7.2.1 General

As has been discussed in earlier chapters, FRP materials are generally two component composites. The first component is the reinforcing fibers which almost exclusively in construction will be carbon, aramid, or glass fibers. In some situations, two or more fiber types can be used, hence the use of ‘generally’ in the first sentence.The second component is a resin, an organic chemical which in the right circumstances will polymerize and solidify into long-chained molecules. The fibers and resin are intimately mixed together before the resin cures. The fibers give the composite strength and stiffness, the resin binds the fibers together and provides protection to the fibers. In this chapter many of the uses of composites involve mass production and the need to drive down cost. Thus glass fibers, the cheapest of the common fibers, will often be used. The resin will be a thermoset, a polymer in which the curing process cannot be reversed. Polyester, vinylester, and epoxy resins are all used in construction products. Polyester resins are cheapest but shrink during curing, vinylesters are more dimensionally stable, while epoxy resins produce the best quality FRP but are the most expensive of the three resin types. A problem with the use of polyester and vinylester resins is the styrene vapor given off during curing, which has been identified as a health risk. Another class of resin is the phenols whose mechanical properties are not as good as the others, but they do offer very good fire performance.

7.2.2 The benefits of using FRP composites

It is worth repeating the benefits from using FRP composite materials. There are four main advantages; high strength, the ability to customize the properties of the FRP, low density, and excellent long-term durability.

FRP composites have high strength; for example, typical glass fibers have a tensile strength of 2400–3500 N/mm². The effective strength of the composite will be much lower (depending on the ratio of reinforcing fibers to resin), but it would still be significantly greater than that of most steels.

The properties of FRP can be tailored as desired by changing the quantity and direction of the fibers. Fabricators have great flexibility in their choice of materials. They can choose the most appropriate reinforcing fibers, both type (glass, carbon, etc.) and also arrangement (chopped strand mat, individual fibers, woven or stitched fabrics of various geometries). Whilst many products can be obtained ‘off the shelf’, there is also the opportunity to have ‘tailor-made’ products in novel situations or for specific (large volume) applications. With careful design, FRP mate-rials can be used very efficiently.

The density of FRP is about one-fifth that of steel and less than two-thirds that of concrete. The density of glass fiber-reinforced polymer composite (GFRP) is typically about 1800 kg/m³, while the density of steel is about 7850 kg/m³. This means that FRP materials require less falsework (scaffold-ing) and heavy lifting equipment than conventional materials and can be handled by a smaller workforce.

Extensive testing has shown that FRP is extremely durable when used appropriately with low long-term degradation and very good fatigue resist-ance. FRP composites are resistant to water, salt, and other chemicals, and are unaffected by oil and other heavy hydrocarbons; as a result they require little maintenance in comparison to conventional materials.

No material is perfect and there can be downsides to the use of FRP. Material costs on a weight for weight basis are higher than those for con-ventional materials, but this should be seen as an incentive to use the FRP in innovative, efficient, and cost-effective ways. FRP materials have low coefficients of thermal expansion (some CFRP has a negative coefficient). This is only a problem when the FRP is used in conjunction with a conventional material and careful design should eliminate any potential problems. The resin properties are temperature dependent; lower temperatures cause the strength to increase but make the resin more brittle. The converse is true as temperature is increased, but at a critical temperature (called the glass transition temperature and in reality a temperature range of which the glass transition temperature is the median) there is a rapid decrease in strength. Problems can be avoided by the choice of a resin whose glass transition temperature is significantly higher than the anticipated maximum operating temperature.

A frequent criticism of FRP composites is that they perform poorly in fire. However, this has been shown not to be the case (Cutter et al,. 2009). In a fire the exposed resin surface chars and produces a tar-like layer which protects the underlying fibers, delaying the onset of failure. Of course, it is also possible to use conventional fire protection on composite material.

It is worth mentioning two further points concerning FRP composites. Stiffness (measured as elastic modulus) can be an important property. Carbon fiber-reinforced polymer composites (CFRP) can have high stiff-ness; an elastic modulus of up to 300 kN/mm² compared to 200 kN/mm² for steel. However, GFRP has lower stiffness, typically in the range 72–87 kN/mm² and it is frequently stiffness rather than strength which drives the design of GFRP. Finally, in many applications surface finish and appearance is important; with composites the use of appropriate fabrication techniques and materials, for example automated systems with high quality moulds and the use of gel coats, will achieve the finishes required.

