GFRC is a composite material that combines the high compressive strength of cement mortars with significantly increased impact, flexural and tensile strengths imparted by the glass fiber reinforcement. According to the particular product and the engineering design, GFRC formulation materials are normally involved with the following:
Alkali Resistant (AR) Glass Fibre Products
AR glassfiber is specially formulated to have a high degree of resistance to alkali attack and high durability in cement. Laboratory testing shows that at least 16% zirconia content is required for adequate alkali resistance. AR glassfibers are available in roving, chopped strand, net and mat forms. The use of roving for hand spray including auto spray and chopped strands for premix are most common, with scrim being used in areas of high stress concentrations and mat for the similar application although more successful in floor screeding.
Ordinary Portland Cement (OPC), Rapid hardening Portland Cement (RHPC), Calcium Sulphoaluminate Cement (CSA) and White Portland Cement are the most commonly used cements. They should conform to the relevant National or International Standards. CSA cement is widely used in the Far East due to its low alkalinity and rapid hardening properties, which consequently reduces the corrosion of fibers and speeds up the mould turnover. White Portland Cement is used in GFRC where a white or light coloured finish is required.
Properly graded silica sand is recommended. The particle shape is preferably round or irregular but having a smooth surface without honeycombing. For spray GFRC, the maximum particle size is generally limited to 1.2 mm; for premix GFRC, the maximum particle size may be 2.4 mm. Sand other than silica sands may be used but the producer should provide evidence of their suitability.
Water should be clean and free from deleterious matter and should meet relevant standards for water to be used to mix concrete. Potable water is normally suitable.
PFA, GGBS, Metakaolin, Microsilica and ground glass powder are a range of pozzolanic materials that can be used to partially replace cement either to have a beneficial effect on the properties of GFRC or for the environmental concerns. They work by reacting with the free lime produced during the hydration process to form further hydration products.
Sand and aggregate for facing
When a facing mix is used to produce an architectural finish special aggregates and sand may be required. They should be clean, hard, strong, durable and inert, and free of staining or deleterious material. Crushed and graded hard rocks like limestone, granite, spar, calcite or marble are particularly suitable.
Admixtures such as water reducers, accelerators, retarders and air entraining agents may be used to impart specific properties to GFRC. They can be standard concrete admixtures or those specially formulated for GFRC manufacture.
Acrylic thermalplastic polymer is added to the GFRC mix to allow for a subsequent dry cure and for property enhancement, particularly the reduction of surface crazing. It is particularly favoured, when white cement is used, to avoid the possible water stain from wet cure.
Powder pigments or dispersions may be used to achieve specific colour effect. They are normally iron oxide based and should be harmless to concrete strength and set, temperature stable, non-fading and alkali resistant. The dosage should not exceed 10% of amount of cement.
Paints and sealers
Paints and sealers may be applied to GFRC. Latex masonry paints and water resistant stains in a methylemethacrylate base offer a wide range of colour choices. A clear coating of silane or siloxane may be applied to the face and back surfaces of a panel to reduce moisture movement and efflorescence.