During the introduction of FRP composites into the building and construction industry in the 1970s glass fibres were used in a polyester matrix as a construction material. Skeletal frames constructed from reinforced concrete (RC) or steel columns and beams were in-filled with non-load-bearing or semi-load-bearing GFRP panels manufactured by the wet lay-up process or by the spray-up technique to form structural buildings.
Applications of FRP Composites
Several problems developed owing to a lack of understanding of the FRP material, mainly arising from insufficient knowledge of its in-service properties relating to durability and the enthusiasm of architects and fabricators for developing geometric shapes and finding new outlets for their products without undertaking a thorough analysis of them.
Consequently, to improve certain physical properties of the FRP some additives were incorporated into the polymers by the fabricators without a full understanding of their effect on the durability of the FRP material, or indeed were omitted in cases where additives should have been added. Advanced polymer composites did not enter the civil engineering construction industry until the middle to late 1980s; polyester and epoxy polymers were used initially and vinylester was introduced in the 1990s.
From the 1970s, universities, research institutes and industrial firms have been involved in researching the in-service, mechanical properties of FRPs and in the design and testing of structural units manufactured from fibre/polymer composites. This was followed by the involvement of interested civil engineering consultants undertaking industrial research and the utilisation of the structural material in practice.
The application of advanced polymer composites, over the past 35 years for the building industry and the past 25 years for the civil engineering industry, can be conveniently divided into some specific areas, which will be discussed briefly in this article:
Building industry: infill panels and new building structures.
Civil engineering industry:
- civil engineering structures, fabricated entirely from advanced polymer composite material, known as all-polymer/fibre composite structures
- bridge enclosures and fairings
- bridge decks
- external reinforcement rehabilitation and retrofitting to RC structures (including FRP confining of concrete columns)
- external reinforcement rehabilitation and retrofitting to steel structures
- internal reinforcement to concrete members
- FRP/concrete duplex beam construction
- polymer bridge bearings and vibration absorbers
All these, other than the first, involve a combination of advanced polymer composites and conventional construction materials and are therefore often termed composite construction. FRP composites are durable and lightweight and consequently they can fulfil many of the requirements of structural materials for many forms of construction.
Ideally when new civil engineering structures are manufactured from polymer composite systems the component parts should be modular to provide rapid and simple assembly. An example of the importance of this is in the installation of highway infrastructure, where any construction or long maintenance period of the infrastructure will cause disruption to traffic flow and will be expensive. The examples of the applications of polymer fibre composites in those areas that we will discuss in this article have been chosen to illustrate all the areas of use listed above.
The building industry
During the 1970s two sophisticated and prestigious GFRP buildings were developed and erected in the UK, Mondial House, the GPO Headquarters in London (Berry, 1974) and the classroom of the primary school in Thornton Clevelys, Lancashire; these are discussed below.
Other FRP buildings that were erected during this period were Covent Garden Flower Market (Roach, 1974; Berry, 1974), the American Express Building in Brighton (Southam, 1978), and Morpeth School, London (Leggatt, 1974, 1978).
These structures played a major role in the development of polymer composite materials for construction. Because of the relatively low modulus of elasticity of the material, all except one of these buildings were designed as folded plate systems and erected as a composite modular system, with either steel or reinforced concrete units as the main structural elements and the GFRP composite as the load-bearing infill panels. The exception to this is the classroom of the primary school, Thornton Clevelys, Lancashire, UK (Stephenson, 1974), which is entirely manufactured from GFRP material.
Mondial House, erected on the north bank of the Thames in London 1974
This building was clad above the upper ground floor level and the panels were manufactured from glass fibre polyester resin. The outer skin of the panel included a gel coat that used isophthalic resin, pigmented white, with an ultraviolet stabiliser backed up with a glass fibre reinforced polymer laminate; the latter used a 3 oz per square foot chopped strand mat and a self-extinguishing laminating resin reinforced with 9 oz per square foot glass fibre chopped strand mat reinforcement.
Some degree of rigidity was obtained from a core material of rigid polyurethane foam bonded to the outer skin and covered on the back with a further glass-reinforced laminate; this construction also provided thermal insulation. Further strength and rigidity were obtained by the use of lightweight top-hat section beams, manufactured as thin formers and incorporated and over-laminated into the moulding as manufacture proceeded.
