Glassfibre-reinforced concrete (GRC) is a material that was developed in the early 1970s by the Building Research Establishment. The standard material is produced from a mixture of alkali-resistant glass fibres with Portland cement, sand aggregate and water.
Admixtures such as pozzolanas, superplasticisers and polymers are usually incorporated into the mix to give the required fabrication or casting properties. The breakthrough in the development of the material was the production of the alkali-resistant (AR) glass fibres, as the standard E-glass fibres, which are used in GRP and GRG, corrode rapidly in the highly alkaline envir -on ment of hydrated cement. Alkali-resistant glass, in addition to the sodium, silicon and calcium oxide components of standard E-glass, contains zirconium oxide. Alkali-resistant glass fibres, which have been improved by progressive development, are manufactured under the trade name Cem-FIL. The addition to GRC mixes of metakaolin, a pozzolanic material produced by calcining china clay at 750–800°C, prevents the development of lime crystals around the glass fibres. In the unblended GRC this leads to some gradual loss of strength. Standard grey GRC has the appearance of sheet cement and is non-combustible.
PROPERTIES OF GLASSFIBRE-REINFORCED CONCRETE
While standard GRC has the appearance of concrete, a wide diversity of colours, textures and simulated materials can be manufactured. A gloss finish should be avoided, as it tends to craze and show any defects or variations. The use of specific aggregates followed by grinding can simulate marble, granite, terracotta, etc., while reconstructed stone with either a smooth or tooled effect can be produced by the action of acid etching. An exposed aggregate finish is achieved by the use of retardants within the mould, followed by washing and brushing. Applied finishes, which are usually water-based synthetic latex emulsions, may be applied to clean, dust-free surfaces.
Moisture and thermal movement
GRC exhibits an initial irreversible shrinkage followed by a reversible moisture movement of approximately 0.2%. The coefficient of thermal expansion is within the range (7–20) X 10–6 °C–1, typical for cementitious
The thermal conductivity of GRC is within the range 0.21–1.0 W/m K. Double-skin GRC cladding panel units usually incorporate expanded polystyrene, mineral wool or foamed plastic insulation. Cold bridging should be avoided where it may cause shadowing effects.
GRC is less permeable to moisture than normal concrete, so it has good resistance to chemical attack; however, unless manufactured from sulphate-resisting cement, it is attacked by soluble sulphates. GRC is unaffected by freeze/thaw cycling.
GRC exhibits a high-impact resistance but toughness and strength does decrease over long periods of time. However, the incorporation of metakaolin (2SiO2·Al2O3) into the mix appears to improve the long-term performance of the material.
USES OF GLASSFIBRE-REINFORCED CONCRETE
GRC is used extensively for the manufacture of clad -ding and soffit panels because it is lightweight and easily moulded (Fig. 11.6). It is used in conservation work as a replacement for natural stone and in architectural mouldings, including sophisticated decorative screens within countries of the Middle East. It is used as permanent formwork for concrete, fire resistant partitioning and in the manufacture of small components including slates, tiles and decorative ridge tiles. Fibre-reinforced cement slates are manufactured to simulate the texture and colour of natural slate. Some manufacturers incorporate blends of other non-asbestos natural and synthetic fibres together with pigments and fillers to produce a range of coloured products with glossy, matt or simulated riven finishes.