Applications of FRC
In this article we describe some typical applications for the various types of FRC. The list is not exhaustive – other FRC could be used for the applications given, and there are countless other applications for FRC – but they are intended to give you an idea of the major uses of FRC in building and construction.
Architectural cladding: glass–FRC
One of the highest volume semi-structural applications for glass–FRC is architectural cladding, particularly where complex surface mouldings or faithful restoration of heritage features such as capitals or cornices are required. Its major competition is traditional pre-cast concrete.
Since glass–FRC has no steel reinforcement and thus no cover concrete is required, elements can be made very thin (> 6 mm), making glass–FRC cladding components extremely light in comparison with pre-cast concrete elements (which generally must be at least 50 mm thick). As well as reducing structural loads – often important in renovation works – this can significantly reduce installation costs, handling complexity and erection time.
The thin sections can also form a wider range of shapes than traditional pre-cast elements, are less susceptible to visible cracking and do not contain any steel to corrode. The low weight reduces both transport costs and cement usage, reducing environmental impact. Glass–FRC for architectural cladding is manufactured using dual-spray systems, giving a high Vf , quality surface finish and good dimensional tolerance.
Panels are normally fixed to the supporting substructure using L-shaped flexible steel anchors bonded to the rear of the panels (GRCA, 2006). The Newcastle Council Chambers building, Australia was refurbished using glass–FRC panels in the 1990s. The original pre-cast reinforced concrete panels had deteriorated, with dangerous spalling occurring on the panel surfaces.
Glass–FRC panels were designed to fit over the existing façade to cover and contain the spalling. Their light weight allowed them be installed with simple scaffolding and manual handling equipment – thus not requiring the building to be closed during installation – and did not add sufficient additional structural load to require strengthening of the building. In addition, the new panels were designed to seal the building to allow more efficient operation of heating and air-conditioning systems (Glenn Industries).
More advanced applications of the same basic system, but using in-situ spraying rather than factory prefabricated panels, can produce extremely complex ‘megasculptural’ structures such as the Merlion and the UK’s Millennium Dome ‘Body Zone’. The Body Zone structure had to transport up to 3500 people per hour through its interior.
Since the resultant live load varied throughout the body of the structure, the FRC skin thickness over the steel sub-frame had to be continuously varied (Glenn Industries).
Tunnel linings: steel–FRC and polymer–FRC
Tunnel boring machines are now increasingly used in preference to other tunnelling methods. Robust and rapidly deployable linings are required to prevent ground settlement, especially in urban areas. Pre-cast RC segments are often used, which are jacked into place after tunnel excavation has finished.
Manufacture of the steel reinforcing cage for these segments is expensive, and during installation the cover concrete at edges and vertices often spalls under the jacking forces, leading to durability and finishing problems. Using pre-cast steel–FRC reduces cost and weight, and eases installation of the lining (Vandewalle, 2002).
In larger tunnels, the lining is normally placed in-situ. An initial lining of rapid setting/hardening concrete or steel–FRC, around 75 mm thick, is sprayed on to support and stabilise the fresh excavation. The inner, structural liner (about 300–350 mm thick) is then either cast or sprayed in place.
Waterproofing membranes may be placed between the two, depending on the system in use. Cast RC liners require temporary lattice girders to be placed at a set distance from the tunnel roof/walls to orient and support the reinforcing steel (since it cannot be attached to the inner liner without disrupting the membrane or other waterproofing system).
This is cumbersome, and installation can present a health and safety hazard as it occurs in an unsupported excavation (Eddie and Neumann, 2004). Using sprayed FRC for both layers can reduce costs and complexity. Advances in admixtures and placing technologies that allow a reduction in water:cement ratio to help protect against explosive spalling during tunnel fires.
Since most tunnel-lining concrete has a low water:cement ratio the capillary porosity is ‘segmented’, i.e. it does not consist of a continuous network of pores, and the permeability of the concrete is very low. During a fire, water vaporised by the intense heat cannot easily escape and large pressures build up, which can lead to catastrophic explosive spalling of the concrete.
