The construction industry is more conservative than other industries despite that new materials and techniques are making their way into bridge construction. Fibre-reinforced polymer (FRP) is one of the most promising new materials in bridge design. FRPs are composite materials made up of a polymer matrix and fiber reinforcement. Glass, carbon, basalt, or aramid fibers have been utilized, but other fibers such as paper, wood, or plant fibers have also been used. Epoxy, vinyl ester, or polyester thermosetting plastics are commonly used. FRP composites play a vital role in repairing and constructing bridge structures. They’ve been utilized to repair concrete and metal bridges that have deteriorated over time. FRP’s have high durability, high specific stiffness, strength, etc. These factors suggest their suitability for integration in hybrid bridges. Grace and Singh (2003) developed a design approach for CFRP Prestressed concrete Bridge girders wherein all reinforcements including tendons were carbon fiber reinforced polymer systems such as leadline tendons supplied by Mitsubishi Functional Products Inc. and carbon fiber composite cables supplied by Tokyo Rope.

Rehabilitation Of Metallic Bridge Beams Using Unstressed FRP Plates
The upgrading or retrofitting metallic bridge structures is not as standard as updating or retrofitting RC bridges because it has a distinct and more challenging set of challenges. Before the 2000s, the research on the application of FRP composites was limited to a minimal amount. But that scenario has recently changed. Bell (2009) has discussed high-level criteria developed by Network Rail UK to use FRP in bridge strengthening. Strengthening the metallic bridge is done by adding steel plates by different methods such as bolting, riveting, welding, and adhesively bonding or by using adhesively gluing FRP composite plates to the structural part. But the second method has advantages over the first method.

The advantages of using FRP composites compared with steel plate bonding can be stated as follows:
– To achieve maximum efficiency, fibers can be injected in a specific position, volume fraction and direction, allowing the composite to be tailored in the required shape and specification.
– The resultant composites offer great fiber-direction strength and stiffness at a fraction of the weight of steel.
– They are easy to transport.
– Plates are lightweight, so there is no requirement for heavy support equipment to polymerize the glue, resulting in less falsework than steel.
– They may be employed in difficult-to-reach regions with minimum traffic interruption.
– The material has a long life expectancy.

Fig. 1: Illustrates the Upgrading of Network Rail CI Brick Arch Bridge (New Moss Road Bridge, on the CLC Liverpool to Manchester Railway, Greater Manchester)
Image Courtesy of Network Rail

Furthermore, CFRP and AFRP composites have superior fatigue and creep qualities and require less energy per kilogram to manufacture and transport than steel. The material of the bridge and their geometric cross-section define the best analysis and design technique for FRP composite plate bonding of metallic structures. The material used is either ultra-high modulus or high modulus CFRP composite.

The manufacture and installation process would be one of the following:
– Pultrusion technique and components bonded with an ambient cured adhesive on the site structure.
– Prepreg sheets were formed into a plate in the factory and cured at elevated temperature before being transported to the site and bonded to the structure with an ambient cured adhesive.
– Factory-made cold-pre-impregnated fiber (prepreg) and suitable film adhesive are wrapped on the bridge and cured under the elevated temperature of 600 for 16 hours or at 800 C for 4 hours under 1 bar pressure.
– Vacuum infusion

Fig. 1 illustrates a typical Network Rail-owned cast iron beam/brick jack arch carrying a public road over the railway in Irlam, Greater Manchester: the supports to hold the FRP composites in position whilst the adhesive cures are clearly shown; these were removed after about one week when full polymerization had been achieved.

The Rehabilitation Of Metallic Bridge Beams Using Stressed FRP Plates
The use of prestressed FRP plates bonded steel girder is similar to that of RC beams. The anchor for prestressed FRP plates is obtained by bolting steel anchorages into pre-drilled holes of steel girders’ flanges. Drilling directly into metallic beams has a risk of cracking the beam. To solve this problem, prestressing force is transferred to the metallic part utilizing a mix of clamps, high friction bolts, and adhesive bonding. At last, grout is applied to improve the durability of the prestressing anchorage system.

