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.