Applications of FRP Composites

Bridge enclosures and fairings

It is a requirement that all bridge structures have regular inspection and maintenance, which will often cause disruption to travellers, particularly if closure of roads and interruption to railway services are required. Furthermore, increasingly stringent standards are causing the cost of closures to be high, particularly if maintenance work is over or beside busy roads and railways.

Most bridges that have been designed and built over the last 30 years do not have good access for inspection, and in Northern Europe and North America deterioration caused by de-icing salts is creating an increasing maintenance workload. The function of ‘bridge enclosures’ is to erect a ‘floor’ underneath the girder of a steel composite bridge to provide access for inspection and maintenance.

The concept was developed jointly by the Transport Research Laboratory (TRL, formerly TRRL) and Maunsell (now AECOM) in 1982 to provide a solution to the problems. Most bridge enclosures that have been erected in the UK have utilised polymer composites. These materials are ideal because they add little weight to the bridge, are highly durable, and as they are positioned on the soffit of the bridge they are protected from direct sunlight.

The floor is sealed on to the underside of the edge girders to enclose the steelwork and to protect it from further corrosion. Once the enclosures have been erected the rate of corrosion of uncoated steel in the protected environment within the enclosure is 2–10% of that of painted steel in the open (McKenzie, 1991; 1993).

The enclosure space has a high humidity; chloride and sulphur pollutants are excluded by seals and when condensation does occur (as in steel girders) the water drops onto the enclosure floor, which is set below the level of the steel girders from where it escapes through small drainage holes.

Figure 3 shows an example of the enclosure on the approach span of the Dartford River Bridge (QE2) where it passes over the Channel Tunnel rail link (CTRL) (before the train rails were laid).

Bridge decks

The development of FRP deck structures has been based generally on the pultruded systems, but occasionally on moulded structures. Recently FRP deck replacements in conjunction with FRP superstructure replacement for road bridges have been carried out. This type of construction is becoming popular for replacement decks of bridges up to 20 m span.

Figure 4 illustrates a typical cross-section of a bridge deck. The reasons for FRP material being used in particular circumstances are:

• the bridge deck is the most vulnerable element in the bridge system because it is exposed to the direct actions of wheel loads, chemical attack, and temperature/moisture effects including freeze– thaw shrinkage and humidity; FRP material characteristics satisfy these requirements
• reduced future maintenance (FRP composites are durable materials)
• quick installation owing to pre-fabrication and easy handling.

In the USA over 100 concrete bridge decks have been replaced by FRP deck installations, most of which have been built using proprietary experimental systems and details. The lack of standardisation is a challenge to bridge engineers, who traditionally have been accustomed to standard shapes, sizes and material properties.

The first FRP European bridge deck and superstructure replacement was conceived and developed under the innovative European ASSET Project led by Mouchel Consulting. It culminated in 2002 in the construction of the West Mill Bridge over the River Cole in Oxfordshire; the beam and deck structures were manufactured by the pultrusion technique.

The first vehicle-carrying FRP bridge deck in the UK to span over a railway replaced the existing over-line bridge at Standen Hey, near Clitheroe, Lancashire; it has a span of 10 metres, weighs 20 tonnes and was completed in March 2008. This is the first of Network Rail’s six trial sites in the country.

The consultants Tony Gee and Partners were responsible for the design of the deck, which comprises three layers of ASSET panel deck units made from E-glass fibres in the form of biaxial mats within a UV-resistant resin matrix. Composite Advantage (CA) has recently built (April 2008) a new ‘drop-in-place’ GFRP composite prefabricated integral beams and deck bridge superstructure, 6.77 m long by 19.0 m wide (22 feet by 62 feet) in Hamilton County, Ohio, USA. No heavy lifting equipment was required and it took one day to install (Composite Advantage, 2008).

A new single carriageway road bridge over the M6 motorway (UK) has recently been completed by the UK Highways Agency. The superstructure comprises a novel pre-fabricated FRP deck spanning transversely over, and adhesively bonded to, two longitudinal steel plate girders. The Mouchel Group designed the FRP bridge deck, which provides general vehicular access to an equestrian centre ; this was designed for unrestricted traffic loading (Canning, 2008).

