What is Reinforced Aerated Autoclaved Concrete (RAAC)?

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What is Reinforced Aerated Autoclaved Concrete (RAAC)? Blog Image


More than 100 schools across England were recently ordered to close buildings because they were constructed using unsafe concrete known as Reinforced Aerated Autoclaved Concrete (RAAC).

About Reinforced Aerated Autoclaved Concrete (RAAC):

  • It is a lightweight, bubbly form of concrete that was used in roofs, floors, and walls between the 1950s and 1990s.
  • It looks like standard concrete, but compared with the “traditional” reinforced material, which is typically denser, Raac is weak and less durable.
  • The material was favoured in construction projects because of its lightweight thermal properties.
  • How is it made?
    • It is made from a combination of cement, lime, water, and an aeration agent. 
    • The mixture is poured into moulds and then subjected to high pressure and heat, known as autoclaving, to create a lightweight, strong, and porous material.
  • Advantages over Standard concrete:
    • It is cheaper.
    • It is also quicker to produce and easier to install.
    • Insulation Properties: RAAC has excellent thermal insulation properties due to the air bubbles within the material.
    • It helps maintain a comfortable indoor temperature while reducing heating and cooling energy consumption.
  • Why is it a risk?
    • The concrete is aerated and “bubbly”, contains no “coarse aggregate” and is less dense than traditional concrete, being around a third of the weight.
    • It could easily absorb moisture, weakening the material and also corroding steel reinforcement within.
    • This means it is more prone to collapse over time.


Q1) What is Concrete?

Concrete is a versatile and widely used construction material composed of a mixture of cement, water, aggregates (such as sand, gravel, or crushed stone), and often various chemical additives or admixtures. It is a fundamental building material known for its strength, durability, and versatility. Concrete is used in a wide range of construction applications, including buildings, bridges, roads, dams, and more.

Source: 100 schools across England ordered to close buildings as they are constructed using unsafe concrete