The issue of defective RAAC elements in public buildings across the UK and the risk of their sudden, catastrophic failure has been headline news in recent days. This post describes what RAAC is, how defects arise, and how they are found and fixed. It goes on to look at what action to find, assess and remediate defective RAAC elements in public buildings has been taken by the Scottish Government and local authorities to date.
What is RAAC?
RAAC is the acronym for Reinforced Autoclaved Aerated Concrete.
It is important to understand that RAAC is very different from traditional concrete, which is a mix of water, cement, sand and aggregate (small stones), that is typically mixed, poured and set in-situ on building sites. RAAC is prefabricated in a factory and delivered to building sites in the form of panels, which can be used in roofs, walls and floors.
Profile of a RAAC panel.
Source: UK Government
RAAC panels have two key elements:
- Aerated Concrete: This is made by adding aluminium powder to a lime or cement based concrete mix, which does not contain any aggregate larger than sand. This mix is cast in a mould. The aluminium powder reacts with the lime/cement and water to produce millions of tiny gas bubbles, substantially increasing the volume of the material. The product is then cured in an autoclave for between eight and 15 hours at high temperature and pressure, to control shrinking and encourage the formation of strongly binding molecules within the concrete.
- Reinforcing: RAAC panels are given added strength by lattices of steel reinforcing rods, which are covered in an anti-corrosion coating. Reinforcement is placed into the mould before the concrete mix is added.
Why use RAAC?
In the UK, RAAC was used as a building material between the late 1950s and late 1990s, often in the construction of flat roof decks – which were then coated with waterproof roofing material. RAAC was used as it is lightweight, has good thermal insulating properties, is relatively cheap, and quick and easy to install.
Potential RAAC defects
RAAC, if it is manufactured, installed, and maintained correctly, poses no more danger to building users than most other construction products. However, concerns that RAAC elements of some buildings could be liable to fail under certain circumstances and that the product has a relatively limited lifespan have been recognised for decades. The BRE (Building Research Establishment) explored these issues in a paper published in 1996 which estimated the usual lifespan of RAAC panels to be around 30 years. An alert issued by the Standing Committee on Structural Safety in May 2019 advised that all pre-1980 RAAC panels in UK buildings had significantly exceeded their expected service life and should be considered for replacement.
The Institution of Structural Engineers categorise potential RAAC defects under three headings, performance, manufacturing, and construction. The list is extensive and the description of defects technical. The key possible RAAC defects identified by the Institute are briefly described below:
- Deflection of panels, in effect they begin to sag over time.
- Cracking on the underside of panels, known as spalling.
- Corrosion of steel reinforcement and/or reduction in the integrity of the concrete due to water ingress.
- Overloading, which can be caused by water ponding in deflections on flat roofs.
- Panels acting independently rather than as a single unit, limiting load sharing across a structure.
- Poor quality manufacture and placement of the steel reinforcement within RAAC panels may leave them susceptible to failure. A particular concern is where steel reinforcing does not extend along the full length of a panel, leaving the load bearing end section weaker than it should be, meaning it is more likely to fail.
- Small gaps (voids) in the aerated concrete caused by gas bubbles coalescing around the steel reinforcement during manufacture.
- Incorrect or poorly applied anti-corrosion treatment of steel reinforcement.
- Builders have cut RAAC panels post-manufacture to fit the required size, compromising their strength and integrity.
- RAAC panels resting on very short sections of supporting beams, increasing stresses on panel ends. This is a particular problem where there are manufacturing defects, such as the incorrect placing (or lack) of steel reinforcement near panel ends.
- Missing reinforcement such as links between the ends of panels that have been butted together.
- Structurally damaging maintenance or building conversion work, such as holes being drilled or cut through panels, reducing their structural integrity. Another maintenance issue is where a RAAC roof has been re-surfaced since original construction – particularly if this increases the load on the roof or where a black finish has been used when the original was another colour, as it will likely retain more heat.
A more general concern is that the manufacture of panels was highly inconsistent and the quality control poor, meaning there can be quite wide variations in the quality and physical characteristics of panels used within a single building.
It is worth noting that since 2008, new standards have applied to RAAC products used in UK construction, removing the scope for failure and improving the lifespan of such products.
Identifying RAAC defects
Advice on identifying RAAC and possible defective RAAC building elements has been produced by several organisations including the Institution of Structural Engineers, UK Department for Education, and NHS Scotland National Services. Much of this advice is highly technical and is aimed at specialist surveyors and engineers. However, advice to building managers is fairly consistent, generally recommending that:
- The possible presence of RAAC should be identified through a desktop review of building plans and documentation, where it exists, and a simple visual inspection of the building by appropriate staff. This can be complicated by the fact RAAC panels are often hidden behind suspended ceilings and under roofing materials, some of which could contain asbestos – meaning they cannot be moved or tampered with by non-specialist staff.
- Where RAAC is found to be present, or it is suspected to be present, a qualified engineer or surveyor should be employed to undertake a detailed investigation of its condition. A risk assessment may also need to be conducted, which will consider whether and how the space beneath RAAC panels should be used in the interim period.
- The surveyor or engineer will report on the condition of any RAAC elements and, if appropriate, recommend remedial action.
Fixing RAAC defects
The Institution of Structural Engineers highlights in guidance that remedial action can include:
- Emergency propping of roofs or ceilings, when panels are deemed to be in a very poor condition.
- Strengthening the support under the ends of RAAC panels, known as end bearing strengthening, to mitigate against known defects or unknown/unproven defects to the load bearing ends of RAAC panels.
- Permanent remedial supports, to actively take the loading from the panels.
- Passive, fail safe supports, to mitigate catastrophic failure of the panels if a panel was to fail.
- Removal of individual panels and replacement with an alternative lightweight solution.
- Entire roof replacement.
- Periodic monitoring of the panels for their remaining service life.
Illustrations of remedial action
Source: Institution of Structural Engineers.
Action by the Scottish Government and local authorities
The Scottish Government has issued guidance on RAAC to local authorities via the Scottish Heads of Property Services and the Association of Directors of Education in Scotland. Local authorities are currently undertaking initial investigations into the presence of RAAC in their buildings, with recent media reports indicating that RAAC is present in at least 35 Scottish schools.
NHS Scotland Assure, which is leading the national RAAC survey programme of the NHS estate on behalf of Scottish Government, issued a Safety Action Notice to Health Boards in February 2023, advising them to launch investigations into the presence of RAAC in their properties. Media reports from late July 2023 indicate that initial desktop surveys of NHS properties found that RAAC was present in over 250 buildings.
The Scottish Government has convened a cross-public sector working group on RAAC, which met for the first time on 14 August 2023.
At present, data on the number of public buildings containing RAAC elements, and the extent of remedial action needed to deal with any defects found, is limited. Public bodies are currently gathering and processing this information. While details of eventual remediation and mitigation costs are not available, it seems reasonable to assume they will be fairly substantial. The Scottish Government stated in answer to a parliamentary question published on 31 July 2023 that:
From our discussions with local authorities regarding RAAC to date, West Lothian Council indicated that addressing RAAC issues across its estate could cost around £76.8m, whilst East Lothian Council advised that costs relating to one affected school are estimated at around £3.8 million. The current information gathering exercise offers the opportunity to identify similar assessment made by other local authorities.
Alan Rehfisch, Senior Researcher, SPICe
Blog image: Sawn cross-sections of typical RAAC structural panel, by Cavalaro, Sergio & Lee, Kelvin & Casselden, Rebe is licensed under CC BY 4.0