By adding polypropylene fibres, which melt at relatively low temperatures (130–160°C), pathways for vapour to escape are provided, reducing spalling without sacrificing strength.
Industrial flooring: steel–FRC and polymer–FRC
Installation of concrete industrial floors is a demanding application, since the large exposed surface area (per m3 of concrete) can lead to increased probability of cracking caused by plastic shrinkage and/or drying shrinkage. Also floors are required to deal with concentrated dynamic and static loads from, e.g. fork-lift trucks or warehouse shelving and so need enhanced resistance to localised cracking.
Thus a form of distributed crack-control reinforcement (in addition to any structural reinforcement required, e.g. for floors laid over pile caps) is required. Traditionally, this has been provided by a double layer of steel mesh. Large sheets of mesh – typically of ~10 mm diameter bars crossing in a 100–200 mm grid – are laid on spacers to ensure that the layers are positioned at the correct depth. The concrete is then poured over the mesh.
Using mesh in this way is cumbersome, especially on multi-storey projects. Large sheets of mesh must be craned and manoeuvred into the building and fixed into place; the pour must be closely supervised to ensure that the spacers are not accidentally knocked out by personnel necessarily walking on or ‘between’ the mesh during concrete pouring; the mesh itself is a trip hazard. Replacing the mesh with steel fibres provides several advantages:
- All handling and placing cost/time associated with the steel mesh is eliminated.
- The reinforcement is distributed throughout the full thickness of the slab, rather than just concentrated in one or two layers.
- Since the shear resistance of steel–FRC is greater than that of mesh–RC, and no cover to the steel is required, the floor thickness can be reduced, reducing dead weight.
- FRC can be used more easily in conjunction with composite floor construction.
The technical performance (with regard to strength and deflection) of FRC is generally claimed to be as good as or better than that of mesh–RC. Fibre manufacturers also claim that 10–40% time savings and 10–30% cost savings can be achieved (Stadlober, 2006). While steel fibres are the usual choice for mesh replacement, polymer fibres are also used.
Some hybrid systems combine steel macro-fibres to control service cracking with a relatively small amount of polypropylene micro-fibres (12 mm long, 18 µm diameter at ~1 kg/m3 concrete) to control plastic shrinkage cracking. Other systems use a combination of crimped polypropylene or PVA macrofibres (2–7 kg m−3 concrete) with polypropylene microfibres (as above) to obtain a similar effect. In lightly loaded floors (or other applications where shrinkage control is of enhanced importance), polypropylene microfibres might be used on their own; independent tests (by the British Board of Agrément, see BBA certificate no. 06/4373 details sheet 3) have shown that such fibres can reduce plastic-shrinkage cracking in slabs by a factor of almost 10.
Sheet materials for building: natural–FRC
The most widespread use for natural–FRC is probably ‘siding’ – external cladding for domestic and light commercial buildings. In this application it competes with plastics, plywood and metallic sheeting, and is a direct replacement for asbestos sheeting. The sheets are made using the Hatschek process. Wood-pulp fibres obtained using the kraft process (where wood chips are treated with sodium hydroxide and sodium sulphide at < 180°C to extract the cellulose fibres from the lignin matrix) are mechanically treated to internally and externally fibrillate the surfaces in order that the fibres can flocculate and retain the cement particles in the green felt (Coutts, 2005).
Products may be air-cured or autoclaved. The strength of typical boards can vary considerably according to the moisture state (typically 10.0 MPa dry strength, 7.0 MPa wet strength; see Anon, 2008).
The siding market is currently dominated by wood-based and polymer products, but natural– FRC now has a ~9% share of a US market estimated at around $500 million (Coutts, 2005). As with glass–FRC, panel products can be made to a wide range of finishes to re-create the look of traditional timber siding, but with much lower maintenance requirements.