Fears over concrete ‘time bomb’ should not stop search for new building materials | Ed Conway
The chances are that, at least up until recently, you hadn’t heard of reinforced autoclaved aerated concrete (RAAC).
Despite having written an entire chapter about cement and concrete in Material World, I confess I was completely unaware of this somewhat obscure type of concrete up until recent months.
But when RAAC began to hit the headlines in the UK recently, my ears pricked up. It transpires that a lot of municipal buildings, notably a fair few English schools, have rather a lot of RAAC in their structures.
And this RAAC has been associated with a risk of sudden collapses of classroom roofs in more than 100 schools – with the upshot that quite a lot of schools are being closed.
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Now in some senses, this episode isn’t ultimately about concrete at all but actually about something far deeper – a broader failure of policy.
Why on earth is this decision landing only days before schools are due to reopen? What does it say about public administration given we have known about problems with this stuff since the 1990s? How emblematic is this of the poor state of public buildings more widely, and the lack of capital investment in state infrastructure?
However, it also illustrates a couple of other points. The first is one I mention in chapter 3 of Material World – that the “hangover left by poor concrete construction is only just beginning”.
From RAAC to poorly-mixed concrete to corroded reinforcement bars that cause highway bridges to collapse, we are about to have to reckon with rather a lot of this kind of thing – especially in China, where record amounts of concrete have been poured in recent years.
But there’s another more unexpected lesson which is also worth teasing out: the RAAC episode illustrates how and why addressing climate change will be trickier than you might have thought.
Before we get to that though, a brief primer on what RAAC actually is.
It’s a kind of “liquid stone” which you make by adding cement to sand and aggregates and mixing it with water. The cement acts as a sort of glue which holds those pieces of sand and gravel in place, creating a very strong form of stone which you can mould into pretty much any shape you fancy.
Add some steel reinforcement bars and voila, you have an incredibly effective building material which is also wonderfully cheap and pretty easy to make.
There are a few problems with plain vanilla concrete. One is that making cement – that glue at the heart of concrete – involves lots of carbon emissions which are very hard to mitigate. By some estimates, cement production accounts for a whopping 8% of global carbon emissions, which is a staggering figure – to put it into perspective, aviation accounts for roughly 2.5%.
And while there are some interesting low-carbon solutions being prototyped, none has been deployed yet at large scale. We pour tremendous amounts of cement worldwide – enough each year to cover the entire landmass of England – and pretty much all of this is old-fashioned cement, which derives from a recipe which goes back about two centuries.
Another issue with concrete is that it’s quite heavy, mostly because of all that stone and sand inside it. The upshot is that while concrete is excellent for many functions, it’s less good for lightweight uses in, for instance, roofs.
And this is where RAAC comes in. Developed in Sweden in the 1920s, autoclaved aerated concrete (AAC) is a super-lightweight version of concrete. Rather than adding stones and aggregate to cement, you mix it with a bit of fine sand or fly ash and water, and then add some aluminium flake to the concoction.
The flakes generate gas bubbles throughout the mixture and once you’ve cured the resulting block in an autoclave – a kind of giant pressure cooker – you get a kind of concrete with lots of bubbles inside, a little like a bar of Aero chocolate.
Add some steel reinforcement bars and you have reinforced autoclaved aerated concrete (RAAC), the variant in all these British schools.
AAC and RAAC are much less strong than normal concrete (actually in some senses AAC isn’t really concrete at all since it contains no stone aggregates) but they’re also far, far lighter, about a fifth of the weight of standard concrete.
Indeed, it’s light enough in some formulations to float in water. So in the decades after, AAC became a very popular type of building block used in many roofs and also, since it is fireproof and an excellent thermal insulator, for some walls.
AAC and RAAC became very voguish building materials in Europe in the middle of the 20th century.
In particular, a lot of flat roofs were made using RAAC, but then in the 1990s structural engineers discovered that many of the RAAC planks (they were mostly fashioned into planks) used in Britain were losing their strength – especially when they came into contact with standing water.
All of a sudden thousands of buildings had a far shorter life than everyone had assumed when they were being built.
Now, it looks as though these problems with British RAAC don’t seem to extend to all types of aerated concrete. Indeed, while this stuff has fallen out of favour in this country, it’s still used widely elsewhere and is often cited as an important building material for the future. Even so, it’s a reminder of one of the most important (but most frequently overlooked) points about concrete.
One of the reasons why concrete has become such an important building material is that we now have centuries of experience in using it and, just as important, century-old buildings we can point to to reassure ourselves that this material can stand the test of time.
When you build something, you generally want it to last more than a few years, and we know from experience that, provided it’s well put together, plain vanilla concrete will last for a long time.
Indeed, the roof of the Pantheon in Rome is still standing after nearly two millennia; there are Portland cement structures still doing their job after more than a century and a half (a good example being some of the Victorian sewage infrastructure in London).
This is what brings us back to that thesis about fixing climate change. There are many exciting formulations for new low carbon concretes being tested. Some involve fiddling with the formulation of cement, some entail a change to the way that concrete is prepared and some involve entirely new recipes. There are promising trials under way, many of them in laboratories.
However, it’s very difficult to know in advance with absolute certainty which of these recipes will deliver genuinely long-lasting structures.
This might seem like a trivial point but success in a laboratory is only one part of the solution to finding a low-carbon version of cement. No architect or builder will opt for a variety of concrete which hasn’t shown an ability to withstand time and the elements.
Traditional concrete – for all its shortcomings – is at least a known, tested building material. New recipes are viewed with understandable caution.
One of the reasons why cement scientists are so interested in a particular variety of low-carbon concrete, made using alkali activated cement, is not just because of the chemistry of the cement itself but because people were making buildings out of it decades ago – so we have decades of proof that buildings made out of alkali-activated cements will actually stay standing.
Ironically, since many of these buildings are in Ukraine, the main risk to them has not been sudden collapse due to a failure of their structural integrity, but Russian missile attacks.
So, while the RAAC episode is in part an example of maladministration and in part an example of what happens when shoddy building work goes wrong, it’s also something else.
It’s a reminder that new recipes of how to make concrete are also likely to carry their own risks – that often we don’t discover the downsides and risks of certain building materials until decades after we’ve coated the country in them.
It underlines the fact that even clever advances in technology can have unexpected side effects that cause problems many years down the line. Today’s wonder material can often become a ticking time bomb.
None of this should deter us from trying to find an alternative to cheap, carbon-emitting conventional concrete. But it’s a pragmatic piece of logic we would do well not to forget.