7.2.3 Fabrication techniques

These have been covered in earlier chapters but it is worth looking at them again in the context of construction applications. The basic methods such as wet-layup are labour intensive and appropriate for one-off products. For mass production, the automated processes are much better. FRP panels are fabricated to a high quality and finish using vacuum assisted resin transfer molding (VARTM) or vacuum infusion. Structural shapes (I-beams, chan-nels, angles, etc.) are produced in large quantities using the pultrusion process; it is ideal for products produced in long straight lengths with a constant cross section and constant reinforcing fiber architecture. Pipes are produced in many different sizes using the fiber winding process, which is a development of pultrusion in which the fibers are wound to a predeter-mined helical shape within the resin.

A further benefit of the automated processes is that they produce the best quality FRP. Composites are inherently variable because of air trapped within the matrix; however, the automated processes squeeze out almost all the air (typically less than 2% air is left in the matrix compared to over 5% in wet layup composites) so that the resulting composite is much less variable. Also it is possible to incorporate more reinforcement into the same volume of composite (reinforcement volume fraction is about 0.6 in vacuum infused composite compared to about 0.4 in wet layup material). Strength and stiffness are proportional to the volume fraction of reinforcement.

7.2.4 Conclusion

The discussion above has emphasized the flexibility in the use of FRP com-posites and the scope to use them in novel or unconventional ways. The designer is able to benefit from the potential to customize the composite properties to suit a particular application.

Even a cursory search of the web will reveal the very wide range of composites applications in construction. In order to give an overview, this chapter will now present a variety of applications for FRP composites in construction. Illustrations will use photographs from particular companies but it should be emphasized that there is considerable choice in manufacturers and fabricators and a potential user should investigate the possibili-ties before making any choice. Applications are presented in sections to give an impression of what is available. Where appropriate, fabrication methods and materials are also discussed.

7.3.1 Building interiors and exteriors

Advanced composite materials have been used in buildings for many years. An early example was the cladding of Mondial House in London (Fig. 7.1) which was completed in 1974. The white cladding was still in excellent con-dition when the building was demolished in 1996. Cladding systems are widely available and come in a variety of textures and colors.

Advanced composite materials are available for almost every aspect of building interiors ranging from floors to doors. Since FRP composites are electrically non-conducting and have hard smooth surfaces, they are particularly applicable to situations where high levels of hygiene are required or where magnetic or electrical machinery is being operated. Figure 7.2 shows a clean room finished almost entirely with FRP composite. In this case, the walls have a smooth finish for cleaning but textured finishes are also available.

Typical construction of wall and ceiling panels has GRP faces as little as 1 mm thick with a polystyrene insulating core, whose thickness will depend on the level of insulation required.

There are various FRP grating type flooring systems on the market. They are in competition with metal gratings but score heavily in aggressive environments where their excellent durability is important. Figure 7.3 shows a grating system which is used in water treatment and offshore applications. This system uses a phenolic resin in the GRP which gives particularly effec-tive fire resistance.

Floor grating systems are frequently used in industrial buildings for mezzanine or intermediate floors. Figure 7.4 shows a typical application in which the handrails are also GFRP.

Doors, door frames, and window frames are now being fabricated in GFRP in factories as far apart as the US and India. These range from internal and external doors for house construction through to heavy-duty industrial doors. Figures 7.5 and 7.6 show typical examples, but many different finishes are available. An Indian manufacturer points out that GFRP doors are termite proof, a significant factor in many parts of the world.

Pultruded composite sections (often similar in shape to standard steel sections) are being used for structures such as the floor support structure shown in Figure 7.7. In some ways this is a strange application of FRP because steel section shapes have been developed to use the material prop-erties of steel which are different from those of FRP. However, FRP requires much lighter lifting gear and is a non-conductor, factors which may be important in a particular project.

Components fabricated from GRP composites are used as architectural features on building exteriors as shown in Figures 7.8 and 7.9, while outbuildings made from composite material are increasingly common (Fig. 7.10).

The potential for the use of advanced composites in construction has been demonstrated by the Eyecatcher Building in Basel, Switzerland. The load bearing structure of this striking five-storey building is entirely fabricated from composite material. The pultruded GRP profiles used were produced using E-glass fibers and polyester resin by Fiberline Composites in Denmark. Figure 7.11 shows the building which was originally produced for the Swissbau99 exhibition.An important feature is that the load-bearing structure forms part of the façade because the good thermal properties of the GRP do not produce any cold or warm bridges.

