Reinforced autoclaved aerated concrete (Raac) has been found in more than 100 schools, forcing them to either close completely or partially.
However, other buildings are believed to contain Raac, with experts warning that offices, courthouses, hospitals and factories could be at risk of collapse.
But what is Raac and why is it a potential security risk?
– What is Raac?
Raac consists of two parts, autoclaved aerated concrete and a steel rebar. Autoclaved aerated concrete is made by adding aluminum to a lime- or cement-based concrete mix.
This reacts to create millions of tiny bubbles that form the bulk of the material.
The steel rebar is coated with a latex or cement mixture before the concrete is poured around it.
The material is mainly found as prefabricated panels on 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 faster and easier to install.
It was also used because of its light thermal properties.
– Where was Raac found?
So far, much attention has been paid to Raac for its use in schools, with more than 100 being fully or partially closed in England because of its presence.
However, experts warn that the problem may be much wider than schools.
Indeed, almost £700 million has already been allocated to NHS hospitals in England with Raac problems.
Harrow Crown Court was closed indefinitely last month and emergency 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 “urgently” check whether their buildings contained the material.
The report said Raac was used “mainly” in offices and schools, but that it had also been found in a “wide range” of other buildings in both the public and private sectors.
– Why is Raac a potential risk?
Raac is less durable than concrete and is prone to collapse when wet as moisture is absorbed into its air holes.
It has a life expectancy of just over 30 years.
This means that buildings built between the 1950s and 1990s that have not been assessed by a structural engineer may be at risk of collapse.
Several roof failures on public buildings have been linked to Raac panels, with the Health and Safety Executive (HSE) warning that the material is now “at the end of its useful life” and “likely to collapse with little or no warning”.
– What can be done with Raac?
Building owners can hire special inspectors to check if Raac is present in their buildings.
If Raac is found, building owners can hire a structural engineer to perform remedial work, such as adding timber or light structures to support the panels.