September 6, 2023

expert reaction to situation with RAAC in school buildings

The Department for Education have announced a change in their approach to managing Reinforced Autoclaved Aerated Concrete (RAAC), a building material found in some school buildings and other education settings.

Comment sent out 06/09/2023:

Prof Cise Unluer, Professor of Engineering Zero, University of Manchester, said:

“A few points need to be considered when assessing the risk involved with any concrete structure, including Reinforced Autoclaved Aerated Concrete (RAAC).

“Firstly, the components that are involved in RAAC and the interaction of those components, as well as the surrounding environment, play a key role in determining the performance and the service life of an overall concrete structure.

“As in all building materials, elements involving RAAC have a certain service life, which is generally shorter than normal concrete. The presence of a high volume of pores within RAAC, and its lower compressive and tensile strength when compared to traditional concrete, limits its uses.

“In addition to its lower load-carrying capacity than normal concrete, larger units involving RAAC are also prone to cracking, which can be critical for load-bearing elements, where sudden crack formation (i.e. with little or no warning) due to the brittle nature of these elements, can have serious consequences.

“Furthermore, considering the porous structure of RAAC that is more susceptible to reaction with moisture, chloride or carbon dioxide, the corrosion of reinforcement can be expected. In the case of corrosion, there will be an increased tensile stress on the concrete and the bond between the reinforcement and surrounding concrete will weaken. These will reduce the service life of structures involving RAAC.

“It is important that any building containing these products is further evaluated by performing a detailed condition survey and a risk analysis. Considering that each structure will behave differently and have a different service live depending on its design, exposure conditions and maintenance; this can help us determine if particular elements within a structure can be strengthened or further actions need to be taken to ensure the safety of all users.

“Considering its widespread application between 1950-1990, it is likely that the use of RAAC has not been limited to schools and can include other structures including public and other residential buildings. Keeping in mind the points made above, the use of RAAC for load-bearing applications needs to be re-considered.

“Going forward, I would like to emphasise the importance of allocating relevant funding for the ongoing assessment and maintenance of concrete structures in order to proactively identify any structural issues at an early stage and develop solutions that would prolong the service life of structures without compromising the safety of their users. Such an approach can prevent the rapid deterioration of structures and any associated undesirable failures.”

Previous comments:

Christian Stone, Professional Scientist and Technical Expert at Concrete Preservation Technologies (CPT), said:

“RAAC is a composite material invented in Sweden in the 1930’s that made its way in the 1950’s to the UK, Europe and eventually spread throughout Asian and North America.

“RAAC is a prefabricated material that was in the most part made by two major manufacturers in the UK with the majority of domestic production in the 60’s, 70’s and into the 1980’s. It is a visibly aerated, lightweight concrete, typically white to light grey in colour containing steel reinforcement which has been coated. It can be found as structural or non-structural capacities in walls, floors and roofs.

“To best understand RAAC we are best spitting it into its two main constituent parts; AAC, and the reinforcement.

AAC:

“AAC or Aerated Autoclaved concrete is still made today in the UK and throughout the world. It is a lightweight form of concrete made by adding Aluminium into a lime or cement based (sometimes with fly ash, etc) concrete mix that contains no aggregates larger than sand (no crushed gravel). This reacts to make millions of bubbles typically 0.1 – 2 mm in size which form the bulk of the material. During curing the AAC is autoclaved (Steam and controlled heating) to control shrinking and push the formation of strongly binding molecules within the concrete.

“This process leads to a material with a compressive strength of around 2-5MPa, much less than concrete, that is very porous (it can hold up to half its own mass in water), that is thermally insulating, and has about 1/6th the density of concrete. These properties made it affordable, easy to build with quickly, and less of a load on the structure.

Reinforcement:

“To protect the reinforcement from corrosion the steel is typically coated with a Latex/cement mix (sometimes powder coated in the 80’s) before the concrete is cast around it. This coating is very well designed and creates an impenetrable layer around the steel sealing out moisture and also keeping the steel so alkaline it will remain passive to corrosion while the coating is intact.