7.3.2 Bridges

There are ongoing problems in the US and elsewhere due to salt-induced degradation of concrete bridge decks. Chloride ions migrate through the concrete and onto the steel reinforcing bar. This causes the steel to corrode and the concrete breaks up because the volume of the corrosion products is greater than that of the original steel. Since advanced composites are very durable and largely unaffected by salt, they offer an alternative to concrete decks.

There has been considerable research into composite bridge decks using pultruded GFRP sections. The original shape considered was the plank developed by Maunsell Structural Plastics in the UK. This section, shown in Fig. 7.12, had rectangular cells which could be connected together in-line or at right angles using adhesive and a connector strip. The planks were used in the innovative all composite Aberfeldy footbridge in Scotland. Unfortunately, Maunsell Structural Plastics no longer exists, but their planks (now called Composolite) are still fabricated by Strongwell in the US.

Although suitable for pedestrian loading, the rectangular cells have drawbacks with vehicle loading. Various pultruded sections have been developed to improve vehicle load-bearing characteristics. Figure 7.13 and 7.14 show, respectively, the Asset and DuraSpan profiles which span transverse to the main girders and can act compositely with the main girders. The Asset profile was first used in the all composite West Mill replacement bridge in Oxfordshire, UK. During its opening ceremony, its integrity was demonstrated by means of a Sherman tank. The four main girders consisted of hybrid GRP/CFRP box beams to which the Asset profile deck was adhesively bonded. The DuraSpan profile has been used in a number of deck replacement projects in the US and abroad, the thirtieth application being the Siuslaw Bridge in Florence, Oregon. Both systems come with a recommended wearing surface which has to bond to the FRP as well as provide traction for vehicles. Other profiles are also available.

Handrail and balustrade systems in advanced composites are readily available and competitive in cost with conventional materials. Figure 7.15 and 7.16 show typical details.

Concerns over steel girders in aggressive environments (coastal or indus-trial) have led to the development of bridge enclosure systems.These struc-tures are attached to the underside of the bridge, protecting the girders and allowing access for inspection or maintenance. GFRP has proved the ideal material for the skin of the enclosure because of its low weight. Figure 7.17 shows the enclosure on one of the approach bridges to the Second Severn Crossing, Avon, UK.

Aramid (trade name Twaron or Kevlar) fibers have high tensile strength and have been formed into cables, one trade name being Parafil. Research at Cambridge University, UK (Burgoyne, 1993) has developed specialized end anchors for the FRP cables. As a result, these cables have been used on cable-stayed bridges, for example the Aberfeldy Bridge, Perth and Kinross, Scotland, shown in Fig. 7.18. This footbridge, already mentioned above, is particularly interesting. It is set in a golf course which straddles the River Tay and was designed by Maunsell Structural Plastics and constructed by a group of Dundee University students under the supervision of Professor Bill Harvey (Harvey, 1993). It was amongst the first all advanced composite bridges to be constructed anywhere in the world and, during almost twenty years in service, has required minimal maintenance.

7.3.3 Infrastructure

The uses of advanced composites in infrastructure applications are diverse and surprisingly large. This section will present a selection which will give an impression of that diversity.

The construction of roads in or close to built-up areas has resulted in the need for noise barriers which mitigate the noise pollution caused by traffic. Traditional barriers have been constructed of timber and concrete. However, FRP barriers are also available and come in two versions. Reflective barriers have solid surfaces and deflect incident sound back towards the road. Absorbent barriers have perforated GRP faces with a fiberglass wool infill. Sound passing through the perforations is absorbed by the infill, while some sound is reflected by the solid parts of the face. Figures 7.19 and 7.20 show both types of barrier, one manufactured in the UK, the other in China.

Pylons for supporting power and other utility cables often have to be installed in inhospitable terrain. The low weight and excellent durability of FRP composites helps to minimize the resulting installation and maintenance difficulties. Figure 7.21 shows typical details. Signs for displaying information on motorways are also being fabricated in FRP composites as shown in Fig. 7.22. In Dubai there are a surprising number of GRP palm trees which conceal mobile phone masts.

FRP composites are widely used in the water treatment and chemical industries. Storage tanks up to 160,000 litres capacity are readily available.