Joining Of Concrete, Metallic And FRP Composite Components
There are two ways in which FRP can be joined with concrete
– By adhesive bonding
– By using nails

Some adhesives that can link concrete and metallic material to FRP composite adherents are epoxy, polyurethanes, acrylics, and cyanoacrylate. The most commonly used adhesive is one of the epoxy groups. Out of the variety of epoxies, the most suitable one is used, which is compatible with the two adherent and the curing conditions. Bridge engineers should obtain manufacturers’ recommendations for the best adhesive to use in certain circumstances.

Concrete Adherents
Most structural adhesives depend upon chemical bonds between adherent surface atoms and compounds constituting the adhesive. Their surface must be prepared before rehabilitating or retrofitting RC and PC structures. The surface preparation of concrete aims to remove the exterior soft and possibly contaminated skin and weakly bonded material to reveal small- to medium-sized aggregate particles. Surface preparation must be done without damaging the layer behind it. After the surface preparation, the substrate is grit blasted. Particles of nominal 0.18 mm are commonly used, and the surface is subsequently degreased. This process is vital because it removes impurities that prevent the formation of covalent bonds. Compared to CFRP composite material, the CFRP textile has more robust compatibility with the adherent and is thus more effective in bonding the composite plate to concrete beams.

Metal Adherents
The formation of chemical bonds is a load transmission mechanism between the adherents. In solvents, degreasing is a vital process; commonly used solvents are ketones (such as methyl ethyl ketone) or trichloroethylene. However, although solvent degreasing offers a clean surface, it does not stimulate the establishment of acceptable surface conditions for longer-term bond durability. A new chemically active surface is essential for a successful adhesive bonding process. The chemically active surface can be achieved by grit blasting or acid etching for particular steel or aluminum substrates using an aqueous acid solution.

The basic principles for surface preparation are that the surfaces to be bonded must be:
– Free from contamination
– Sufficiently chemically active to enable the formation of chemical bonds between the adhesive and the adherents
– Resistant to environmental deterioration in service, mainly due to hydration

FRP Composite Adherents
When the FRP composite plate is made from the prepreg or pultrusion process, it generally has peel-ply on either side. One of the peel-ply layers is removed just before bonding to adherents; the peel-ply is a sacrificial layer of glass fiber and polymer material. If the polymer surface does not contain a peelply layer, the surface preparation procedure would be to abrade the bond-side of the plate using medium sandpaper or a sandblaster and wipe clean with a dry cloth to remove any residue. Finally, the surface would be wiped with acetone or equivalent. The adhesive is then applied to the prepared grinder surface and the CFRP plate. For enhanced steel structures, a critical worry is the potential bond failure between FRP laminate and steel surface. The primary failure mode is an adhesive failure. Steel and FRP both have higher failure strength than adhesive bonds. Specific approaches have been prepared to anticipate bond failure. The easiest way is to utilize the greatest stress in the bond-line as the failure criterion. The benefit of this method is that the bond failure load of the FRP enhanced steel beam may be computed if the bond strength of the FRP-steel joint is known. This technique is only applicable for the case of elastic deformation, and the consequence of varied geometries of the bond line is not explored.

Composite Patch Repair For Metallic Bridge Structures
Composite material patching is a unique approach and highly promising for mending and reinforcing metallic structures by gluing CFRP strips to military aircraft after being damaged. It is possible to increase the service life of aluminum aerospace components. The same benefits may not apply to bridge patching as aircraft patching, as there are some fundamental distinctions between aerospace applications and those steel bridges. The differences are:
– The steel is stiffer than aluminum
– The difference in geometry between aerospace and large bridges
– The diverse loading instances and the different-in-service operating and environmental conditions

Furthermore, there are more significant differences in the usual repair costs. Research has been performed, and it has been proved that CFRP patches can increase the lifetime of fractured steel members. The composite patches inhibited crack propagation and extended the lifetime of the repaired structure. It is also possible to boost the reinforcement efficiency by prestressing the combined patch; it increases the lifetime by a factor of roughly 5. It provides compressive forces that induce a fracture closing effect and lowers the stress intensity range at the crack-tip. Patches perpendicular to the crack route restrict the crack opening, and the critical parameter is the intensity factor range and not its highest value.