“Sadly the coating is made from natural materials (Latex) which due to time, heating and cooling, and flexing, degrade. Corrosion can then initiate on the reinforcement and cause issues due to rust needing to expand, rust has up to 6-7 times the volume of steel, causing cracking and stress in the plank around the steel.

“These issues can be addressed with a corrosion management solution if the plank is still sound. Damage cannot be reversed but it can be slowed or even halted.

Other issues:

“The other issues found in RAAC are a product of age such as planks becoming deformed or weakened from moisture, and that the steel reinforcement is not where it is most needed.

“When a plank of RAAC is sitting between two beams (or bearings as we like to call them) the position of the steel over the beam is vital. AAC alone is relatively strong when being crushed however it is easily pulled apart. That is where the steel is critical. The reinforcement gives the RAAC the tensile strength of steel, holding it together in a welded cage when forces try to snap or pull it.

“What this means is that if the cage of steel is not sitting over the beam supporting the roof plank, but instead the end of the beam is just the bubbly concrete, when a force is applied there is a risk it will snap (or sheer). Therefore, a critical question for engineers is where the steel (especially those that go across the plank forming the end of the steel cage) sit within the plank and how much of the plank is sitting on the beam.

“It should be no great surprise that planks made in the 1960’s did not have the same levels of quality control as we would expect today and therefore the position of the steel and indeed defects in the coating vary from plank to plank. Planks may also have been situated with a small bearing (overlap) or moved off the beam as it expanded and contracted with temperature over time. An engineer will need to assess this risk and in some cases supports may be added to further support these planks. Some planks will be adequately situated in a location where they are safely supporting themselves and the roof structure, others may require support and corrosion management, others may be beyond safely saving.

“The UK has likely hundreds of thousands or even millions of these planks in place on public buildings past their intended design life and there have been few failures. However, a single failure in the wrong place could be a catastrophe. Therefore, it is very understandable that caution is being taken, many of these planks may have considerable more life if properly surveyed, managed, and maintained.”

Dr Sam Adu-Amankwah, Lecturer in the Department of Civil Engineering, Aston University Birmingham, said:

“Autoclave aerated concrete (AAC) is mortar (binder plus fine aggreagate – sand) which derives its low density from specially formed air bubbles. Concrete is made from 3 primary constituents – binder (commonly referred as cement), sand (also fine aggregate, typically less than 5mm) and coarse aggregate (gravel larger than 5mm). They have been around since 1920s, but widespread usage was after World War II. They are lightweight, with a typical density less than 1000 kg per cubic metre compared to normal concrete which has about 2400kg per cubic metre density. Unlike normal lightweight concrete which is produced from lightweight aggregate (see the example here https://eprints.whiterose.ac.uk/193274/1/V56I09P08.pdf), RAAC derives low density from voids formed by gases (mainly hydrogen gas) from alumina reacting with lime or other alkalis. Strength is gained through heat curing (up to 180-degree Celsius). Due to these processes, RAAC is always prefabricated in factories with advantages of faster site assembly and insulation against fire and sound. 

“Every built asset has a designed service life, which is typically 50 years for buildings and, 120 years for monumental structures including bridges. Autoclave aerated concretes are known to be generally weaker than normal concrete. For example, normal strength concrete is expected to achieve between 20 -50MPa strength by 28 days whilst RAAC typically has 2.5 – 5MPa strength after autoclaving (depending on density) which does not increase over time. More importantly, tensile strength which is even critical for resistance to cracking is much lower for RAAC. It is important to point out that various sizes e.g., as 150mm blocks (AAC blocks) to larger and reinforced wall, slab, or roof panels (reinforced AAC = RAAC) can be found in some buildings.  Susceptibility of large RAAC panels to cracking have been known since the 60s.  The question of whether this would have been known or recognized as a design problem would depend on what standards these were manufactured to. If designed for fire resistance, it is unlikely the resistance to cracking would have been considered as a critical design criterion or even its performance under saturated conditions.     

“RAAC, which were factory manufactured were widely used around the world after the second world war.  A report dating back to the early 90s showed that RAAC and associated products were used extensively in the Soviet Union, Czechoslovakia, Poland and Japan. At the time, the UK was the 5th largest user of RAAC. 

