If you were to read this BBC report on RAAC (Reinforced AAC panels), you could be forgiven for thinking that RAAC is a terrible, unsafe building material that is coming for your children. You could be forgiven for thinking your incompetent government has been hiding its "risks" since "1961" - the horror! Generations of nefarious greed (because it is "cheaper," "quicker," and "easier," even though it is "weaker," "crumbling," and "less durable"), thus putting public safety at risk! It's even being linked to the ancient nightmare, asbestos, in the article that provided the featured photo (copyrighted to NORAC, 2024).
Let's start with the fact that there's no asbestos in RAAC. The concern would be that if repairs have to be made, it could disturb other materials that contain asbestos. That's true of any repair of any building material and really isn't the fault of RAAC. Maybe just don't use asbestos? That's not to say any concrete or rock won't also produce dust you don't want to breathe (e.g., silicosis), but dust from cured concrete isn't much of a concern if you leave it alone. In perspective, you're currently eating a credit card's worth of plastic every week, your body is riddled with PFAS chemicals... there's a lot bigger problems to worry about.
So back to the UK's crisis. I've heard a lot of hysteria coming from this little island (and one science center in Canada) about RAAC. So is the sky (uh, roof...) falling?
Let's address the biggest fear first. Yes, it is weaker than regular concrete. Probably by a factor of 10! So instead of 34 MPa (5k psi), residential AAC may be 4 MPa (5.8k psi). Quick! What's the MPa of a frame wall? Wouldn't it be better to make a house with a material that has 10,000 MPa? How much MPa do you need? Can we agree that weaker doesn't mean bad as long as it's strong enough to do what it needs to do?
Second, the strength of any concrete, including AAC and RAAC, is in compression. But all the RAAC that we're concerned about are panels that make the roof, which are mainly in tension. While the term RAAC is developing a reputation of rotted garbage, not a single wall panel (or even floor panel, which is also in tension!) has failed. Our issue is water.
As the BBC article states, "...RAAC is often coated with another material, such as bitumen..." Note: every other type of concrete and frame roof is also coated with something like bitumen. The article then says bitumen "can also degrade." Incorrect. It will degrade. If there's any scandal at all here, it's that apparently these buildings weren't maintained. Can someone in the construction industry please chime in with what happens if you never maintain a flat roof??
All concrete can spall and expose its steel reinforcement to corrosion. Most bridges (made with the high! strength! normal! concrete!) are given 75 year lifespans in the US. Nothing is forever. And yes, it's sometimes harder to detect spalling in RAAC (not good) because all AAC is just so. dang. good. at absorbing and localizing impacts. Regular concrete has capillaries (long tubes) that allow for deep water penetration (and cracking). AAC has bubbles and their "shells" create a matrix that resists water penetration. So saying, "the bubbles allow water to enter the material" is bonkers. Yes, water will eventually get in, but do you know anything about regular concrete, where water penetration is much worse?
A little simple math to perhaps reduce the panic. All materials have a service life. I have seen this "30 year" lifespan tossed around by many sources, including structural engineers in England & Canada. I have been unable to find a source for this claim. Apparently, one manufacturer said this at some point?
I hardly think the UK government made a bad decision choosing RAAC if that's the case. RAAC has been used in the UK since the 1950s. Consider a panel installed in 1975. That panel is now 50 years old. And there has been one failure across hundreds, probably thousands, of buildings that use RAAC as their roof. Some panels are 70 years old, others 35. They quickly and inexpensively rebuilt a country devastated by war. I would say they've done a good job. It seems a little crazy to say they last 30 years when your own reporting is evidence they last much longer.
I'm not saying I would recommend them. I like materials that are designed for their purpose. If I need a material that can handle compression, I like concrete & rock. If I need a material for spans (i.e., compression & tension), I like steel or wood trusses.
If you want a reasoned and non-hysterical take on the situation, I recommend this podcast from Engineering Matters (episode 236 on Spotify).
All buildings need to be maintained. If you want to build something that will last 500 years, don't build in the US. Some RAAC will have to be replaced in the near future because the buildings were poorly maintained. Other RAAC will have to be replaced in the next decades because nothing lasts forever and it's a weaker concrete. It's being asked to do something that no modern concrete does: cover spans (yes, the strength of concrete covering a span is coming from its internal reinforcement, not the concrete. And for the sake of all that's holy, if you want it to last, stop using steel, which corrodes).
No, this does not mean that AAC should not be used as a building material. For many builds, AAC is the far superior material. Don't transmogrify well-known limitations of RAAC into misplaced criticisms of AAC. Everyone stay in your lane!
(Apologies to the BBC, journalists can't be expected to be experts on everything, but the engineers who are grinding an axe and failing to put reality into terms a layperson can understand, who are failing to provide context? No apology will be forthcoming.)
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