The Lemon Fool

I comment only from an uneducated position but I seem to recall a similar level of concern about HAC (high alumina concrete) in the late '60s~70s as I was moving house at that time and an elderly family were wanting to buy my house financed from the sale of two flats.
Their sales were thwarted by the threat of their flats maybe having been built using HAC.
Nearly 50 years on those flats seem to be still standing and occupied though I've not checked wrt any transactions within those low-rise blocks.

gnawsome wrote:I comment only from an uneducated position but I seem to recall a similar level of concern about HAC (high alumina concrete) in the late '60s~70s as I was moving house at that time and an elderly family were wanting to buy my house financed from the sale of two flats.
Their sales were thwarted by the threat of their flats maybe having been built using HAC.
Nearly 50 years on those flats seem to be still standing and occupied though I've not checked wrt any transactions within those low-rise blocks.

On the other hand, a few roof structures appear to have collapsed in the last five years or so in schools etc.

The bit that puzzles me is cement is pretty stable once set so why is RAAC turning into a problem after 50 years? What changes with the passage of decades?

They seemed to know about it from the start too, hence the (reported) 'design life' of 20 to 50 years.

And another thing. RAAC was sold as non-structural roof panels apparently, but the collapses I've read about were described as "beams". Beams are by definition, structural supports.

Mike4 wrote:The bit that puzzles me is cement is pretty stable once set so why is RAAC turning into a problem after 50 years? What changes with the passage of decades?

They seemed to know about it from the start too, hence the (reported) 'design life' of 20 to 50 years.

And another thing. RAAC was sold as non-structural roof panels apparently, but the collapses I've read about were described as "beams". Beams are by definition, structural supports.

I wonder if subsequently loads have been applied to those panels? I think the High Alumina cement is a different problem, because a feature of the RAAC is the proliferation of bubbles, combined with the use of lightweight aggregate, like fly ash. This gave low panel weight.

Solutions range from support below the panels to replacement. Quick and dirty or a lasting cure? As I understand it, alarm was first raised by material falling inside a school. Maybe introduction of a supporting layer, maybe GRP panels, would be adequate?

A report from two years ago, about the RAAC problems in hospitals and schools, and about how property administrators were being called upon to conduct checks. It's still a good question as to why this problem is so new in 2023 that it's come as a last-minute shock to the authorities, just as the kids are going back to school?

https://www.newcivilengineer.com/latest ... 5-08-2021/

I'm not an engineer by any stretch, but the gist of this article seems to be that RAAC problems have some common ground with the high alumina cement issue, even though they had different ways of affecting strength. (HAC was hard all right, but it could go crystalline after contact with water and then shatter, which was how bridges could be weakened.) The communal problem, it says, is that designers should have amended their specifications (i.e. made beams etc thicker), and that contractors for their part had often skimped on quality.

And, we could add, that nobody in authority took responsibility. One thing I did read, though, is that the UK has generally made far less use of RAAC than much of continental Europe. It would be interesting to know whether the French or the Germans employed different construction techniques with the new wonder material?

BJ