“It is important to point out the difference between AAC and RAAC and whether the product was used as loadbearing or not. Specification as load-bearing elements example in roof or floor slabs and or wall element in single or multi-storey buildings were common. In such cases, the sudden crack formation can be catastrophic. There is evidence that when RAAC is carbonated or it has prolonged exposure to moisture or chlorides, corrosion of the reinforcement can be rapid due to lower resistance to moisture movement. In such a case, the reinforcement does not act compositely with the concrete (which already has low tensile strength). Upon cracking, there is no load transfer from the concrete to the reinforcing bar. This can lead to collapse.  

“In regard to dealing with these buildings, it must be informed by extensive condition survey, proper risk, and cost benefit analysis. Each situation is unique in terms of condition of the structure as a whole and its exposure environment. If the element e.g., a wall is non-load bearing or the condition permits, strengthening could be an option and a wide range of options are available.  The emphasis so far has been on public buildings – including courts, hospitals and schools. The numbers are already staggering. But these are not the only buildings that were built in RAAC. They are likely to be found in tower blocks and industrial buildings. A national audit is needed urgently to quantify the risk.  This raises questions about our priorities as a nation and whether we recognize the challenges and how we manage our existing building stock. 

“The construction industry is already risk-averse and very slow to innovation. Whilst the problems with RAAC may be wrongly linked to the material being inferior, we must recognize the importance of concrete and the fact that reinforced concrete structures have designed service-life. These may be shortened or extended depending on the exposure conditions and how the structure is maintained. Adequate funding for regular surveys and maintaining/retrofitting strategy is required to achieve the designed life.”

Dr Wei Tan, Senior Lecturer at Mechanical Engineering, Head of Mechanics of Composite Materials Group, Queen Mary University of London (QMUL), said:

What is Reinforced Autoclaved Aerated Concrete (RAAC)? When was it first used?

“Reinforced Autoclaved Aerated Concrete (RAAC) is a type of building material that combines autoclaved aerated concrete (AAC) with reinforcing materials, typically steel bars or mesh, to enhance its structural strength. AAC is a lightweight precast concrete material that contains numerous small air bubbles (like porous foams), making it lightweight and insulating (for sound and heat). RAAC was developed to improve the structural properties of AAC, making it suitable for load-bearing applications. The exact date of its first use is not readily available. It was used in buildings between the 1950s and 1990s.

How long have we known about issues of longevity and collapse? Would this have been known at the time of building?

“Issues related to the longevity and potential collapse of buildings constructed with AAC or RAAC have been known for some time. The specific concerns include deterioration of the reinforcing materials and AAC over time, especially when exposed to moisture or environmental factors. Moisture can not only deteriorate the concrete but also corrode the reinforced steel rebar. The knowledge of these issues may have varied depending on the region and the local construction industry. In some cases, it might not have been fully understood at the time of building, especially in older structures. However, with advancements in construction technology and research, these issues have become better understood over the years.

Was it a common building material outside of the UK?

“AAC and RAAC have been used as building materials in many countries around the world, not limited to the UK. AAC gained popularity due to its lightweight and insulating properties. Its prevalence as a common building material can vary by region and time period.

How dangerous are buildings built with this substance?

“The safety of buildings constructed with AAC or RAAC depends on various factors, including the quality of construction, maintenance, and the specific conditions they are exposed to. Over time, if not properly maintained, these materials can deteriorate, potentially leading to structural issues. However, not all buildings made with AAC or RAAC are inherently dangerous. It is crucial to assess the condition of each structure individually to determine any potential risks.

Do these buildings need to be completely demolished and rebuilt?

Whether a building constructed with AAC or RAAC needs to be completely demolished and rebuilt depends on the extent of the structural issues, local building codes, and the feasibility of making necessary repairs or reinforcements. In many cases, structural engineers can assess the condition and recommend appropriate remediation measures, which may include reinforcement or partial reconstruction.

Could more buildings be affected? What is the scale of this issue?

