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Local Government, Housing and Planning Committee 03 October 2023

03 Oct 2023 · S6 · Local Government, Housing and Planning Committee

Item of business

Reinforced Autoclaved Aerated Concrete

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I will start on the material, and then pass over to Martin Liddell to talk about how it impacts on structural design, which is a different field. The name “RAAC” is a good one, bar the “concrete” bit, which is misleading. For something to be called concrete, you need aggregate in there, which is the big 10mm or 20mm stones. RAAC does not have those, so it is not really concrete; it is really a mortar. It has fine sand in there. It is aerated, which means that it has air in there, which is fine, because that means that it is a very good insulator. For a builder, it is lightweight and great to build with, and it is cheaper and easy to transport. The “autoclaved” bit is just about how the material is manufactured, which is in a pressure cooker. You literally cook this stuff at temperature and under pressure, which means that it gains strength more quickly and can be manufactured more quickly. It is reinforced, which back then involved mild steel bars, which I will come back to. Then there is the kind of fancy ingredient. It has cement in there, just like normal concrete and mortar, but it also has aluminium powder that, under temperature, creates hydrogen gas and the material expands. I remember that, at school, we dropped aluminium into water and found that it fizzes and gives off a gas. The underlying chemistry is very basic and is similar to that of normal cement and concrete. There is nothing too fancy and there are no magic ingredients. However, from a materials property point of view, the big difference is that the AAC—the concrete bit—is very low strength, at about 3 or 4 megapascals, whereas normal concrete is around 30 to 40 megapascals. Therefore, we could say that is about 10 or 15 per cent of the strength of normal-strength concrete. However, before you get scared and think, “How could this be possible?”, I point out that that is okay, because the structural engineers type those numbers into their calculations. We know its strength, and that is fine. For me, it is similar to wood, which is really weak—you can snap a pencil or a ruler in your hand. That is fine, because those are the numbers that we use for the design. The reinforcement is the basic mild steel that is used in traditional reinforced concrete. However, it has a coating on it, because one problem with the aerated nature is that water can get through it quite easily. The material can soak up water, which adds to its self-weight. That is a problem, but the water can also get to the steel in the middle, and I guess that we all know that steel, water and oxygen are not a good combination and can create rust. The bars are coated with a fine waterproof coating when they are manufactured. In most cases, we have found that the coating is still pretty intact after 40 years or so but, in some cases, it has deteriorated, cracked and come away a bit, so the bars can rust a little. I will pause there and pass over to Martin Liddell, who can maybe comment on how that affects the structural engineering or the design.

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Local Government, Housing and Planning Committee 03 October 2023

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