Reinforced autoclaved aerated concrete: material behind raft of school closures

Material was used widely in public buildings between the 1960s and 1980s

Reinforced autoclaved aerated concrete was a popular choice for walls and roofs
Reinforced autoclaved aerated concrete was a popular choice for walls and roofs

RAAC, or reinforced autoclaved aerated concrete, is a type of concrete widely used in buildings during the second half of the 20th century.

‌It was a common building material in public buildings between the 60s and 80s, and was used for roofs and walls in many schools and hospitals.

The material consists of lightweight concrete planks reinforced with steel rods. ‌The raw materials are cement, blast furnace slag and pulverised fuel ash or silica flour, and these are mixed with water and aluminium powder.

‌This slurry is poured into a mould and the chemical reaction between the aluminium and other reagents creates hydrogen gas. This makes the slush swell in size as the gas is generated.

‌It can grow to five times its original size following completion of the reaction. These blocks and planks can then be used in buildings.

‌However, the material has small air pockets throughout, which poses long-term durability concerns. Air inside the material makes it vulnerable to moisture, which leads to corrosion and jeopordises the material’s tensile strength, posing a risk of collapse. 

The material can appear fine and then suddenly fail catastrophically without warning.

Water ingress is a problem for buildings with flat roofs, but other problems have been identified specific to poor installation, such as inadequate reinforcement and bearings that are too small.

‌The cost-effective material has a functional lifespan of around 30 years, beyond which its strength can deteriorate.

‌An example of RAAC failure occurred in 2018 where the roof at a school fell in without warning, triggering an investigation by the Government.

‌RAAC panels are typically 600mm (approximately 2ft) wide and are typically up to six metres in length. The panels typically have a chamfer along their edge, meaning there is a distinctive V-shaped groove every 600mm in the surface of the roof, floor or wall.

In early 2019, the standing committee on structural safety issued an alert following the near-miss in a school, alerting all affected building managers to be aware of the RAAC problem. School bosses were alerted, as well as leaders of NHS Trusts, dioceses and parishes, and the Ministry of Defence.

‌The Office of Government Property sent a safety briefing notice to all property leaders in September last year, stating that “RAAC is now life-expired and liable to collapse”.

The material is extremely widespread in public buildings, but was also used by the private sector, including in offices and industrial estates.

Chris Goodier, professor in construction engineering and materials at Loughborough University, told The Telegraph that there are a few options when it comes to replacing RAAC roofs, which are all costly.

Additional strengthening of existing RAAC can be performed to make it safe and this has been the preferred method for afflicted NHS hospitals, he said.

“Schools are simpler buildings and there’s less things happening there,” Prof Goodier said. “They are easier to replace the roof. But it’s very building specific.

“It depends on the scale, but it would cost a school hundreds of thousands of pounds to replace the RAAC roof. It might reach £1million for a secondary.”

Professor Leon Black, professor of infrastructure materials at the University of Leeds, said that poor maintenance of the RAAC planks leads to carbon dioxide and water spreading through the tiny holes in the concrete.

Acid is created by the CO2 which changes the properties and pH level of the concrete, and thus makes the steel reinforcement beams prone to corrosion.

“The corrosion is expansive and causes cracking and bits to fall off,” Prof Black said.

Experts said the material was very useful and practical in public sector buildings because it was lightweight and cheap. It was also good for thermal and sound insulation because of its porosity.

“The problem is they’ve got a design life of 30 - 40 years. That doesn’t mean that they’re going to start failing in 30 - 40 years, but that’s the period that the manufacturers are going to guarantee them for,” Prof Black said. 

“It might well be that, under perfect conditions, they’re going to last up to 60 years.

“But these materials were installed for anything from the 50s to the 90s and now is the point where they’ve gone way beyond their design life and people are starting to get failures.”

Not all failures are catastrophic and sudden, he added, as small chunks could fall off cracking concrete planks owing to corrosion. 

But identifying if a building is at risk is no easy task. It is hard to know if a building was made using RAAC, difficult to find it and challenging to test its structural integrity. 

“One of the issues is that a lot of old buildings, certainly from 50, 60, 70 years ago, estate managers might not know precisely what was used for roofing panels.

“So it’s not just that all the RAAC panels are potentially failing, it is that they don’t even know if the panels are there or not.”

Prof Goodier also said that RAAC can last for decades if looked after properly.

“The problem is it has been used in a lot of flat roofs and flat roofs are notorious for not being looked after properly. They are often not waterproofed and overloaded,” he said. 

“Water gets in and, because it is aerated, gets soaked up and this increases the weight. Another problem is these flat roofs are not meant to be loaded up, they are designed to hold water and a bit of snow. 

“But it is tempting to overload it because there is so much space, often with air conditioning units, materials and other services. 

“Another problem has been that some RAAC wasn’t built properly and many things weren’t built properly back in the 60s and 70s. Quality control was not as good.

“If you have a perfect storm of it not being built properly, not being looked after and you’ve overloaded it, then you’ve got a problem.”

‘RAAC looks like normal concrete, so it’s tempting to abuse it’

RAAC is still used globally, but not in the UK any more. 

“It’s not inherently a bad material. It’s similar to timber. Timber, if used properly, is fine. But imagine leaving timber in the rain and then overloading it. It will bend and it will crack,” Prof Goodier said.

“But the problem with RAAC is that it looks like normal concrete. So it’s tempting to abuse it a bit like you would normal concrete which is a bit more robust.”

He said anybody that owns or runs a building built in the 60s and 70s should be aware of the risks of RAAC. 

“If you have a flat roof from the 60s and 70s. You might have it. Much of the public sector, but also some of the private sector. Think of offices and old industrial estate from the 60s and 70s, some of them might have this.”