Raac and ruin: the dizzying rise and fall of concrete

Hadrian’s Pantheon was covered in concrete and has lasted for 2,000 years. So why has crumbling Raac become an acronym for collapse after mere decades? Architectural critic Jonathan Glancey sifts for clues in the wreckage and towering glories of public buildings

Emperor Hadrian’s thin concrete stretched up and over the Pantheon’s 43m diameter rotunda has endured for nearly 2,000 years (Getty/iStock)

Imagine an assembly of all the Roman gods meeting to choose the roofing material for a new temple Emperor Hadrian is building in their name. Jasper, jacinth and chalcedony are all half-jokingly mentioned and divinely dismissed. The wood of date palms from distant Parthia? Sweet idea, but no go. Gold? What do you think Juno Moneta? Too costly and probably too soft, you say. Well, then, why not stick with tried-and-tested marble? Reflects the sun. Keeps buildings cool and all that. Emperor Augustus, you were keen on the stuff. Transformed Rome you said, before becoming one of us. Any thoughts, Vespasian?

What must these divinities have thought when Hadrian and his architect – rumoured to have been Apollodorus of Damascus, expert in the design of domes – chose concrete to roof over the ambitious dome of the Pantheon? And thin concrete at that. In recent decades, and certainly over the past fortnight, mortals in Britannia have been talking of little else it seems. They call their thin concrete Raac, short for reinforced autoclaved aerated concrete, an acronym difficult to disassociate from ruin.

But where potentially life-threatening Raac came with a 30-year lifespan, Hadrian’s thin concrete stretched up and over the Pantheon’s 43m diameter rotunda has endured. For nearly 2,000 years. All the more remarkable, the Pantheon’s thin concrete gets thinner still as it reaches the crown of the dome. At the very top it dissolves into thin air in the guise of an oculus, or circular opening, shining beams of sunlight and full moonlight around the marble-lined interior of the circular temple.

Remarkably, too, rain falls through the oculus and – a rare occurrence – some of us have even witnessed snow. Rain and snow. These, surely, are arch enemies of concrete. And, so they have been, insinuating their way into nicks and crevices in slabs and stretches of reinforced concrete, of which Raac is an example, causing them to expand and spall.

The Roman gods must have been looking the other way when shortly before midday on 14 August 2018, a 200m section of the viaduct carrying the A10 Autostrada over the Polcevera River and central Genoa collapsed killing 43 people and making 600 homeless. Pollution, salt sea air and a lack of dutiful maintenance appear to have been the main villains rotting prestressed concrete-covered cables.

In Genoa, first structures of the new bridge designed by Renzo Piano are built and placed on pillars (Getty)

Famous for their skilled use of concrete, Italian engineers were crestfallen. The designer of the bridge, Riccardo Morandi, is a famous name in their talented circle. Opened as recently as 1967, the Morandi Bridge is not old for a concrete structure.

Its replacement, realised in less than two years, is by the world-famous Genoese architect Renzo Piano. Robots on rails run the length of the bridge constantly checking its robust structure. No one is taking chances. Simple and elegant, with its subtle references to the design of ships – its 18 elliptical pillars supporting a hull-like deck – it is constructed in concrete.

As is the core of the Burj Khalifa – the tallest building in the world – pointing like a finger up from Dubai to the realm of the one and only god of the Arabic world. True, the very highest, needle-like stories of this American designed and engineered 828m skyscraper are framed in steel, yet Adrian Smith (architect) and Bill Baker (engineer) had concrete pumped as high as they wanted to go.

Concrete is such a contradictory material. Formed well – the Romans mixed a workable hydrate of lime with pozzolanic ash, pumice and small pieces of rock – it can produce results as impressive as the Pantheon’s unreinforced dome, the Roman port of Cosa – its underwater sections in fine order after more than 2,100 years – or, from the era of steel-reinforcement, the Burj Khalifa, the Hoover Dam and the Barbican Estate in London.

Insufficiently prepared, improperly used and poorly maintained concrete can, even when reinforced with steel bars and technically strong, cause roofs and bridges to fall. Certainly, it has been around for a long while. Nabatean Bedouin tribes made underground water systems from concrete as early as 700BC, and they certainly couldn’t afford leaks. The strength and reliability of their concrete was a matter of life and death, as it might yet be with Raac.

A walkway under the Brutalist architecture of the Barbican complex in London (Getty/iStock)

Raac is certainly proving to be a drama, especially in the case of British buildings dating from the 1950s to the 1990s when it was commonly used and too often installed by cheap, untrained and unskilled labour.

This week even the National Theatre was billed as potentially problematic. A spokesperson told the press “The National Theatre is a grade II-listed building made predominantly from traditional reinforced and post-tensioned concrete; there are a small number of select backstage areas where Raac is present. Our structural engineers are in the process of surveying these areas. Initial indications are that they are safe and do not currently require remedial works.”

Talking to Sir Peter Hall, the theatre’s former director, years ago, its architect Sir Denys Lasdun said, “Concrete is a very intractable material, but it can be a beautiful material if it is used in the way its own nature intends it to be used ... it [the National Theatre] is a sort of sculpture that you can only do with reinforced concrete, but you need to work to a certain scale ... it is not a cosy little material.”

