Is RAAC a safe building material?

Some engineers argue it can be useful in construction projects, if used properly...
08 September 2023

Interview with 

Chris Goodier, ISE, Loughborough University


Aerial view of square spiral staircase with metal mesh


Reinforced autoclaved aerated concrete - or RAAC, for short - was used as a cheap, lightweight building material between the 1950s and 1990s. Many public buildings across the UK contain it - but questions have been raised this week as to its safety. Some schools containing RAAC have been forced to shut their doors over fears that it poses a risk to children and their teachers. Hospital bosses have also warned that they are ready to evacuate workers and patients in the event that any weakened concrete shows signs that it is at risk of collapse.

Chris Goodier is a professor in construction engineering and materials at Loughborough University and has been conducting research on RAAC.

Chris G - RAAC is actually quite well named in that every letter is important. So it's reinforced - it's got steel reinforcement in it - and that's what gives it the structural capacity or the strength. It's autoclaved, which means uniquely it's baked at pressure and temperature in an oven when they make it. Aerated, it means it's got air in it, which makes it lightweight and a good insulator. And it's concrete, so there's nothing particularly different or fancy or strange about it from a concrete point of view. It's got cement in there and it's the cement that makes it hard.

James - You would suggest that it has some particular weaknesses as opposed to traditional concrete when it comes to exposure to the elements. And that's the root of why alarm has been sounded in some of the public buildings in this country.

Chris G - Yes, to a degree. So I probably need to set a couple of things straight. So you are right on that point. Because it's aerated, it can soak up water more than traditional concrete. And if left unattended or unaddressed, it can get to the steel inside sometimes and start to corrode that. It's a very low strength compared to traditional concrete and that is fine. So when you're designing it and you're building it, you know it's got low strength. It's the steel reinforcement that gives it the strength. So the fact that you can crumble it and break it quite easily is fine. Similar, I describe, to wood and timber, which we build lots of our homes out of. You can break a piece of wood over your knee if it's thin enough. The other thing is the 30 year design life, this is also something that isn't true but has got out there in everyone's consciousness and they're talking about it. No building lasts forever and no building material lasts forever; steel, concrete, timber, bricks. So we design them with what we call a typical design life of 30 to 40 years. That means we expect, around the 30-40 year mark, we need to look after them, maintain them. And if we want them to last longer, probably remediate them, repair them, prolong their life a bit. This has kind of got out there that it lasts for 30 years and then at year 31 it collapses. But that is simply not true.

James - I think that's the key point, isn't it? It's not so much the issue with the material, but the way it's been used or left unattended. Why has it all come to a head right now and why are we scrambling to survey not just schools and hospitals, but now courts and even parliament?

Chris G - There's a couple of reasons. The country in general, and most countries, don't spend enough on maintaining their infrastructure and their buildings. The maintenance of a building is a fairly easy thing to reduce the budget on short term because, for the next six months to a year, people wouldn't really notice. If you do this year on year on decades, then you will notice. And instead of just spending a bit every year, it gets to a point where you have to spend a lot. And then we found in our research that, in some of the RAAC panels, the biggest worry is that they can look okay. So when you look up at the ceiling, that looks fine. You can't see many cracks, but it looks okay. But 50 years ago it was possibly manufactured poorly or installed poorly by the builders. Poor building is no new news story. Every country in the world, you'll find some builders who don't do it as well as they could. So this happened with RAAC and it's gone unseen and unknown about until now. But, in our investigations there is some RAAC out there that a structural engineer considers critically unsafe.

James - And you made the key concern apparent there, which is you can't always tell from looking at it. How do we address this in the short term or what's the timeline for when all that critically unsafe RAAC will be identified?

Chris G - Million dollar question in a way. Hospitals now are in a good place, but there's a relatively small amount of hospitals. Schools, 22,000 as we know, with a big variety of location ownership and building types. So that's a very different challenge. They're making progress. Maybe not fast enough, but they're getting there. But then you've got all the other government departments, including the Commonwealth office tomorrow, the MoD I talked to, you mentioned justice, police. I've been to an ambulance station that had RAAC in the roof. And then there's private sector offices and factories and commercial properties from the 60's and 70's. So, timeline, this will be years. I think we we're moving to a situation of living with RAAC as a country, learning how to look after it, because we can't get rid of it quickly and it'll be years before we probably get around the whole of the estate.

James - I think it's probably fair to say, you might disagree, that we've offered a lukewarm defense of RAAC as a building material. It's something that, in the right context, can be used very safely. But, it has stopped being used. You don't suppose it'll ever be making return as a building material or start being used again? Or maybe I'm ignorant and it still is being used,?

Chris G - I'll boldy say it never went away. We got worried about it in the 80's in the UK, but it's still manufactured and used all around the world. In fact, I'm reporting here from Prague in Czechia. So I'm attending the seventh international conference on AAC (autoclave areated concrete.) There's people all around the world here, including China, India, Indonesia, manufacturing RAAC and installing it now, still. Thousands of factories around the world. So it comes back to where I started. There's nothing wrong with it as a building approach. You design it properly, manufacture it properly, and install it properly, it'll be fine. Cut corners, cut costs, do it wrong, it can fail. Coming back to the UK, I think we've gone too far now with public perception that you'd have to be a very bold builder to try and do it because no one would want you to, I think.


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