Two teams of architects employ very different strategies to reinvigorate a pair of ambitious 1960s apartment projects, one in the north of England and the other in Paris.

The enthusiasm with which Britain and France took to the construction of Mid-Century Modern social housing is equaled only by their present appetite for its demolition. In 2003, the French government announced a 10-year urban-renewal plan in which 200,000 dwellings would be replaced; in Britain, Alison and Peter Smithson's Robin Hood Gardens, completed in London in 1972, is one of many projects that once enjoyed international prestige and are now facing the wrecking ball. But an alternative course is plotted by the rehabilitation of two 1960s buildings'the 170-foot-tall Bois-le-Pr'tre in the northwest of Paris, extended by Lacaton & Vassal and Fr'd'ric Druot, and Park Hill, a Brutalist megastructure in Sheffield, in the north of England, where architects Hawkins\Brown and Studio Egret West have completed the first phase of a comprehensive renovation.

Park Hill, Sheffield, England

Designed by Jack Lynn and Ivor Smith, Park Hill has dominated Sheffield's skyline since 1961. Remarkably, the bridge-linked chain of slab blocks that snakes across the 32-acre sloping site constitutes a single building of almost 1,000 apartments. Below the datum of a horizontal roofline, it ranges from four stories at the southern end to 13 stories at the north, where the block renovated in the first phase is located. Outdoor corridors, or 'streets in the sky,' occur every three stories, serving duplexes at deck level and above, and single-story apartments below. All are dual-aspect, with bedrooms facing north and east on the corridor side and living areas facing south and west, where the grid of the expressed cast-in-place concrete frame is enlivened by double-height bays and sheltered balconies.

Though initially well liked, Park Hill by the 1980s was a byword for failed public housing; it was plagued by poor maintenance, while postindustrial decline brought social problems. Nevertheless, in 1998 the building was given protected status for its architectural significance, precluding demolition. To secure its future, Park Hill was sold to Urban Splash, a commercial developer; two-thirds of all renovated apartments will be for sale from around $150,000 for 550 square feet, with the remainder'materially identical'providing subsidized rental homes owned by a nonprofit housing association. With 79 of 261 first-phase apartments complete, residents began to move in at the end of 2012.

The building's protected status 'covered everything down to the door handles,' says David Bickle, project director at Hawkins\Brown, so the developer's desire to signal a break with the building's past had to be weighed against legally binding conservation requirements. After painstaking element-by-element negotiation, the supervisory body, English Heritage, agreed that the building could be stripped back to its frame, and it devised a 'squint test' as a principle for reconstruction: faithful reproduction of its timber-framed windows and brick infill panels was not required; rather, through narrowed eyes, from a distance, the reconditioned building should be recognizable as Park Hill.

Further intense discussion covered 5,500 individual repairs to the spalling concrete frame. Conservationists initially insisted each repair should be both visible'to acknowledge the building's history'and carefully matched to its immediate surroundings, while the developer wanted a uniform appearance with less evident scarring. A 'reasonable compromise' was agreed on, says Bickle. Decayed concrete was removed with high-pressure water jets, leaving a neat geometric outline at each repair site. The exposed steel reinforcement was treated with an anticorrosion agent before these areas were patched with ready-mix mortar. Finally, the whole outer face of the frame was washed with a pale-ocher semi-opaque mineral paint. Up close, the repairs can be read, but from a distance the structure looks new.

The building's distinctive precast-concrete balustrades were beyond salvation'their 2.4-inch-square-section balusters had provided insufficient coverage of steel reinforcement. These were replaced with acid-etched precast units whose balusters taper from 2.2 inches to 1.4 inches from front to back, increasing daylight to apartments.

Internally, the apartments share much of their DNA with the original layouts, though enlarged kitchens and bathrooms and open-plan living areas reflect current preferences and enhance views and cross-ventilation. As before, homes are grouped in three-story, three-bay 'clusters.' However, two bedrooms have been removed from each cluster to leave pairs of a two-bed duplex and a one-bed apartment on either side of each H-shaped stair core.

Along with the need to improve thermal performance and the quality of interiors, the desire to change perceptions of the building informed the choice and detailing of facade materials. Within the expressed concrete grid, drab-hued, rough-textured brickwork and intricate windows are supplanted by a crisp, graphic composition of glass and brightly colored aluminum panels. On the bedroom elevations, the ratio of infill to glazing across each structural bay was inverted, from 2:1 to 1:2. Floor-to-ceiling powder-coated-aluminum-framed windows, fitted with low-E glass, are nonoperable to achieve an unfussy appearance and maximize daylight penetration. A laser survey revealed numerous irregularities in the apparently regular concrete frame'the tops of some verticals are out of line with the base by up to 1.5 inches'giving sufficiently wide variation in bay widths that the subcontractor was required to produce several sizes of each window type. 'What appeared in the drawings to be maybe 20 types overall actually meant 60 variants for the manufacturer,' says Hawkins\Brown architect Greg Moss.

To ventilate the rooms, adjacent 1.7-inch-thick insulated aluminum panels slide behind the windows. The red, orange, and yellow panels, each identified with a different deck, refer loosely to the coloration of the original building, but for the architects it was important that the facade material be more radiant than 'light-sapping' brickwork, says Bickle. Anodized aluminum was selected for its metallic sheen and its behavior in sunlight: the appearance of the building changes subtly but distinctly from different angles and throughout the day. The organic pigments used in the anodizing process are stable, and panels were subjected to accelerated UV testing to ensure that colors will not fade.

Luster is added by glass elevators and a mirror-polished, steel-clad helical escape stair set within the frame. The material also forms the soffit of a new four-story entrance at the northwestern corner, intended to improve the sense of connection with the city. The face Park Hill now presents to its neighbors is certainly brighter and more open, but the aversion felt by many citizens remains. Nevertheless, architect Roger Hawkins is confident that perceptions are changing. 'It is difficult to get people to believe that transformation is possible, but Park Hill shows that it is,' he says.