Manufacturing of FRC
By mass of FRC produced, casting of premixed FRC is probably the most common method, especially for secondary or tertiary FRC. Fibres are normally supplied in ‘pre-batched’ bags, with each bag suitable for direct addition to one cubic meter of concrete to provide the correct volume fraction Vf . Steel fibres for traditional secondary FRC and polypropylene fibres intended for plastic cracking control (tertiary FRC) are almost exclusively supplied this way. Glass fibres may also be supplied in pre-batched bags for many applications.
For applications where large quantities of ready-mix or pre-cast FRC are to be manufactured on an ongoing basis, or more precise control of Vf is required, continuous batching equipment is available. Once the fibres have been added to the concrete, it can be installed using any of the normal concrete placing methods (pouring, pumping, vibration etc.).
The performance of cast premix FRC is limited by the effect of increasing Vf on workability and compaction. Steel–FRC suppliers place a typical limit on Vf of 2% (equivalent to 160 kg fibres m−3 concrete (Nemegeer, 1998)), above which balling of fibres in the mixer, or poor compaction leading to air voids and decreased strength and durability, will occur.
Guidelines for premix glass–FRC manufacture suggest an upper limit of 3.5% (GRCA, 2006) and the use of two-stage mixing in high-shear mixers to ensure fibre dispersion. Polypropylene fibres for tertiary FRC are normally added at 0.91 kg m−3 concrete (i.e. Vf ~0.1%), although some commercial products that offer a degree of secondary reinforcement may be added at up to 5 kg m−3 concrete.
In many systems, after premixing the fresh FRC slurry is placed by spraying onto a mould or substrate. For steel–FRC, standard concrete spraying/shotcreting equipment and methods are used, with few restrictions except that the nozzle diameter should be at least 1.5 times the fibre length. Both dry-mix and wet-mix systems can be used.
Mix design guidelines suggest that rather lower fibre contents are typically used than in premix, up to 70 kg m−3 concrete (Vandewalle, 2005). For glass–FRC, specialised equipment (including peristaltic pumps, high-shear mixing and purpose-designed spray guns) are used to spray premix. Fibre volume fractions of up 5.5% are claimed to be attainable (Peter, 2008).
Some spray systems deliver the matrix and the fibre to the spray gun separately, using either a twin or concentric double nozzle. This allows closer control and monitoring of Vf during manufacture. Fibre is delivered as a continuous roving to the gun and cut to the required length by an internal chopper. Mixing and consolidation occur as the sprayed constituents impinge on the mould or substrate.
The sprayed layer is then generally further compacted by hand rollers (for in-situ work) or automated production machinery (for factory pre-fabricated components). This arrangement is the main system in use for producing primary glass–FRC, and volume fractions well in excess of 5% are attainable by skilled operatives.
The strength of dual-sprayed GRC is generally superior to that of premix by 50–100%, owing to the higher Vf attainable and better compaction. Production rates of up to several tons of glass–FRC per day can be achieved.
Textile reinforcement lends itself to hand lay-up techniques similar to those used for FRP production. It allows more control of fibre placing than the spray methods and can potentially produce very high volume fractions, perhaps > 10%, but it is labour intensive and comparatively low output. The steps are very similar to FRP lay-up.
First a thin ‘gel coat’ of modified matrix slurry is used to coat the inside of the mould and provide good adhesion and surface finish. Next, alternate layers of matrix and textile are added, each layer being compacted using hand rollers, until the required component thickness is obtained. After preliminary curing, the component is then released from the mould and finished.
Several automated systems for FRC manufacture are available. The most well established is the continuous ‘Hatschek process’, developed for asbestos– FRC sheets from paper-making processes (Fig. 1). Fibre, fillers and cement are formed into a dilute slurry with water (about 6% solids by weight).
The slurry is drained through a continuously rotating porous roller, depositing a layer of wet solids on the surface, which are then picked up by a continuous loop of permeable mat called a ‘felt’. This felt, with the layer of green FRC attached, passes over a vacuum dewatering machine, where most of the remaining water is sucked out, consolidating the wet solids into a dense but flexible green-sheet product.
Several layers are built up before it is taken off the felt mat. It is then cut to size and further shaped (e.g. into corrugated forms) if required, before curing at either normal or elevated temperatures. Fibre contents of Vf = 9–30% can be achieved depending on the application. Once the equipment has been installed, the production process is very cheap.
The widespread outlawing of asbestos–FRC has spurred interest in using other fibres within the Hatschek process. Polyethylene fibres and cellulose fibres (i.e. processed natural fibres derived from woody materials), both in pulp form, are the most widely investigated for this application.
In principle, any fibre that can be dispersed within cementitious slurry to produce a dense fibre mat can be used. The controlling factor tends to be whether the fibre size and surface morphology are suitable to ‘trap’ cement particles in the slurry and prevent washout (Coutts, 2005). Since the properties of such fibres tend to be inferior to those of asbestos fibre, the FRC properties are correspondingly reduced.
Several systems are available that use robotic or quasi-robotic systems to control sprayed FRC nozzles. These range from factory-based prefabrication systems for glass–FRC components to computer controlled steel–FRC guns for in-situ tunnel lining fabrication. Several other methods, mainly arising from FRP production technology, are also in development, including (Brameshuber, 2006):
- Pultrusion: textiles are passed through baths of slurry and then pulled through shaped rollers to consolidate the matrix and fibre and form FRC laminates.
- Filament winding: rovings or tapes are passed through slurry baths and wound onto a mandrel to form, e.g., pipes and other cylindrical vessels.
- Extrusion: FRC premix is extruded under pressure into a die, either to directly form components or to produce green shapes for compression moulding.