Why are concrete schools crumbling in the UK — and what can be done?

More than 100 schools and public buildings in the United Kingdom are facing closure because of reinforced autoclaved aerated concrete (RAAC), a type of concrete used extensively throughout the country and in many others between the 1950s and 1990s.

Last week, just days before the start of the school year, the UK government announced that school buildings that contain RAAC would need to close or delay opening owing to a risk of collapse.

“We’re going to start having similar problems very soon with the rest of our infrastructure,” says Theodore Hanein, a materials scientist at the University of Sheffield, UK. “It is a big deal.”

Nature spoke to researchers about why the building material is causing safety concerns, and how they can be addressed.

What is RAAC?

RAAC is a type of concrete, invented in the 1930s, that was widely used in the United Kingdom and countries in Europe, Asia and North America in the decades after the Second World War. Made of materials including cement, lime and sand, the concrete is heated to 200 °C under high pressure, a process called autoclaving. Aluminium flakes added before autoclaving react to produce hydrogen, forming air bubbles. This process results in a material that is much cheaper and lighter than regular concrete, and less than half as dense.

“At the time, it was a bit of a wonder material,” says Christian Stone, a research scientist at Concrete Preservation Technologiesin Nottingham, UK. “You get to use a sixth of the really expensive building materials. It’s thermally insulating. And you get it in big white blocks that you can quickly stack on top of each other.”

Why is RAAC now unsafe?

Inside RAAC blocks, most commonly used in roofs, steel bars provide support. This steel is encased by a protective layer, often a mix of latex and cement or sometimes acrylic powder, to prevent corrosion if water gets into the concrete’s pores. But over time, this reinforcement can erode. When this happens, the concrete can “fail catastrophically and suddenly”, says Alice Moncaster, a sustainable-construction specialist at the University of the West of England in Bristol, UK.

If water seeps into the concrete and comes into contact with the steel, it can cause the metal to rust. And as the concrete absorbs carbon dioxide over many years, its pH drops, which also increases the risk of corrosion. Corrosion can increase the volume of the iron by “up to seven times”, says Hanein. The expanding iron can then push and crack the surrounding concrete, causing it to snap or fail. “Sometimes you will not see this failure,” says Hanein. “It might not swell or crack from the outside. It might all happen inside.”

Overloading RAAC structures, or cutting the concrete to make room for skylights and ventilation, can also increase the chance of failure, says Chris Goodier, a construction specialist at Loughborough University, UK. “Like any material, if you overload it its going to bend a bit more,” he says. “You’ll get long-term durability issues, and it will crack more.”

What is happening at UK schools?

In 2018, a primary school in Kent suffered a RAAC failure, resulting in a collapsed ceiling. It happened on a Saturday and there were no injuries, but the incident sparked an inquiry from the Department for Education and, later, surveys from organizations including the Standing Committee on Structural Safety and the Institution of Structural Engineers into public buildings such as schools and hospitals.

Goodier says that most of the RAAC in the nearly 2,000 hospitals in the country has since been “made safe” by the addition of extra support, but the much larger number of schools — around 22,000 — presents a challenge. In mid-August, the Health and Safety Executive announced that RAAC was “now life-expired” and “liable to collapse with little or no notice”, leading to the immediate closure of buildings at dozens of schools.

How extensive is the use of RAAC? Are other buildings likely to be affected?

In the United Kingdom, RAAC was used extensively in public buildings, including schools, hospitals and universities, at a time when budgets and materials were tight. Philip Purnell, a materials specialist at the University of Leeds, UK, estimates that “between one and five per cent of public buildings built between 1950 and 1990 will have some of this material”, equating to “certainly hundreds, possibly thousands, of public buildings.”

The prevalence of RAAC in commercial buildings is unclear, but likely to be similarly widespread. Schools “are the tip of the iceberg here”, says Stone. “We’re going to find this in factories and office blocks. I bet it’s in airports and council offices. It’s going to be all that post-war reconstruction building”.

Further afield, RAAC is thought to be in many buildings in Europe, Asia and North America. But damp, rainy conditions in the United Kingdom mean that RAAC’s durability issues have become apparent earlier than they would in most other places. “The UK is pretty much the wettest place in Europe, so it’s not surprising that it got to us first,” says Stone. “But it’s only a matter of time before the rest of the world starts facing problems.” Climate change is likely to have exacerbated the issue of RAAC failing, says Moncaster — for example, drought can cause cracks to form in the concrete, which then allow moisture in.

What can be done to make buildings safe?

There are some short-term fixes that can be used to reinforce the concrete. Surveys can determine whether any structures are at risk of failing, which can sometimes be shored up somewhat easily. “Depending on the weight of the [concrete] plank, a relatively short plank of timber support may be perfectly adequate,” says Stone. For longer planks, steel can be installed to provide support.

But these measures are temporary, and in the longer term, RAAC might need to be replaced. “This is not a bad material,” says Purnell. “It is behaving exactly as it would have been expected. This is a failure of maintenance, refurbishment and rebuilding budgets.”