Reinforced autoclaved aerated concrete (RAAC) is a lightweight material that was used mostly in flat roofing, but also in floors and walls, between the 1950s and 1990s.

It is a cheaper alternative to standard concrete, is quicker to produce and easier to install.

It is aerated, or "bubbly", like an Aero chocolate bar.

But it is less durable and has a lifespan of around 30 years.

Its structural behaviour differs significantly from traditional reinforced concrete.

Moreover, it is susceptible to structural failure when exposed to moisture. The bubbles can allow water to enter the material.

If that happens, any rebar reinforcing RAAC can also decay, rust and weaken.

Because of this, RAAC is often coated with another material, such as bitumen on roofing panels. But this material can also degrade. Source

This is an international issue btw.