If you mean RAAC concrete , it stands for reinforced autoclaved aerated concrete : a lightweight, bubbly concrete used in some buildings, especially older roofs, floors, and walls, and it has raised safety concerns because it can deteriorate over time.

Quick Scoop

RAAC is not standard concrete. It was made to be lighter and easier to install, but its lower durability means some buildings now need inspection, repair, or replacement where it was used.

In plain English

  • Reinforced means it contains steel reinforcement.
  • Autoclaved means it is heat-and-pressure cured during manufacturing.
  • Aerated means it has lots of air bubbles, making it lighter than normal concrete.

Why it matters

RAAC was used in many places from roughly the 1950s to the 1990s, but later reports found that moisture and aging can make it more prone to failure than traditional concrete.

If you meant something else by “rack concrete,” tell me the context and I’ll narrow it down.