Avoiding RAAC and Ruin

What is RAAC?

Reinforced Autoclaved Aerated Concrete (RAAC) is a form of steel-reinforced lightweight concrete. Autoclaved Aerated Concrete (AAC) was first created in Sweden in the 1920s as a significantly lighter alternative to traditional concrete mixes and was utilised to create lightweight masonry blocks of the type still commonly used today. By the early 1930s, steel reinforcement had been incorporated to create RAAC, allowing for significantly larger structural units such as wall panels, and floor and roof planks.

RAAC products are formed by mixing cement, lime, extremely fine sand or pulverised fuel ash and calcined gypsum. Aluminium powder and water are then added to form a concrete slurry, which is cast in a mould containing the steel reinforcement. The aluminium powder chemically reacts with the lime and water to produce small bubbles of hydrogen gas, which causes the mixture to foam, more than doubling its volume. The hydrogen evaporates into the atmosphere when this foaming reaction finishes, leaving a lightweight cellular concrete mixture. Finally, the mould is removed, and the product is cured at both high temperature and pressure in an autoclave for around 12 hours, which causes the sand and lime to fuse into a form of calcium silicate hydrate crystal, increasing the strength of the finished product.

RAAC products were first introduced into the UK market in the 1950s and, until the mid-1980s, were increasingly used in the construction industry. Due to its lightweight, fire resistance and thermal performance, RAAC became popular in roof decks and was extensively used in both commercial buildings and, particularly, in public sector buildings such as schools, colleges and hospitals.

Problems with RAAC

Unfortunately, RAAC’s lightweight properties came at the cost of its structural strength. RAAC does not perform in the same way as traditional reinforced concrete, being significantly more prone to deflection. Due to its porous nature, reinforced steel is more vulnerable to corrosion, so a protective coating of latex cement or bitumen is needed. The ACC also bonds poorly with the reinforcement steel, requiring welded crossbars to secure it in place.

By the 1980s, RAAC roof planks installed in the 1960s had begun to fail. Subsequent case studies revealed significant underlying issues with the RAAC roof planks. The steel reinforcement was often insufficiently covered, allowing corrosion to occur. Many roof planks had a high span-to-depth ratio, meaning they did not have suitable capacity to span between bearing points. Often, the steel reinforcement did not extend throughout the complete length of the product and contained an inadequate number of welded crossbars.

In 1996, the BRE published an Information Paper stating that excessive deflections and cracking had been identified in many RAAC roof planks, and there was evidence that reinforcement corrosion had occurred. In 2002, the BRE released further information on the performance issues of RAAC roof planks, advising that, although in-service performance was judged to be satisfactory, it would be prudent to monitor their actual performance after a number of years. It was determined that the lifespan of RAAC roof planks was about 30 years. As this construction was last used in the 1980s, all RAAC roof decks have now exceeded their estimated lifespan and may require to be replaced in their entirety.

Recent Developments – RAAC Roof Failures

In late 2018, the Local Government Association (LGA) and the Department for Education (DfE) alerted all school building owners about the partial collapse of a flat roof at an Essex school constructed from RAAC roof planks, which occurred with little warning. Fortunately, this happened at a weekend when the building was unoccupied. The school had recently had a new roofing membrane installed, which may have increased the load across the roof. It was also suspected that its darker colouration may have increased the amount of thermal heating to which the RAAC roof planks were exposed, contributing to the failure. All schools and education estates were advised to immediately assess their assets to identify any RAAC roofs, perform a risk assessment and take remedial action where necessary.

The Standing Committee on Structural Safety (SCOSS) released an Alert in May 2019 highlighting that this is a broader issue, as RAAC roof planks were used across many types of buildings. They highlight a case of damage that occurred to RAAC roof planks at a retail premise in early 2019.

Following this Alert, NHS England identified several hospitals with RAAC roof planks and serious structural concerns. West Suffolk Hospital, for example, has now had to install 27 metal supports under the RAAC roof planks to mitigate the immediate risks. The Government then set aside £100 million to continue to carry out this urgent remediation work at hospitals affected by RAAC roof plank issues.

Identifying a RAAC roof

How to tackle RAAC