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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

Martin Liddell Watch on SPTV

The low-strength nature of the material means that it behaves very differently, in two ways. The first is to do with the way that the reinforcement gets anchorage at the end bearing of the plank. Normal concrete does that by bond between the steel and the concrete, but the very weak nature of this material means that it cannot achieve that. That was known in the 1950s and 1960s. A transverse bar therefore needs to be welded on to the end of the reinforcing cages to anchor the reinforcement, and the transverse bar needs to be positioned over the support for the panel. One of the biggest risks associated with RAAC goes back to the time of construction, when panels were allowed to have 45mm end bearings. In other words, they had only a very short bearing length, and the transverse bar had to be positioned over the top. The biggest risk that we are finding is that, due to manufacturing control in the 1950s, the 1960s and even the 1970s, those transverse bars were sometimes not positioned correctly; indeed, in rare cases, they are not there at all. Moreover, during the same period in construction control, the panels were not given the end bearings that they were meant to have, which means that, in some instances, the transverse bar can be in front of the bearing rather than over it. According to the research, that does not affect the panel’s strength very much, but if the panel fails, the failure is not a ductile failure, where it just slowly sags and deflects until it fails, but a brittle failure mechanism. It can therefore fail very suddenly and that is what instigated the SCOSS report. Someone has corrected me; that report came out in 2019—I think that I might have said an earlier year—after a failure at a school. Subsequent to that, a handful of failures have been identified; in some cases, the panels crashed to the floor, while in others, they were caught on something below them that stopped them actually failing. That is the biggest difference between RAAC and concrete and the biggest risk associated with RAAC. The other thing that affects RAAC is how it ages. As Chris Goodier pointed out, RAAC does not protect the reinforcement from corrosion as normal concrete does. That was taken care of during manufacture—the issue was known about at the time of manufacture—with a coating that was put on to the reinforcement before it was cast into the panels. With time, however, that coating is breaking down and we are finding in quite a lot of locations that that is allowing corrosion to develop. Corrosion has two impacts on panels. First, it can actually push out the cover concrete and lead to the panels spalling, which is where parts of the cover concrete over the reinforcement fall away from the rest of the concrete. That, in itself, presents a risk to people, but the spalling process can affect the bond between the reinforcement and the concrete, too. Where the bond strength might be very weak to begin with, that reduction in bond can lead to concerns about reduction in strength of the panels. I am sorry—that was quite a long answer. Does it help to answer your question?

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Local Government, Housing and Planning Committee 03 October 2023

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