And why reinforced autoclaved aerated concrete crumbles
building boom that began in the 1960s, a novel form of construction material began to be used. Reinforced autoclaved aerated concrete, known as RAAC, is pre-cast in a factory, typically into planks, before being taken to a construction site where it can be quickly assembled into part of a building. Although RAAC is still used in many parts of the world, Britain is havingwith it. In some buildings the material is crumbling; in 2018 it caused a school roof to collapse.
RAAC is formed by mixing cement with other materials, such as pulverised ash, to create a concrete slurry. This is poured into a mould and steel reinforcing bars are added. A chemical reaction creates small air bubbles which results in the slurry turning into a foam, greatly increasing its volume. It is then cooked at a high temperature to harden. Being lighter and cheaper to install than regular steel-reinforced concrete, RAAC is often used in roofs and floors.
With the right care and attention, some building materials can survive for a long time—centuries, even, in the case of Roman concrete. But RAAC has a particular vulnerability. Because of its “bubbly” nature, water can get into its pores and cause it to decay rapidly. The steel reinforcement inside will then rust, weakening its structure further.
To protect it against decay, RAAC needs to be shielded from rain. On a flat roof that might typically mean waterproofing it with coatings of bitumen. But these coatings can fail, especially if not maintained properly, says Chris Goodier, an expert in construction materials at Loughborough University in Britain. And as many public buildings in Britain are short of cash, maintenance often gets overlooked. Other problems may stem from the material being manufactured or installed incorrectly.
Regular inspections will now be required to ensure RAAC is not decaying and making more buildings unsafe. Some may have to be replaced with more conventional timber or steel flooring. But buildings do not necessarily have to be demolished and rebuilt, says Phil Purnell, a professor of materials at the University of Leeds, since RAAC was only supposed to be used for roofs and floors. At least that should make rectifying Britain’s RAAC mess slightly easier.
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