The effect of the beams was transferred to the front of the panel by means of glass-fibre reinforced ties or bridges formed between the polyurethane foam at the base of each beam. The face of the beam was reeded on the vertical surfaces in order to mask any minor undulations and to provide channels off which the water ran and thereby cleaned the surface. The reeding also gave the effect of a matt panel without reducing the high surface white finish. The structure was visually inspected in 1994 by Scott Bader and the University of Surrey and the degradation was found to be minimal. It was demolished in 2007 to allow for redevelopment of that area.
A part of the composite material from the demolished structure was analysed at the University of Surrey for any variations in the mechanical properties due to the degradation of the composite material during its life (Sriramula and Chryssanthopoulos, 2009).
An ‘all-polymer composite’ classroom of primary school, Thornton Clevelys, Lancashire, UK, 1974
The classroom, is an ‘allcomposite’ FRP building in the form of a geometrically modified icosahedron, and is manufactured from 35 independent self-supported tetrahedral panels of chopped strand glass-fibre reinforced polyester composite.
Twenty eight panels have a solid single skin GFRP composite and in five of these panels circular apertures were constructed to contain ventilation fans. In the remaining seven panels non-opening triangular windows were inserted. The wet lay-up method was utilised to manufacture the E-glass fibre/polyester composite skins. The inside of the panels has a 50 mm thick integral skin phenolic foam core acting as a non-load bearing fire protection lining to the GFRP composite skins.
The icosahedron structure is separated from the concrete base by a timber hardwood ring. The FRP panels were fabricated onto a mould lining of Perspex with an appropriate profile to give a fluted finish to the flat surfaces of the panels.
The edges of the panels were specially shaped to provide a flanged joint, which formed the connection with adjacent panels. Sandwiched between two adjacent flanges is a shaped hardwood batten, which provides the correct geometric angle between the panels; the whole is bolted together using galvanised steel bolts placed at 450 mm intervals.
The external joint surfaces between the adjacent panels were sealed with polysulphide mastic. The glass windows were fixed in position on site by means of neoprene gaskets. The classroom was designed by Stephenson (1974). When the classroom structure was under construction in 1974 a fire test at the BRE Fire Research Station was undertaken on four connected GFRP panels, with the integral skin phenolic foam in place.
At the same time, tests were also undertaken on an identical geometrically shaped school system used at that time. The results demonstrated that the GFRP classroom had over 30 minutes fire rating whereas the existing school system had only 20 minutes. These two descriptions of the Mondial House and the school classroom at Thornton Clevelys have been based on Hollaway (2009).
The civil engineering industry
The ‘all-polymer composite’ structure systems – like those of the building industry produced to date – have tended to be single prestigious structures, manufactured from ‘building blocks’, Hollaway and Head (2001). The advantages of this are:
- the controlled mechanised or manual factory manufacture and fabrication of identical structural units
- the transportation to site of the lightweight units, which can be readily stacked; it is more economical to transport lightweight stacked FRP units than the heavier steel and concrete units.
McNaughton (2006) said: ‘The majority of the Network Rail’s bridges in the UK are 100 years old and are constructed in a variety of materials, for example cast iron, wrought iron, steel, reinforced concrete, brick, masonry and timber. Future construction is likely to use more complex forms of composite construction, in particular fibre reinforced polymers, which are already being used to strengthen bridges’.
Examples of some of these ‘more complex structures’ are the Aberfeldy Footbridge, Scotland (1993), the Bonds Mill Single Bascule Lift Road Bridge, Oxfordshire (1994) (Head, 1994), Halgavor Bridge (2001) (Cooper, 2001), the road bridge over the River Cole at West Mill, Oxfordshire (2002) (Canning et al., 2004), the Willcott Bridge (2003) (Faber Maunsell, 2003), the New Chamberlain Bridge, Bridgetown, Barbados (2006) and the Network Rail footbridge which crosses the Paddington–Penzance railway at St Austell, UK (2007). An innovative £2 million Highways Agency super-strength FRP composite bridge (The Mount Pleasant Bridge) was installed in 2006 over the M6, between Junctions 32 and 33; the structure won the National Institution of Highways and Transportation Award for Innovation in June 2007.
All these structures were of modular construction, manufactured utilising advanced composite materials; for the construction to be successful the material had to be durable, and assembly of the units had to be rapid and simple with reliable connections.
As we have already seen advanced polymer composite materials are durable and lightweight and consequently they fulfil these requirements, provided that the initial design of the basic building modular system is properly undertaken and the material properly installed. A number of bridges have used the concept of the Maunsell structural plank, shown in Fig. 2.