VOICE OF THE INDUSTRY
GRC – ARCHITECTURE’S MOST MISUNDERSTOOD MATERIAL? It’s been around for nearly 50 years, but glass-reinforced concrete (GRC) remains an enigma to many architects and contractors in the UK, says James Butler of Pura Facades. PURA FACADES
JAMES BUTLER IS A COMMERCIAL DIRECTOR AT PURA FACADES
FC&A – JANUARY – 2021
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t’s widely recognised that the first significant architectural use of glassreinforced concrete (GRC) was as a facade on an office building known as 30 Cannon Street in the city of London. Designed by Whinney, Son & Austen Hall, the iconic structure was granted Grade II Listed status in 2015 due to its “expressive and assured design, its splayed plan and canted profile”. Given the fact that GRC has been around for so long, it’s surprising how little is still understood about the performance, structural properties and various types of the material now available to architects and specifiers alike. While the basic components of GRC – fine silica sand, cement and glass (fibres) – go back to Roman times, new innovations and technical improvements have led to a relatively poor understanding of the material in its current forms. In my role as an independent facade specialist, I work closely with architects in the area of cladding specification. The improved technical performance and range of different types of GRC currently on the market is understandably confusing for those not dealing with the material on a regular basis, so below are the four main types of GRC on the market, along with their pros and cons. 18
In short, GRC can be categorised into four main sections; pre-cast, sprayed, lightweight GRC and ultra-high performance concrete (UHPC). Each have their own merits, and when specified in the correct way, will give many decades of low-maintenance worth, as aptly demonstrated by 30 Cannon Street.
Cast in stone Cast GRC, also known as poured GRC or premix, has all the ingredients mixed together before the material is poured into a mould. The fine silica sand cement and alkali-resistant glass fibres are mixed with water and polymers in an industrial mixer. The mix is poured into a mould in much the same way as ordinary concrete. This type of GRC closely resembles stone. It’s substantial, heavy and dense – and offers probably the widest range of surface finishes. Cast concrete most closely resembles natural stone – but it’s also the most expensive option in terms of both manufacturing and putting it onto the building. Specifiers also need to plan around the potentially long lead times cast GRC can sometimes take to produce.
Spray it all over Sprayed GRC is manufactured using a less viscous, thinner cementitious mix than traditional cast. This is sprayed into a designed wooden or GRP mould. The material is then built up in thin layers until the desired thickness is achieved. The product is left to cure in a controlled atmospheric environment until the required strength is achieved.
