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Material that broke the architectural mould

CONCRETE QUARTERLY - TECHNICAL

Introduced commercially only 30 years ago, use of glass reinforced concrete (GRC) quickly became established as designers realised the potential of its light weight, strength and versatility As the name implies, glass-reinforced concrete comprises two essential components: alkaliresistant glass fibres and cement.

These are mixed with fine aggregate, water and chemical admixtures to provide a construction product of considerable strength and versatility that offers surprising lightness.

The glass fibre has a tensile strength three to four times greater than the equivalent steel fibre, making GRC a robust material with high impact resistance.

Typically manufactured to a thickness of 10mm, GRC cladding panels, for example, are much lighter than their precast equivalents.

GRC can be formed with a wide variety of textures and colours to replicate natural materials, reconstructed stone or concrete finishes. It is not a single material but can be manufactured and engineered for a wide range of architectural and civil engineering projects.

Manufacturing process GRC is generally manufactured by either spray process or premix vibration casting, normally dictated by factors such as strength requirements, size of mould or specification.

As a general rule, larger items such as cladding panels are sprayed, whereas smaller items are manufactured from pre-mix GRC. Sprayed GRC is generally stronger, due to its higher glass fibre content of 5-6 per cent, with longer fibre length and planar orientation, together with a lower water/cement ratio.

Sprayed GRC With the spray approach, the water and admixture (and polymer if used) are placed in a high-shear mixer, and the sand/cement are added slowly until a smooth creamy slurry is achieved. The consistency of the slurry can be checked using a simple slump-test kit.

When ready, the mix is pumped at a regulated rate of flow to the spray gun where fibre, in the form of a roving, is added, chopped to a length of approximately 32mm.

The two materials are then projected on to the mould surface, using an air supply from a compressor.

The GRC material is built up in thin layers until the required thickness is achieved - normally 10-15mm, with simple hand rollers used to compact the material between layers. The mould is covered with polythene to prevent moisture loss until the next day when the product is then demoulded.

The units are allowed to cure for seven days, once again covered with polythene, although if a polymer curing compound is used in the mix, units can be exposed to the atmosphere immediately. It is advisable, however, to keep them protected from direct sunlight or severe external conditions for a day or two.

Pre-mix GRC With pre-mix GRC, the sand and cement are mixed dry before the water/admixture and polymer (if used) are added.

Generally, a two-speed slurry/fibre blender mixer is used.

The fast speed first creates a smooth creamy slurry. The mixer is then switched to slow speed, and fibre in the form of chopped strand of 13mm in length is added slowly and blended for about a minute.

Once the mix is ready, it is poured into moulds, which are vibrated using a vibrating table. The material is allowed to set, ready for demoulding the next day.

After demoulding, the product is cured under polythene sheets to maintain moisture for up to seven days. Alternatively, a polymer curing compound can be used.

Lightweight versatility Both designers and constructors appreciate the lightweight versatility of GRC. This is especially important on congested inner city projects where GRC panels can cover three to four times the area of precast concrete panels of the same weight, enabling lower-capacity cranes to be used to handle units.

The material can be moulded into a considerable range of architectural and engineering applications, from cladding and architectural features to soundabsorption barriers and sewer linings.

Cladding panels GRC offers a wide range of opportunities for cladding. The sculptural nature of the material encourages innovation in shape, form and texture.

GRC cladding panels, both single-skin and sandwich, are being used increasingly for industrial and warehousing projects because of the cost savings in transport and erection.

One of the most important developments has been the adaptation of the 'stud frame' construction technique to GRC facades on high-rise buildings. Here, single-skin GRC cladding panels are attached to the stud frame by means of special joints. The result is a far lighter cladding system.

Temporary works GRC is particularly well suited to use as both permanent and temporary formwork.

The material has been used successfully in parapet shutters, bridge decking and in the construction of retaining walls and sewer linings.

Refurbishment GRC is also used widely in the repair and renovation of old concrete and stone buildings, where it scores on mouldability and its ability to be formed in different finishes. GRC is particularly suited to simulating stone and brick.

The future Already a widely used material, GRC is predicted to increase its application as concerns over the material's long-term performance are overcome by the positive real-time experience in a variety of world climates, as reported at the last GRCA International Congress, 'GRC 2003', held in Barcelona last October.

Iain Peter is chairman of the Glass Reinforced Concrete Council (GRCA). For more information on GRC and the GRCA visit: www. grca. co. uk

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