Reinforced autoclaved aerated concrete (RAAC) is a reinforced version of autoclaved aerated concrete, commonly used in roofing and wall construction. In Europe, it gained popularity in the mid-1950s as a cheaper and more lightweight alternative to conventional reinforced concrete.
Structural issues involving RAAC began surfacing publicly in the United Kingdom in the 1990s. Concerns were amplified in 2023 following reports of an earlier roofing collapse at a British primary school, which fell without warning in 2018. The material is still present in the UK in public buildings such as schools and hospitals. The material's nature makes it difficult to identify these issues before collapse, leading to the Department for Education issuing emergency closures to a number of schools due to the risk to pupils.
Overview
RAAC was used in roof, floor and wall construction due to its lighter weight and lower cost compared to traditional concrete, and has good fire resistance properties; it does not require plastering to achieve good fire resistance and fire does not cause spalls. RAAC was used in construction in Europe, in buildings constructed after the mid-1950s. RAAC elements have also been used in Japan as walling units owing to their good behaviour in seismic conditions.
The first structural reinforced roof and floor panels were manufactured in Sweden, soon after the first autoclaved aerated concrete block plant started up there in 1929, but Belgian and German technologies became market leaders for RAAC elements after the Second World War.
Deterioration concerns in the United Kingdom
RAAC has been shown to have limited structural reinforcement bar (rebar) integrity in 40 to 50 year-old RAAC roof panels, which began to be observed in the 1990s. The material is liable to fail without visible deterioration or warning. The material is not the root cause, rather inadequate roof maintenance, which permits water infiltration, and decisions by building owners as to repair or the replacement of existing roofs, which is a part of cost-benefit analysis.
Professional engineering concern was publicly raised in 1995 about the structural performance of RAAC following inspections of cracked units in British school roofs. Structural deficiencies and signs of corrosion were observed in 1996. It was then recommended that owners of buildings, including schools, arrange for RAAC roofs to be inspected. Concern regarding the material was subsequently raised in July 2018, following a roof collapse at Singlewell primary school in Gravesend, Kent. In August 2022, the UK Government Property Agency stated that "RAAC is now life-expired and liable to collapse".
2023
In 2023, the UK government raised concerns about the structural integrity of buildings containing RAAC, and ordered remedial action to be taken. An analysis by The Guardian in October 2023 showed that three-quarters of schools hit by the RAAC crisis are in Conservative Party areas.
In May 2023, the Secretary of State for Health and Social Care, Steve Barclay, confirmed seven hospitals in England, which were constructed largely with RAAC, were not safe to operate beyond 2030. The hospitals were surveyed in 2022. His list included Airedale General Hospital in West Yorkshire, for which a rebuild was approved that month, as 83 per cent of the hospital was constructed of RAAC, with the ceiling held up by props.
In August 2023, more than 100 school buildings in England were ordered to be closed while safety measures were installed, after a beam collapsed at a school. The figure was later revised to 174. The Health and Safety Executive warned that RAAC could "collapse with little or no notice". Other public buildings such as Harrow Crown Court (which opened in 1991) and Royal & Derngate theatre in Northampton were also closed indefinitely. A number of other theatres have subsequently been found to have RAAC. The Scottish Government subsequently confirmed that RAAC had been found in 35 of Scotland's council-run schools. This figure was later revised to 40 following continued inspection, with the First Minister, Humza Yousaf, confirming that fire stations and NHS buildings were also affected. In Wales, RAAC had been found at four schools by 14 September – two in Anglesey, one in Denbighshire and one in Conwy. One of the Anglesey schools had reopened following inspection, while the other three were closed while they underwent inspection.