Composite patch repairs remove the disadvantages of standard restoration procedures to enhance bridge structures. The advantages are:
– There are minimum temporary falsework requirements
– Patches may be put directly onto corroded steel elements by completing a simple surface preparation technique
– Patches may be applied fast to the bridge structure
– Patches demonstrate excellent fatigue resistance
– Patches do not produce stress concentrations
– Patches result in minimum extra weight

All-Fibre-Reinforced Polymer (FRP) Composite Bridge Superstructure
Two different types of bridge concepts are:
– The old bridge one with material replacement
– The new material one The number of bridges being constructed using the second concept with all FRP composite material is limited; however, it is expanding. Ideally, the bridge with all FRP composite material is easy to build and has reliable connections; the advanced polymer composite fulfills these requirements as they are lightweight, have high specific stiffness and strength. The first FRP composite bridge is Maunsell plank which consists of several interlocking fibre-reinforced polymer composite units which could be assembled into an extensive range of different high-performance structural units in the construction industry. The panels were joined together by bonded connectors, and GFRP toggles were employed to keep the sections together while adhesive polymerized; these toggles remain in situ after polymerization. Some of the more instances of building all-FRP composite bridges across the world are in Spain and Russia.

*Fig. 2: Lleida Bridge, Spain, Over an Electrified Railway Line Image Courtesy of Fiberline, Composites A/S*

Spain
The bridge is situated at Lleida, Spain, over the Madrid-Barcelona high-speed rail connection. The bridge is made of E-glass fibers coupled with woven and complex mats; the minimum glass fiber is 50% by volume. The main reasons for choosing GRP (glass reinforced polymer) material were that.

The material is an electrical insulator that avoids magnetic interference with the electrified train. – The bridge could be constructed on-site and then maneuvered into the place by crane.

The bridge is a U–shaped beam with transverse ribs. It is a load-bearing, joint less, single structure using carbon fiber-reinforced epoxy polymer. The bridge is planned and produced using an injection infusion process with lay-up to fill and strengthen the epoxy-bonded prefabricated ribs; the epoxy method employed has enhanced toughness.

Fig. 2 shows the design aspects of the GFRP footbridge crossing the Madrid–Barcelona high-speed rail link at Lleida, Spain. The bridge is manufactured from E-glass fibres combined with woven and complex mats; the minimum glass fiber is 50% by volume.

Fig. 3: The First Russian Composite Bridge Manufactured by Vacuum Infusion Technology

Russia
The bridge was designed by Lightweight Structures BV, the Netherlands and Applied Advanced Technology (ApAteCh), Russia, who installed it. The vacuum-infused technology provided a reduction in manufacturing steps, thus avoiding assembly operations on-site; the potential of utilizing one mould for bridges of varied diameters allowed for a decrease in the cost of the structure. Furthermore, the bridge manufacturing process allows new aesthetic design options and the construction of new uncommon shapes.

Fig. 3 shows the first Russian composite bridge manufactured by vacuum infusion technology for small rivers with spans of 15 to 30 meters and an expected life cycle of 100 years; the structure was designed by Lightweight Structures BV, the Netherlands and by Applied Advanced Technology (ApATeCh).

New Bridge Construction With Hybrid Systems
A new novel hybrid structural system can be produced by connecting advanced polymer composite with traditional building materials, for example, combining concrete which is weak in tension but strong in compression, with FRP composites in plate form, which are strong in tension but weak in compression, could take advantage of dominant properties of both by joining the two to form a hybrid structural member.

A hybrid system may be categorized as:
– Structural composite products with hybrid fibers.
– Structural systems are composed of hybrid composites and traditional materials.

Fig. 4: Footbridge Erected in the Khimki, Moscow Region

The first category is where the FRP composites create rebars, gratings, and flat plates. When designing the Second category, the designer aims to place two or more components in their most strategic position in the structural system to take full advantage of their unique superior properties.

The successful use of these systems demands that hybrid system should meet the following three requirements:
– The FRP composites should be used in regions exposed to tension.
– Fire resistance should not be necessary if the Structure is an open space. Advanced Composites Group Ltd. (ACG now Cytec) has released a new resin system with outstanding fire performance to mass transit. The operating temperatures are within the range of -55o C to 80o C.
– Cost-effectiveness is the most favorable combination of whole-life cost and good quality and performance.

Fig. 4 shows Footbridge erected in the Khimki, Moscow region, designed and erected by ApATeCh was a lightweight pedestrian footbridge, shaped to form a cross; the four spans, each of length 33 m, are integrated into one unit.

