In England, autumn term 2023 began inauspiciously for some parents and their offspring. The UK Government released a report on the last day of August, confirming that, as of then, 156 educational “settings” in England contained RAAC (Reinforced Autoclaved Aerated Concrete) in buildings on their premises. Out of these, 52 settings had “mitigations” in place, referring to ongoing or completed repair works, while mitigations were “being put in place” in the remaining 104. The pupils of these 104 schools would thus have to either attend temporary facilities, such as Portakabins, on their school premises, travel to other schools, or be taught remotely and online, just as during the pandemic.
Autoclaved aerated concrete was developed in Sweden during the 1920s and introduced to the UK by the late 1950s. Being cheap, lightweight, and heat resistant, RAAC was used by municipal architects, in place of traditional concrete, for various public sector construction projects between the 1960s and 1990s-including schools, colleges, hospitals, courts, and office buildings. RAAC planks were commonly used for roofs, mostly flat but also pitched, as well as in eaves, floors, walls, and cladding systems.
RAAC appears light grey or off-white in colour, and has a smooth surface, free from visible irregularities. Gas pockets contribute to an internal cellular honeycomb-like structure, which has been likened to the inside of an Aero chocolate bar. RAAC has a crumbly feel and is easy to gouge into with hand tools. RAAC planks bear such trade names as Celcon, Durox, Siporex, and Thermalite. Individual planks are rectangular in cross-section, have chamfered edges, and are 300 to 750 mm wide and 100 to 250 mm thick.
Autoclaved aerated concrete was made from cement and silica flour (fine sand), which were mixed with water and aluminium powder. A chemical reaction produced small bubbles of hydrogen gas, accounting for the “bubbly” appearance. A coating of latex or bitumen protects against corrosion. The absence of coarse aggregates-which includes gravel, crushed stones, and slag- makes it weaker than standard concrete. The porous nature of RAAC allows water ingress, which further weakens the concrete while also damaging the reinforcing steel rebars (reinforcing bars). These changes can lead to structural failure, which can either be sudden and catastrophic, or may be heralded by excessive deflections between adjacent planks or by the appearance of transverse cracks. Concerns over limited life-span and the potential for delayed structural failure meant that RAAC production in the UK ended in 1982.
The presence of RAAC in public sector buildings has apparently been flagged up for the government’s scrutiny ever since 1994. The sudden collapse of a roof at Singlewell primary school at Gravesend in Kent in July 2018 injected some urgency into the matter. Following this incident, schools have been advised to have “adequate contingencies” in the event that school buildings were to require evacuation. A report issued by the Standing Committee on Structural Safety, an independent body, in May 2019 confirmed a significant risk of structural failure of RAAC panels.
RAAC has also been used to build hospitals. A National Audit Office report on ‘Progress with the New Hospital Programme’, published in July 2023, identified 41 buildings at 23 hospital trusts to have been constructed with RAAC. Seven hospitals have been deemed “structurally unsound” as a result and have since been added to the government’s New Hospital programme, due for completion by 2030.
RAAC is now recognised as having a life expectancy of around thirty years, making inspection and ongoing maintenance of RAAC-containing buildings a priority. Under normal circumstances, estates managers in hospitals and schools would be expected to undertake the bare minimum of visual inspection of buildings wherever feasible and safe to do so. Information about the presence of RAAC may also be obtained from design documentation wherever available. More often, the specialised knowledge of building inspectors, chartered surveyors, and structural engineers is required to confirm the presence of, and assess the risks from RAAC, and to then respond proportionately.
The initial users of RAAC may not have foreseen all the problems that were to emerge with their chosen product and can be absolved of at least some of the blame, which relates to the actions, or inaction, of their successors. The present spate of school closures, although small in relation to England’s total of 24, 413 schools, testifies to shortcomings in the public sector buildings maintenance programme as well as to delays in the official oversight of the same.
Ashis Banerjee