Friday 8 September 2023

CRAAC propagation

I failed my Physics "O" Level, not least because I live in my head rather than in the real world of materials and how they interact together. I can hammer a nail; but not with 100% reliability; I just confessed to be unable to drive 3 screws out of 12; I have installed sheep-wire upside-down because I haven't a clue. I like bodging (chairs from tea-chests;wooden-tops) but wouldn't trust me to make something structural

It's not that I don't try. A few years ago The Beloved sent me on a dry-stone walling course up in Westmeath. It was a piece of pee! we had a shed full of limestone slabs which are sedimentary rocks so flat on parallel sides. The one rule of thumb was to ensure never to have two joints between rocks one above the other. If you do, it makes that part of the wall intrinsically less stable. Overlapping the joints - like in regular brick-work - prevents cracks propagating through the structure. Building with granite is much harder [ho ho! granite is 6-7 on the Mohs scale; limestone only 3-4] because parallel faces are a matter of luck [or trimming] rather than due to the inherent molecular structure. See [R] how a crack has propagated down about 1m from the wall-plate of one of our shed until blocked by a sufficiently long & hefty stone [under the ] in the course below.

The same principal plays out on a smaller scale in concrete which is a mix of sand, cement and aggregate. The chunks in the aggregate serve the same purpose in concrete as stones in walls. In amorphous solids like glass or aluminum sheet, small cracks will propagate across the whole sheet over time. One emergency way to halt this is to drill a hole at the end of the crack which dissipates the energy which is sundering the material.

All that is back-story for a lamentable fiasco across the water in England. After 13 years of Tory misrule, it has come to the attention of the government right at the start of the school year that many of the country's school are unsafe for children. The reason is the widespread use of RAAC reinforced autoclaved aerated concrete slabs in the construction of schools in the 50s-80s of the last century. RAAC is cheap-and-cheerful and aggregate-free and so is more likely to fail. It has/had a design shelf-life of 30 years anyway, and it is 30 years since RAAC was deprecated by engineers and replaced by regular concrete in the 1990s. So any school, any building, which still has RAAC is out of warranty.

Failure could be catastrophic cracks through the whole beam or it could 'only' involve spalling [prev] where chunks of concrete shear off the ceiling and rain down on the desks. The reality of this problem was highlighted by an actual collapse at Singlewell School in Kent 5 years ago. It goes against Tory ideology to properly resource state schools because only proles go there. Resourcing includes regular inspections to determine whether the buildings are fit for purpose. The Irish government is stoutly maintaining that RAAC is not present in any schools in the Republic. 

Much gleeful RAAC schadenfreude rained down on Education Secretary Gillian Keegan who wanted a medal for closing school before she had to explain away a repeat of Aberfan 1966. She didn't actually want a medal because that's only for soldiers. Her rhetorical question was: 'Does anyone ever say: you know what, you've done a fucking good job because everyone else has sat on their arse and done nothing?'. imo, Keegan is by no means the worst cabinet minister in London.

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