More than 100 schools and colleges in England have been told to close areas of their buildings by the government as they are thought to have been built with reinforced autoclaved aerated concrete (RAAC).
It is a lightweight form of concrete that was used construction from the 1950s until the mid-1990s.
RAAC is weaker than regular concrete which is used as a building material.
As seen in this footage, some formulations of RAAC are light enough to float in water.
This clip explains what RAAC is and why its use in schools is so concerning.
