Is it the same or simlliar to what used to be known as Concrete Cancer? The steel reinforcement within the concrete rusts and expands thus destroying the cement, so I would guess it can't really be stabilised.

On this mornings news it was stated that the lifespan of this material is about thirty years, that leaves many questions to be answered.

Romans used mass concrete, no steel rebars. This lasts for a very long time if you get the chemistry right. The Glenfinnan_Viaduct in Scotland is over 100 years old and still going strong. The Romans were doubly clever, with big arched structures like the Pantheon they embedded amphorae in the concrete to reduce the mass.

Conventional reinforced concrete has steel embedded in it. Eventually the steel starts to corrode and the concrete spalls off. Lifetime depends on how deeply the steel is embedded in the concrete. Typically 50 to 100 years. See this for a picture

The cement used in RAAC is porous and so the steel corrodes much more quickly. It's not 'concrete' as there is no aggregate, they add an agent which causes bubbles to form in the final product, so it is lightweight. That's why they used it.

Problem is that when used in roof spans, the reinforcement is at the bottom and the underside is hidden behind cladding, so it goes un-noticed until it fails catastrophically

Edited By duncan webster on 02/09/2023 15:18:27

According to the Times (so it must be right) this stuff is far more common in mainland Europe. The stuff itself is fine as long as it's kept dry, but even then it had a design life of 30 years. Government has known it's a problem for years, but kept on sweeping it under the carpet. No doubt we will now have a panic reaction and waste a lot more money than it would have cost to do the maintainance, and to replace in a planned phased manner.

Building like the Atlantic wall, ie over engineering, would not have been sensible. Do you want 50 schools which last 30 years, or one that lasts 100 years. They probably wanted some windows as well, which encourages lightweight roof construction

Edited By duncan webster on 02/09/2023 18:54:58

Exactly the same happened with Spaghetti junction flyovers which involve 559 concrete columns being replaced, also large overhead sections of the M5 at Oldbury near West Bromwich caused long delays for many months

Normal load-bearing concrete has a compressive strength of around 40 Newtons per square mm. This stuff never had any substantial compressive strength – 4Newtons per mm^2 – so should really only have been used as supported panelling.

It would appear that its light weight was too much of a tempting idea to use it – such that the supporting structure could also be down-specced. IMO, it should never have been used as a ‘roofing material’.

Your typical garage base would be made with ‘20N’ concrete – and need to be 150mm thick, probably with some form of reinforcement in the base – either a layer of weld mesh, or stainless steel needles (or even plastic fibres) in the mix) if expected to be loaded heavily. That concrete would be laid over a well compacted sub-base, too.

RAAC concrete – if one could call it concrete – was not only lightweight (more like a mortar sponge) but was also much cheaper to produce (the virtual final strength was achieved in less than a day compared to a month for normal concrete). Normal concrete would be made based on the 28 day strength, but would continue to slowly gain strength – possibly over several years.

One thing that is not suspect is the cement. Cement, made to BS12 was perfectly adequate for the purposes for which it was specified, in structural concrete. Your average sectional concrete garage panels are stronger than this stuff. I never came across this stuff and cement was proper cement until the industry started to blend in things, such as fly ash, in the 1970s (to compete with cheap imports from Europe).

This problem is down to architects along with cheap building design and construction. What do they say? Buy cheap, buy twice. It is coming home, now….

The compressive strength of RAAC structures is exactly the same as when they were built. There is no problem with this. What has become a problem is that the re-bar used for RAAC beams in tension is liable to corrosion because it is mild steel without any protection from moisture ingress. Light weght concrete blocks are a large part of builder's' armory across Europe. Relying on mild steel rebar to give long-term tensile strength without protection from wet, not so much.

Many thanks to OpenOffice Writer for the spelling checks, which have battled with the Late Bottled Vintage Port

Evidently .. the Science continues: **LINK**

https://www.sciencedirect.com/science/article/abs/pii/S2352710221009645

MichaelG.

John,

You are probably right to identify the different mechanisms by which the water ingress starts, but the ‘bursting’ mechanism is similar for both products … Rust has a bigger volume that the metal it replaces.

MichaelG.

the US put out and alert for a roofing concrete used in public building of the eighty's……..

Wythenshawe council did nothing for years until a child was hit or nearly when a big lump fell out of the roof of the local swimming pool……

they had just spent 100's of tousands upgrading it……

stayed empty for a few years before demolition……

Have heard in maj construction like bridges etc they are now using ST/Steel rebar…..????.

when u drive around look at the conc bridges etc…..most have at least a meter of Bitumen painted at the bottom…

To help protect them……

a good salt water soaking does them a lot of harm……

Mind it keeps the money flowing to those at the top of the gravey train………