Fibreglass (the material to make boats) is really glass fibre reinforced plastic--that is, glass fibres in a polymer matrix. Instead of using polymer, use cement and sand to get fibre-reinforced concrete (GFRC). GFRC is used to make concrete products such as ornamental concrete structures, domes, planters and fountains. It can also be used for concrete countertops and decorative panels.
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Fibre-reinforced concrete weighs much less than regular concrete--as much as 75 per cent less in some cases. This allows for reduced shipping costs. GFRC has a very high strength-to-weight ratio and can be used to make complex shapes, since it is reinforced internally. It can also be sprayed into forms and moulds, making a better finished products, as there is no chance for air bubbles to form. It also does not crack as easily as regular cement and does not chip when it is cut.
Hand sprayed glass fibre-reinforced concrete can be sprayed into a mould. Most precast panels and ornamental pieces are made by using a direct spray-up method. A concentric chopper gun is fed from a spool of GFRC, and the mix is blended right at the nozzle as it is being sprayed into the mould.
Premixed GFRC can be poured into a watertight mould, then vibrated to get the consolidation. This method cannot be used for rock moulds because of the complexity of the shape of the mould.
Spraying premixed glass fibre reinforced cement that has chopped fibres already in the mix gives you a better strength than vibration casting and does not require as much experience as the traditional hand spray-up but requires a peristaltic pump and a special spray nozzle.
Decorative pieces are sprayed in two layers. The backing contains the glass fibres, and the top layer--the layer that is visible--is a thin layer of concrete only.
Glass fibre reinforced concrete was first made in the 1940s. This mixture lost strength because alkaline breaks down silica. Concrete is a very high alkaline environment, and glass is made primarily of silica. In the 1970s, Owens-Corning and Nippon Electric Glass figured out a way to perfect alkali-resistant glass fibres.
Since rock is not abundant in all parts of the country, it can be very expensive to ship it in. With the advent of GFRC, since it is lighter, it can be shipped for a lower cost to the consumer. One type of GFRC, VCAS 160, is even environment friendly, since it uses byproducts (E-glass) instead of concrete.
Traditional glass fibre-reinforced concrete used for spraying has a low water-to-cement ratio mix. It is used for most decorative products, with the exception of artificial rocks. The sand and cement are mixed at a one-to-one ratio. It may cure too fast, which leaves a weak finished product. It needs seven days to cure fully.
By adding an acrylic polymer additive that works like a curing compound, evaporation of the little bit of water in the mix is slowed down. GFRC made with the acrylic polymer takes only three days to set.
Silica fume may also be used in the mix, making the concrete more water resistant. It also reduces the alkalinity so that it does not affect the glass. One type of GFRC made by Vitro Materials is called VCAS 160, which is made from E-glass. This is a "green" material, as it replaces cement with an industrial byproduct.
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