Last week, Education Secretary Gillian Keegan revealed that more than 100 schools, colleges, and nurseries across England have been ordered to close after their buildings were found to include a material known as RAAC that was prone to collapse.

The Department for Education has now revealed that a total of 147 schools have been affected by the crumbling concrete, including more than 10 London schools.

Treasury officials have since stated that money for repairs will come from the Department for Education‘s (DfE) current capital budget, and Chancellor Jeremy Hunt has rushed to reassure parents that the Government would “spend what it takes” to fix the issue.

The Government has been urged to provide clarity on the impact of this lightweight and unreliable concrete’s use in hospitals and other public buildings, with the Labour Party calling for an “urgent audit”.

According to The Guardian, an investigation has also been launched into whether RAAC has been used in the construction of the Houses of Parliament.

Amid all of this, many have found themselves unsure about what RAAC is and when exactly they were being used in construction. Here is a comprehensive look at RAAC.

What is RAAC and why was it used?

RAAC stands for reinforced autoclaved aerated concrete. It is a lightweight building material that is mostly used in flat roofing, and sometimes in floors and walls.

It has an aerated, bubbly texture, and was preferred as a cheaper alternative to standard concrete, as it was faster to produce and easier to install. The air bubbles in RAAC mean it’s lighter and can provide good thermal insulation. However, the speed and ease come at a cost. RAAC is less durable and its lifespan is only around 30 years.

Plus, if it is exposed to moisture, it is prone to collapse, as the bubbles can allow water to enter the structure.

How is RAAC made?

According to the Standing Committee on Structural Safety (SCOSS), RAAC is structurally different from concrete because of the way it’s made.

SCOSS said: “Although called “concrete”, (RAAC) is very different from traditional concrete and, because of the way in which it was made, much weaker.

The Trades Union Congress (TUC) explains that RAAC is manufactured by mixing raw materials before pouring the mixture into a mould. A high-pressure autoclaving process — which essentially means exposing it to steam and pressure — is then used to create RAAC. This results in the formation of air bubbles in the structure.

When was RAAC used to build structures?

Reinforced autoclaved aerated concrete was actively used to construct buildings between the 1950s and the 1990s.

The risks of RAAC aren’t a new discovery. RAAC’s differences from traditional concrete were outlined back in the 1960s.

And, in the 1980s, a report by the Institution for Structural Engineers revealed that short-term exposure to moisture reduced the material’s strength by around 13 per cent, while long-term exposure to air pollution reduced it by 40 per cent.

A 1996 government-funded report by the Building Research Establishment also found that RAAC panels had been cracking in a housing development, and there were cracks and bends in panels installed in schools.

In 2003, a Labour government aimed to refurbish all of England’s secondary schools but, in 2010, the Tories dropped the project as not cost-effective.

The Government concluded that while there were no immediate safety risks, RAAC panels should be inspected every year.