The refurbishment of Olso’s Health Council building shows the enduring appeal – and performance – of Erling Viksjø’s naturbetong
Mad arkitekter has completed the refurbishment and retrofit of Oslo’s 1960s Health Council building – one of the finest examples of Erling Viksjø’s naturbetong (“natural concrete”) architecture. Known as the Trekantblokka, or triangle block, the building has been modernised to accommodate new speculative office space, as well as public-facing functions at street level and a plant floor at the top of the building. But at the heart of the project has been the restoration of the rugged concrete loadbearing facades, and the reinstatement of Viksjø’s raw interiors.
“They were partly covered by plasterboard and bright, colourful elements before the transformation process began,” says project architect Karen Selmer. “During the construction period, all the concrete was exposed – including the parts that had previously been hidden behind retaining walls.”
Naturbetong was a casting process, pioneered by Viksjø, that involved filling formwork with large pieces of rounded aggregate – often river gravel – before piping in a mortar of cement, water, fine sand and various admixtures. The mortar would form a smooth surface to the concrete, and crucially would still be fairly soft when the forms were removed. This offered a range of finishing possibilities, chiefly involving varying degrees of sandblasting to reveal the aggregate. Local artists such as Carl Nesjar and Kai Fjell used huge rubber stencils or light projections to map abstract line patterns onto this rough canvas, while Picasso contributed two vast murals to Viksjø’s Government Building (1969). “In the hands of the right person,” wrote CQ at the time, “[the technique] has as many possibilities as the pencil or graver in the hands of the artist.”
At the Trekantblokka, Mad’s refurbishment has uncovered abstract stem-like patterns of smooth mortar on the naturbetong walls, now dramatically emphasised with downlighting. Throughout, the concrete has been complemented with a minimal palette of oak and black detailing. New energy-efficient windows in the upper floors have a uniform grey colour, close to the original, while all windows, doors and fittings on the ground floor are black-brown, slightly darker than the original, to add definition.
The team found that the concrete was still in good condition both inside and out, says Selmer. For the most part, it simply needed cleaning with water, with only small repairs to the more exposed surfaces: “After the work with the facade was finished, we had experts hanging on ropes from the roof for weeks, checking the whole facade and replacing bits and pieces of concrete that weren’t in top shape.”
A concrete-coloured dust sealant has been applied to the rough concrete beams, while the original terrazzo floors in the lobby and stairs have been sanded and treated with a matt varnish.
The main interventions have taken place at the top and bottom of the building, she adds. At street level, the facade has been opened up, in collaboration with the Oslo Landmark Preservation Committee, to create a more engaging urban presence. “We added big, openable windows to parts of the ground floor, and removed and replaced some of the old steel structure from the mezzanine and the bay windows in the ground-floor cafe to let the light in.”
Meanwhile, the original plant space at the top of the building has been removed to make way for an offset office floor. A new smaller technical floor on the floor above, further offset and in the original hexagonal style, houses new and updated plant installations. This is barely visible from ground level.
A precast concrete spiral staircase has been inserted, leading up to the new ninth floor. This was lifted into the building in two parts through the roof. Selmer says that special care was taken to echo the colour and mix of the original naturbetong stair, although the pebbles are smaller and a more conventional casting method was used.
When CQ first wrote about Viksjø and naturbetong in 1960, it noted the material’s impressive strength credentials – in one instance, cubes were returned from the testing laboratory in Oslo because they could not be crushed with the equipment available. This durability will be the key to the building’s second act, says Selmer. “The concrete structure and facade will probably live a very long life. The technology, the interior, new elements and materials have all been chosen to ensure the longest possible lifespan.”