“The extent of this issue can vary by region and the prevalence of AAC or RAAC construction in different areas. It’s possible that more buildings could be affected, especially if they were constructed with these materials several decades ago and have not undergone regular maintenance or inspections. Assessing the scale of the issue would require thorough inspections and evaluations of buildings in question.

How concerning is this issue?

“The concern regarding buildings constructed with AAC or RAAC lies in their potential structural deterioration over time. It emphasises the importance of regular maintenance, structural assessments, and adherence to building codes to ensure the safety of occupants. The level of concern can vary depending on the condition and maintenance history of specific buildings.

Any other comments?

“The issue of RAAC-related risks in school buildings is a serious concern that demands immediate attention and action. Ensuring the safety of students and staff should be the top priority, followed by thorough inspections, necessary repairs or replacements, and long-term measures to prevent such issues in the future. This situation also serves as a reminder of the importance of maintaining and monitoring the structural integrity of public buildings to safeguard the well-being of those who use them.”

Prof Xiangming Zhou, Head of Department of Civil & Environmental Engineering, Brunel University London, said:

Can you explain what this sort of concrete this is?  

“RAAC represents Reinforced Autoclaved Aerated Concrete. The root cause of the RAAC that we see today is actually the Autoclaved Aerated Concrete (AAC) itself. AAC is cured under heat and pressure in an autoclave so it can gain strength quicker than under traditional curing conditions. Aerated means air is added to concrete to make it lighter. When rebars are used to reinforce AAC, we get RAAC. In this sense, RAAC is nothing different from the traditional reinforced concrete.  

When was it brought into use and why?

“After the 2^nd World War due to the lack of quality concrete and the demand for fast construction for recovery, especially from 1960 to early 1990s when there was a great push of the concrete industry from labour-intensive to industrialised. RAAC is lightweight, cheaper, and easy to mould into panels for floors and walls for precasting and modular construction. Due to autoclave curing, AAC gains strength quicker. Lightweight also means less load on building foundations, therefore further reducing the construction cost or requiring smaller columns that leave more space for usage. Indeed, RAAC does possess a feature superior to traditional concrete, that is it has a lower thermal conductivity coefficient so it is a good insulator and it likely has a higher thermal capacity than traditional concrete, all together meaning it is more energy efficient than traditional concrete. 

Why now is it risky? 

“Autoclaved Aerated Concrete’s aerated structure means higher porosity and bigger pores inside this type of concrete. It has a lower strength (compressive, flexural or tensile) and, therefore, has a lower load-carrying capacity and is easier to crack. Moistures can easily get into RAAC, and detrimental ions like chloride ions in moisture make it easier to reach the rebar’s surface, therefore causing corrosion. Corroded rebars will expand with corrosion products having a volume of a few times more than the original. Corrosion products will enforce tensile stress on surrounding concrete. Due to its much lower tensile stress, the aerated concrete is easier to crack. Besides, due to the lack of gravels, i.e. coarse aggregates, which are used for making traditional concrete, the bond between rebars and aerated concrete is much weaker than that between the rebars and traditional concrete with coarse aggregates. Autoclaved Aerated Concrete tends to be much more brittle than traditional concrete. Therefore, it gives less warning before a catastrophic failure happens or is observed. The service life of RAAC is generally between 30 to 50 years while that of traditional concrete can easily reach 50 to 100 years.   

Is there a way to make it safe other than rebuilding? 

“Short-termly, there may be some measures to repair and strengthen RAAC elements to extend their life, such as surface render to apply a dense cementitious coating layer to block surface voids and block the pathway of moisture penetrating to concrete body. Or use FRP (fibre-reinforced polymer) strips to retrofit and strengthen RAAC elements. Long-termly, due to the poor bond between rebars and AAC, it is better to rebuild those structural elements made of RAAC. 

Are there not laws and regulations protecting people from things like this? 

“The commonly used codes of practice of reinforced concrete design (RC) do not cover RAAC design. However, the industry has been mimicking the code of RC design for RAAC with minor modifications on safety factors, design parameters, detailing requirements, etc. largely based on experience. It has been notably realised that RAAC has a shorter service life than RC.     

