Reinforced autoclaved aerated concrete (Raac) has been found in more than 100 schools, forcing them to either fully or partially close.

However, other buildings are believed to contain Raac, with experts warning that offices, court houses, hospitals, and factories could be at risk of collapse.

But what is Raac, and why is it a potential safety risk?

– What is Raac?

Raac is made up of two parts, aerated autoclaved concrete, and a steel reinforcement. The aerated autoclaved concrete is made by adding aluminium into a lime or cement based concrete mix.

This reacts to make millions of tiny bubbles which form the bulk of the material.

The steel reinforcement is coated with a latex or cement mix before the concrete is then cast around it.

The material is mostly found as precast panels in roofs, as well as floors and walls.

– When was it introduced?

Raac was invented in Sweden in the 1930s. It was used in British buildings from the 1950s to the mid-1990s.

– Why was Raac used?

Raac is cheaper than traditional dense concrete, and is quicker and easier to install.

It was also used due to its lightweight thermal properties.

– Where has Raac been found?

So far a lot of the focus on Raac has been on its usage in schools, with more than 100 fully or partially closed in England due to its presence.

However, experts warn the problem could be far wider than just schools.

Indeed, almost £700 million has already been allocated for NHS hospitals in England with Raac issues.

Harrow Crown Court was closed indefinitely last month, and urgent concrete tests have been ordered on courts built in the 1990s.

A report by the Collaborative Reporting for Safer Structures published in April 2020 urged its members to check as a “matter of urgency” whether their buildings contained the material.

The report said that Raac was used “primarily” in offices and schools but that it had also been found in a “wide range” of other buildings in both the public and private sector.

– Why is Raac a potential risk?

Raac is less durable than concrete, and is prone to collapse when wet, as moisture soaks into its aerated holes.

It has a life expectancy of little more than 30 years.

This means that buildings built between the 1950s to the 1990s that have not been assessed by a structural engineer could be at risk of collapse.

Several roof failures in public buildings have been linked to Raac panels, with the Health and Safety Executive (HSE) warning that the material is now “life-expired” and “liable to collapse with little or no notice”.

– What can be done about Raac?

Building owners can hire specialist surveyors to check if Raac is present in their buildings.

If Raac is found, building owners can hire a structural engineer to carry out remedial works, such as adding timber or lightweight structures to support the panels.