what is rack concrete

Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight form of concrete used in roof, floor, cladding, and wall construction in the UK from the mid-1950s to the mid-1980s. It is an aerated lightweight cementitious material with no coarse aggregate, and its material properties and structural behavior differ significantly from traditional reinforced concrete. RAAC has an open texture with open-sided bubbles often visible, and the surface is slightly crumbly when touched and easy to gouge with a screwdriver. However, if the concrete has been heavily painted or coated, it may be harder to identify. RAAC panels have low compressive strength, being around 10-20% of traditional concrete, meaning the shear and bending strength is reduced. This strength is further impacted by water saturation. It is very porous and highly permeable, which means that the steel reinforcement within the panels is less well protected against corrosion than steel reinforcement in traditional concrete. The reinforcement within RAAC panels is less well bonded to the surrounding concrete, and the bearing of planks is often insufficient, by comparison to modern standards, which presents a significant risk. RAAC has reduced stiffness characteristics resulting in high displacements, deflections, and sagging.