Reinforced Autoclaved Aerated Concrete – handily shortened to RAAC – is essentially a lightweight form of concrete.
It was used to build roofs, schools, colleges and other buildings from the 1950s until the mid-1990s, according to GOV.UK.
In comparison to traditional concrete, RAAC is weaker. It is made in factories using fine aggregate, with chemicals to create gas bubbles and heat.
Both the material properties and structural behaviour differs significantly from traditional reinforced concrete.
In 2019, the Standing Committee on Structural Safety highlighted the significant risk of failure of RAAC planks.
Three years later in 2022, the Office of Government Property sent a safety briefing notice to all property leaders, saying that “RAAC is now life-expired and liable to collapse”.
Chris Goodier, professor of construction engineering and materials at Loughborough University, said: “It is RAAC from the 1950s, 60s and 70s that is of main concern, especially if it has not been adequately maintained.
“RAAC examples have been found with bearings (supports) which aren’t big enough, and RAAC with the steel reinforcement in the wrong place, both of which can have structural implications.”
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