What is RAAC?

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Lightweight and bubbly are probably not words you would usually associate with concrete. But reinforced autoclaved aerated concrete -- known as RAAC -- has previously been used in construction projects, and is now causing major problems for some schools across England. So why has this old building material suddenly become such a big issue? And why was it used in the first place? For this episode of 5 Questions On, Owen TJ gets some answers from Chris Goodier, a Professor of Construction Engineering and Materials at Loughborough University, who knows pretty much everything you need to know about RAAC. (Image credit: Aliaksandr Litviniuk)