Related: Britain's brutalist playgrounds – in pictures  An assortment of strange concrete forms stand in a sunken pit at the foot of the Balfron Tower in east London – a little tableau of walls, ramps and an odd lookout tower that could be the fenced-off ruins of some ancient civilisation. The civilisation in question was known as the welfare state, and this was one of its playgrounds, lying at the base of what was once a great slab of social housing designed by Hungarian emigre architect Ernő Goldfinger. The building is now being scrubbed up before being sold off as luxury brutalist apartments.  “The tower itself might have been saved, but we’re interested in what happens to the landscapes around these postwar housing estates, and what they represent,” says Jane Hall of young architecture collective Assemble, the group recently announced as a wildcard entry on the Turner prize shortlist. They are currently rebuilding full-sized chunks of these brutalist playgrounds inside the Royal Institute of British Architects in London, in one of the most radical exhibitions ever staged in the prim surrounds of its Portland Place HQ.  Goldfinger’s lookout platform now pokes through the ceiling of the gallery, popping up into a skylight to provide an unusual, worm’s-eye perspective on the RIBA’s backyard. Reached by scampering up some steps and crawling through a tunnel, from where a slide whisks you back down, it also gives a vantage point down into the gallery space, now filled with an assortment of curious bits and pieces sampled from other estates in London, forming a brutalist bricolage of soft play.  In a brilliant move of material alchemy – typical of Assemble, who often spend months tracking down samples and developing new materials – the bush-hammered concrete that is so synonymous with these brutalist structures has been substituted by reconstituted foam, with a grain of squishy coloured chunks that matches the mix of rocks and pebbles in the gnarled concrete aggregate.  “It’s a sort of in-joke,” says fellow Assembler Joe Halligan, “that in order for these postwar structures to meet current safety standards, everything has to be squidgy.”  Squidgy brutalism, it turns out, also comes in a gentle pastel palette of pale pink, blue and green – the cushioning industry’s standard colours for denoting the foam’s density, which are usually hidden behind covers of wipe-clean vinyl in lurid primary shades. Left exposed here, it transforms the estates’ hard-edged, knee-grazing terrain of cement, brick and tiles into an innocuous, candy-coloured world, consciously removed from the negative associations of stained concrete and failed utopias that have become ingrained in the public psyche. Shed of such connotations and transposed into a Mr Soft universe, the playful power of these landscapes can be seen afresh, freed from political baggage.  “We’re not passing comment on whether we think these brutalist playgrounds were good or bad,” says Hall. “But it’s about how attitudes to play have changed. Today we have so many specialist play designers that architects don’t really get involved.”  “In the examples we’ve looked at,” Halligan adds, “the playgrounds are often an offshoot of the architecture. It’s the stuff Goldfinger couldn’t do on the tower, so he let his mind go wild in the playground. I don’t know how much he actually thought about how good it would be to play on – which is the beauty of it really.” Fenced off for decades, deemed unsafe due to the prospect of crumbling concrete falling from the tower, the Balfron playground has become a popular spot for free-runners and skaters, who are happy to make use of its steep ramps and lethal drops.  Related: The street that might win the Turner prize: how Assemble are transforming Toxteth  These landscapes were the product of an era when children were used to playing in the accidental playgrounds provided by bomb sites, appropriating the ruins of wartime devastation for adventurous fun and games. While today’s sanitised playgrounds are full of risk-averse equipment made for designated play activities, the postwar attitude was often more of an imaginative make-do-and-mend, building unusual, often purposeless structures that could then be played on, in and under, and inhabited as children saw fit. They were places for them to make their own.  It’s a theme Assemble have picked up in their own work, from a public square in Croydon’s New Addington estate, featuring ramps and ledges for skaters that were initially developed using temporary structures to gauge how people would use the space, to a wild adventure playground in Glasgow’s East End, built from the rubble of the Commonwealth Games.  Of the postwar period, perhaps no estate playground was as weird and wonderful as the one at the centre of the Churchill Gardens estate in Pimlico, central London. Churchill Gardens was one of many social housing schemes proposed by the ambitious Abercrombie plan for rebuilding London after the second world war, but ultimately the only one that was realised. It was built between 1946 and 1962 to designs by Powell and Moya. Featuring crenellated castles with spiralling staircases leading down to dungeons, along with rope walkways and a concrete ocean liner, the playground had as its centrepiece a gigantic flying saucer, tipped at a precipitous angle and accessed by an outcrop of hexagonal blocks shaped like the basalt columns of the Giant’s Causeway in Northern Ireland.  Related: 10 of the most bizarre homemade playgrounds and attractions  The five metre-wide disc has now been faithfully rebuilt in the centre of the RIBA gallery, supported by a half-tonne steel frame clad in green foam and finished off with a bright yellow balustrade. There will also be 40 hexagonal foam basalt blocks for visitors to sit on and build their own structures with, along with a surreal soundscape running through the installation produced by Halligan – who has a small recording studio at Assemble’s Stratford HQ and is currently in the process of setting up a record label, Sugarhouse Records.  “It’s a bit like the playgrounds are getting their own back,” says photographer Simon Terrill, who lives in the Balfron Tower and collaborated on the project. His photos of postwar estates, along with images from the RIBA archive, are projected across one wall of the gallery. “Before, they were dominated by the buildings around them, but here it’s the playgrounds that really overwhelm the architecture.”  Intentionally designed to feel almost too big for the space, the fragments – which also include a series of terraced ledges and ramps sampled from the Brunel estate in Paddington – have all been remade at full scale, measured from archive drawings and photographs since most of them are no longer standing.  Related: The incredible hulks: Jonathan Meades' A-Z of brutalism  “We were inspired by walking through the V&A cast courts,” says Halligan, “and the idea that these precious things from around the world would be copied and rebuilt in the museum at full size, as a sign of their value.” He adds that contemporary playgrounds in housing developments are all too often the result of Section 106 payments, the “planning gain” tax that developers reluctantly pay to local authorities for added community benefits. And the results are often as dispiriting as the procurement process would suggest.  By lovingly remaking these forgotten structures from a lost era of socially minded housing estates, Assemble have highlighted the value such places can bring, with a dose of wit and charm. And they’ll really have achieved something if they can convince some of the older members of the RIBA to take off their shoes and have a tumble.  • The Brutalist Playground is at the RIBA, Portland Place, London W1, from 10 June until 16 August.
Many postwar housing estates came with knee-skinning concrete slides and uncompromising cold steel climbing frames. Now young architecture radicals Assemble are paying tribute – by putting the bounce into brutalism