The crumbly concrete problem

We all know about this from the UK news so I thought I would see if our (your) combined knowledge could save the day for crumbling Britain

I noticed that the concrete allows water ingress which can make the bars rusty and cause issues

So does this not mean you can drill a hole and pump a hardening agent right into the porous structure and give it a new lease of life?

(They've only had 50 years to figure it out )

Edited By Ady1 on 02/09/2023 14:27:31

Roman concrete seems to last a bit longer than the modern stuff, more than 2000 years longer in some cases.

Mike

Edited By Mike Poole on 02/09/2023 14:39:51

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.

You really couldn't make some of this stuff up, was it a case of 'cheap' construction today and in 50 years time we will not be around to fix the almighty mess££££? I'm pretty sure steel rebar is still widely used in construction, any experts out there?

Tony

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

One wonders if the lightweight aerated thermalite concrete blocks with a high insulation value, that replaced the concrete breeze blocks in house building, will suffer the same problems with water causing structural degradation. The internal side of cavity construction, where they are used, takes all the stresses in the construction, the outer brick is just mainly decorative. Is this another time bomb waiting to happen. Dave W

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

What really gets me is that they have no records of where the stuff was used.

Nothing more than a "grade A" farce as regards how this has been handled.

Regards

Gray,

The stuff they cobbled together and poured in the war still seems to be fine, lots of gun emplacements and storage huts still around

The stuff Adolf poured into all his dastardly engineering seems to almost indestructible...

Only in Britain could we get soluble concrete...

I think the Grenfell disaster may have saved us from a repeat performance with everything going up now being made of brick again instead of those panel things

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Yes in principle … but the special feature is that this stuff has similar structure to a Crunchie bar in the first place.

MichaelG.

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Image credit is in my album.

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Edited By Michael Gilligan on 02/09/2023 17:56:50

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

The problem AAC concrete is caused by the fact that in manufacture it is aerated. Cut it in half and it looks like Aero chocolate, but does not taste so nice. It has a number of benefits on which it was sold.

Lightweight: It floats in water.

Cheap to produce.

Great thermal efficiency, 

Great noise insulation value, so ideal for multi occupancy dwellings.

Can be easily cut if necessary.

However in their haste to use these characteristics some body in just about building design/architects practice forgot to check and compensate for its reduced compressive (load bearing) strength.

Edited By JasonB on 03/09/2023 06:51:32

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

AAC has no problems whatsoever. when it is used in compression (walls etc.), not in tension (roofs). It's still commonly used throughout Europe with no issues. RAAC was used, mostly in the UK as a cheap, quick, fire-resistant construction material with a limited lifespan.

Had the beams been sealed against humidity, or had a (more expensive) stainless steel reinforcing bar been specified, the stuff would be good for centuries. As it is, the problem's been known about for thirty years and some buildings have been replaced or patched because of that knowledge.

I might be getting to a certain age, but I'm getting increasingly irritated by the likes of the BBC dumbing down engineering stories and getting them hopelessly wrong in the process.

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….

Looking at some of the pictures I’ve seen of the stuff online it looks more like what I would term a cement block. I was always under the impression that “Concrete” had aggregate of some type in it, normally very noticeable size stones. Looking up “Aerated Concrete” though you get descriptions like: “It is composed of quartz sand, calcined gypsum, lime, portland cement, water and aluminium powder”. No mention of aggregate at all.

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

This Wikipedia page about AAC is worth a look: **LINK**

https://en.wikipedia.org/wiki/Autoclaved_aerated_concrete

and it includes a link to the RAAC page.

MichaelG.

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P.S. __ This page [which is linked from the above] provides a history of the development of AAC

https://web.archive.org/web/20101104001651/http://www.hebel.co.nz/about/hebel%20history.php

Edited By Michael Gilligan on 03/09/2023 04:38:29

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

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

MichaelG.

Is RAAC failure the same as "concrete cancer"? My understanding of the current problems of with RAAC is water ingress leading to corrosion of the steel reinforcement.

Concrete cancer is a chemical reaction within the concrete, alkali vs silica(?). This causes spalling of the surface of the structure that, in turn, eventually exposes the reinforcement which then corrodes.

The media seemed to have latched on to the term concrete cancer when it is, in fact, another mode of failure.