It is hard to imagine Sir Denys specifying anything other than rigorously tested and solid materials in a building that is more like some sculpted geological outcrop than a brittle PFI-financed contemporary arts venue. But if Raac is found hidden behind the arras or ceilings in parts of this monumental structure, it is not because the material was considered cheap and nasty at the time. What it did require was skilled installation.

The National Theatre is as solid as the ancient Greek theatre of Epidaurus on which its auditoria are based, although, of course, this being England, they have roofs over their tiered seats. And roofs need regular maintenance as do all parts of any building’s structure of any era. It is too easy to say, “Ah, but they built much better in the good old days when brick was brick, stone was stone” and reinforced concrete had yet to be invented.

People walking along the concourse of the Royal National Theatre in London (Getty)

Thirty years ago, I was very nearly killed by a handsome 18th-century building that imploded the moment I pressed the shutter release of my Canon A-1 to record it on film. This was in Old Havana. The building was the Colegio Santo Angel, and I was about to step inside it with one of Cuba’s city’s leading conservation architects.

My story for The Independent (19 October 1993) began, “To say that Havana is collapsing is to state the obvious. It is even more obvious when the building you are about to enter falls down in front of you. One moment Victor Marin, one of the city’s leading architectural conservationists, is reciting the faded glories of the arcaded 18th-century merchant’s house that occupies the north-west corner of Plaza Vieja, one of the oldest and most elegant city squares in the Americas. The next, the building crumbles and falls as quickly and quietly as a house of cards.”

Victor Marin’s first thought was that termites had finally had their way with the building. Their appetite for architecture in tropical climes is insatiable. Juliet Barclay, then working with the Office of the Historian of the City of Havana, wrote to the paper to say it was the removal of timber scaffolding supporting the angelic college by local people in need of firewood that caused its fall.

“After this article”, Gabriel Fuentes – founder and director of New York’s Design Action Studio for Research, Architecture, and Urbanism – has written, “along with images of the collapse spread across European media, president Fidel Castro purportedly met with city historian Eusebio Leal to discuss restoration strategies in Old Havana.”

I tell this story here as a reminder of how architecture of any era, traditionally built and without a jot of Raac, can be subject to sudden failure to the point of collapse.

While the angels were with me that memorable day in Havana, they must have flown elsewhere on the day in 1284 when the newly installed and daringly high vaults of the choir of Beauvais Cathedral, then under construction, hurtled to the ground.

The gothic 13th-century Beauvais Cathedral, located in the northern town of Beauvais, Oise, France (Getty/iStock)

Nor were they there in 1573 when the cathedral’s central tower collapsed. This may have been due to high winds causing vibrations to resonate through the vertiginous stone structure. France’s cathedral builders were highly skilled, yet at Beauvais they ascended too high towards the heavenly host.

Today, all - all! - that stands of this breathtaking design is its choir and transepts, two sections that have tried to pull apart from one another over the centuries, while the tall, slender buttresses flying around the choir are held once again in subtle and necessary tension by iron rods.

Beauvais was built by master masons as was the Pantheon. But, in ancient Rome, shoddily built rent-slab apartment blocks (insulae) collapsed all too frequently, as did good-looking though cheap Georgian terraced housing raced up by fast-buck city developers. No, the old ways were not always good.

Equally, some of the most rewarding buildings and structures of the past half-century have been constructed from lightweight materials based in part on the design of spiders’ webs and other natural phenomena.

The tensile roof over the 1972 Munich Olympic Stadium designed by the German engineer Frei Otto and architect Gunther Behnisch. The tent-like roof structure of the Mound Pavilion at Lord’s Cricket Ground (1987) by Michael Hopkins, and the same architect’s – with engineers Tony Hunt and Arup – ethereal roofs covering the Schlumberger Cambridge Research Centre of 1985, on the fringe of the university city.

True, such fabric structures need careful maintenance and even replacement at specified intervals, but their very lightness ensures they can do little harm even if neglected – and neglect is easy to spot as their workings are clearly exposed.

For all the immediate concern with Raac and the opportunistic claims and counterclaims of point-scoring politicians who, of whichever and whatever hue, have landed Britain with successive blights of hastily built and questionably financed buildings since the 1950s, there is another threat to the built landscape of this country.

The rush to build new homes of little or no architectural value on inappropriate sites the length and breadth of Britain is a sure way of stacking up a big problem for the future. The failure of Ronan Point, the 22-storey East London concrete slab housing block, one corner of which collapsed in 1968 just two weeks after residents moved in, was a clear case of poor design and construction. The most dismal of the horrid new houses spawning across the country will not fail in such a spectacular and fatal manner, yet, like Raac-infested schools, but they will need a lot of work to shore them up in the future.

History shows that buildings of any period or material can fail. That odd Victorian poet William McGonagall was not wrong when he ended his “The Tay Bridge Disaster”with the lines,

“For the stronger we our houses do build/ The less chance we have of being killed.”

That strength, though, as Hadrian, his architects, engineers and skilled builders proved so extraordinarily well in the design and making of the Pantheon, came through the use of lightweight concrete.

How we choose to specify, use and look after the array of building materials available to us today is only highlighted by the Raac affair. It is not in the lap of the gods.