Published: 7th December 2023

Construction sector should remember Raac is not the only problem concrete

By Richard Tucker, Associate Director, Green Box Thinking

Construction industry specifiers will be following the scandal concerning reinforced autoclaved aerated concrete (Raac) but shouldn’t forget that this is not the only material of its type to be avoided in the future.

In reality, the use of all forms of traditional concrete should be informed by whole life carbon analysis. This is because their manufacture and deployment generate substantial carbon emissions and proven, cost-effective, environmentally friendly substitute materials are now widely available.

Raac hit the headlines in September 2023, when the Department for Education told more than 100 English schools to shut buildings known to contain the material, as, alarmingly, it had been found to be at risk of sudden collapse.

Then, in late October, it was announced that 214 English schools and 42 hospitals had now been identified as housing Raac, while media reports suggested that at least 14 British entertainment venues had been forced to shut because the concrete had been found in their buildings too.

Green Box Thinking regards these revelations about Raac in public buildings as shocking and strongly supports remedial action being taken urgently, to prevent possible injuries or worse to groups such as pupils, patients, and theatregoers.

Construction sector should remember Raac is not the only problem concrete

As journalists such as Ed Conway, Sky News Economics and Data Editor, have reported, Raac was originally developed as a lightweight alternative to traditional concrete, as the latter is often too heavy for areas such as roofs. Raac typically consisted of ingredients such as cement, water, sand or fly ash and aluminium flake, thus avoiding some usual elements such as aggregates and stones.

The resulting mix was then cured in a steam steriliser. That produced a material filled with air bubbles, which was reinforced with steel bars, cast in factories into planks or panels and taken to building sites.

Construction sector should remember Raac is not the only problem concrete

Raac became popular with public building specifiers between the 1950s and 1990s for reasons including that, compared to most alternative materials, it cost less and shortened construction projects.

The material was cheaper, partly because the air bubbles meant it used less cement, usually the most expensive of concrete’s elements.

Raac also speeded up construction projects because, by coming ready-made, it removed the need for conventional on-site processes such as: pouring wet concrete, which then took several weeks to cure; installing shuttering, to support and stabilise this material before it became solid; and deploying steel cages to reinforce it.

Officially, Raac was therefore classed among the Modern Methods of Construction – though plenty of experts would now dispute that it fits the current understanding of the term.

Raac was also proven to be fire resistant and to offer effective thermal insulation. This meant that, although it was used mainly in roofs and sometimes in floors, it was also often seen as suitable for walls.

So far, so positive. But, as experts such as Alice Moncaster, Professor of Sustainable Construction, in the School of Architecture and Environment, at the University of the West of England, in Bristol, have pointed out, Raac has now become a major issue for several reasons. Some are related to climate change and present problems across the building stock, others affect all forms of traditional concrete, a third set of explanations relates to Raac’s composition, and a fourth concerns associated systemic and administrative failures.

Construction sector should remember Raac is not the only problem concrete

Firstly, the changing climate. All buildings are constructed according to codes based on their locations and expected weather patterns. However, no-one specifying Raac, say 30 years ago, will have foreseen the heavier rainfalls, which have led to increased failures of roofs and cladding systems, or drier summers, the cause of ground shrinking and foundations moving, which we now experience. It’s almost inevitable that these factors will have helped cause Raac to deteriorate in some buildings.

Secondly, Raac, like all concretes, is subject to a process called carbonation. This means it absorbs carbon dioxide from the air after installation, which increases its acidity and reduces its pH value, leading to a higher risk of corrosion, which produces cracking in its microstructure.

All traditional concretes are also brittle and can fail suddenly, sometimes with catastrophic results. In contrast, many other common construction elements are more likely to deteriorate slowly and more visibly, therefore providing greater opportunities for remedial action.

Raac-specific reasons for the current crisis include the material being much less heavy than traditional concrete. This made Raac alluring for some applications but also meant it was significantly weaker than many alternatives, especially if water entered and corroded the steel bars, which was not uncommon, as it was often used in flat roofs.

Raac was also found over time to have a lifespan totalling only about 30 years at best.

On top of all this, investigators wrestling with the current crisis often find it difficult to confirm Raac’s presence until it actually starts to crack, as it looks virtually identical to other forms of concrete.

All these flaws have been compounded by some systemic and administrative failings. The choice of materials for public buildings in recent decades has often been left to contractors, who knew their liabilities were usually limited to one year for defects and 12 years at most for overall designs, for example. This meant there was little downside for them in saving cost and time by specifying materials such as Raac.

In addition, few school buildings have been replaced, and existing constructions have often been poorly maintained, since the last Labour government’s Building Schools for the Future programme. This neglect has provided opportunities for problems to occur such as rain entering structures containing Raac and causing long-term damp problems.

Construction sector should remember Raac is not the only problem concrete

We hope very much that where Raac is identified in public buildings, it’s not replaced by traditional concrete without consideration of whole life carbon impacts. This is because of facts such as making cement alone accounts, according to some estimates, for as much as eight per cent of global carbon emissions. If you want to see that in context, bear in mind the much-criticised aviation sector is thought responsible for less than three per cent.

In addition, many low-carbon substitutes for traditional concrete are now widely available. Some are wholly original – such as Hempcrete, formed by wet-mixing hemp shiv with a lime binder – while others involve amending concrete’s preparation or changing its formulation, through increasing recycled aggregate content, for example.

It’s also now outdated thinking for professionals to continue regarding all such low-carbon substitutes as new and unproven.

There’s one variety of low-carbon concrete, made using alkali activated cement, for example, which has been used safely literally for decades.

Nonetheless, the “we’ve always done it this way” mindset among many construction industry specifiers remains a problem. Groups like architects and developers – whose caution is understandable up to a point – often continue to favour ingredients, such as traditional concrete, of which we have literally centuries of experience and which they see as proven to be durable and secure.

But it’s also a fact that, with the built environment supplying almost 40 per cent of the UK’s carbon emissions, largely due to the use of traditional cement, the construction industry must cut its discharges dramatically and quickly if the country’s legally binding target of being net zero by 2050 is not to become Mission Impossible.

So, what are the overall lessons of the Raac crisis? One is that it arose largely because of historical verdicts based on short term cash savings. To ensure we don’t find ourselves in a similar position in another 30 or 40 years, government and industry need to make decisions based on whole life thinking. This may cost more in the immediate term but will lead to savings when considering buildings’ expected lifespans.

Another takeout of the Raac crisis is the vivid reminder it supplies about the importance of retrofitting buildings generally, not just to replace dangerous materials but, given the current climate emergency, to reduce their embodied carbon emissions. We say much more about retrofitting in another dedicated post on this site.