External reinforcement to reinforced concrete (RC) structural members

The repair, upgrading and strengthening/stiffening of deteriorated, damaged and substandard infrastructure has become one of the fastest growing and most important challenges confronting the bridge engineer worldwide. It is generally much less expensive and less time consuming to repair a bridge or building structure than to replace it.

Civil infrastructure routinely has a serviceable life in excess of 100 years. It is inevitable that some structures will eventually be required to fulfil a role not envisaged in the original specification. It is often unable to meet these new requirements, and consequently needs strengthening. Changes in use of a structure include:

• Increased live load. For example, increased traffic load on a bridge; change in use of a building resulting in greater imposed loads.
• Increased dead load. For example, additional load on underground structures owing to new construction above ground.
• Increased dead and live load. For example, widening a bridge to add an extra lane of traffic.
• Change in load path. For example, by making an opening in a floor slab to accept a lift shaft, staircase or service duct.
• Modern design practice. An existing structure may not satisfy modern design requirements; for example, owing to the development of modern design methods or to changes in design codes.
• Design or construction errors. Poor construction workmanship and management, the use of inferior materials, or inadequate design, can result in deficient structures that are unable to carry the intended loads.
• New loading requirements. For example, a structure may not have originally been designed to carry blast or seismic loads.
• Material deterioration. For example, concrete degradation by the alkali–silica reaction or corrosion of steel reinforcement in marine or industrial environments or from the de-icing salts used on highways.
• Structural deterioration. The condition of a structure will deteriorate with time owing to the service conditions to which it is subjected. In some cases this deterioration might be slowed or rectified by maintenance, but if unchecked the structure will become unable to perform the purpose for which it was originally designed.
• Fatigue. This is a secondary cause of structural degradation, and it can govern the remaining life of a structure.

Structural degradation can also result from hazard events, such as impact (for example, ‘bridge bashing’ by over-height vehicles), vandalism, fire, blast loading or inappropriate structural alterations during maintenance. A single event may not be structurally significant, but multiple events could cause significant cumulative degradation to a structure.

The following discussions and examples illustrate the strengthening of members by external bonding of FRP plates or members. These will be considered as un-stressed at the time of bonding onto the structural beam. It is however possible to pre-stress the plate before bonding it onto the beam; this is known as active flexural strengthening.

This topic will not be discussed here but further reading on it may be found in Teng et al. (2002) and De Lorenzis et al. (2008), and a practical example is cited in Hollaway (2008). Many experimental and analytical research investigations have been undertaken on reinforced concrete beams strengthened by FRP composites; some of these are discussed in Triantafillou and Plevris (1991), Hollaway and Leeming (1999), Teng et al. (2002), Concrete Society Technical Reports (2000, 2003), Oehlers and Seracino (2004) and Hollaway and Teng (2008). Both flexural and shear upgrading can be undertaken using FRP composites.

Rehabilitation of degraded flexural RC structural beams using FRP plates

Within the scope of ‘strengthening’ concrete, it is essential to differentiate between the terms repair, rehabilitation, strengthening and retrofitting; these terms are often erroneously interchanged but they do refer to four different structural upgrading procedures.

• Repair to an RC structural member implies the filling of cracks by the injection of a polymer into the crack.
• Rehabilitation of a structural member (of any type) refers to the improvement of a functional deficiency of that member, such as caused by severe degradation, by providing it with additional strength and stiffness to return it to its original structural form.
• Strengthening of a structural member is specific to the enhancement of the existing designed performance level.
• Retrofit is used to relate to the upgrading of a structural member damaged during a seismic event.

Bonding of FRP plates to the adherend: As with all bonding operations the adherends must be free of all dust, dirt and surface grease. Consequently, the concrete or steel surface onto which the composite is to be bonded must be grit blasted to roughen and clean the surface.

