bridge ^[34][60].

2.1.14. Pont y Ddraig Bridge or the Dragon’s Bridge, Wales, 2013

Pont y Ddraig Bridge or the Dragon’s Bridge is a double bascule FRP footbridge in a raised position. The bridge was installed at Rhyl Harbour, North Wales in 2013. It contains two mirroring 30 m long decks connected to a central stainless-steel tower with lifting cables. The bridge deck can be lifted by the cables for navigation purposes. It combines glass and carbon resin-infused FRPs. The deck is curved in both plan and elevation. Each deck span weighed 10.6 tonnes. The deck used mainly glass fibre sandwich shells with carbon fibre unidirectional longitudinal plates at highly stressed regions. It also used Corecell M-Foam as core material and Ampreg 21 resin. The lightweight of the bridge enabled two composite decks or dragon wings to be raised ^[8][35][36][8,61,62].

2.1.15. Eindhoven University of Technology Pedestrian Bridge, The Netherlands, 2016

Eindhoven Pedestrian Bridge is a bio-composite footbridge over the river Dommel in Eindhoven, The Netherlands. The bridge was completed in 2016. This 14 m span and 1.2 wide bridge was designed and installed within a year. The structural elements in the bridge used a mix of hemp and flax fibres in epoxy resin with a 56% fibre volume fraction. The non-structural parts employed an aliphatic thermoplastic polyester made of renewable resources. The bridge deck had a rectangular cross-section near abutments and a nearly triangular section in the middle. To facilitate this geometry the vacuum-assisted resin infusion process was used to produce the bridge. The railings of the bridge also used bio-composite material and resembled grass cutters. The Fibre Brag sensor technique was used to embed sensors in the bridge for structural health monitoring ^[37][63]. The data was monitored by Blok et al. ^[37][63] in their paper continuously for two years. The sensors applied to the woven flax material on the tension side proved to be successful and gave reliable results until the writing of their paper in 2019. The sensors embedded in non-woven hemp fibre composite on the compression side failed after initial readings. This bridge is an excellent example of using eco-friendly sustainable bio-based materials for structural elements.

2.1.16. Dover Sea Wall Footbridge over a Rail Line, Dover, Kent, UK 2017

The storms of 2015 completely damaged the existing steel footbridge near Dover Sea Wall, Dover, Kent, UK. The steel bridge was replaced with a hybrid–FRP bridge with pultruded and resin-infused parts. The bridge deck, top chords and anti-slip phenolic wear plates for stairways were resin infused. On the other hand, the truss members, landings, stairwell and parapet were fabricated from pultruded FRP profiles and plates. The sections were prefabricated, and the completed bridge was 31 m long and 2.415 m wide. It consisted of two simply supported 14.5 m bridge spans over concrete abutments with one span on top of the railway track. The bridge design life was for 120 years. The bridge weighed about a third of its steel equivalent [8]. Human-induced vibrations, dynamic properties and serviceability of the bridge were studied in a recent paper by Russell et al. ^[38][64].

2.2. Examples of Hybrid–FRP Bridges

Hybrid–FRP bridges have been used for the past 20 years. FRP composites can be used in combination with traditional materials, such as steel and concrete. Main examples of these bridges include road bridges with CFRP girders with concrete slabs on top and supported on concrete piers. Another common example relates to an FRP deck supported by steel or concrete girders or steel cables. In the sections to follow, real-life examples of hybrid–FRP bridges are described.

2.2.1. Asturias Bridge, Spain, 2004 (Hybrid–FRP Beam—Concrete Slab)

Asturias Bridge is the first FRP vehicular bridge constructed in Spain in 2004. It is a four-span bridge with CFRP girders supported on three intermediate supports. The length of the bridge is 46 m with two middle spans of 13 m and two end spans of 10 m. The concrete slab was supported on a permanent GFRP formwork, which was connected to the CFRP girders underneath. A polyurethane mould was wrapped by CFRP prepregs to produce the girder. The girder was a trapezoidal box section with a 1.2 m top flange, 0.8 m bottom flange and 0.8 m web. The core of the box girder was filled with polyurethane ^[39][40][27,65]. To validate the serviceability and ultimate load capacity limits of the 46 m long bridge, the 13 m full section of the bridge was tested in the laboratory by Gutiérrez et al. ^[41][66].

2.2.2. M111 Bridges, Madrid, Spain 2007 (Moulded FRP Girders and Concrete Deck)

The M111 twin bridges are located near Madrid, Spain along the M111 motorway. Built in 2007 by Acciona construction, Spain, the bridges had three simply supported spans of 10, 14 and 10 m and a 20.4 m wide box girder deck. The deck slab is supported by four FRP girders. The girder had a reverse omega shape. Its top and bottom flanges are made of hybrid carbon and glass fibre laminates. The girder webs consist of sandwich panels having glass fibre skins and a polyurethane core ^[39][40][42][27,65,67].

