Exclusive:RAAC school closures: why was aerated concrete so commonly used - and how many buildings could be affected?
A construction engineering expert says all buildings with suspected RAAC, which is so soft you can stab a screwdriver into it, should be inspected
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Low-slung government office buildings and ambulance and fire stations built in the 1960s are among those most likely to be built with a lightweight - and prone to collapse - form of concrete.
The Department for Education suddenly told 104 schools and colleges to close off buildings as students prepare to return from their summer holidays, due to the presence of RAAC - reinforced autoclaved aerated concrete - at "critical" risk of sudden collapse.
Many hospitals across the UK are also built with the lightweight concrete, which contains air bubbles, but the NHS has been ahead of the game, having already replaced some of its buildings that contained RAAC and making plans to "eradicate" the others.
However, the situation has raised concerns about just how many other UK buildings might be at risk of sudden collapse.
NationalWorld spoke to Chris Gorse, a professor of construction engineering and management at Loughborough University, about what other types of buildings RAAC was commonly used in, and why it was so popular during the 1950s and 60s.
What other buildings are likely to have been built with RAAC?
Professor Gorse said that around the 1950s and 60s, RAAC was a quick building system, which offered itself quite well to a lot of public buildings - although his department had worked with a number of private sector owners as well.
"Most public sectors are affected, and most public sectors are engaged with us," he said. The lightweight building material was common in roof and floor construction, and he said outside of schools and hospitals, public sector and government office buildings - as well as local council buildings - were some of the worst offenders.
Professor Gorse also noted it was common in ambulance and fire stations, and he had seen it in auxiliary buildings like nursing accommodation.
However, apartment dwellers need not worry too much, he said, as it was rare to find it in any building more than three storeys tall. Similarly, there were some public sector buildings which needed to be made of sturdier stuff, that would be unlikely to have RAAC in them by design.
"It's something [where] we could pick up a sharp implement, such as a screwdriver, and push it into the concrete," he said. "So you wouldn't expect it to be used in prison cells."
Professor Gorse said any building from the 50s, 60s or 70s with RAAC in it should be inspected, and the public sector inspections underway at the moment were "absolutely necessary" to make sure they were safe.
"We should certainly be checking our buildings, and if you suspect you've got RAAC... you ought to get it inspected, and establish whether it's got sufficient bearing or not."
Why was RAAC so popular during this period?
Professor Gorse said there were a few factors playing into RAAC's popularity in public buildings back in the 1950s and 60s, including how quick, easy and malleable it was to work with, at a time when a lot of buildings needed to go up fast.
AAC's bubbly structure meant it was light, and could even be "manually lifted into place", he said. "It uses less material, and when you put the reinforcements in it - as we've done with the reinforced autoclaved aerated concrete - you can then use it in a beam or a floor or a roof structure."
It was also cheap. "It was a cheaper form of construction," Gorse added. "But it is cost-effective, and we will still see this material being used in the future."
Aerated concretes in general are still considered an eco-friendly construction solution. They are usually made of natural minerals with little to no pollutants, and their light weight means they uses less energy and create less emissions to transport. It is also thermally efficient, meaning less heating may be required in buildings with RAAC.
"During the 50s, there wasn't an energy crisis the same as there is now," he said. "[But] it does have properties... that do have benefits over traditional concrete.
"Traditional concrete is a lot stronger and a lot more robust, but it's one of the less environmentally friendly products we have to deal with."
RAAC still had a purpose in construction, Professor Gorse said, and a lot of the problems coming up now in structures from the 50s and 60s owed it to three things; poor-quality manufacturing at the time, poor-quality reinforcement or beam placement, and abuse over the years.
"Because it looks similar to concrete, people treat it similar to concrete, and they damage it," he said. "Just because of the nature of the material, that's creating a problem for us where we've not had the due level of inspections and maintenance programmes that we probably should have done."