When super-thin panels are needed, GRC can provide an answer.
To make it look like it's floating on a procession of polished concrete columns, Harlow's new council building has embraced a cladding rarely seen in the UK. Super-thin glassfibre reinforced concrete panels proved the only way to clad the exposed steel beams that support the facade without expanding their girth to the sort of dimensions that would have torpedoed the floating design.

Part of a redevelopment of Harlow town centre, the three-storey Civic Centre is a £9m design and build project. To help make the offices appear to float on top of the floor-to-ceiling glazing of the recessed ground floor, the architect wanted the steel stilts clad in curved and polished concrete panels, and insisted that the resulting circular columns be no thicker than 560mm. As each stilt already measured 465mm at its widest point (the diagonal of the 350mm by 350mm steel), that left just 95mm for the cladding.

However, the amount of steel cage needed within concrete panels to give them enough tensile strength to survive demoulding, transporting and craning into position meant that project precaster Histon Concrete couldn't make reinforced panels for the columns thinner than 75mm. As the panels would completely encircle the steel stilts, that would have added 150mm to the column width. For a design pivoting on the slenderest of pillars, that was far too much.

Peter Dorkings, Histon's technical services manager, says they considered but then rejected fibre-reinforced concrete panels. "The absolute minimum thickness of each panel would be 50mm," he explains: tantalisingly close, but no cigar. "Plus, nothing is ever exactly to grid, so we would have had to add a tolerance between the steelwork and the back of the cladding."

The solution came from one of Histon's recent introductions: glassfibre reinforced concrete (GRC). The layer of glassfibre reinforcement required by a GRC panel is so much thinner than rebar that Histon can make them just 16mm thick. As roadsweeper vehicles would be moving around the pillars of the Civic Centre, Dorkings decided to make the panels 35mm thick to provide better impact resistance.

Packed at the back of each curved panel, the long thin strands of glassfibre can't be seen on the face. Dorkings made the panels by spraying 4mm of the desired concrete and aggregate finish into the mould face, followed by a much deeper backing layer of chopped up glassfibre mixed with cement. Spraying gives a more even finish and can easily follow a curve to cover a semi-circular mould. The back of each panel was further reinforced with structural GRC ribs.

"GRC is never going to replace reinforced concrete," says Dorkings, "but it is an excellent complement. There will always be occasions, like when you need a slender parapet, where it will work well."

Five things you didn't know about GRC

  1. Developed by the Russians and the Chinese in the 1960s, GRC only stopped falling apart when an Indian scientist at Britain’s Building Research identified a chemical-resistant fibreglass. Before then, it corroded under the high alkaline content of lime-based hydrating cement.
  2. Fibreglass is made by pouring molten glass into a large platinum box. The glass drips through holes in the box and is stretched out and wound onto a bobbin before being chopped up.
  3. A GRC panel weighs around 75% less than precast concrete.
  4. Steel reinforcement has to be enclosed in cement to protect it against corrosion. Glassfibre needs no protection, so the panel can be thinner.
  5. GRC’s poor reputation in the UK derives from its failure in large sandwich panels with insulation cores. The jumbo panels had too few joints to cope with the bowing pressure caused by differential moisture content between the inner and outer skins, and ended up cracking. GRC has traditionally been used for large panels in steel frame buildings in Japan, the US and the Middle East.