Reinforced Aerated Autoclaved Concrete (RAAC)

Reinforced Autoclaved Aerated Concrete (RAAC) was developed in the 1930s and incorporated steel reinforcement with Autoclaved Aerated Concrete (AAC) in order to produce larger structures. RAAC is highly aerated with different material properties making it a substantially lighter alternative to conventional concrete mixtures. 

RAAC was introduced into the UK construction industry in the post-war era and, due to its lightweight, fire resistant and thermal performance, became widely used in commercial buildings and, particularly, public sector properties such as hospitals and schools. It was also used in the construction of many residential properties. It is usually found in roofs and occasionally in walls and floors.

RAAC’s lightweight properties came at the cost of its structural integrity and can be affected by factors such as moisture, exposure to high temperatures, and impact. Due to RAACs porous nature, reinforced steel is more susceptible to corrosion causing structures to be compromised. 

Cracks and signs of movement may indicate that the structure is under stress and there may be issues with foundations and/or damage to the material. Moisture and water absorption can weaken and reduce its performance causing problems such as mould growth, deterioration, and reduced insulation properties.

In recent experience, roofs have failed with no warning signs. This suggests that the problems and risks associated with RAAC could be more serious than once thought.

Where there is concern that RAAC exists, it is extremely important that a competent and qualified Structural Engineer with specific knowledge of RAAC is appointed to carry out an assessment of the building. 

Without a proficient understanding of RAAC possible structural failures, which should have been prevented, may be missed.