Is it probably in other buildings too like homes and hospitals?  

“It is very likely that RAAC has been used for other buildings like industrial buildings, public buildings, hospitals, apartment buildings, leisure centres, etc., i.e. those built by the governments. In fact, AAC without reinforcement is still widely used for residential buildings as the inner leaf of the cavity wall and partition wall of blocks of residential towers and office buildings. It can be used generally safely for such non-load-bearing building elements. AAC and rebars do not make a good match for load-bearing building elements due to the reasons elaborated earlier. The problems we have seen from the schools recently are roof panels which are reinforced AAC panels. Those panels are horizontal elements and rely on rebars to hold concrete. As AAC has a porous structure that leads moisture with detrimental ions penetrating through the concrete body and reaching the rebars causing them to corrode, the RAAC panels lost their strength and the concrete was compressed to crack internally by expanding corroded rebars. 

Is there anything else important you’ve not heard mentioned? 

“It is important to note that AAC is still a good construction material for many purposes. The industry is still using AACs nowadays, largely for non-load-bearing partition walls. However, reinforcing AAC to make RAAC for load-bearing building elements, like traditional reinforced concrete elements, can be problematic.” 

Prof Alice Moncaster, Professor of Sustainable Construction, Open University, said:

“The main issue is that concrete gradually carbonates over many years when it is exposed to air, meaning it absorbs some of the CO2 from the air (CO2 having been originally released in the process of cement manufacture).  This reduces the pH of the concrete, leading to a higher risk of corrosion of any steel reinforcement (see Monteiro et al, 2012).  Where the concrete has been aerated the concrete is clearly more vulnerable to carbonation therefore. 

“Moisture also increases the risk of corrosion.  If school buildings and roofs have been poorly maintained due to lack of money, there are likely to be long-term damp problems in walls and ceilings which they weren’t originally designed for. 

“In addition, last winter saw some exceptionally heavy downfalls, one of the many examples of climate change affecting our weather patterns.  Schools, like other buildings, will have been built to the building codes of their era, which would have been suitable for the expected weather for the region back then.  Our changing climate means that many existing buildings are likely to need retrofitting to cope with the new climate.

Finally, most manufactured construction products have a ‘design life’ of around 50 years even under expected conditions, so it would be expected that schools built in the 1970s are now getting to the end of their life.”

Adrian Tagg, Associate Professor in Building Surveying at the University of Reading and Managing Director of Tech DD Ltd, said:

“RAAC planks are prefabricated lightweight concrete slabs typically used for the roof structure of single storey, low-cost buildings. Developed post WW2 and widely used in the 1960’s to 1980’s this is typical of the ‘experimental’ construction techniques evident in post war Britain. However, it is not just a UK issue as I’ve identified their widespread use in mainland Europe (particularly France and Belgium). We typically discuss the lifecycle of building materials or design life of buildings and concrete is no exception. However, if concealed and protected from the weather, concrete can last decades or even centuries. Reinforced concrete tends to decay when exposed to moisture and pollution, with RAAC, the highly porous nature of the material means that when it gets wet it absorbs moisture which increases the density as well as propagating corrosion of the steel reinforcement. While the decay itself may takes some time to manifest, the critical point of failure can be rapid and sudden.

“RAAC planks have been classed as a deleterious material for a number of years and the nature of their failure is well publicised in professional surveying or engineering journals. The buildings containing this are only dangerous if the planks are damaged and typically failure to maintain roofs permits water infiltration and can therefore enhance the risk of collapse. I’ve seen schemes designed to build an external supporting structure over such roofs to reverse deflection but this is a costly compromise. The best solution would be to replace the roofs but this becomes a cost / benefit exercise as a new roof on an older, likely poor energy performing building makes little long term strategic sense, particularly with a drive to net zero by 2050.

