Investigations are under way into whether the Houses of Parliament have any reinforced autoclaved aerated concrete (Raac), the material at risk of collapse that has led to the closure of more than 100 schools. Maintenance teams are trying to establish whether the dangerous concrete is present in any parts of the parliamentary estate, Bloomberg reported. A source told the Guardian that parliamentary authorities are unsure whether Raac is present and are trying to find out. Raac is a lightweight material used from the 1950s up to the mid-1990s, but is now assessed to be at risk of collapse. It is made up of aerated autoclaved concrete and a steel reinforcement. The aerated autoclaved concrete is made by adding aluminium into a lime and cement-based concrete mix. This reacts to make millions of tiny bubbles that form the bulk of the material. The steel reinforcement is coated with a latex or cement mix before the concrete is then cast around it. The material is mostly found as precast panels in roofs, as well as floors and walls. It is cheaper than traditional dense concrete and is quicker and easier to install. Since last week, more than 100 schools in England have been told by the government to fully or partially close at the start of term because of the presence of Raac in their buildings. Headteachers have been scrambling to find temporary teaching spaces ahead of the new academic year, while others have been forced to replace face-to-face lessons with remote learning. About 5% of schools, or the bodies responsible for them, have not responded to a questionnaire sent out by the Department for Education about Raac on their sites, according to the education secretary, Gillian Keegan. Meanwhile, a theatre in Kent has been closed with immediate effect in order to carry out work on Raac in its roof. The Orchard Theatre in Dartford will suspend performances until the end of September as further investigations get under way. A joint statement from Dartford borough council and Trafalgar Theatres states that following a routine inspection on Monday, the latest industry guidance meant closing the theatre for further surveys to be done. Keegan, who apologised on Monday for swearing after an interview with ITV, told schools that have not responded to a survey about crumbling concrete to “get off their backsides” and inform the government if they are affected. NHS bosses have also told hospitals across England to be ready to evacuate staff and patients if buildings containing Raac start to fall down. Jeremy Quin, the Cabinet Office minister, was appointed to lead cross-departmental work looking into the possible existence of the material in other public buildings such as hospitals and courts. A House of Commons spokesperson said: “Routine ongoing in-house investigations are currently taking place as to whether any parts of the parliamentary estate have Raac.”