The dangerous concrete forcing schools to close: Video reveals how lightweight RAAC – once hailed a wonder material in the 1950s – is prone to collapse and FLOATS in water

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  • Reinforced autoclaved aerated concrete was a cheap post-war building material 

The dangerous building material now forcing more than a hundred schools to close just days before the start of the new term was once hailed as something ‘of a wonder’. 

Reinforced autoclaved aerated concrete (RAAC) was used as a cheap post-war building material in public sector construction projects.

The lightweight, aerated cement-like material was used as a cheaper alternative to concrete but unlike traditional concrete it has no coarse aggregate – the chunks of quarry rock that are used to give concrete its strength and volume.

Instead, it is made with fine aggregate and a chemical agent creating gas pockets to bulk out the material, giving it a texture that resembles the inside of an Aero chocolate bar.

Over time, it has been found to lose its structural integrity as moisture creeps in, with a lifespan of just 30 years. 

More than 100 schools and colleges have now been told to partially or fully close buildings as children prepared to return after the summer holidays because of fears the RAAC could suddenly collapse. The DfE has been considering RAAC as a potential issue since late 2018 but the timing of the decision to issue guidance just days before the start of term has angered unions and parents. 

Professor Chris Goodier, an expert on construction materials at Loughborough University, says the material was widely used in the 1960s and 70s but, decades on, is showing concerning signs of wear. 

Hundreds of schools across the country were built with reinforced autoclaved aerated concrete, known as RAAC, between the 1960s and 1990s

Professor Chris Goodier, an expert on construction materials at Loughborough University, says the material was widely used in the 1960s and 70s but, decades on, is showing concerning signs of wear

Professor Chris Goodier, an expert on construction materials at Loughborough University, says the material was widely used in the 1960s and 70s but, decades on, is showing concerning signs of wear

A demonstration shows how lightweight RAAC isn't as strong as modern day concrete

A demonstration shows how lightweight RAAC isn’t as strong as modern day concrete 

A total of 104 schools have been instructed to keep buildings shut if they are made with a type of concrete that is prone to collapse, the government announced

A total of 104 schools have been instructed to keep buildings shut if they are made with a type of concrete that is prone to collapse, the government announced 

The difference between concrete and RAAC under water

The difference between concrete and RAAC under water 

He said earlier this year: ‘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. 

‘Prolonged water ingress (not uncommon on old flat roofs) can also lead to deterioration.’

‘If you keep the RAAC nice and dry then you’re fine, but if you let water get to it, it can soak up the water and if that water gets to the reinforcement that’s inside it can corrode it and make it rusty.’ 

This is potentially why RAAC could be prone to collapse. 

RAAC is also said to be much weaker than modern concrete.  

RAAC can still be – and often is – used all over the world as a construction material, and Prof Goodier says there is no reason why it cannot be used if it is properly designed, installed and maintained. 

But for the RAAC put into British schools, hospitals and other public buildings decades ago, he added: ‘This is often not the case.’

A collapsed RAAC roof at a Kent primary school

A collapsed RAAC roof at a Kent primary school

Reinforced autoclaved aerated concrete (RAAC) was used as a cheap post-war building material in public sector construction projects

Reinforced autoclaved aerated concrete (RAAC) was used as a cheap post-war building material in public sector construction projects

Parents can be assured it is safe to send their children back to school because the vast majority of questionnaires on the structural soundness of buildings say ‘there is no RAAC (reinforced autoclaved aerated concrete)’, the schools minister has said.

Nick Gibb told Times Radio: ‘Parents can be assured that if they haven’t heard from schools, that it is safe to send their children into school.’

Asked how he can give assurances when not all questionnaires have been returned, Mr Gibb said: ‘Yes I can, because more than any other governments in the world, we have been the most proactive in dealing with this issue. It’s been around since the mid-1990s over successive governments.

Read more: Parents ‘devastated’ as more than 100 schools ordered to close on eve of new term over concrete safety concerns leaving pupils facing Covid-style online learning

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‘But as evidence has emerged, we have taken a very proactive approach in trying to identify in the 22,500 schools – and indeed across the public sector as a whole – where RAAC is.

‘And those questionnaires when they come back, the vast majority of them say that there is no RAAC in the schools and we’ve only been surveying schools in the period that they have been built or extended between the 1950s and the 1990s.’

The Government will cover ‘all capital costs’ over disruption to educational building from the use of a type of concrete prone to collapse, the schools minister has said.

Asked on Sky News who will pay if schools need to either fully or partially relocate, Nick Gibb said: ‘We will pay for that. We’ve made it very clear we will cover all capital costs.

‘So if in the worst-case scenario, we need portacabins in the school estate for an alternative accommodation, we will cover all those costs.

‘So there has been some speculation that we won’t cover those costs. We absolutely will.’

On whether all schools affected have now been informed, Mr Gibb said: ‘The vast majority have. We have been calling them yesterday, but there are a few more that we’re calling today, and those schools are now talking to parents about what’s going to happen in their school.’

Mr Gibb also said the Government will release a list of the schools in ‘due course’.




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