Hybrid Columns
Hybrid FRP/concrete structural columns (Fig. 5) filled with concrete where most fibres were placed in the hoop direction, providing confinement to the concrete with only minimal fibre volume fraction being arranged longitudinally.

*Fig. 5: A series of Possible Column Cross-Sections for Confining the Concrete and a Section for the Beam, Utilizing Steel, Composite and Concrete*

The hybrid systems have various advantages over steel-jacketed methods:
– Lightweight
– Corrosion-resistant
– Resistant to lateral force on column
– Retains the cracking of the concrete.

The disadvantages of FRP/concrete hybrid columns are:
– Brittle failure in bending
– Difficulty in detailing connection details when joining column to beam
– Poor fire resistance, but this is not significant concerning bridge columns.

A new hybrid column consists of an outer FRP tube and a concentric steel tube within; the annulus is filled with concrete (see Figs. 5-7). The ‘Teng’ column intends to develop a high-performing structural component by integrating three materials’ benefits and delivering the advantages indicated above. The ‘Teng’ column may quickly be transformed to a beam by shifting the inner Steel tube close to the FRP outer tube

Hybrid Bridge Beams
One of the first hybrid bridge projects was the King Storm Water Channel Bridge on California State Route 86 near Salton Sea, USA (see Figs. 8-9). The carbon shell bridge design is the Composite Shell System (CSS), composed of the two-span continuous beam-and-slab bridge with five intermediate column piers. The six concrete-filled carbon tubes with an interior diameter of 344 mm. They constituted the longitudinal beams and joined their tops to a slab consisting of an E-glass GFRP deck system (Fig. 11). Depths and geometry is decided mainly by geometric limitations and structural performance. An innovative hybrid beam of the rectangular cross-section was extended to form a composite/concrete duplex beam for both a standard rectangular and a Tee beam cross-section. NECSO launched an R&D project and created an advanced composite/concrete beam element. This method resulted in a novel structural concept that is corrosion-free with outstanding damping and fatigue qualities. The Knickerbocker Bridge is another variation on the hybrid beam system. Fig. 8 shows the elevation of the composite bridge while Fig. 9 shows the soffit of Knickerbocker Bridge. The HCB consisted of an FRP shell which was shaped in the form of a U and was manufactured by the vacuum infusion technique; the beam interior is lined with interlocking sections of two to four 3 mm thick fiberglass textile fabric.

Fig. 6: Elliptical FRP-Concrete-Steel Double-Skin Tubular Columns

Fig. 7: Concrete-Filled Single and Double-Skin uPVC Tubular Columns

Fig. 8: Elevation of Knickerbocker Bridge, Boothbay, Maine, USA

Fig. 9: Soffit of Knickerbocker Bridge, Boothbay, Maine, USA

Fig. 10 shows the cross-section of the bridge. The HCBs were designed to match the recommended 838 mm depth box beams in order to maintain the required vertical under-clearance; the HCB framing system was limited to two 18.3 m end spans and six 21.3 m interior spans, resulting in an eight-span bridge with a total length of 164.3 m. Fig. 11 shows a hybrid bridge of steel girders and a GFRP sandwich deck including GFRP shear webs, joined with bolts; this bridge is located at Rijksstraatweg, De Meerin, the Netherlands.

It may be noted that Prof. Shamsher Bahadur Singh and his team at Civil Engineering Department of Birla Institute of Technology are working on development of advance composites materials for structural applications. Materials developed are FRP rebars, FRP plates, Hybrid plates, and functionally graded composites plates for examining their performance as structural elements. Most recently, Singh and his research team has started working on natural fiber based composites for material characterization, fabrication and examining its potential for structural plated elements primarily its strength and stability. Some of the technical papers referred for preparation of this article has been presented in the reference section for better understanding of the current scope of the advanced FRP composites for structures in general and innovative bridges in particular.

Conclusion
In this article, various elements of using modern polymer composites in bridge engineering have been discussed. The benefits of FRP composites are recognized from their physical properties. The reduced weight of FRP composites might result in decreased building construction time, high construction speed and fewer material requirements to satisfy equivalent performance standards as conventional materials. Thus resulting in less wastage of materials. The advantages of using FRP composites are the extended life of structures and more resistance to environmental changes, aging, corrosion and degradation.

References

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