BAR GUIDANCE NOTE: REINFORCED AUTOCLAVED AERATED CONCRETE (RAAC)

BAR has published a guidance note on Reinforced Autoclaved Aerated Concrete (RAAC) following increased concerns over the material’s structural stability and safety.

RAAC is an aerated lightweight cementitious material with no coarse aggregate – a key ingredient for concrete. It is aerated using chemicals to create gas bubbles. This means that the material properties, strength and structural behaviour differ significantly from ‘traditional’ reinforced concrete. Indeed, the Standing Committee on Structural Safety (SCOSS) notes that: “Although called ‘concrete’, RAAC is very different from traditional concrete, and, because of the way in which it was made, is much weaker.”

Schools, hospitals and various other public buildings from the 1950s to the mid-1990s were built using RAAC. It was withdrawn from British design standards in 2001 and is not included within current UK and European specification standards for concrete.

Although also classified as a concrete and is visually similar, RAAC panels differ from traditional reinforced concrete structures in a number of ways:

• The concrete material is aerated giving it the benefit of being considerably lighter than traditional concrete. Typically, aerated concrete has a density of 600-800kg/ m³ compared to 2400kg/m³ for traditional concrete. The aerated nature and reduced density influences reduces compressive strength to typically, in the range of 2-5N/mm². This is much lower than traditional concrete.

• Flexural, shear, and tensile strengths are also similarly reduced compared with traditional concrete.

• Due to the aerated nature of the material, it will not form adequate bond strength with the reinforcement. The reinforcement within is also smooth and not ribbed. Tensile forces are therefore predominantly transferred to the reinforcement via transverse reinforcement bars being welded to the longitudinal reinforcement with bars over the bearings of the panels for end anchorage. The position and effectiveness of the transverse reinforcement over the bearing is critical to the shear capacity of the panels at their bearings.

• Unlike reinforced concrete, the aerated material is highly permeable. As a result, cover to the reinforcement does not protect against environmental conditions as with traditional concrete and the cover zone can be expected to be highly carbonated.

• The aerated nature and lack of coarse aggregate means that the elasticity and creep characteristics of AAC are substantially inferior to traditional concrete. This can have a detrimental impact on long term deflections of the RAAC panels.

It is important to note, that RAAC is not the same as traditional reinforced concrete. The composition of RAAC is very different and its 30-year design life is significantly less than reinforced concrete. The BAR Guidance Note: Reinforced Autoclaved Aerated Concrete (RAAC) may be downloaded from the publications section of the BAR website: www.uk-bar.org