It will then be air blasted to remove any loose particles and wiped with acetone or equivalent to remove any grease before the bonding operation. The surface preparation of component materials of FRP composite plate bonding to concrete surfaces is described in Hutchinson (2008).

The thickness of the adhesive and FRP composite plate would generally be about 1.0–1.5 mm and about 1.2 mm, respectively; the total length of the FRP plate as delivered to site would be of the order of 18 metres. It is possible to roll the material into a cylinder of about 1.5 metre diameter for transportation and for bonding the plate onto the beam in one operation.

Power actuated (PA) fastening ‘pins’ for fastening FRP composites: This method, which has been recently developed, is known as the Mechanically-fastened unbonded FRP (MF-UFRP) method and is a viable alternative to the adhesive bonding of a preformed pultruded or a prepreg rigid plate.

It mechanically fastens the FRP plate to the RC beam by using many closely spaced steel power-actuated (PA) fastening ‘pins’ and a limited number of steel expansion anchors. The process is rapid and uses conventional hand tools, lightweight materials and unskilled labour. In addition, the MF-UFRP method requires minimal surface preparation of the concrete and permits immediate use of the strengthened structure.

The advantage of using multiple small fasteners as opposed to large diameter bolts, which are generally used for anchorages, is that the load is distributed uniformly over the FRP strip and this reduces the stress concentrations that can lead to premature failure.

The method was developed by researchers at the University of Wisconsin, Madison, USA (Bank, 2004). Bank et al. (2003a, 2003b) have discussed the strengthening of a 1930 RC flat-slab bridge of span 7.3 m by mechanically fastening the rigid FRP plates using the MF-UFRP method.

Unstressed FRP plates: Figure 6 shows an FRP composite flexural plate bonded in position. The plate material used for the bonding or the MF-UFRP operations is generally the high-modulus (European Definition) CFRP, AFRP (Kevlar 49) or GFRP composite. These will be fabricated by one of three methods:

• the pultrusion technique, in which the factory made rigid pre-cast FRP plate is bonded onto the degraded member with cold-cure adhesive polymer
• the factory made rigid fully cured FRP prepreg plate, which is bonded to the degraded member with cold-cure adhesive polymer
• the low-temperature mould prepreg FRP prepreg/ adhesive film placed onto the structural member and both components are cured simultaneously on site under pressure and elevated temperature.

The third method for the bonding operation is superior to the precast plate and cold-cure adhesive systems (first and second methods) as the site compaction and cure procedure of the prepreg and film adhesive ensure a low void ratio in the composite and an excellent join to the concrete.

The current drawback to this method is the cost; it is about twice as expensive as the other two, and the currently preferred manufacturing system for upgrading is either the first or the second method. With these systems the plate material cannot be reformed to cope with any irregular geometry of the structural member. In addition, a two-part cold-cure epoxy adhesive is used to bond the plate onto the substrate.

This is the Achilles’ heel of the system, particularly if it is cured at a low ambient temperature since without post cure the polymerisation of the polymer will continue over a long period of time; this incomplete polymerisation might affect the durability of the material.

Near-surface mounted (NSM) FRP composite reinforcement technique: This is another method for the rehabilitation of RC structural members. CFRP, AFRP and GFRP composites can be utilised and generally the cross-section of the member is either circular or rectangular.

Grooves are cut into the surface of the member, generally into the soffit of the concrete beam, but if the cover to the steel rebars is insufficient for this the grooves may be cut into the vertical side of the beam as near to the bottom of the section as is practical. The NSM FRP reinforcement is embedded and bonded into this groove with an appropriate binder (usually high-viscosity epoxy or cement paste).

Figure 7 shows the position of NSM bars in an RC structural member. The NSM reinforcement can significantly increase the flexural capacity of RC elements. Bond may be the limiting factor to the efficiency of this technique as it is with externally bonded laminates. A review of the technique has been given by De Lorenzis and Teng (2007).

