Typically, when creating a hard-wearing, robust building structure, the primary construction material is reinforced concrete, where concrete’s compressive strength is mixed with metal rebar to provide additional tensile strength and prevent structural issues.

However, in the 1950s, a relatively new alternative began to see use in Britain by the name of reinforced autoclaved aerated concrete (RAAC) that was lighter, cheaper and had excellent fire-resistant properties without plastering, making it popular for use in academic facilities and schools.

However, concerns have been raised after a roof with the material suddenly collapsed, and just before the start of the 2023 autumn term, at least 100 schools that use RAAC have been forced to close some or all of their buildings for safety reasons.

It is important to know what RAAC is, why it has caused issues now, and to what extent the construction material is safe in normal operating conditions.


Defining RAAC

Reinforced autoclaved aerated concrete is a type of reinforced concrete that was made without any aggregates. Instead, its construction process using a device called an autoclave created pockets of air which created a bubbly texture.

This made it lightweight, strong and at the time a reliable construction material. It was also at the time cheaper to produce than conventional reinforced concrete which led to its use in major rebuilding projects after the Second World War.

It does not function the same as reinforced concrete, and in some use cases, there are benefits to RAAC in comparison to conventional concrete. The most notable example is its use as a walling material for buildings in Japan due to its resistance to damage in earthquakes.

One of its biggest issues is a significantly shorter lifespan than conventional concrete. Whilst concrete can last between 50 and 100 years depending on how well it is maintained, RAAC has a lifespan of closer to 30 years, and deterioration can happen exceptionally quickly.


Why Has It Caused Concerns?

In the 1990s, concerns were raised about flat roof panels made from RAAC, as they could fail without warning, as opposed to the slow deterioration of reinforced concrete that allows time for surveying, maintenance and repairs.

It must be emphasised that in a lot of cases, the material itself is not at fault necessarily. Instead, a lack of effective maintenance, poor installation of the rebar elements as well as an inability or unwillingness to replace RAAC elements no longer fit for purpose.

Due to air bubbles and less density, water ingress can more easily reach the rebar elements, causing corrosion and more significant compromises to its structural integrity.

This was concerning because many public buildings including schools constructed from the 1950s to the 1990s used RAAC as a flat roofing material.

This reached a tipping point after decades of delays on the issue when a roof over a staff room in a Kent school showed signs of distress. Less than 24 hours later it collapsed, triggering concerns about other time-expired public buildings.

After several roof collapses in August 2023 alone, several schools have been closed and classes relocated as engineers rush to fix the issue.

The material itself in normal conditions is safe, but left neglected it deteriorates far faster than conventional reinforced concrete.