Reinforced Autoclaved Aerated Concrete: What Are The Risks?

Reinforced autoclaved aerated concrete (RAAC) was used in the construction of UK school buildings until the 90s and has been in the news due to safety concerns.

Reinforced autoclaved aerated concrete (RAAC) has been in the news recently, as at least 100 schools that contain this material have been forced to close just days before the start of term due to safety concerns. Here’s a closer look at what RAAC is, and why it’s now considered to be a safety risk.

RAAC is a lightweight form of concrete that is mixed with fine aggregates and has chemicals added to create gas bubbles. This gives the centre of the concrete the appearance of an Aero chocolate bar. As you might expect, RAAC is weaker than normal density concrete, and it also bonds less well with other materials.

RAAC was first developed in Sweden as a lightweight building material in the 1920s, and was widely used in UK construction from the 1950s to the 1980s. It was mainly manufactured in the form of thermal blocks or ‘planks’, most typically a width of 100-250mm, and used to form the roof decks of school buildings.

At the time, RAAC was considered to be an affordable and effective building material. However, by the early 1980s, concerns were raised about structural performance, and after concrete testing, the life expectancy of RAAC was subsequently revised to 30 years.

This has led to potentially thousands of structurally unsafe buildings still being used in schools and other public sector buildings such as hospitals, offices, and police stations. Alarmingly, the Health and Safety Executive has said that buildings incorporating RAAC are ‘liable to collapse at any moment.’

It is thought that a small number of cases of RAAC failure in schools have been reported over the summer in the UK, although at the time of writing, the exact numbers and locations remain undisclosed. This has led to a change in policy from only the most high-risk schools being closed to the closure of any school buildings containing RAAC.

Professor Goodier, an expert in Construction Engineering and Materials at Loughborough University, explained: “Like many countries, the UK has an old building stock, which needs to be adequately repaired and maintained. In the post-war period the country built numerous new buildings with a variety of different methods, many of which are now feeling their age.”

He added: “One innovative construction material and process was RAAC: Reinforced Autoclaved  Aerated Concrete, which is an aerated lightweight cementitious material with no coarse aggregate; the material properties and structural behaviour therefore differs significantly from ‘traditional’ reinforced concrete.”

RAAC panels can be identified by a distinctive V-shaped groove between the panels, and a light grey or white colour (unless painted). Warning signs of RAAC failure include cracks around the bearings, water ingress and ponding on roofs, and sagging of the roof structure.

Expert guidance from a qualified structural engineer should be sought if any defects are suspected.