Special lining resins are used to provide exceptional chemical resistance. Figure 7.23 shows a typical tank. Another possibility is the GRP sectional tank which is made up of panels connected together, possibly with an internal supporting structure, as shown in Fig. 7.24. FRP tank covers for storage tanks are a cost-effective alternative to conventional covers. The covers can be freestanding or supported by an FRP truss system allowing spans up to 30 m. Figure 7.25 shows a freestanding cover on an underground tank.

A common sight in built-up areas is the excavations made by utility companies as they repair or update their services. The resulting trenches can cause access problems for residents and delivery companies. A simple but very useful solution has been the GRP temporary access covers shown in Figs 7.26 and 7.27. These can be 1 m square and about 25 mm thick, and are suitable for light traffic or heavy duty capable of supporting lorry traffic. A useful feature is that they can be colored so that they are easily recognizable by their owner.

This section has presented a selection of advanced composite artefacts that are now being widely used in the construction industry. Most are competing with conventional materials such as steel or concrete and are gaining a market position because of their durability and low weight.

7.3.4 Railway infrastructure

Railways have particular infrastructure needs which differ from mainstream construction. GRP composites have been used in a variety of applications. Figure 7.28 shows a station platform system made of GRP and with a non-slip wearing surface. Other applications have included trays and frames for carrying cables and electrical components (Fig. 7.29), water collection pits for drainage systems (Fig. 7.30) and ballast retaining supports.

7.3.5 Geotechnical applications

FRP systems are being used in geotechnical applications in increasing numbers. The long-term durability of FRP can be very attractive in aggressive environments which would corrode steel and its low weight is an advantage in areas such as steep cliffs where access is difficult. This section will look at typical current applications.

Sheet piling has traditionally been constructed using steel sheets. GRP is now providing a viable alternative and Fig. 7.31 shows the components of a typical system.

A common requirement in construction is for slope or rockface stabilization, whether for relatively small embankments or large cliff faces. A typical approach is the use of soil nails or rock bolts with a structural grid attached to the nail heads. Holes are drilled into the face to be stabilized, the nails or bolts are inserted to the required length and are then grouted in place. Finally, if necessary, a net is attached to the nail or bolt heads. GRP systems can have significant advantages because traditional steel nails and bolts need corrosion protection and are heavy. Figure 7.32 shows typical GRP soil nails and rock bolts, and Figs 7.33–7.35 show their use in embankment, cliff face (with difficult access), and retaining wall stabilization.

An interesting development reported by Ortigao (1996) is the use of geobars as temporary soil nails in tunneling applications. A GRP geobar is a tube which may have valves at intervals along its length, which may be as long as 30 m. The geobars are inserted into drilled holes at the tunnel drive face and grouted in place by pumping resin through the tube and valves; they stabilize the drive face but are sacrificed as driving proceeds. The advantage of the hollow tubes is that they offer less resistance to the tunnel boring machine than solid steel or GRP rock bolts.

7.3.6 Pipes

Traditionally, water pipes have been made from cast iron (actually very early pipes were timber). FRP pipes offer considerable advantages; they are fabricated by the pultrusion/fiber winding process shown in Fig. 7.36 which results in an accurate cross-section shape and a very smooth internal finish with low friction. Hydraulically, FRP pipes are more efficient because they suffer little head loss due to internal friction so that for a given application smaller diameter GRP pipes are needed. Hence, it is frequently possible to replace older pipes by pushing the FRP replacement through the original. New pipes can range from 50 mm to 4 m in diameter and are used in the water, chemical, petroleum, and gas industries up to internal pressures of 175 bar. Figure 7.37 shows a large diameter water pipe.

A frequent problem with underground clay drainage pipes is that they are damaged and blocked by debris. Contractors can usually clear the blockage, but the pipe needs to be repaired. An ingenious system uses a pitch-based GFRP liner which is threaded through the pipe like a flat hose. The liner is then inflated using water or air pressure until it fits tightly to the inside of the pipe. At this stage the resin catalyst activates the curing process resulting in a solid lining within a few hours. This reduces the effective pipe cross section by about 6% but this is compensated for by the increased hydraulic efficiency. Figure 7.38 shows the benefit of this type of repair. Other localized pipe repair systems use GFRP tape wrapped around the pipe and cured in situ; some of these systems can even be used underwater.

7.3.7 Conclusion

The intention behind this section has been to give an impression of the wealth of applications for advanced composite materials within the construction industry. The examples presented have come from all over the world, thanks to the Internet! Fabricators are always on the lookout for new applications and would welcome enquiries from potential customers.

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