NSM FRP reinforcement has also been used to enhance the shear capacity of RC beams. In this case, the bars are embedded in grooves cut into the sides of the member at the desired angle to the axis. Utilising NSM round bars, De Lorenzis and Nanni (2001) have shown experimentally that an increase in capacity as high as 106% can be achieved, thus when stirrups are used a significant increase can be obtained.

Flexural strengthening of pre-stressed concrete members: Limited research has been undertaken on strengthening pre-stressed concrete (PC) members; the fib have reported that less than 10% of FRP-strengthened bridges as of 2001 are pre-stressed (fib Task Group 9.3, 2001). Strengthening usually takes place when all long-term phenomena (creep, shrinkage, relaxation) have fully developed, which may complicate the preliminary assessment of the existing condition.

As in RC strengthening, the required amount of FRP will generally be governed by the ultimate limit state design in PC members. Additional failure modes controlled by rupture of the pre-stressing tendons must also be considered, and consideration should be given to limitations on cracking.

Seismic retrofit of RC columns: The properties of FRP composites (their light weight and tailorability characteristics) provide immense advantages for the development of structural components for bridges and buildings in seismic regions. The retrofit of RC structures improves the strength of those members that are vulnerable to seismic attack.

The seismic retrofit of RC columns tends to change the column failure mode from shear to flexural failure, or to transfer the failure criteria from column to joint and/or from joint to beam failure, depending upon the strengthening parameters. This technique is used in existing reinforced concrete columns where insufficient transverse reinforcement and/or seismic detailing are provided; three different types of failure mode can be observed under seismic input. These are:

• Column shear failure mode: This mode of failure is the most critical one. The modern seismic column designs contain detailed transverse or shear reinforcement, but the shear strength of existing substandard columns can be enhanced by providing external shear reinforcement or by strengthening the column through composite fibres in the hoop direction.
• Confinement failure at the flexural plastic hinge: Subsequent to flexural cracking, the cover-concrete will crush and spall; this is followed by buckling of the longitudinal steel reinforcement, or a compression failure of the concrete, which in turn initiates plastic hinge deterioration.
• Confinement of lower ends of columns: Some bridge columns have lap splices in the column reinforcement; these are starter bars used for ease of construction and are located at the lower column end to form the connection between the footings and the columns. This is a potential plastic hinge region and it is advantageous to provide confinement by external jacketing or continuous fibre winding in this area.

None of these failure modes and associated column retrofits can be viewed separately since retrofitting for one deficiency may shift the seismic problem to another location and a different failure mode without necessarily improving the overall deformation capacity.

The confinement of RC columns can be undertaken by fabricating FRP composites using techniques such as the wet lay-up, the semi-automated cold-melt factory-made pre-impregnated fibre or the automated filament-winding processes. The fib have discussed the use of prefabricated (pre-cured) elements in the form of shells or jackets that are bonded to the concrete and to each other to provide confinement (fib Task Group 9.3, 2001).

The wet lay-up and the prefabricated systems are generally placed with the principal fibre direction perpendicular to the axis of the member. The concrete column takes essentially axial load therefore the ratio of the areas of the circumferential to axial fibres of the composite is large thus providing confinement to the concrete. This allows the tensile strength in the circumferential direction to be virtually independent of the axial stress value.

A review of the effectiveness of FRP composites for confining RC columns has been given in De Lorenzis and Tepfers (2003). 

Shear strengthening of degraded RC beams

Shear strengthening of RC beams and columns may be undertaken by bonding FRP laminates to the sides of the member. The principal fibre direction is parallel to that of the maximum principal tensile stresses, which in most cases is at approximately 45° to the member axis. However, for practical reasons it is usually preferable to attach the external FRP reinforcement with the principal fibre direction perpendicular to the member axis.

Various researchers – El-Hacha and Rizkalla (2004), Triantafillou (1998) – and current design recommendations – El-Refaie et al. (2003) and Ibell and Silva (2004) – have shown that an FRP-shear-strengthened member can be modelled in accordance with Mörsch’s truss analogy. Further information on this topic can be found in Lu et al. (2009).