“The construction sector is littered with conceptual materials that have failed with time, a prime example being asbestos, even the relatively recent notion that aluminium composite materials (ACM’s) for cladding would improve existing concrete clad buildings was tragically exposed with the Grenfell fire. The UK is no doubt leading in the sense of surveying assets and adopting a safety first, risk averse approach. However, it is not leading in the terms of public sector investment in schools and proactive maintenance may have delayed or averted this current ‘crisis’. However, with a low tax economy compared to countries such as Belgium, it’s a struggle to see where investment will be made available for this essential reactive work. Compounded to this is the present construction sector skills shortage which makes planning remedial works almost impossible within the window of non-term time.

“There is no doubt a need to adopt a risk averse approach and surveyors will have led on this, it’s better to avoid a catastrophic roof collapse and any subsequent fatalities. But this defect is a dormant issue, common in other buildings of similar age and design. It can be mitigated by good proactive maintenance, however, as with asbestos it is necessary to eventually phase out and remove this defective material. Furthermore, and as with the fallout from Grenfell Tower and the need to replace dangerous cladding…ultimately who pays?”

Dr Theo Hanein, a cement scientist at the University of Sheffield, said: 

“RAAC is a type of concrete which is produced in a factory (pre-cast) and transported to the building site. It is most used for its light weight and thermal insulation characteristics. The production process means that it has much larger pores. These pores provide its desirable characteristics of being lightweight (so less dead weight in a structure) and good thermal insulators (the pores are filled with air and air is the best insulator). 

“However, these pores also make the concrete less durable as more water can fill these pores and foreign species such as CO2 from the air can react with the concrete and alter the chemical equilibrium; the mild steel then reacts with oxygen and/or water to form rust. The increase in volume of the steel can be up to 7 times the initial volume and this causes the concrete to crack. Generally, RAAC have a shorter lifetime than standard concrete, so problems can arise in the RAAC before the rest of the concrete parts (which have smaller pores) in a structure.”

Prof Phil Purnell, Professor of Materials and Structures, University of Leeds, said:

“RAAC is a foamed concrete that is “cooked” at high temperature to set the foam, reinforced with steel bars. The bars are not protected from corrosion as they are in normal reinforced concrete, so pieces of RAAC (normally “planks”) must be coated with bitumen or similar to prevent water getting in. When this coating goes because it’s not maintained, the plank starts to crack.

“We have known about the issues of longevity and collapse since about 1992. The BRE produced a report in 1996 that effectively killed any further use in the UK.

“This was a common building material outside the UK and still is. A new European standard for its use was issued in 2016, the trade association (EAACA) is based in Germany, and the best-known form is Aircrete in the Netherlands.

“The current buildings are no more dangerous than any other building that is not maintained properly. The problem here is that because roofs are not properly maintained, water gets in a corrodes the steel bars. However, the material can fail more suddenly than e.g. steel, timber or normal concrete with very little warning.

“These buildings do not necessarily need to be demolished and rebuilt. RAAC will only have been used for roofs and occasionally floors, not the main structural parts of the buildings. But the roof and floor planks/beams will need to be replaced if they are load bearing.

“More buildings may well be affected. RAAC was used extensively between about 1950 and 1990 in the UK for public buildings because it was cheap and could be imported as “ready made” roof planks. Many courts, hospitals, council buildings, university buildings etc used this material as well as schools. The material is well past its design lifetime in most cases; it should have been replaced like timber roof battens and rafters would be periodically replaced, but this was not done.

“This is a concerning situation, but the root cause is not the material, but poor maintenance and squeezed school budgets for building upkeep. Similar problems will exist for steel, timber and other roof material types. All materials deteriorate much more quickly if maintenance is skipped.”

https://educationhub.blog.gov.uk/2023/08/31/new-guidance-on-raac-in-education-settings/

Declared interests

Christian Stone: “We primarily work towards extending the lifetime of structures by managing and monitoring corrosion of steel within structures. We also undertake research projects and have over the last 5 years led research into corrosion in RAAC for an NHS trust and also worked as part of the Loughborough University RAAC research team doing research into 60’s and 70’s RAAC elements in the NHS.”

Prof Alice Moncaster: “No conflicts of interest.”

Adrian Tagg is an associate professor and practitioner undertaking Planned Preventative Maintenance surveys.

For all other experts, no reply to our request for DOIs was received.