White Collar Factory, a 17-storey new-build office building situated on Old Street’s “Silicon Roundabout” on the edge of the City, is the latest and perhaps purest expression of the style pioneered by Derwent London and its partner, architect Allford Hall Monaghan Morris.
Much of its in-situ concrete construction – including columns, walls, cores and ceilings – is in clear sight, and the concrete finishes, with timber markings and tie holes visible, tell the story of its construction. Particularly striking is the floor-to-ceiling height.
“This is one of the features of industrial-style buildings that tenants really love,” says Stephen Taylor, associate director with AHMM. “Here we have a 4m floor-to-floor height, incorporating a 350mm reinforced concrete flat slab and a 150mm raised floor. That leaves 3.5m floor-to-ceiling – considerably more headroom than a standard office, which has more like 2.7m.”
With so much visible concrete the mix was of paramount importance. The construction team aimed for (and achieved) an Outstanding BREEAM rating and 50% of the cement was replaced with ground granulated blast-furnace slag (GGBS) to reduce the carbon content. But, says Taylor, the GGBS resulted in a “whitish” finish: “We wanted a warmer, more traditional concrete look, so 200kg/m3 of FA (fly ash) was added to darken the mix.”
The final recipe, which also included recycled aggregate, was only arrived at after casting a dozen 1m2 medallions of various mixes, and then constructing a pair of 3m-high mock-ups of the two preferred options. “These also allowed us to try out working joints and corner details as well as test the effects of different species of timber formwork,” says Taylor.
Central to how the building works is the flat slab. Its thickness, says Taylor, is principally to enable it to cope with spans of up to 12m. Slimmer ribbed slabs or post-tensioned solutions were considered, but rejected for several reasons. “The 350mm helps give the building the thermal mass we wanted to help it stay cool in summer and warm in winter.
The passive cooling offered by the slabs is boosted by miles of plastic water pipes embedded within it. The water pipes at White Collar Factory benefit from an advanced building management system (BMS) to control the water temperature.
Five sensors on every floor monitor slab temperature and air humidity and the BMS incorporates weather forecasts to work with the thermal-mass-based temperature control system. Little of this sophistication is apparent to the visitor as the soffits have a simple finish revealing the standard plywood boards used for the formwork.