2.2.3. Standen Hey Overbridge, UK, 2007 (Deck)

Standen Hey Overbridge was installed by Network Rail, UK. It is a built-up bridge deck made of all–FRP pultruded panels in 2007 near Clitheroe, Lancashire. This was perhaps the UK’s first purpose-built deck for a road bridge. The bridge had a clear span of 9.5 m with 3 m wide single-lane rural road access. The deck was designed using ASSET profiles consisting of two triangular cells creating a rhombus shape. The ASSET profiles were produced in Denmark and transported in required lengths to the UK. The original bridge had cast-iron beams supporting a timber deck. The new FRP bridge deck had double layers of ASSET profiles and a third layer to act as a plinth for the parapet. The deck was installed in the standard 8 h rail closure ^[29][26]. Each profile was 225 m deep and 300 mm wide, and the webs were inclined at 60° to the horizontal. The profiles were bonded with epoxy adhesive. The 10 m long built-up deck weighed 20 tonnes, spanned between original abutments and was supported on new precast concrete beams. The deck used E-glass fibres in the form of bi-axial mats ^[43][69]. A similar deck with a single layer was used on Klipphausen Bridge, Germany [8].

2.2.4. Gądki Footbridge, Poland 2008 (Deck)

Built in 2008, Gądki Footbridge is located over road no. 11 (Poznan—Kornik expressway) in Gądki, Poland. This hybrid steel arch bridge has FRP composite deck in the main central span, and steel–concrete composite and reinforced concrete access spans. The complete bridge is 260 m long. The main span containing the FRP deck has in-plane curved girders supported by an inclined arch. The 40 m span main arch girder was a steel pipe section with 1200 mm diameter and 16 mm wall thickness. The deck girder was a 660 mm diameter and 20–30 mm thick steel pipe section. The deck is curved with an 80 m radius and the walkway is made of pultruded FRP profiles. The footbridge is supported on spot footing and the main arch is supported on prefabricated reinforced concrete piles ^[44][23].

2.2.5. Moss Canal Bridge, UK, 2011 (Deck)

The use of the largest FRP profile as a deck on Moss Canal Bridge, Rochdale, UK was a step change for FRP bridges in 2011. The FRP profile was a double web beam about 900 mm deep and 450 mm wide. The bridge was 12 m long and 3 m wide. The FRP deck used multi-axial pultruded sections of E-glass fibres in vinylester resin to support the 9 m bridge span. The deterioration of the existing reinforced concrete deck required deck replacement. Again, the lightweight of FRPs reduced both installation and fabrication costs ^[45][72].

2.2.6. St. Mateus—GFRP–Steel and São Silvestre Footbridges, Portugal 2013

St. Mateus Footbridge is a GFRP–steel hybrid footbridge constructed in Viseu, Portugal. This represents a typical GFRP deck combined with a steel girder. The span of the bridge was 13.3 m and the width was 2.5 m. The GFRP slab was fabricated using thin-walled multicellular deck panels. Experimental, numerical, and analytical study of St. Mateus Bridge is presented in papers ^[46][47][73,74]. São Silvestre Footbridge is a 10.5 m span hybrid bridge in Portugal constructed using pultruded GFRP profiles and a very thin concrete deck made of steel fibre-reinforced self-compacting concrete (SFRSCC) pre-cast slabs. The prototype of the bridge with a 5.5 m span was tested in a laboratory by Gonilha et al. ^[48][75].

2.2.7. Mapledurham Footbridge, UK, 2015 (Deck)

Mapledurham Footbridge was a replacement bridge, which was installed on the river Thames in Mapledurham, Oxfordshire, UK in 2015. This 13 m bridge uses a sandwich system with GFRP skin and foam core. Due to limited access to the site, the bridge was floated to the site on a barge along River Thames and the installed FRP bridge, with no physical joints. The bridge span was fabricated as a single unit. The FRP bridge decks were one-third the weight of equivalent steel or concrete slabs. The prefabrication and lightweight of FRPs allowed easy transportation of the bridge on a freight boat [8].

2.2.8. Sedlescombe Footbridge, UK, 2015 (Deck)

Sedlescombe Footbridge was installed in Sedlescombe village in East Sussex, UK to replace an existing timber bridge in 2015. The lightweight, quick installation time, low maintenance and whole-life cost controlled the design. The bridge was designed for 60 years. The bridge used a resin-infused FRP composite deck with powder coated steel parapet. It was 8 m long and 1.35 m wide, and the depth of sections was 250 mm. The bridge weighed only one tonne compared to the four-tonne original timber bridge [8].